Timber Bridges & Foundations Forestry Commission
Content
INNOVATIVE TIMBER ENGINEERING FOR THE COUNTRYSIDE - InTeC
Timber Bridges and Foundations
Timber Bridges and Foundations A report produced for the Forestry Commission
PREPARED BY:
G Freedman - FCE (InTeC chairman) C Mettem, P Larsen, S Edwards - TRADA Technology T Reynolds, V Enjily - BRE November 2002
BRE Ltd, Bucknalls Lane, Garston, Watford, WD2 7JR 01923 664000 TRADA Technology Ltd, Stocking Lane, Hughenden Valley, High Wycombe, HP14 4ND 01494 563091 Forestry Civil Engineering, Greenside, Peebles, EH45 8JA 01721 720 448
© Building Research Establishment Ltd 2002 © TRADA Technology Ltd 2002 © Forestry Civil Engineering 2002
Front Cover Picture - Footbridge at Garpenburg, Sweden, with timber caisson foundations (photo BRE)
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EXECUTIVE SUMMARY Bridges are one of the highest forms of civil engineering - few other structures command the same combination of functionality and visual impact. In the United Kingdom bridge building in timber has been very limited. This is in marked contrast to the initiatives which have taken place in North America (USDA Forest Service Timber Bridge Initiative), Canada, and Northern Europe (Nordic Timber Bridge Programme). World-wide, the use of timber for bridges is experiencing a major revival. In most industrialised countries other than the UK, timber is widely and increasingly being used, for vehicular, as well as for pedestrian bridges. The strength, lightness in weight, energy absorption and environmental features of timber make it highly desirable for bridge construction. Although there is an established history, and a continued use, of timber for bridges in the United Kingdom applications tend to be limited both in span and capacity, than is merited by the virtues of this aesthetic, sustainable material. Experience elsewhere in the world is showing that with correct design, timber is also a durable material for vehicle carrying bridge structures and, additionally, piled foundations. Nevertheless, this aspect remains a significant query in the minds of many mainstream designers, both engineers and architects, who advise UK clients. Revitalised timber bridge activities elsewhere are impressing UK specialists. Nevertheless, there is a great need to disseminate awareness and knowledge to mainstream designers, commissioners of projects and the public. At present, timber bridge producers in the UK are a small, niche sector of the UK timber industry, and some firms are really only representatives of producers that are adding the main value elsewhere in Europe. Timber engineers have the expertise to provide aesthetically exciting, well-protected, and durable bridge structures. To achieve impact, economic drivers must be harnessed, to unlock consumer and specifier indifference. Key motivators include: • • • • • • •
National cycle routes City regeneration, calling for aesthetically exciting, well-performing links. Canal and rail regeneration Marina and dockside development Housing developments, with associated bridging needs. Forest roads and infrastructure maintenance in remote regions. Linking to value-added forest products.
The use of sustainably grown and locally produced timber for bridge, foundation and sea defence engineering will increasingly be seen as favourable. In addition there are concerns and moves in Europe away from the use of timber treatments such as creosote and Copper Chrome Arsenic. Applied research and development, demonstration projects, and benchmarking involving the use of domestic grown timber are seen as vital. Above all, however, well-informed promotion is recognised as of paramount importance in unlocking demand for timber bridges as flagship projects in sustainable development, environmental protection, and improvements to the quality of life.
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Contents
EXECUTIVE SUMMARY 1.0
INTRODUCTION AND PROJECT BACKGROUND
1
2.0
THE HISTORY OF TIMBER BRIDGES
5
2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10
Bridges in ancient history Mediaeval bridges The Renaissance and the growth of trade Long spans - the triumphs of bridge carpentry The dawning of industrialisation Laminated timber - from mechanical to reliable adhesive technology The railway era Protective design lessons from history Maintenance of historic timber bridges in Britain New materials
5 6 6 7 8 8 9 11 11 12
3.0
THE OVERSEAS DEVELOPMENT OF TIMBER BRIDGES
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3.1 3.2
Relevant history The way forward to make use of international research
14 18
4.0
CURRENT UK POSITION
20
5.0
CATEGORIES OF TIMBER BRIDGES
24
5.1 5.2
Categories of use Locations
24 24
6.0
STRUCTURAL FORMS
26
6.1 6.2 6.3 6.4 6.5 6.6
General Beams, including bowed types, no arch action Arches Girder beams & trusses Lift & swing bridges Further design fundamentals
26 29 29 29 29 29
7.0
MATERIALS
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7.1 7.2 7.3 7.4
Principal elements Decks & decking – UK current practice Parapets & handrails Connections
31 36 37 38
8
DURABILITY
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8.1 8.2 8.3
Detailing Natural durability Preservative treatments
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9.0
TIMBER FOUNDATIONS
47
9.1 9.2 9.3 9.4 9.5 9.6
History and overseas Use Durability of timber piles Traditional timber species and treatments Marine structures Pile driving and design Other geotechnical uses for timber
47 48 49 49 50 52
10.0
BRIDGE DESIGN PRACTICE
55
10.1 10.2 10.3 10.4
General practice for design of bridges in the UK Deflection limits Eurocode 5 Overseas practice - Decks:
55 60 60 61
11.0
FUTURE CHALLENGES
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11.1 11.2 11.3 11.4 11.5
High efficiency composite materials New adhesive bonding technologies Steel reinforced timber Timber concrete composites Deck protection systems
65 65 66 66 66
12.0
PRIORITY WORK AREAS
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12.1 12.2 13.0
Innovative Timber Engineering for the Countryside prEN Eurocode 5, Part 2 CONCLUSIONS
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REFERENCES AND BIBLIOGRAPHY
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1.0 INTRODUCTION Bridges are one of the highest forms of civil engineering - few other structures command the same combination of functionality and visual impact. In the United Kingdom bridge building in timber has been very limited. This is in marked contrast to the initiatives which have taken place in North America (USDA Forest Service Timber Bridge Initiative), Canada, and Northern Europe (Nordic Timber Bridge Programme). World-wide, the use of timber for bridges is experiencing a major revival. In most industrialised countries other than the UK, timber is widely and increasingly being used, for vehicular, as well as for pedestrian bridges. The strength, lightness in weight, energy absorption and environmental features of timber make it highly desirable for bridge construction. Although there is an established history, and a continued use, of timber for bridges in the United Kingdom applications tend to be quite limited - although some very fine short span timber footbridges are constructed. Experience elsewhere in the world is showing that with correct design, timber is also a capable material for vehicle carrying bridge structures and, additionally, piled foundations. Nevertheless, this aspect remains a significant query in the minds of many mainstream designers, both engineers and architects, who advise UK clients. Revitalised timber bridge activities elsewhere are impressing UK specialists. Nevertheless, there is a great need to disseminate awareness and knowledge to mainstream designers, commissioners of projects and the public. At present, timber bridge producers in the UK are a small, niche sector of the UK timber industry, and some firms are really only representatives of producers that are adding the main value elsewhere in Europe. To illustrate the extent of use elsewhere, the United States Department of Agriculture reports that approximately 41,700 road bridges of over 6 m span are made of timber, and improvements are continually being introduced, through the federal Highway Administration Timber Bridge Programme. Also in North America, a number of significant modern timber bridge innovations were first introduced in Canada, in the 1970’s. These included the stressed laminated deck, details of which were added to the Ontario Bridge Code at that time. Since then, use of the material has continued, and the technologies have further improved, with several additional innovations such as new types of structural deck, and prefabrication systems. North American experience has been that in situations where salts and other de-icing chemicals are extensively applied, modern timber bridges are more durable than concrete structures. In Finland, about 700 timber bridges are owned by the Finnish Road Administration, and along with other Nordic countries, (Denmark, Finland, Norway and Sweden), a development programme has been in progress since 1994, to extend relevant techniques and experience. Due to the investments into research activities applied to timber bridges in the Nordic Region, and the increase of the general interest in the use of natural materials, timber bridges have become again a real alternative in bridge engineering (Figure 1). In continental Europe, particularly but not exclusively the alpine regions, impressive modern bridge structures are also to be seen, and major contributions have been made to developing harmonised codes and guidance documents, spearheaded by the new bridges Eurocode itself.
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Figure 1: Modern timber road bridge - Evenstad, Norway; 5 spans of bowstring trusses; 180m total length; creosote treated pine glulam; internally flitched steel gusset plates, attached with stainless steel dowels. (photo CM/Trada)
The advantages of timber for bridges is also recognised by quite a large number of emerging countries, such as the West African territories, notably Ghana; countries in Central and South America, as well as a number of Asian regions. In developing countries, the revival of interest in timber bridges in the fully industrialised zones of the world encourages a futuristic view, rather than a “poor material” attitude. For those with rapidly growing populations, this is eminently appropriate, not only from an environmental viewpoint, but also in order to be able to avoid expensive imported technologies and materials. Bridge clients, engineers and architects are beginning to become aware once more that bridges using this traditional material can be designed, fabricated and constructed in exciting new ways, as well as being created in forms sensitive to past traditions. Developments such as new, efficient connection techniques, and the introduction of modern wood-based composites which can be preservatively treated in environmentally acceptable ways, are further encouraging innovations. To re-establish timber bridges in the UK, a great deal needs to be done, especially in terms of “Knowledge Re-Packaging” and technical dissemination. Architecturally
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pleasing solutions need to be backed up by the ability and confidence to provide good protective design measures and to overcome prejudices about lack of longevity. Modern timber bridges need to be seen as more than just a routine, and possibly poorer alternative to concrete or steel bridges. Timber is a renewable construction material with impeccable "green" credentials. Trees, while they grow, absorb carbon dioxide and release oxygen. 1 cubic metre of dry softwood represents around 611kg of carbon dioxide that has been removed from the atmosphere. In addition, forests also provide areas for wildlife and recreation. Timber is light to transport, easy to handle and work with on site, and has a natural empathy with the landscape. To ensure a viable future timber supply chain for engineered, exterior structures, including bridges, the industry needs to grow both the high-profile, spectacular projects, and also the bread-and-butter access structures and smaller bridges that are of great amenity and community value. Producers and advocates of timber bridges also need to establish, sustain and grow their abilities to meet exacting performance requirements, in terms of safety, serviceability, and design life, as well as providing client satisfaction through elegance, tactility, warmth and craftsmanship. Contractors, looking for rapid delivery, and even faster erection, seek standard solutions. The importance of minimising road or track closures is paramount, and competing answers, especially steel footbridges, are fully geared up to these demands. Softwood timber production in the UK has doubled in the last 10 years and is about to double again in the next 10 years but pulp, paper and board markets are so saturated that new markets need to be developed. The structural market is poorly penetrated by home grown products and timber is the UK’s second biggest import. The focus of this research is to utilise poorer quality home grown timber for the high quality structural market. The timber-housing sector is growing steadily but needs a boost and any structural developments will be welcome. Rural structures e.g. bridges, towers and crash barriers are high profile uses will help move timber into the public eye and act as a catalyst for other developments. Research is essential to support these uses in the UK. To compound an already serious situation the value of the raw material has dropped dramatically over the last 5 years and this has led to a drop in harvesting and a general weakness in the industry at a time, ironically, when Forestry has been granted ‘Industry Cluster’ status in Scotland. This means that it is one of the country’s 5 ‘core industries’ employing a large number of people and as such, requires to flourish for the sake of the economy. These factors point to the desperate need for the creation of new initiatives, in the knowledge that they will be well supported by Government agencies. Innovative Timber Engineering for the Countryside: Against this background Forestry Civil Engineering (FCE) of Forest Enterprise (FE) and the two major players in timber research, Building Research Establishment (BRE) and Timber Research and Development Association (TRADA), came together to gather ideas. It was during initial meetings that agreement was quickly reached on the focus being “Countryside and Highway” and that to utilise lower quality home grown timber in high value added products “Engineering” would be required. Innovative ideas for research projects were put forward and immediately some common factors surfaced. Timber has some very desirable properties but it is relatively low in stiffness compared with steel. In the interest of sustainability and optimal utilisation of existing forest and woodland resources, there is a desire to include the use of lower grade material. This led us to accept that composites with
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steel, high quality timber or fibre reinforced polymer composites (FRPs) would be required to develop a product in which timber could display its best value. The objective of the InTeC project is to stimulate by research and demonstration the use of timber for road bridges as well as pedestrian traffic, and the use of timber for other related civil engineering including abutments, retaining walls and foundations.
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2.0 THE HISTORY OF TIMBER BRIDGES 2.1 Bridges in ancient history Timber is a traditional bridge building material, with examples in authenticated records dating to as long ago as 600 years BC. It is suspected that even before this, ancient cultures, including those in China, Persia, the Asian subcontinent and around the Mediterranean rim, had quite sophisticated timber bridge structures. Roman bridges are recorded in works quite accessible today. Julius Caesar himself, for example, records a large timber bridge in Italy, whilst Padillio (1518 – 1580) discusses another big bridge which was used by the Romans to cross the Rhine into Germany. There is also some evidence that the Roman bridge in London was by no means a crude or simple structure (O'Connor, 1993). One of the largest and best documented of the Roman timber bridges was built over the Danube, in what is now Bulgaria, in 104 AD. This is often known as “Trajan’s Bridge” (Figure 2), because its images are recorded on Trajan’s Column, now standing in Rome. This bridge consisted of 20 piers up to 45m high, each joined by a semi-circular timber arch of about 52m span. The thrusts in the triangulated timberwork, correctly transmitted into the masonry piers according to modern engineering concepts, seemed to be fully understood by the Roman engineers, who constructed and rapidly erected this prodigious feat. Methods of timber conversion and treatment for durability were also recorded in contemporary Latin texts.
Figure 2: An arch of Trajan's bridge, modelled by architectural historians, Florence University. (photo CM/Trada)
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2.2 Mediaeval bridges The oldest timber bridges that still exist in Europe date from the late mediaeval period, that is from the 14th to the early 16th century. Many of these are covered bridges, owing their longevity to this simple structural protective device. Several examples of these ancient bridges are in Lucerne, for example the Kapell bridge and the Spreuer bridge. Other such timber bridges, which are very important from both the historical and also the technical point of view, are those built by the State of Berne during the 16th century. These include bridges at Neubrugg 1532, Gummenen 1555, Wangen 1559 and Aarberg, 1568. These are still in good condition, most of them having their original main elements, and some still carrying heavy traffic. It is to be emphasised yet again that all of these bridges follow the same structural principle; that is protection of the timber against direct wetting from rain, sleet and snow, by means of a duopitched roof with a large overhang.
2.3 The renaissance & the growth of trade A large number of timber bridges which are still on record, and sometimes still in use, were built from the 16th through to the 18th century, when increasing trade and transport needs resulted in the construction of new and better roads. As a result of the beginnings of an understanding of engineering principles, during the spread of learning after the Renaissance, more technically advanced designs began to appear, and new construction techniques were introduced. These included arches, trusses and suspension bridges. Palladio, mentioned earlier as the recorder of Roman bridges, also documented and illustrated a series of his own “Inventions”. The sites of some, such as the oftenillustrated Cismone Bridge, have been rediscovered and archeologically investigated. Less well known are some ably-conceived timber trussed arch bridges, also by this same influential architect. Leonardo da Vinci, (1452-1519) Italian painter, sculptor, architect, engineer and scientist, was one of the greatest figures of the Italian Renaissance. He was active in Florence, Milan, and, from 1516, in France. Amongst his design sketches and notes are a series of ingenious timber bridges, several of which have been modelled in recent exhibitions of his life and works (Figure 3).
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Figure 3: Bridge designs by Leonardo da Vinci (photo CM/Trada)
2.4 Long spans – the triumphs of bridge carpentry During the 18th century, very long timber bridge spans were achieved through the use of arched trusses. Typical European examples include a Rhine bridge, constructed at Schaffhausen in 1758 by Hans Ulrich Grubenmann. This had an overall span of 119m, with the construction including a redundant pier at mid-span, which this famous bridge builder was obliged to include at the behest of the dubious “Bergermeisters” of the commissioning town. The structure had laminated arched ribs, each with a depth of about 2 metres and comprising seven courses of timber, notched and banded together. This same pioneer of timber bridging constructed a number of other impressive structures, all of which had complex end-jointing details, and many other advanced features. Expansion of trade and business in North America also gave rise to some very large timber arched spans, one of the most noteworthy being the “Colossus Bridge” over the Schuylkill river at Philadelphia, USA. This was constructed in 1812 by Lewis Wernwag, and had an amazing free span of 340 feet (102 m). The laminated arch elements each comprised six 6 x 14 inch (150 x 350 mm) heart-sawn baulks of softwood, separated, but strongly linked together with iron bands and threaded rods.
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2.5 The dawning of industrialisation The next stages in the evolution of timber construction saw a gradual transition from carpentry to engineering principles. This entailed the greater use of metallic fasteners in the form of bolts, rods, spikes, straps and other devices, such as special keys. These developments also involved the greater use of side-lapped members, rather than members intersecting in a single plain through mortises, tenons and other such carpentry joints. There was also an increasing reliance upon triangulation, and in some instances standard designs accompanied by published calculations. Truss systems started to be introduced for timber bridging, particularly in North America, where the European custom of roofing timber bridges had been adopted. Entrepreneurs such as Palmer, Town, Long and Howe introduced “Patented” Truss Systems. Town and Howe trusses in particular were very successful, owing their popularity to their simplicity and ease of construction from a relatively standardised range of member sizes. Many covered bridges of these types have remained in use in North America for over a hundred years. They are now regarded as part of the historical industrial heritage, and even have “Preservation Societies” dedicated to their upkeep. A few bridges of the Howe type were also built in Europe, and some of these too remain in use. Although the records are generally difficult to obtain, it seems likely that early suspension bridges used timber walkways and support gantries, along with other natural materials as the cables and suspenders. Such bridges must have pre-dated arches and trusses, but by their nature they would have been regarded as less permanent affairs. However there do exist 19th century photographs by Forrest, the Scottish plant collector, of suspension bridges in China, using timber and other materials, which are probably directly similar to centuries old designs. It is also evident that the suspension bridge goes back long into history, from some of the forms of such bridges that are still built in remote regions of Asia and the South Pacific, without the benefit of any metal parts or cables. In the 19th Century, impressive suspension bridges created very long spans using steel cabling along with stiffening trusses and decking in timber. A good example is the footbridge in Ojuela, Mexico, which was built in 1892. This has a span of 278m, and is still in use today. Through European development aid, particularly from Switzerland, impressive modern steel and timber suspension bridges, for which a series of design manuals is available, have been constructed in Nepal. The mountainous terrain, use of pack animals, and extreme inaccessibility of some regions, makes these structures a continued necessity of life.
2.6 Laminated timber – from mechanical to reliable adhesive technology Very early applications of mechanical laminating are discussed by Newlands (1857). For example, he cites the knowledge, on the part of Col. M. Emy, in France, commencing in 1819, of the much earlier mechanical laminating system of Philibert De Lorme. Newlands also discusses a report for the Society for the Encouragement of National Industry in 1831, by Emy, publishing his laminating inventions and techniques. He illustrates a roof for a shed at Marac, near Bayonne, and a “ridinghouse” (cavalry training structure) at Libourne. Newlands then shows a Gothic church roof at Grassendale, near Liverpool, which he states followed the Emy system. It is Nov 2002
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not at present known whether this still stands. He also gives quite elaborate details of various forms of bending apparatus, manufacturing for curved roof laminations (“of the bending of timber”). The Timber Development Association (forerunner of TRADA) historic photographic archives contain several examples of mechanically laminated worsted mills, in the Bradford region of England. Booth (1964) discusses mechanically laminated railway station roofs, such as GWR, Bath (by Brunel), as well as dealing extensively with railway bridges, as indicated below. James (1982) provides densely annotated lists of potential primary sources for those able to pursue early American and other international (e.g. Russian) mechanically laminated bridges. Developments in the use of glued, as opposed to mechanically laminated timber, began surprisingly early, and in Europe it was established by the start of the 19th century. During 1807 – 1809 a Bavarian engineer named Wiebeking developed horizontally laminated timber arch bridges with spans of up to 60 m. Most of his bridges used very thick iron bolted or rod-connected laminations of oak. However in 1809, the first major glued laminated timber bridge structure was built by Wiebeking, at Altenmarkt. This had ribbed laminations fabricated in situ, working (presumably with great difficulty) from scaffolding and temporary piling. Thinner spruce boards were used for this bridge than with the mechanically laminated oak types that he had built previously, and there was an appreciation of the benefits of staggering end-joints, relative to adjacent laminations. Further evidence of the well-established nature of glulam is the mid-nineteenth century Congregational Sunday School roof in Manchester, 1864, documented by Booth (1971) and surviving until demolition in 1963. A former schoolroom, now used as the Southampton Register Office, in Southampton, 1860 is documented by the GLTA, and is also corroborated by Booth, as the earliest known use of glulam arches in a building. Yeomans cites the “German Gymnasium” in London as another stillstanding structure with more than one hundred years service. Private correspondence and photographs, courtesy of P. J. Steer, show a nineteenth century music hall in Nottingham during recent restorations, that is glulam roofed, and still in use. By the start of the twentieth century, patents were being taken out for glulam in Germany. In Switzerland, certain structures, laminated with casein adhesive, were constructed that still stand today (Chugg, 1962). In 1939, in the USA, a landmark technical publication appeared that strongly influenced subsequent North American codes. This was entitled “The glued laminated wooden arch”, by T. R. C. Wilson (1939) of the USDA. Evidently, glued laminated softwood bridges were well established by then, since a footbridge of such construction is included in this reference.
2.7 The railway era The civil engineering construction associated with the rapid 19th century development of the railways made extensive use of timber bridging. Some of the finest examples included Brunel’s designs, although there were also other successful British railway pioneers using timber for bridges and for other structures, including the Greens (John, and his son Benjamin) in the North East of England. For the Newcastle and North Shields Railway, these engineers continued the use of mechanically laminated timber for structures such as Ouseburn Viaduct.
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Returning briefly to the famous Isambard Kingdom Brunel, there is only space to say that he made extensive use of timber for many railway viaducts, which were built across the valleys of south-west England and South Wales. Surprisingly sophisticated concepts were involved, including the use of timbers that were preservatively-treated using chemicals applied under pressure. An early process of this type was Kyanising (1832, Kyan’s patent, using chloride of mercury, Newlands p. 106). Brunel planned his designs to allow maintenance to be carried out on these structures without interrupting the passage of trains. He built forty-three viaducts in Cornwall alone, spanning a total of eight kilometres. The last of Brunel’s timber railway viaducts were only dismantled in South Wales the 1930’s, and generally these structures were replaced only to construct bridges able to carry much heavier traffic, rather than because of deterioration through decay. At Barmouth estuary, in North Wales, a timber railway viaduct designed and constructed according to similar principles remains in use today (Figure 4), with pitch pine piles having been replaced by the extremely durable greenheart timber.
Figure 4: Barmouth Bridge - one half mile long timber trestle pile viaduct completed in 1867, the only large timber viaduct in Britain still in use. It spans the Mawddach estuary on 113 short spans. There are two steel girders at the north end, one of which used to swing to allow ships up river. For a wealth of further information on timber railway bridges in England in the nineteenth century, the copious and scholarly work of Booth (1996) is an essential starting point for the serious historian as it contains many secondary reference sources, including Booth’s own. These would lead to many prime source references, many of which are available in UK libraries such as the Science Museum (Imperial College) and archives such as those of the Great Western Railway. As mentioned above, the “Kyanising Process”, and similar methods, were known to Brunel’s contemporaries. Other chemical treatment processes of that era are described by Newlands. These include, for example, Sir William Burnett, 1838, chloride of zinc; Payne’s patent. 1841, sulphate of iron and muriate of lime; the early use of tars and essential oils (Newlands cites as an example a 1737 patent by one Alexander Emerson); Bethell, who gave the basis for modern creosote treatments, in 1838.
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Early patents for American bridges, during the “Palmer, Town, Long and Howe” era, often involved the co-incident publication of patents concerning timber treatments. Later, in North America, in the 1930s, the widespread industrial introduction of pressure preservation processes, using substances such as creosote, is said to have led to an expansion of the use of timber for large truss and girder bridge forms. An impressive example was constructed at Sioux Narrows, in Kenora, Ontario, where large, preservative treated Douglas fir members were arranged in a box-Howe Truss pattern, to create what was for many years the world’s longest single-span highway traffic bridge. At 64m main span, this bridge still remains in service.
2.8 Protective design lessons from history Although decay has always been one of the factors affecting the service life of timber bridges, they have more often been destroyed by war, natural disasters and fire. It is known from the durability records of ancient timber churches, cathedrals and houses, as well as roofed bridges, that preserving timber structures with adequate protective design measures considerably reduces decay risks. The importance of good protective design detailing is a lesson from history that cannot be emphasised too strongly, in the context of modern timber bridges.
Figure 5: Good protective design features - in 1,000 year old Norwegian stave church. Left; Stone sill elevates timber ground sill, sacrificial boarding protects exterior of corner posts and cladding. Right; Elevated post base, drip moulding at cladding bottoms. Note also extensive use of pitch preservative in both cases. (photo CM/Trada)
2.9 Maintenance of historic timber bridges in Britain Many historic timber bridges may still be found throughout Europe, including Britain, and fortunately, nowadays, restoration work is undertaken to preserve them. The
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continuous maintenance and replacement of timber elements, using like-for-like carpentry, in the Lucerne covered bridges has already been mentioned, as has the restoration work on Barmouth viaduct. Until quite recently, the Scottish East Coast main line railway crossed one of the few surviving timber viaducts, over a peat bog, near Inverness. The live track no longer passes over this structure, which has, however, been conserved. In 1915, John Saner, engineer to the Weaver Navigation System in North West England, designed a structure known as Dutton Horse Bridge. This has twin elliptical spans skewing across the River Weaver’s sluice channel, which leads eventually to the Mersey. The bridge is constructed from mechanically laminated greenheart, and is believed to be the oldest such structure remaining in service in the world. Greenheart planked caissons and many other impressive engineering features are to be found in and around this outstanding structure. The durability of this structure is unsurprising however, since greenheart is renowned for its longevity. In Guyana itself, its country of origin, marine jetties of over one hundred years in age stand, even in such adverse, warm sea-water and termite ridden conditions, whilst the cathedral of Georgetown is claimed to be the tallest (greenheart) nineteenth century ecclesiastical building in the world. Two examples of King post truss vehicular bridges in the river Spey region of Scotland were used as lecture examples by TRADA until quite recently, and were said to be still in service. There has not yet been opportunity to verify their current status.
2.10 New materials Following the epoch making construction of Ironbridge in Shropshire, England, (with a framework arrangement based on contemporary timber designs!) the 19th and early 20th centuries saw the rapid spread of the industrialised use of the ‘new’ materials, iron, steel and later on, reinforced and pre-stressed concrete. This completely altered the concepts of bridge construction, making the increased requirements regarding longer spans, larger roadways, and higher loads achievable with ease. However with higher frequency, heavier traffic, and the need to guarantee an all-year-round use of the roads, problems have arisen with these ‘modern’ materials. Besides faults due to inadequate design and execution, which may happen with all materials, high maintenance costs have been incurred as a result of the use of salt as a de-icing agent on roads. This has caused corrosion problems with reinforcing bars and prestressing steels in concrete bridges, as well as deterioration of paints and member surfaces in steel structures. The lesson has gradually been learnt that adequate protective measures against direct and indirect hazards of the climate are also necessary for these so-called “durable” materials, and that such measures invoke considerable penalties in terms of whole-life costs. The complete replacement of quite new bridges necessitated by poor durability has also demonstrated the high costs of dismantling concrete structures. In the face of this situation, timber engineers have recognised new opportunities, proposing their material to solve some of the problems that bridge engineers have been encountering. Timber engineering itself also has become armed with “new” materials, several of which have very high performance, low variability and excellent reliability, thus offering additional advantages over the “traditional” version of solid sawn timber.
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3.0 OVERSEAS DEVELOPMENT OF TIMBER BRIDGES The USA, Canada, Australasia and the rest of Europe are well ahead of the UK in the design and production of “modern” timber bridges: • • • •
Glulam timber and transverse decks Longitudinal glulam decks Dowel-laminated, longitudinal panel decks Stress laminated decks
A common factor between modern bridge designs is load sharing through composite action which distinguishes them from the old “stick” designs. Timber for bridges has advantages over other structural materials which have been recognised overseas but ignored in the UK. Timber is:• • • • • • • • • •
Durable and long-lasting - with modern treatments bridges are expected to last at least 50 years. Simple construction ~ Construction usually demands low skills and simple equipment available locally. Maintenance is also within the scope of local labour Prefabricated Components ~ Modern timber bridges are either entirely factory made or factory component manufactured thus assuring good quality Wood has high strength to weight ratio ~ This saves in foundations and gives confidence to reuse old foundations. Crane loadings are reduced and money is saved. Competitive ~ Small span rural bridges can be built in timber at a significantly lower cost than from steel or concrete. Aesthetics ~ Timber is natural and is appreciated by everyone and looks as good in the countryside as in an urban location. Chemically stable ~ Timber is not affected by de-icing salts as is steel and concrete Expansion ~ Timber does not expand and contract much with heat so road surfaces can be continuous over them without the need for troublesome joints. Renewable and Sustainable ~ This is important to the economy Removes Carbon from Atmosphere and locks it on the Ground ~ This is of ultimate important in today’s environmentally conscience world.
There is a lot of catching up to do and much development is needed to enable modern bridge ideas to be imported to the UK and then assimilate them to UK practice, codes and materials.
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3.1 Relevant History Scandinavia: Around 1990 the Norwegian/Nordic Timber Council took steps to plan the introduction of more timber bridges to the public road network (notable examples are shown in Figures 6 to 8). Their reasons were not in the first instance economic but more to use indigenous materials and later to assess the whole value when practice had produced the best solutions. Otto Kleppe, Chief Bridge Engineer for the Government in Oslo, travelled the world to study old timber bridges in order to gain insight into efficient design and durability. He learned lessons from 100 year old covered timber bridges as well as the latest forms of modern stress laminated decks. He returned to Norway and has engineered the development of some remarkable new timber structures, not only on the public road network, but also over motorways. Norway is fortunate in having vast reserves of very high quality timber available which makes the task of producing elegant long spans much easier. The Norwegians have worked on many fronts with a view to providing a full range of timber bridge solutions and included experimental work with preservatives. A very successful design is a combination of longitudinal glulam beams with CCA treatment subsequently stress laminated and treated with creosote. They have developed very high quality jointing systems which permit large king post and truss structures and have innovative ideas allowing timber crash barriers. The high quality structures are protected using copper sheeting on the structure and bitumen compounds on the deck. In both Norway and Sweden simple stress laminated decks are factory made for minor road and forestry road bridges. These low cost options are treated with preservative but not protected in any other way. Even with a shorter life these structures will have a very competitive whole life cost.
Figure 6: Vihantasalmi bridge, Finland - Glulam king-post trusses each spanning 42m; composite concrete-steel-glulam deck. (photo Nordic Timber Council)
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Figure 7: Evenstad bridge, Norway - glulam truss beams each spanning 36m with stress laminated timber deck. (photo Nordic Timber Council)
Figure 8: Sinettäjoki footbridge, Finland - glulam king post trusses spanning 18.8m with lumber deck. (photo Nordic Timber Council)
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USA and Canada: Timber, which was readily available in enormous quantities, played a major role as a construction material in the development of North America since the early pioneer days. Indeed, it is estimated that by 1900 half of the total forest area of the continent was felled. Timber trestles were used extensively to span gorges and rivers for the transcontinental railways. Many century old covered road bridges are still in service and are considered to be heritage items. With a main span of 64m, the Sioux Narrows bridge, built in 1936 in Kenoria, Ontario, is one of the worlds longest single span wood highway bridges (Figure 9)
Figure 9: Sioux Narrows Bridge - Howe trusses formed from solid sawn Douglas fir (photo Canadian Wood Council) Currently in the USA there are nearly 600,000 bridges, 7% of which are timber and a further 7.3% have timber decks. Recent studies have shown that 240,000 of these bridges are classified structurally deficient or functionally obsolete. This critical state of affairs prompted Congress into introducing the Timber Bridge Initiative (TBI) in 1989 and another similar programme which promotes demonstration bridges, research and information transfer. Under the programme a 50% grant in available to build a bridge which demonstrates modern technology. Timber structures declined in number from 50 years ago when large trees became scarce and concrete technology became reliable. However the modern materials, concrete and steel, have not been without their problems and since the middle 70’s much research effort has been undertaken to utilise smaller wood sections to build large structures. It was in Canada in the 1970’s the real pioneering work was carried out on the stress-laminated decks which has become important to the new bridge initiative in the USA and could become a very useful concept in the UK. Timber engineering and technology has benefited greatly through these programmes and many hundreds of bridges have been built although much remains to be done.
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There are so many avenues of help in the USA especially as these initiatives were taken by government agencies but personal contacts will be crucial to ensure the accelerated programme necessary in the UK, in order to catch up.
Australasia: In Australia and New Zealand the increased production of plantation timber has generated a modern timber engineering industry. The UK should have been shadowing these recent impressive developments. Many are the same as those in America and are necessary in the UK to catch up and create some high value markets for our new increasing production. Although there are many initiatives in transportation structures, there are also significant ideas in building, from which UK practice could benefit.
Developing World: There has been substantial experience involving the UK timber research organisations, and TRL (see website http://www.trl.co.uk/bridges.htm), in the overseas development uses of timber for bridges. These have been carried out through assistance provided via organisations such as United Nations Industrial Development Organization (reported in Anon 1985) and the UK Department for International Development (DFID, formerly ODA). For example, prefabricated modular timber road bridges have been successfully introduced into a significant number of developing countries on four continents. The first of a series of standard designs for modular timber road bridges was prototyped in Kenya, some thirty five years ago. Further development work continued in Central America the Far East and elsewhere. This development included contributions of local expertise, and associated professional training. Similar road bridges are still being produced, in accordance with well-tried design manuals and drawings, using local timbers and labour, to the great advantage of rural communities in more than two dozen countries, in all of the tropical continents. Extensions to the original designs, and substantially new types of standard design, have subsequently been added.
Figure 10: UNIDO prefabricated bridge (photo UNIDO)
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It has been found that by giving ministry departments, and their associated professionals, renewed confidence in timber in communally important, heavy-duty applications such as these, there is spin-off resulting in an enormously improved usage for smaller-scale applications. These are initiated and executed entirely by communities themselves, on their own initiative. This is another important lesson that the timber industry needs to take on board, here in our own country.
3.2 The Way Forward to Make use of International Research When specific areas of research are identified and funding is in place for individual projects, the initial literature search will be extended using references and contacts. Research partnerships will be explored and when the missing knowledge is identified work will begin. Much will be assimilation of past international work to accord with UK timber species. There are a number of codes of practice in existence which will be of use but climatic conditions, safety regimes, species differences etc will create many transfer problems. The mission of InTeC is not just to carry out the research, solve the problems and show that things can work but also to produce the codes and guidance so that the ideas are taken up. This part of InTeC’s work will be time consuming, but if the past successes of concrete and steel as construction materials are to be emulated then the information circle must be closed. Young designers will not adopt timber unless it is made easy, logical and sensible. Some Specific International Ideas likely to be Transferred to the UK: Stress laminated bridge decks are certain to become a useful solution for minor rural bridges and they are a timely product of recent ability and need. Accurate sawing is essential. Safe bacteriological treatment is demanded and high tensile stressing of steel tendons is the key to the structure. All of these are now available at low cost. The technology level is not high and their production could become a cottage industry. This idea has arrived at a time when UK softwood timber production is about to double again for the second successive decade and funds for rural bridging are low. Abutments for bridges and retaining walls have traditionally been constructed from ‘permanent materials’ like concrete and masonry but the question of ‘life cycle’ needs to be addressed. A public road bridge in the UK is designed for a 120 year life but forestry bridges are designed for 50 years as that is the economic cycle of the industry, being the growth time from plant to mature tree. We have, in the past, expected buildings to last forever given enough maintenance, but supermarket buildings are now financially appraised over 7 years, that being the predictable trading projection limit. Perhaps it is time to look at structures with a shorter life, provided that short life gives a unit cost per year less than the more permanent structure. A timber bridge deck and abutments could be constructed for a 20 or 30 year life, require no maintenance and be replaced within the unit annual cost of the ‘permanent structure’. Appraisal of Timber Structures: Overseas countries have realised that timber structures enhance the environment, local economies, society, aesthetics etc. factors which must be introduced into any appraisal. Energy values are used in appraisals, where the inputs to refine the component materials and the energy to carry out the construction and demolition are evaluated to calculate a whole life cost. Timber structures excel in all of the components of appraisal. A local timber industry creates stable rural employment which tourism can latch onto. Timber structures are popular with rural people and Nov 2002
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their visitors. The western society accepts that timber structures in the countryside make sense. An international research check has proved that we are behind the rest of the developed world in this area of applied engineering. This does, however, mean that with careful planning and study of current research we can catch up quickly and then concentrate our efforts on the most relevant areas. It means that much of our research will be to assimilate ideas developed for other species. The most valuable gain however will be in finding the ways to extend already good ideas. This is much easier when a fresh mind takes up a partly developed piece of work. InTeC researchers bring that quality and with the correct funding many exciting extensions of existing work that could bring benefits to the UK. A programme similar the Timber Bridge Initiative in the USA would be welcomed in the UK and could become the cornerstone for future development while bringing forward the many bridge replacements necessary in the UK.
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4.0 CURRENT UK POSITION Unfortunately, the official bridge design scene, viewed from the position of the average civil and structural consulting engineer in the United Kingdom, includes nothing to encourage the use of timber. There is no British Standard dealing specifically with design. The BS 5400 series only covers steel, concrete, and steel-concrete composite bridges. This absence of a British Standard is thought to have inhibited the specification of timber as the structural medium for many footbridges, as well as having resulted in a number of designs whose performance has not been entirely satisfactory. Although authorities such as Highways Agency (HA) have their own standards, recognising timber in footbridges to a small degree, the absence of a main code of practice and accompanying support standards, is a serious deterrent. Awakening awareness by some influential specialists within authorities of the development of new Eurocodes relating to bridges, is likely to provide better hope for the future, provided that this is seized as an opportunity by the timber industry itself. Timber interests in the UK were extremely impressed by the manner in which the US National Timber Bridge Initiative was launched (USDA 1983), and by its subsequent success. Their programme involves many demonstration timber bridges, together with research and technology transfer. Starting from a relatively small financial basis, it was difficult to see how anything comparable could possibly be started in the United Kingdom. However, there are now some positive signs. Work was carried out about seven years ago, with support by DETR. This led to two preliminary study reports (Mettem 1993) and (Mettem 1994). A pilot project, termed “Innovative Timber Engineering for the Countryside”, has been initiated involving BRE, TRADA Technology, and Forestry Civil Engineering, with support from the Forestry Commission, and this report relates to this particular project. The second positive step is that active work has now been started on an EN version of Eurocode 5: Design of Timber Structures Part 2: Bridges. This is scheduled for issue for public comment in 2003, with the target of a final draft for printing in 2004. The principal Eurocode 5 for timber structures, to which the bridges part refers for all of its main technology, is ahead of this, and has already been strongly promulgated and supported by design guidance, involving TRADA Technology and its various industrial and research partners. BS DD ENV 1995-1-1 was issued in 1994 and is already used in practice mainly by more experienced timber engineering designers. A BS EN version is expected to be published early in 2004, before which, training will be given to all practising designers and new students. Current Requirements: Current requirements for bridges are generally formulated independently of the materials to be used. In general terms, bridge design has to fulfil certain main requirements which can be related to timber and wood-based materials as follows: Load Capacity & Vehicle Clearances: Modern timber bridge designs for vehicular traffic are perfectly possible, and are in fact already being designed and constructed in a number of countries and regions. Appropriately designed timber and composite deck systems can provide for increasing traffic loads. Clearances, given in regulations, are taken into account wherever necessary. For road bridges, vehicle size obviously affects the design of
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both the carriageway widths, and also, in the case of covered and arched bridges, the overhead clearance. Long Spans: Earlier limitations of timber brought about by its availability only in the sawn form no longer apply. Glued laminated timber, structural timber composites - STCs (Mettem 1996) improved strength grading procedures, reliable connection techniques and the use of other materials acting compositely in conjunction with timber, all help to make long spans possible. Roadway Surface Conditions: Normally there is a requirement that there shall be no difference in the surface conditions and levels between the bridge and the connecting road pavements. With appropriate deck systems and sealed wearing surfaces, such requirements can be fulfilled using timber structures. Routing of the Bridge: Modern bridges have to be integrated into the general route-planning scheme. Consequently skew, cambered and curved deck bridges are often required. Such forms are attainable with timber bridges. Recent Developments in Timber Bridge Decks: Timber & Concrete Composite Decks Timber and concrete composite decks have existed in regions such as New Zealand and North America for decades. Early systems comprised nailed laminated decking with un-reinforced concrete and a thin asphalt surface. More recently, thicker reinforced-concrete layers and shear connectors have been added, giving greater composite action. The effective width of the concrete flange is determined as for a concrete T-Section. All of the shear force transmission between the two materials takes place via special, strength-calculated connectors, and not by natural bonds. No tensile strength is recognised within the concrete layer. Some design rules for this form of construction are given in Eurocode 5 Part 1-1, the general design document, whilst supplementary rules are contained within prEN Eurocode 5 Part 2, Bridges. Developments in the composite timber/concrete deck continue, for example: • •
•
More stable laminated timber decking, using post-tensioning systems. More efficient shear connecting systems between concrete and timber, to achieve more reliable composite action during the service life of the deck. Better systems to seal the concrete surface, and to provide protective and hard wearing road surfaces.
Laminated timber deck plates are made of individual laminations which are held together by nailing or adhesive bonding. In the case of pre-stressed plates, discussed below, there is in addition a permanent lateral pressure, which guarantees continued friction between the faces of the laminations, and according to the type of construction, between any un-bonded adjacent faces which may exist between the individual slabs.
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Pre-Stressed Timber Decks Pre-stressing in a timber bridge deck is defined as a permanent effect due to controlled forces and/or deformations imposed upon the structure. The plates are normally pre-stressed by means of steel bars or tendons. Pre-stressed decks in timber bridges first appeared in Canada, where they were introduced as a repair method for nailed timber-laminated decks. The correct choice of materials and specification of moisture contents overcame early problems with loss in pre-stressing force due to timber shrinkage. These forms of deck are now very common in North America, and their use has been spreading to other regions where modern timber bridge developments are occurring. These include Australia, and Finland, Norway and Sweden. The additional step of using glued laminated timber rather than solid sawn timber for the decks was probably taken first in Switzerland. Recently in Australia and Scandinavia, progress has been made in utilising other modern STCs such as Laminated Veneer Lumber (LVL), for which reliable pressure preservative treatment processes have now been developed. Glued laminated timber and STCs are always supplied at low, factoryconditioned moisture contents, and with these, early problems in loss of pre-stressing have been completely overcome. Decks with no re-stressing requirements are being achieved by following design recommendations such as those given in prEN Eurocode 5 Part 2, Bridges. Pre-stressing bars are also now sometimes bonded in, resulting in a high degree of corrosion resistance and good load carrying capacity. Dowel-Type Fasteners & Mechanically Laminated Bridge Structures: The term “dowel-type fastener” is used throughout the structural timber Eurocodes, and in the latest edition of BS 5268 (BSI 2002), to refer to fasteners whose crosssection is essentially of a cylindrically prismatic form, and whose function is to transmit forces in lateral shear between adjacent layers of the timber. In the context of current codes, such fasteners essentially consist of steels of adequate and defined strength, although research is now in progress on the use of non-metallic, and in particular Fibre Reinforced Plastics (FRP) dowels. Bolts, lag screws and plain steel rods acting in transverse shear are all examples of dowel-type fasteners that are commonly employed in bridge design. Over the past twenty-five years, timber engineering researchers have extensively explored the design theories associated with these types of device, and the theories are now well adapted to reflect real fastener behaviour in actual structures. Essentially, the theories depend upon a knowledge of the behaviour of the fastener as a rod-like device, which tends to embed itself elasto-plastically into the surrounding timber. The response of the latter is modelled as a yielding elastic foundation. At the same time, allowance is made for the tendency for the steel of the fastener itself to yield plastically, and to form plastic hinges at various points, whose locations depend upon the exact interface arrangements. The development and use of such theories for the design of dowel-type fasteners, along with methods enabling designers to predict changes in the effective section modulus, due to slip between adjacent layers, has enabled the accurate design of large and impressive modern mechanically laminated timber bridge designs. Mechanically laminated timber bridges are designed and constructed throughout Europe, but are particularly prevalent in the UK, Netherlands and Germany. Very dense, durable tropical hardwood timbers are normally used for these designs, with
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one particular timber , being found to be very successful. This is the hardwood named “Ekki” in the UK, and “Azobé” in Continental Europe (the same species of timber in either case, namely Lophira Alata).
5.0 CATEGORIES OF TIMBER BRIDGES Nov 2002
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5.1 Categories of use Three broad categories of use can be considered, as follows: a) Highway and adopted road bridges. b) Footbridges. c) Footbridges with occasional vehicular access (e.g. farm, golf-course and parkland bridges). Category a) in timber is extremely rare in the UK, and in the main restricted to special and historic structures. The present volumes of use in categories b) and c) are modest, but growing, with opportunities for bridge and associated timber suppliers and engineers, especially in the light of the positive factors mentioned in the introduction. Even in category b) however, there are significant obstacles to the procurement of timber, where influential authorities are involved. The Highways Agency inventory for England, for example, contains only one timber pedestrian crossing bridge over the roads for which they are responsible. It is the intention of the present project to conduct more thorough market research, costing studies and business potential investigations.
5.2 Locations Generically, locations for footbridges and light vehicular bridges can be divided into the following four types of crossing: • • • •
Over roads – general access. Over rivers, canals and other water features. Associated with the leisure industry, various crossings, including the three types above. Over railways– general access.
Bridges associated with alternative modes of transport, such as cycling, might arguably be regarded as a separate category. However, provision of routes and facilities for serious and mundane access to work, education, and other aspects of daily life by such means can hardly be argued to have reached the stage to warrant separation from the leisure category. a) Road crossings Many footbridges are used to provide safe pedestrian crossings. Timber is permitted, as well as steel and concrete. However, the Highways Agency (formerly, through a Department of Transport document, currently undergoing revision) points out the following, in its Standard BD 29/87 (DoT/HMSO 1987): "A footbridge is the least suitable form of crossing for disabled people and should only be provided when other forms of crossing – e.g. a crossing at grade or a subway are deemed to be unsuitable." Timber has only a small share of this market. Furthermore, its share is probably even smaller, as a result of some unfortunate instances of glulam bridges de-laminating during the 1980’s. These were manufactured by firms that were not members of the Glued Laminated Timber Association (GLTA). Lack of independent third party manufacturing control and certification is recognised to have been part of the
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problem. However, the failures achieved notoriety for the timber industry as a whole, causing a major setback to information dissemination and promotional exercises. b) Crossing rivers, canals and other water features This is an important market, with timber footbridges, boardwalks and piers having a sizeable portion of the total. Often the surroundings and environment are such as to suggest the choice of timber as the most sympathetic material. Timber weathers particularly well in marine environments compared with steel or reinforced concrete. c) Associated with the leisure industry. This is generally an expanding and promising market for timber bridges and other landscape features. Example applications include golf courses, theme parks, visitor centres, wildlife and animal sanctuaries and nature reserves. d) Crossing railways Timber has only a very small share of this market. Historically, the extensive facilities for iron and steelwork available to railway builders tended to facilitate the choice of metal in the first instance. There are some modern examples of timber station structures, and a few of timber footbridge crossings. Not all of the engineers concerned are opposed to this material, but railway structural engineers have a cautious approach, and need assistance to specify in performance terms, rather than by prescription. Reorganisation of the administration of the national rail network has also made it difficult, in recent years, to decide where best to focus impact.
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6.0 STRUCTURAL FORMS 6.1 General To describe the majority of footbridge and light vehicular access types, the following five categories of structural form have been devised. The forms refer to the principal structural elements of the bridge: • • • • •
Beams, including bowed types, no arch action. Arches. Girder beams and trusses. Lift and swing bridges. Cable stayed and suspension types.
These five categories of bridge based upon the form of the principal members led to the summary shown in Figure 11 and Table 1 (below). This classification also relates to the usual static system for the principal members, connected in turn to the structural analysis that will be required in the design. Also shown in Table 1 is the form, or shape alternatives for the principal members, and an indication of the materials which are commonly chosen for each of the types.
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A wide range of structural forms is available for bridge solutions in timber. Within this report, the most common and appropriate are broadly categorised into five main types, as follows: a) Beams, including bowed types, no arch action.
b) Arches.
c) Girder beams and trusses.
d) Lift & Swing Bridges.
e) Cable Stayed & Suspension Types.
Figure 11
Five principal types of timber bridge.
(Not to scale).
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Table 1: Structural forms of timber bridge a) Beams
b) Arches
c) Girder trusses
Usual static system
Single simply supported span; flat or bowed (positive precamber), but no arch action.
3-pinned, round & parabolic.
Single simply supported span; Triangulated, e.g. King post trusses. Bow-string trusses.
Additiona l forms
Multiple simply supported spans. Cantilever side spans supporting suspended central span.
Two-pinned, multiple spans.
Multiple simply Single-leaf supported spans. cantilever Multiple bow-strings.
Single or twin towers with side spans.
Form of principal member s
Straight, lightly curved or precambered.
Circular or parabolic (wide range of radii).
Parallel or nearparallel chorded (often Warren or Pratt trusses).
Deck beams straight or tapered
Structura l form
d) Lift & swing
e) Cable Stayed & Suspension
Figure 1
Two-leaf cantilever.
Main (lower) beams straight or singletapered
Timber masts; steel cables/links.
Towers, parallel masts or A-frames.
Balance (higher) beams straight or double-tapered.
Common materials
Sawn timber – softwood or hardwood. Timber poles – natural, debarked or turned/ profiled.
Glulam. Mechlam.
Sawn timber– softwood or hardwood.
Beams – sawn timber – softwood or hardwood.
Glulam. Glulam. Mechlam. Portals – glulam, mechlam.
Towers and masts – Sawn timber, glulam, timber poles – natural, debarked or turned/ profiled. Decks as in 1.
Glulam. Mechlam.
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6.2 Beams, including bowed types, no arch action Beam bridges range from a single, simply supported span, to multiple spans and cantilever arrangements. In laminated construction, pre-camber, and slightly bowed forms (without expressly designed arch action) are quite common. Span ranges for beam bridges may be from as little as 3m for a very small solid timber footbridge, to about 24m in bowed laminated construction.
6.3 Arches Site, terrain and clearance considerations may lead to choice of the arched form, which is architecturally very striking. Much larger spans are possible than with beams, in the order of 12m to 70m being feasible. Various deck arrangements and positioning levels can also be provided.
6.4 Girder beams & trusses Trussed girders provide greater load carrying capacity and stiffness than simple beams. Various trussing arrangements are possible. Girders are often formed from several lines of trusses. These require to be cross-linked with bracing, and the design may involve other lateral members, such as transoms. Deck levels may also be varied. Camber and light curvature are often applied. Well-designed timber girder bridges are architecturally pleasing. Viewers “read” the structural forms, and appropriate designs can be conceived for both urban and rural situations. Individual spans for bridges formed from girders of this type are likely to range from about 9 m to 45 m. Modern timber engineering versions of several traditional timber bridge forms have also appeared recently in the Nordic regions, for example. Both “bow string” and “King post truss” types have been given an updated treatment through the use of new connections technologies, innovative deck types, and environmentally sensitive timber treatment processes. Use of these forms has been extended into multiple spans creating some of the longest timber bridges constructed in modern times.
6.5 Lift & swing bridges There are several practical and available moveable bridge forms in timber. These include bascule bridges, which can be lifted by tilting, and swing bridges. It is of practical importance in dockland, harbour, and inland waterway situations to be able to obtain clearance for waterway traffic. Recently, in such areas, many regeneration and refurbishment schemes have been undertaken. These continue to be needed with expansion of walking and cycle routes, and linking-up of riverside districts. Modern timber design methods, materials and fabrication concepts can provide similar solutions to those used in the past for industrial duties. Either traditional or contemporary architectural styles are possible. Spans for this type of bridge tend to be fairly modest, with those in excess of about 24 m being uncommon.
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6.6 Further design fundamentals Once the principal structural form has been selected, further fundamental structural considerations are necessary. The elevation of the deck in relation to the remainder of the structure is another very important design issue. The option of roofing the bridge must also be taken into account at the early design and cost estimating stages. According to the authorities concerned, regulations exist which may affect several other key elements of the bridge structure, both at ground level and above. These include for example abutments, piers or other supports, deck width, roof clearance (if provided), stair or ramp accesses, and parapet height and design. a)Deck levels There are usually several options in choosing the elevation of the deck. For most forms of bridge shown in Table 1, these are fundamentally low-level, mid-level or highlevel decks. The choice of deck level has considerable influence on the architectural form and engineering design of the structure. It also relates to planning considerations and functional aspects. The former includes for example headroom for vehicles or vessels beneath the bridge. The latter include the measure of protection provided by the deck to the remainder of the structure. Possible elevations of the deck are interpreted in relation to the principal structural forms in Table 2. This second table also incorporates some notes on variations on the basic forms. It mentions for example roofed bridges. Although these are uncommon in the UK, they are not unknown.
a)
Beams
a)
Arches
a)
Girder trusses
Structural form
a)
Lift & swing
a)
Cable Stayed & Suspension
Figure 1
Deck elevations
Over beams. Between beams.
Additions & variations
Roof. Beams as part of parapet.
Over the crown of arch(es).
Over girders.
Mansard arch with stairs.
Roof. Roof supports as part of main girders.
Tied arch, tangent at deck or other level.
Trusses as part of parapet
Skewed plan form.
Deck itself, as for 6.
Deck itself, as for 6.
Generally keeps to classical form of Dutch drawbridge.
Single cable, central in plan, from Aframed tower(s), or similar principle using stays.
Between girders.
Parallel or pitched top chords
Vessel passage also achievable by swing-bridge (another cantilever form).
Table 2: Forms of timber bridge, deck elevations and variations
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b) Materials and durability Correct choice of materials, also understanding and applying best practice options to achieve durability, are such vital aspects of timber bridge design that these topics are discussed under major sub-headings below.
7.0 MATERIALS 7.1 Principal elements In relation to the categories of principal structural forms shown in Table 1, the materials that are commonly used as elements are shown in the table. In essence, these are as follows: a) Sawn timber. b) Timber in round and pole forms. c) Glued laminated timber (glulam). d) Other structural timber composites e) Mechanically laminated timber (“mechlam”). a) Sawn Timber: This may be used for all of the forms in Table 1 which do not involve significant curvature of the members and for which adequate lengths can be obtained to meet the main spans and to provide the other structural spanning requirements. Sawn timbers can range from small sections of softwood or hardwood, suitable for the simplest of short-span beam bridges, through larger sections, more usually hardwood, to very long lengths of specialist hardwoods that can be used for the biggest members, such as masts for cable stayed bridges and for pilings. The latter are still in some cases hewn, rather than sawn sections, although technically this has little effect to the designer. The types of hardwood used for the intermediate applications include temperate species such as oak, often British grown, and established tropical hardwoods that are available for structures in the UK. BS 5268: Part 2 lists data for oak, and also for twelve tropical hardwoods. Typical examples of the latter are Iroko (West Africa, e.g. Ghana) Strength class D40; Keruing (South East Asia, e.g. Malaysia) D50 and Ekki (West Africa e.g. Cameroon, Ghana) D60. For the largest lengths and crosssections, including big beams and masts, Greenheart (Guyana) D70 and Basralocus (Dicorynia guianensis- not listed in BS 5256) are used. Sustainability: Mention of tropical timbers clearly immediately raises the issue of sustainability. The key organisation for producers is the ITTO (International Tropical Timber Organisation) which facilitates discussion, consultation and international cooperation on issues relating to the international trade and utilisation of tropical timber and the sustainable management of its resource base. Regarding certification, it has this to say, in one of its most recent (2000) reports: “Certification is a process which has, to date, not found favour with many producer members, largely because it is seen to be discriminatory. But as countries make progress towards sustainable management, certification may become increasingly
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attractive. Indeed, Malaysia has already established its own national certification scheme, and in Indonesia the project ‘Training development on the assessment of sustainable forest management in Indonesia’ (PD01/95) is being operated jointly with the Indonesian Eco-labelling Working Group, one of the aims of which is to develop training materials for the inspection of sustainable forest management.” Principal ITTO exporter countries are listed in Table 3. Ghana is working towards a similar position to Malaysia, in having its own, nationally-based scheme. Good progress towards Certified sustainability is also being made independently by some small tropical producing countries such as Fiji, Honduras, and several African and Caribbean countries that have plantations of non-indigenous species, such as teak. This material is particularly suited to bridges and other landscaping applications.
ITTO exporters of tropical timber Malaysia Indonesia Brazil Cameroon Papua New Guinea Gabon Côte d’Ivoire Ghana Table 3: ITTO tropical hardwood producers and exporters Softwoods: Where smaller sawn cross-sections and lengths are required, there are better opportunities in bridges than in the building market generally for specially valued British grown softwoods. The great majority of British softwood production is of Sitka and Norway spruce, non-durable species that are hard to treat with preservatives, and unsuited to prolonged external exposure. Those British softwoods worthy of consideration include Scots Pine (SS grade = C22), which has good preservative retention, and Larch (three British grown species, SS grade = C24), which has a good degree of natural durability. Douglas fir grown in the UK also has a degree of natural durability, and sufficient availability to be considered in some regions. In comparison with imported Douglas fir (SS grade = C18), it seems to have suffered from an unduly cautious down-rating of its strength properties. However, this has recently been rectified, for larger cross-sections at least, by a Code amendment approved but awaiting printing. b) Timber in round and pole forms. All three of the British softwood species or groups mentioned above have a long record of accomplishment of good service in exposed conditions as power-line and telegraph poles. Suitable treatment regimes for these are well established, and these have been supplemented by further quite recent research into the topic of durability. Poles themselves are also used, as the main beams for small or medium span footbridges, and occasionally for small masts. They are usually of softwood, with a preference for species with a degree of natural durability such as Larch and Douglas
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fir. Debarked, straight poles are a simple version of this class of element. A limit on taper, such as 10mm per metre, is recommended. Strength grading rules for round timber in uses other than power lines and telegraph poles are not yet fully standardised. In the past, similar ad hoc grading rules and stresses have been derived for other projects. These have included applications in developing countries, where the use of plantation softwood poles is of great economical importance. Advantages of using round timbers include the retention of the natural strength of the tree form itself, and, in appropriate situations, a good appearance. Disadvantages include response of the round timber to drying, and difficulties in connecting such elements neatly and efficiently. Both of these have, to some extent, been overcome, as a result of applied research and developments of the technologies concerned. Treatment techniques for poles are well understood and documented. In countries such as Switzerland and Austria, where a great deal of use is made of timber for bridges, round timbers are also partially shaped. Profiles that are used include circular with one slot, to relieve radial drying stresses and eliminate splitting; circular with one or two flats; and circular with one V-shaped segment removed. Connection systems have also been developed for these more sophisticated forms of round timber. c) Glued laminated timber (glulam). Glued laminated timber (glulam) bridge elements are manufactured to BS EN 386, and other supporting standards, to which this principal document refers. Others cover strength grading of laminations; adhesives; and end-joint testing. Strength classes for glulam are contained in prEN 1194. Both softwood and hardwood laminations are used for bridges, the latter to a far greater extent than in glulam beams for buildings. The British timber code and its related standards used a system of grading laminations and performing design calculations which was peculiar to the UK, but which stood the test of time (8). Some of the procedures and requirements described in the former British Standard for glulam, BS4978 are still followed for hardwood glulam, since the European documents have been developed principally with softwoods in mind. An introduction to glulam production and strength classes is available in STEP/EUROFORTECH Volume 1, Lecture A8. Glulam bridge beams are possibly more common in one particular laminated hardwood, namely Iroko, than in any of the softwoods. This timber has found favour for its combination of good durability, the ability to be bent and glued, and its good joinery properties. Substitutes are now being considered, because Iroko is under pressure through perceived sustainability issues, and may even be coming into genuine shortage from some forest regions. Alternatives might include Dahoma (Piptadeniastrum africanum). This has been used successfully in several vehicular bridges in its country of origin, to demonstrate the concept of sustainability through choice of alternative (“Secondary”) species. Since these bridges were built in a highly termite-susceptible region, pressure preservative treatment was used in addition to selecting a durable species. Where laminated softwoods are specified, European redwood rather than whitewood is preferred for external structures, by Nordic glulam producers, due to its greater amenability to pressure preservative treatment. Douglas fir was hitherto more widely used in these situations in the UK. Also, there are no technical reasons why larch should not be chosen. Indeed, one particular specialist timber engineering manufacturer prefers this timber, finding that it bonds very well. It was the preferred
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choice for Scottish fishing boats, including laminated keels, but is now uncommon in the timber engineering industry. Unlike the case in Germany, laminations for external structures such as bridges are not restricted, in Britain, to a maximum of 33mm. In that country, this is understood to be a precaution to ensure that full clamping pressure is applied throughout the depth of the elements. However, a prescriptive standard of this nature would no longer be likely, within the harmonised European system. The matter would be left to the discretion of individual manufacturers, under third party quality assurance controls. The normally permitted maximum of 45mm for straight laminations is quite common for glulam in both softwoods and hardwoods. Permitted adhesive types are of course selected from the most rigorous exterior/high hazard exposure category, and this normally indicates a phenol/resorcinol formaldehyde type. Provided that the adhesive specification and manufacturing procedures are correct, including quality control tests in relation to the finger joints, there seems to be no reason to believe that 45mm thick laminations, including those from selected hardwoods, are unsuitable. d) Other structural timber composites. The 'family’ of structural timber composites (STCs) is growing. Glulam is really the best known, and longest established, structural timber composite, but it has been joined by other products that are manufactured from veneers, strands and flakes. These are dried, graded and reconstituted, using modern synthetic adhesives, applied under heat and pressure. The exact processes vary, but long, prismatic structural sections always ensue, as opposed to wide, flat boards. The newer STCs are still not as widely known as glulam, by generalist engineers and architects. These tend to take a long time to become aware of such changes, but publicity by major European producers, and support work on codes and standards, is beginning to take effect. STCs are manufactured using well established techniques and materials that have been developed over many years for the production of structural wood-based board materials. Indeed, each member of the ‘family’ of STCs has its ‘relatives’ in the structural board materials range, most of which have long-standing references by codes such as BS 5268. Several types of STC are suitable for bridges, and instances of their application for this purpose can already be cited. The following outlines the three main types of STC, commenting on their potential for this purpose : Laminated veneer lumber (LVL): Bonding together dried, graded, spliced and trimmed veneers that are peeled from a log, in much the same way as making plywood, produces laminated veneer lumber (LVL). Once pressed and trimmed, the resulting long panels are sliced into prismatic structural-sized sections. Unlike plywood, successive veneers are generally orientated in a common grain direction, although a hybrid product, that has every fifth veneer laid orthogonally, has also been found useful for diaphragm applications, including bridge decks. European LVL is manufactured in Finland, from Norway spruce, by Finnforest Oy under the name Kerto LVL. The standard (all veneers longitudinal) type is named Kerto S, and the special (cross-veneered) type is Kerto Q.
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LVL is also manufactured elsewhere in the world, the great majority of the manufacturing machinery emanating from Finland. In Australia, for example, LVL made from Radiata pine is applied in bridges. LVL is found to be amenable to full penetration by pressure-applied preservatives, and is thus one of the best established of the new STCs for exterior structures. Parallel strand lumber (PSL): Parallel strand lumber is manufactured from peeled veneers that are cut into long strands. These are then coated with adhesive and combined under heat and pressure, in a quasi-extrusion process, to form structuralsized sections in long lengths. PSL is manufactured in the USA, by TrusJoist MacMillan, under the name Parallam. It is made mainly from Southern yellow pine and Douglas fir. PSL is also understood to be amenable to pressure-applied preservatives, but there is less experience in its use for exterior structures in Europe, possibly due in part to pricing differentials, compared with LVL. Laminated strand lumber (LSL): Laminated strand lumber is produced by bonding together flakes of wood, again under heat and pressure, to produce structural sections. LSL is also manufactured in the USA, by Trus Joist MacMillan, under the name Intrallam. It is produced mainly from Aspen strands, a timber that is perishable (extremely non-durable), rendering this an unlikely choice for bridge structures, even if preservatives were to be introduced. Advantages of STCs: The major advantages of STCs are that large dimensions are available, with higher characteristic strength values than those of the raw material itself. This is brought about by defect dispersal within the manufacturing processes. These products are manufactured at a low timber moisture content, their dimensions are accurate, and when installed, moisture-related movements, such as shrinkage, twisting and warping, are virtually eliminated. The strength of solid timber sections depends largely on the influence of defects, such as knots and irregular or sloping grain, rather than on the inherent strength of the clear straight grained species. Clear, kiln dried timber is normally at least two and a half times stronger than average quality commercial sawn timber, at air dried moisture content. When making composites, the veneers, strands or flakes are recombined. This dispersal of defects produces a material, which has significantly more consistent structural properties than solid timber. The longer spans that can achieved often mean that fewer intermediate supports are required, and simpler structural systems are possible. Construction times can be significantly reduced, by taking advantage of these features, in combination with a number of innovative, partially prefabricated techniques for element and connection formation. Guidance and standards for STCs: The design of elements and components using STCs may, in general terms, be undertaken in accordance with the rules given in BS 5268, or with those in DD ENV 1995-1-1 Eurocode 5 Design of timber structures Part 1.1. There are at present no British or European Standards for STCs, and as a matter of principle, no materials have their design data included as part of the Eurocodes. In fact, it is considered an advantage of the newer Eurocodes that they are more “open” to the introduction of such innovative products.
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Suitable information on generically classified STCs is shortly to be provided, in intended European Standards (ENs). Examples of existing Technical Approvals that address STCs such as Kerto are the British Board of Agreement Certificates. Meanwhile, procedures are also being developed to establish European Harmonised Technical Approvals that will address materials such as these. e) Mechanically laminated timber (“mechlam”). Mechanically laminated members are termed "mechlam" as a convenient abbreviation in this review, although it should be clarified that this is not a universally recognised term. Recently encountered has been an interesting example of a mechanically laminated Greenheart bridge, which was built in Cheshire in 1915 and which, having remained in good condition, has just been refurbished. The modern manufacturing process, which was developed in Germany, and used quite extensively there and in the Netherlands, has become quite familiar in the UK. Numerous examples of bridges containing members of this type are to be found, ranging from simple short-span beam bridges, to the more ambitious types such as arches and cable stayed structures. Formerly, the timber used was almost exclusively Ekki, or Azobé, as it is known in Continental Europe. Recently, experiments and a few actual applications have occurred using oak. The design of mechlam structures involves some special considerations involving slip between the layers that leads to incomplete composite behaviour. This affects ultimate limit states, as well as serviceability design. Some of the fundamental principles are provided by Eurocode 5 Part 1-1, and STEP Lecture B11 also explains the basis of the computations, with elementary examples, based on this code. However, mechlam timber bridges are now offered in Western Europe, including UK, by some half a dozen firms, on a design, supply and erect basis. All of these types of supplier tend to guard precise details of the full basis of design from mainstream practitioners, as well as from organisations such as BRE and TRADA Technology.
7.2 Decks & decking – UK current practice Structural diaphragm decks do not at present form part of the British timber bridge designer’s vocabulary. They are an extremely significant item that needs to be brought forward for their attention, in order to improve efficiency, as outlined in the Introduction. For convenience however, the following only discusses decks that span as secondary or tertiary items between transoms or stringers, and which do not act as composite diaphragms. The latter are briefly introduced in Section 10.5 of this report, dealing with Overseas Practice – Decks. The commonest form of simple one-way spanning, non-diaphragm deck uses spaced sawn planks. These are usually laid transverse, but are sometimes placed longitudinally. The deck planks can be softwood or hardwood, with certain hardwoods preferred for maximum wear and durability. In connection with wear, designers are in either case advised to discount a proportion of the section, as newly-placed, when calculating for strength and stiffness. This point is addressed specifically by prEN Eurocode 5 Part 2. Softwood decking planks can be specified as either GS or SS grade to BS 4978. Suitable preservative treatment may be considered. This would tend to lead specifiers towards timbers such as Scots pine/European redwood, Douglas fir and larch.
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Hardwood decking planks are usually from a naturally durable species, such as Iroko, Jarrah or Ekki, and are specified as HS grade to BS 5756. Slipperiness of timber decks in general is recognised as something to avoid. It is an issue also related to maintenance and upkeep. It is an aspect of timber footbridges that would merit attention that is more specific to these structures, where the question of incline of ramps and arches also enters into the equation. Where foot grip is especially important, profiled decking planks provide a good solution. Hardwood profiled planks, in timbers such as ekki, are marketed extensively as a separate purchasable item, by timber bridge suppliers, for customers’ use in landscaping and crossing structures beyond the realm of bridges. In footbridges specifically, at higher gradients, profiled planks are sometimes used in conjunction with kick-plates, which are nailed down to the deck. Unless these are well executed however, they are a notorious source of early wear and hence maintenance cost. Recently a proprietary form of profiled, treated softwood decking board has also become available. This has embedded inserts of non-slip material, which are grooved into each castellation of the profile. It is generally felt that the gaps between simple decking in rural footbridges should not be less than 5mm, in order that dirt and debris can pass through the deck. This also allows air to circulate around the planks, thereby avoiding damp pockets where fungal decay can start. Larger gaps are sometimes used, and in remote country areas, deliberate gaps of up to 25mm have been specified. For certain bridges over roads and railways, particularly in more urban environments, gaps in the walkway are not permitted, due to concern over vandals dropping objects onto vehicles or persons below. This has led some designers to use glulam beams, which can provide the spanning medium for the bridge, as well as the deck. Such laminated decks are abutted together, to provide the walkway. An alternative solution has been to use plywood decking with additional non-slip surfaces, but this does not seem to have had a good record. Wear has been rapid and it has become evident that plywood decking requires special attention to drainage details. LVL decks, with appropriate pressure preservative treatment and added wear protection, have started to occur in a few instances, in the UK. This type of progressive solution is moving towards the concept of treating the deck both as a walking surface, and as a structural diaphragm. As mentioned above, this is discussed further in section 8.5.
7.3 Parapets & handrails The primary function of the handrails and parapets is of course the protection of bridge users. Occasionally, in very remote areas such as forests and moorland trails, bridges are built with no parapet, or with only one handrail. A pair are however the norm. Various configurations are used, with the choice primarily depending on the following: a) The type of footbridge user (for example – pedestrians only, or cyclists and pedestrians). b) The nature of the site and locality, for example whether it is a rural or urban location, and whether it passes over a main road, railway or a stream.
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The first item dictates the height and strength requirements for the parapet, as discussed in more detail below. The second affects the degree of openness that is permitted for the handrail, intermediate rails (if any), spindles and posts. Common solutions for bridges in rural locations involve cantilevered handrails. A better, and very traditional arrangement in these types of location may be for the vertical post to be triangulated, by projecting out the decking members in the vicinity of the post, and adding a diagonal raking member. This type of design is often adopted for short-span bridges, where the main beam is of limited depth. For suburban bridges, it is not uncommon for mesh infill, or even solid, surfaceprofiled metal sheet, to be fixed to the handrail, to prevent children from falling through the gaps. The cheap, but somewhat inelegant solution is mesh infill. A better alternative is light section, close centred spindles with adequate intermediate longitudinal rails for stiffness. In fully urban areas, an altogether lesser degree of openness is usually required, whilst for bridges over trunk roads, motorways and railways, there is serious concern over objects being accidentally kicked, or deliberately dropped, onto the highway, rails, or traffic. In applications such as these, authorities will invariably stipulate the required dimensions of enclosure that will normally prevent an open solution. This has resulted in a number of bridges where the deck is located near the centre, or towards the base of the main beams. These then provide the lower half or two-thirds of the parapet. This type of arrangement is common in glulam footbridges with through decks. Larger, girder truss bridges ( for example the type illustrated in Figure 11 c), also often incorporate part of the structural girder depth into the parapet. This of course has further detailing implications, but such structures tend to be offered by specialists who have evolved practical and acceptable solutions that comply with the rules and customs of the various European countries in which they operate. Both softwoods and hardwoods are used for handrails, with the latter, in a suitable species, preferred for durability and smoothness to touch. Most, if not all, of the smoothest-to-hand timbers used in joinery are of tropical origin. External weathering tends to aggravate splinter pick-up in open grained species. Hence, this is a particular issue that needs to be clarified, in relation to user-inhibitions through preference for avoiding tropical timbers, because of perceived sustainability questions. Besides sustainability, however, is the matter of toxicity to skin of the splinters of some species. Good detailing of parapets and handrails is thus undoubtedly an aspect of the furtherance of timber ridges that requires co-operation between timber engineers and wood technologists. Regarding grade and strength class specifications, softwood handrail members will again be GS or SS grade to BS4978, assigned to the appropriate strength class, and with suitable preservative treatment, if required. Hardwoods are usually from a naturally durable species, such as Iroko or Opepe, and are HS grade to BS 5756 and thus to the appropriate hardwood strength class (D Classes). Parapet and handrail detailing to achieve protective design against long-term deterioration, through weathering and decay, is addressed by several Nordic and German-language publications. Some indication of the type of guidance available can be seen in STEP Lecture E17 (Fisher 1995). This is another aspect that will be well worth further attention for UK design guidance.
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7.4 Connections Connections within modern timber bridge structures is an enormous subject, that could quite easily occupy a two year applied research and “knowledge repackaging” project, in its own right. TRADA Technology is currently engaged in a separate project to this, under DETR Partners in Innovation funding, entitled “Connections IT Toolbox”. Good introductory texts for engineers on mechanically fastened timber joints in general are also contained in STEP Volume 1, Lectures C1 to C19. The TRADA Technology Eurocode Design Guidance Documents already published also address the topic extensively. Mechanical fasteners and connectors for bridge structures are at present normally of steel, and quite often of stainless steel specifications, rather than from plain carbon steel. In virtually all instances, some form of corrosion protection is required on other that stainless items. Where flitched or spliced joints involve the use of steel plates, these are usually specified with a thickness of not less than 6mm, following steel bridge design practice. Again corrosion protection is essential and the Eurocodes 5 (both Part 1-1 and Part 2), supported by the documents described above, provide an entry point. Signposts to research on bonded-in connections, a lot of which is highly relevant to bridges, are given in the state of art reference cited above. Work has also been started on bonded-in non-metallic (Fibre Reinforced Polymer, or FRP) connections that may in future be relevant, especially in view of their potential corrosion resistance, as well as their high tensile strength (Bainbridge et al, 2000). Fatigue within timber structural connections for bridges is another aspect that researchers have started to address. Some success has already been achieved, showing this to be not a hyper-critical issue, but one that can be handled using established timber research and code formatting techniques.
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8.0 DURABILITY Timber, when suitably protected, can be remarkably durable and can outlast in certain conditions other materials such as metals, brick, stone and concrete. Timber is invulnerable to salt water, either from sea or de-icing salts, and freeze-thaw action. In a timber bridge, elements which are not covered will frequently attain moisture contents above 20% - the threshold for fungal decay. The threshold for insect attack is even lower, at 12%, although only sapwood and decayed heartwood is vulnerable. Preservative treatment will be necessary only if the natural durability of a timber is insufficient to meet the required service life.
8.1 Detailing Bridges are a particularly exacting application, and ensuring that the timber members have adequate durability is a vital consideration. Before considering this item from the perspective of material selection, it is important to note that much can be achieved in terms of increased durability by means of improved detailing. Indeed the converse is also unfortunately true, in that if poor detailing is provided then premature failure of timber components can occur. Table 4 identifies seven susceptible parts of a timber bridge in general. Most of these points apply to all types of bridge, irrespective of the precise form of the structure. The table then exemplifies poor detailing aspects and gives better alternatives. At this stage, the items in the table are regarded as pointers for guidance, and as suggestions for closer attention, rather than definitive solutions. It is anticipated that it will be necessary to pay considerable attention to detailing, and that these aspects will require discussion by timber experts and bridge manufacturers, in conjunction with the analysis of the survey results. The benefits of effective and well maintained finishes have been very apparent in the survey work which has already been performed and which continues. Modern water repellent finishes offer a considerable measure of protection to exterior timber structures such as bridges. The prevention of weathering of the timber surface itself has an important role in this respect. A high specification of finish and a good maintenance programme for the same would always be advocated in addition to the correct detailing and choice of durable species mentioned above.
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Part of the structure End grain of members in general e.g. beams
Upper edges of exposed members e.g. beams and handrails
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Examples of poor detailing Exposed end grain, leading to fissures, unattractive and ultimately a seat of decay
Flat upper edges where water lies and which trap dirt, especially when weathered/ fissured
Examples of better detailing Protected end grain eg: by attaching other timber members having side grain, or by ventilated capping/sealing Chamfered and sloped upper edges which freely drain Edges protected by ventilated capping
Joinery details e.g.: handrails, parapet to beam connections
Details which trap moisture in mortises, fixing holes, recesses etc.
Freely draining, ventilated, flush details Raise parapet above splash level with separate drained kerb
Decking and its attachments
DPC between deck and beams
Member intersection points, column bases, especially with steelwork Bearing points, supports, bank seats etc.
Deck which is tight jointed or with a sealed surface but which merely traps moisture Attachments to beams which form traps DPC between deck and beams Intersection points can easily form moisture/dirt entrapment regions. Intersection points can trap moisture and remain damp.
Poorly ventilated, susceptible to silting up, dirt and debris entrapment
Deck which freely drains, laterally and longitudinally, even when worn Drip mouldings beneath deck boards
Not easily avoided, but detail for maximum ventilation and drainage eg: by drilling/arranging gaps Design steelwork to allow drainage and ventilation. Avoid details which allow the collection of water . As well raised from surroundings, eg: by masonry and supporting steel, as possible
Table 4: Susceptible parts of timber bridge structures, with examples of detailing weaknesses and improvements
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8.2 Natural durability The biological natural durability of timber is due to the anatomy of the timber species and in some cases the presence of naturally occurring extractives within the heartwood. Each timber species has its own characteristic set of these chemicals, some of which are toxic to wood-destroying organisms. Even when the detailing is as good as possible, for an exacting, fully exposed application of timber such as a bridge, it is advisable to consider the use of a timber which falls into a natural durability category which is at least as good as "moderately durable", Table 5. It is important to note that the biological natural durability of a timber refers only to its heartwood. Such classifications are well-established in Britain for all of the better-known construction timbers, both softwoods and hardwoods, including all of those listed in BS 5268: Part 2. Table 6 shows the natural durability classifications of the twelve tropical hardwoods listed in the code, together with European redwood and Douglas fir, for comparison. These classifications are based principally but not exclusively upon traditional ground contact stake tests. It should be noted that the ratings relate to UK conditions, which do not include a termite hazard, but which represent a high risk from fungal attack. Exposure trials have been conducted using EN 330 "L-joint" type specimens, both to assess natural (untreated) durability, and to evaluate various forms of preservative treatment beneath a coating. In due course, the information from this project will be of value to bridge designers, especially when they consider the joinery items such as parapets and handrails. Certified, sustainable and naturally durable timbers, including, for example, plantation teak from various sources, may be admitted for the most exposed parts of superstructures, such as handrails and parapets, on the basis of evidence from such tests.
Durability Category
Approximate life in ground contact, 50mm x 50mm section (years)
Very durable
More than 25
Durable
15 – 25
Moderately durable
10 – 15
Non-durable
5 – 10
Perishable
Less than 5 Table 5: Natural durability categories
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Timber, Standard Name; Species
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Region of Origin
Natural durability
Balau Dense Shorea spp
SE Asia
Durable
Ekki (Azobe) Lophira alata
W Africa
Very durable
Greenheart Ocotea rodiaei
Guyana
Very durable
Iroko Milicia excelsa
W Africa
Very durable
Jarrah Eucalyptus marginata
W. Australia
Very durable
Kapur Dryobalanops spp
SE Asia
Very durable
Karri Eucalyptus disersicolor
W. Australia
Durable
Kempas Koompasia malaccensis
SE Asia
Durable
Keruing Dipterocarpus spp
SE Asia
Moderately durable
Merbau Intsia spp
SE Asia
Opepe Nauclea diderrichii
W Africa
Very durable
Teak Tectona grandis
SE Asia
Very durable
Douglas fir Pseudotsuga menziesii
N America
Moderately durable
European Larch Larix decidua (L. uropaea)
Europe, incl. UK
Moderately durable
Scots pine/European redwood Pinus sylvestris
Europe, incl. UK
Non-durable
Durable
Table 6: Natural durability classifications of the twelve tropical hardwoods listed in BS 5268: Part 2, and of Douglas fir and European redwood
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8.3 Preservative treatment In the modern philosophy of designing for durability, the use of chemicals to treat the timber, normally through pressure application, is regarded as the third line of defence, following good detailing and species selection. Increasingly, though, the most sustainable use of our principal commercial timbers (i.e. non-durable softwoods) is to extend their service lives through preservative treatment. This allows more than sufficient time for forest growth to compensate for the consumption of timber. However, evidence from around Europe suggests that traditional preservative active ingredients are going to come under increasing environmental scrutiny and legislation. In Denmark and the Netherlands, legislation has already instigated restrictions on the use of copper/chromium/arsenic (CCA) - the most widely used wood preservative. Amendment to the EC Marketing and Use Directive for arsenic will limit CCA to a few derogated uses including bridges. Creosote is due to be withdrawn for public/domestic use in the EC in 2003, but will still be available for industrial applications such as utility poles and bridges. Of the 150 bridges constructed recently under the Nordic Timber Bridge Project (Nordic Timber Council, 1999), the majority were either creosote or CCA treated. Clearly up to date guidance is needed for timber bridge designers and certain clients may demand "environmentally friendly" solutions. In the United Kingdom only formulations approved under the Control of Pesticides Regulations 1986 by the HSE Pesticides Safety Directive are used for timber treatment, and formal authorisation procedures are in place to ensure that operations comply with legislation relating to aspects such as employee health, safety, material control and waste disposal. Treated timber is, of course, far more widely used in applications such as fencing, decking and utility poles than bridges. The latter may, however, come under greater scrutiny because of their siting over sensitive watercourses. For timber bridge members the principal chemical preservative treatments applicable are either Copper Chrome Arsenic (CCA) or creosote, applied under pressure, although there are some alternatives on the market. In general, hardwoods either do not require treatment or are difficult to treat due to poor penetration, whilst most softwoods are treated. Spruce and hemlock are difficult to treat, although penetration can be improved by incising. BRE Digest 429 (1998) gives guidance on both natural durability and resistance to preservative treatment. Copper Chrome Arsenic: CCA (marketed under the trademarks of "Celcure" or "Tanalith") is a water-borne preservative with a particular application for softwood glulam bridge beams, usually in conjunction with European redwood as the timber. Both pre-treatment of individual laminations and treatment of the entire member after complete manufacture and machining are known. Pressure cylinders of up to 25m length are available. Timber is dried before application of CCA under high pressure/vacuum process and allowed to dry again for between 7 to 14 days during which fixation occurs. The advantages of CCA treatment are that the timber is free from odour and that paints and varnishes can be applied. CCA treated timbers (such as off-cuts) should not be burnt in an open fire, since this releases the preservative elements in the ash and smoke. Waste CCA treated wood should either be burnt with flue gas recovery or put into landfill.
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Creosote: Creosote is a complex mixture of over 300 substances derived from the distillation of coal tar and tends to have a poor image due to an association with odorous garden fences and weeping telegraph poles. In fact, creosote is a very long serving and effective wood preservative which has low water solubility and is biodegradable when dispersed in soil. There are many instances of creosoted timber structures and wood piles still giving good service after 100 years of ground contact, and timber currently treated by the pressure process can offer a service life of at least 40 years. Although fresh creosote will burn the skin, requiring gloves to be worn during handling of treated lumber, it is not a systemic poison. Creosote also protects wood from the development of splits. Creosoted timber should not be used for parts of the bridge which come into contact with unprotected skin, such as handrails. Freshly creosoted timbers may cause the formation of on oil sheen if in contact with water. There are two forms of creosote treatment, full cell and empty cell. In the full cell process all the available voids in the wood structure are filled as far as possible with creosote by first applying a vacuum to the timber, then flooding the pressure cylinder with preservative. After the vacuum is released atmospheric pressure forces the creosote deep into the structure. Further application of pressure after this stage achieves even greater penetration. At the end of the cycle, a second short period under vacuum is applied to withdraw a small amount of preservative from the surface of the timber leaving it dry and in a reasonable state for handling. In the empty cell process a longer period under vacuum is applied to remove a greater amount of preservative, leaving the voids in the wood only partly filled but with the internal walls of the wood cells coated. Although creosote is used undiluted by solvents, freshly treated timber is normally allowed to dry for up to 7 days to allow the more volatile components to evaporate. Alternatives: Many preservative products have been developed over the last 10 years that have aimed to provide alternatives to CCA whilst providing equivalent performance in the field. A number of these have focused on removing the arsenic compound from the preservative formulations, such as CCB systems that use boron. There are also systems available that are both chromium and arsenic free where tebunconazole and boron based preservative have replaced those ingredients. These preservatives still provide the timber with its characteristic green colouration, and are increasing used in markets such as garden decking. All these products are approved for use in Pesticides 2001. Selection system for timber bridges: Selection of an appropriate system for timber bridge component should follow the European Standards below: 1. Design bridge (timber components in contact with the water, soil and out of ground contact) 2. Assess the hazard class of the end use of the timber (EN 335-1). For example a timber in freshwater contact is Hazard Class 4. Out of ground contact timber is Hazard Class 3 and is a less challenging environment for the timber component. 3. Select wood species (EN 350-1 Natural durability) Nov 2002
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4. Is wood species natural durability sufficient for the performance required for the Hazard Class? (EN 460 Links durability to Hazard Class) 5. If species is insufficiently naturally durable then select and specify the preservative (EN 599-1 and required treatment result EN 351-1 and DD 239) Alternatively a similar philosophy is passed through in BS 5589 and BS 5268 with specifying based on preservative treatment schedules applied to particular timber species being fit for purpose.
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9.0 TIMBER FOUNDATIONS Timber piles are a highly suitable choice of foundation, given appropriate ground conditions, for many structures including bridges. Timber piles are economical, easy to transport, handle, cut to length and work with on site. They are particularly suited for locations with access difficulties or where minimal disturbance is a priority such as structures in the countryside and canal-side and shoreline sites where excavations and the delivery of concrete would pose problems. Short driven timber piles can be the solution for foundations in ground with a high water table or where firm strata exists below surface material of loose sand, soft clays, highly organic soils or fill. Timber piles are also resistant to acidic and alkaline soils, and soils with high sulphate or free carbon dioxide content. Treated or durable timber can also be used for the construction of wingwalls and bank seats, as well as for foundation pads and footings. Timber is a sustainable construction material with obvious environmental advantages over both steel and concrete. Trees, while they grow, adsorb carbon dioxide and release oxygen. Forests provide areas for wildlife and recreation. One of the suggested methods of reducing global warming has been to create carbon sinks - to lock up carbon for long periods of time. Using timber for foundations would effectively achieve this.
9.1 History and overseas use Timber has been used for piled foundations for centuries. Before 1900 nearly all piles were either untreated wood or stone. Old London Bridge was founded in 1176 on stone filled starlings constructed from elm piles, and lasted 600 years (Nash, 1981). The City of Louisiana is founded on timber piles, so too is Pont Notredame bridge in Paris, The Royal Place in Amsterdam, The National Theatre of Finland, The Dome of Utrecht and The Reichstag in Berlin. In 1902 the Campanile Tower in Venice was rebuilt on the 1000 year old piles, still in excellent condition, which supported the original structure (Haldeman, 1982). More recently, Graham (2000) reports on the use of 30 tonne capacity timber piles for the foundations of the Cargo Terminal at John F. Kennedy Airport. Timber piles were also used for the 210m diameter Louisiana Superdome supporting 130,000 m 3 of concrete and 18,000 tonnes of steel. Timber piles with 70 tonne design loads are in use on a 300m long viaduct near Winnemuca, Nevada. In Canada alone over 30,000m 3 of treated wood piles are used annually. Most of the deep foundation support for highway bridges in North America comprise of treated timber piles. The US Army Corps of Engineers used over six million timber piles to construct the locks and dams for the Inland Waterway System.
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Figure 12: Timber trestle piles supporting a concrete deck bridge (Bellingen, NSW) (photo Kardon Piling)
9.2 Durability of timber piles Timber piles, when driven in below the ground water level, are virtually immune to biological degradation and can have an almost indefinite life. Timber piles have been recovered from the remains of Roman and medieval constructions in a state of perfect preservation. The section of a pile above ground water level is, however, vulnerable to decay and one option is to terminate the pile below the water table and continue the foundations in a different material such as concrete. In the past this was accomplished with stone or masonry. The timber piles of historic buildings may decay if the local water table is lowered below the tops of the piles for long periods, either by abstraction, drainage or de-watering for nearby excavations. Both York Minster and the Mansion House in London were built in marshy ground on timber foundations which subsequently degraded due to drainage. Bouteje and Bravery(1968) report on similar problems with buildings in Stockholm. Above the water table in soil, fungi attack will lead to severe deterioration of untreated non-durable timber in a matter of months only. Under fresh water such as in rivers and lakes but also in soil below the water table the outer layer of sapwood of untreated timber will become infected by anaerobic bacteria which can degrade the strength properties over a time period of decades. In central Europe untreated species of non-durable softwoods such as Scots pine and Norway spruce were formerly used extensively. Timber piles are highly resistant to both acid and alkaline soil conditions. In Austrailia, at the Ulan coal mine, treated timber piles were chosen for a bridge carrying ore trucks because high free carbon dioxide levels and extreme acidity in the soil would have destroyed both steel and concrete piles. Timber piles were also used for the foundations of the Brambles Container Terminal in Burnie Tasmania (soil pH 11.5) and the Auburn, N.S.W., Waste Transfer Station (soil pH 2.5).
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9.3 Traditional timber species and treatments BS 8004 (1986) gives guidance on the use of timber for piles, and states that in the United Kingdom Douglas fir imported in sections up to 400mm square and 15m length is the most common softwood used for piles. Pitch pine is also available in sections up to 500mm square. Greenheart was formerly the most commonly used hardwood, imported rough-hewn in sections up to 475mm square and up to 24m long. Other suitable tropical hardwoods given by the Code include ekki, jarrah, and opepe. In the past, domestic grown hardwoods such as oak, beech, ash and sweet chestnut have been used for piles. Elm is also durable below ground, so much so that it was used for water pipes and also coffins. In Scandinavia and Central Europe Norway spruce, Scots pine and to a lesser degree fir and larch have historically been used (Peek and Willeitner, 1981). In the US and Canada southern pine is used extensively as well as larch and western red cedar. The Romans are known to have treated timber for pilings by smearing the wood with cedar oil, pitch and then charring. Pressure injection of coal-tar creosote began in England in 1838. Following the use of pressure impregnated railway sleepers (or railroad ties) in the United States the process was first applied to foundation pilings in the early 1880's. Today, pressure impregnation of creosote or copper-chromearsenic (CCA) are the two main types of chemical wood preservation applied to timber used for piles. Only softwoods are suitable for chemical impregnation, with spruce and hemlock being difficult to impregnate. Both types of preservative are applied during a high pressure/vacuum process. In the case of CCA preservative, which is water-borne, the chromium acts as an oxidising agent and the metals become highly fixed into the wood structure. Creosote, which is derived from the distillation of coal tar, may be applied as a "full cell" or "empty cell" process. In the latter case a vacuum is applied post application to remove surplus creosote which might otherwise bleed under the influence of sunlight. For softwood timber piles timber selected with a thick sapwood layer which adsorbs preservative treatment better than heartwood is beneficial since this provides a thick protective layer of well impregnated material. Spikes or hooks should not be used to handle treated timber piles since this may expose less well protected wood in the inside of the log. All cutoffs and drill holes should be liberally applied with preservative. For hardwoods the vulnerable sapwood is removed and the timber is normally supplied squared off.
9.4 Marine structures Timber is favoured for marine works because of its ability to absorb impacts, its ease of handling over water, and the poor performance, historically, of cast iron, steel and reinforced concrete. Timber is used for groynes and sea defence works as well as jetties and dolphins. In seawater and brackish estuary waters untreated timbers are liable to attack by marine borers, around the British Isles principally the mollusc Teredo (the shipworm) and crustacean Limnoria (the gribble). Teredo bores circular tunnels 15mm in diameter and up to 150mm long horizontally and vertically in timbers leading, ultimately, to severe weakening. Occasionally Teredo damage is observed in timbers which have been floated in marine waters prior to sawing, the damage being characterised by lack of bore dust and the chalky white calcareous tunnel linings (Desch and Dinwoodie, 1996). Limnoria creates shallow tunnels approximately 2.5mm in diameter and penetrating less than 15mm in depth, the extensive nature of
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which leads to erosion. Another crustacean Chelura, is associated with attack by Limnoria, but cannot by itself burrow very far into timber. The Sea Action Committee of the Institution of Civil Engineers (ICE 1947) found Limnoria and Chelura to be active in British waters, with Terodo active south of the Mersey and Humber. Greenheart, kauri and jarrah were found very resistant to marine borers, while oak and untreated softwoods were not resistant. Borer attack was found to be limited in polluted water such as in docks, although this observation may not be relevant nowadays. Greenheart was found in excellent condition after 60 years service in Liverpool and similarly Danzig fir after 52 years service in the Thames at Northfleet. Creosoted Baltic pine (i.e. slower grown Scots pine) was recommended for British ports on the grounds of its useful economic life. Other suitable softwoods include Douglas fir, Western hemlock and European larch. The Handbook of Hardwoods (HMSO 1972) lists a number of tropical hardwoods recognised as being resistant to marine borers such as basralocus, belian, okan as well as the Australian hardwoods jarrah, ironbark, southern blue gum and turpentine, the latter being particularly long favoured. Currently there are no FSC approved sources of greenheart or ekki which were traditionally used for marine piles, although possible alternatives include Acariquara and Purpleheart. In tropical waters untreated timber piles of non-durable species can have a useful life of only a few months. For softwoods, combined treatment of CCA and creosote has been found very effective. Hardwoods, such as terpentine, will also benefit greatly from the provision of an outer barrier layer of treated timber. Timber piles may also be protected by providing copper or aluminium sheaths, and there has been some development of the use of PVC (Heinz, 1975). Methods of repair of marine borer attacked piles include jacketing with concrete. The principal inspection method for marine timber piles is by diver. Abrasion resistance is an important design consideration for marine structures such as sea defences and groynes, particularly on shingle beaches. Timber can withstand wear in the marine situation better than either steel or reinforced concrete, with tropical hardwoods being particularly durable. Dense softwoods such as Douglas fir and pitch pine also perform well. Timber structures can be protected from scour simply by providing a sacrificial layer of planking.
9.5 Pile driving and design All timber piles are displacement piles, therefore suitable ground conditions must exist for their use. Conventional pile drivers are used to insert timber piles with the normal weight of a drop hammer being 1.5 times the weight of the pile. A long narrow drop hammer increases the chance that the pile is hit axially, avoiding damage to the pile and maximising the downward impulse. Diesel hammers are also suitable, including those that run on bio-diesel. Care must be taken with all hammers not to over-stress the pile or to cause splitting of the pile toe. A helmet or cap protects the pile head from fracturing or brooming, and in addition the pile may be banded to prevent splitting. Hard driving should not be continued in an attempt to meet a prescribed set, since this may result in the pile head disintegrating. Where there is a surface layer of hard fill a pre-bore may be performed. Timber piles can also be inserted using vibratory methods. Groups of timber piles inserted into soft clays may need to be loaded temporally to prevent the effect on soil pore water pressures causing buoyancy. Timber piles can be spliced and extended in length using short sections of steel tube, angle or plates to reach loadbearing strata.
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Figure 13: Timber pile driving (photo Kardon Piling)
Timber piles support loads by end bearing, shaft friction or combined end bearing and friction depending on the nature of the strata into which they are inserted. Driven thin end down, trees make natural tapered piles enhancing shaft friction. Timber has a high strength to weight ratio, and is particularly strong in compression parallel to grain. The timber selected for piles should be straight grained and free from defects and, in general, suitable material is obtained from SS grades and better. The centreline of a sawn pile should not deviate by more than 25mm throughout its length, and for round piles a deviation of up to 25mm on a 6m chord may be permitted.
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Piles are designed as columns, but consideration should be given to bracing for unsupported lengths above ground level. BS 5268 (2002) may be used to calculate the allowable compression parallel to grain. Stresses on the timber during insertion are usually far higher than those encountered in service. Similar equations exist for the loadbearing capacity of timber piles to those of steel and concrete, including those based on dynamic pile driving formulae. Appropriate factors of safety for foundation design are given in BS 8004 (1986) and BS 6349 (1988). To reduce the need for excessively hard driving the working loads are often limited to 300kN (30 tonnes) on a 300mm x 300mm pile. The American Wood Preservers Institute (2000) and Canadian Wood Council (1991) also give guidance and design examples on the use of timber piles.
9.6 Other geotechnical uses for timber Timber is attractive and its use for earth retaining structures such as bridge abutments is suitable in sensitive countryside, in particular for forest tracks. In addition, there is the obvious benefit of utilising an inexpensive, locally produced building material in situations were the delivery of heavier materials would pose problems. Timber can easily be combined with soil anchors and geotextiles in the same way as concrete or steel. Round timber and sheet piles can provide an economical wall for moderate heights of retained material. Examples of interlocking timber sheet piles are given in BS 6349-2 (1988). Further demonstration of the suitability of treated timber as foundation material is provided by the permanent wood foundation or PWF (CWC, 1997). PWF is a load bearing wood-frame system designed as a foundation for light-frame construction for residential houses, commercial premises such as hotels and factories, and agricultural buildings. Its use dates back to 1967 in Alberta, Canada. Water-based pressure treated timber is laid directly onto a granular drainage layer 300mm deep. This drainage layer prevents hydrostatic pressure building up against the foundations, and allows timber framed basements to be constructed in suitable locations. All connectors are corrosion resistant. A polythene moisture barrier extends over the outside of the walls below ground level terminating at the top of the drainage layer (Figure 14). A separate moisture barrier exists under the floor of the basement, either over-site concrete or a suspended timber floor. There is no need for a damp proof course between the timber footing plate and the bottom rails of the wall panels which, like timber frame, comprise the main support of the structure.
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Figure 14: Permanent Wood Foundation (illustration Canadian Wood Council) Stanchions and columns can be founded on footings comprising two layers of nailed treated timber running at 90 degrees to each other (Figure 15). The timber footing is laid on a thin layer of sand over undisturbed soil. A steel plate placed over the top of the footing helps to transfer the load from the column over the timbers. The principal advantages of these treated timber foundations are that they are economical, fast, require less plant, and do not need measures to protect them from freezing - this last aspect being particularly important in Canada. Timber foundations also make excellent usage of an abundant, renewable and local material. Timber foundations can also take the form of embedded poles with concrete pads and collars. Examples of these types of foundation for countryside structures, together with half round timber footings and an example of a simple wood foundation for a footbridge are given by Jayanetti (1990).
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Figure 15: Timber column base (illustration Canadian Wood Council)
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10.0 BRIDGE DESIGN PRACTICE 10.1 General practice for design of bridges in the UK Bridges constructed from steel or concrete, whether highway or footbridges, are generally designed using the BS 5400 series of Standards. This series comprises ten parts, as follows: Part 1
General statement giving design objectives and definitions.
Part 2
Specification for loads.
Part 3-5
Codes of practice for design of steel, concrete and composite bridges.
Part 6-8
Specifications for materials and workmanship for steel, concrete and composite bridges.
Part 9
Specification for bridge bearings.
Part 10
Code of practice for fatigue.
The partial factor design process for a bridge will primarily involve only two of the above parts. Firstly the loads, and the partial safety factors for the loads, are obtained from Part 2. Secondly the design (including partial safety factors for materials) is undertaken in accordance with Part 3,4 or 5 depending on which construction material is being used. In the case of road bridges or bridges over roads, the Highways Agency (formerly Department of Transport) has produced a number of Department Standards which occasionally override the requirements of the BS 5400 Series. One such Department Standard BD29/87 gives directions to engineers on how to design footbridges, including timber types.
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An Outline Procedure For The Design Of Timber Footbridges Over Roads Or In Urban Areas In outline, the procedure may be considered in three stages, as follows: 1.
Establish the general arrangements for the bridge, taking note of the requirements for layout and minimum dimensions given in DoT Departmental Standard BD29/87.
2.
Evaluate the loads acting on the bridge, using the unfactored loads of BS 5400: Part 2, unless these are made more onerous by an HA Standard.
3.
Design the members of the bridge in accordance with BS 5268: Part 2 which is a permissible stress code, used principally for timber members in buildings. BS 5268 does however contain sufficient basic materials properties, fastener design information and member design procedures for simpler types of timber bridge, as explained above in the section dealing with materials.
An Outline Procedure For Design Of Timber Footbridges In Suburban Or Rural Areas Many engineers specialising in timber bridges consider the BS 5400/HA provisions for minimum dimensions and loading too severe for lightly trafficked footbridges in suburban and rural areas. For such footbridges, typical alternative procedures are exemplified as follows: 1.
Establish the general arrangements for the bridge taking note of the minimum dimensional recommendations given in publications such as "Footbridges in the Countryside, Design and Construction" (Countryside Commission for Scotland, 1989).
2.
Evaluate the loads acting on the bridge using the unfactored loads of BS 5400: Part 2, or consider making them less onerous on the basic of recommendations given in publications such as the above.
3.
Design the bridge members in accordance with BS 5268: Part 2.
Examples of how the "Countryside Commission for Scotland" publication recommends less onerous minimum dimensions and loadings are given in Tables 7 and 8 below:
Source of data Location of bridge
DoT Standard BD29/87 Urban area
Pedestrians only 1800 People with disabilities 2000
"Countryside Commission for Scotland" publication Accessible Inaccessible rural area rural area (one(two-way way traffic) traffic) 1200 900 1700 1200
Table 7: Recommended deck widths (mm) for alternative bridge locations
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Source of data
BS 5400: Part 2
"Countryside Commission for Scotland" publication
Location of bridge
Urban Area
Rural Area
Vertical imposed uniformly distributed load on bridge deck (kN/m‘)
5.0
2.3 – 3.2
Horizontal load per metre (kN/m) perpendicular to handrail
1.4
0.74 – 1.4
Table 7: Recommended imposed loadings for alternative bridge locations
Highways Agency criteria for layout and dimensions of footbridges Layout of Footbridge The HA (former DoT) Standard BD29/87 stipulates several criteria relating to the layout of footbridges, some of which are quite fundamental to the bridge design. These may be summarised as follows: Access: Access to footbridges located adjacent to carriageways should be sited so that pedestrians walking down the access face on-coming traffic. Plain ramped access is preferred to stairs as it is more satisfactory for people in wheelchairs and pedestrians pushing prams. However wherever possible both forms of access should be provided. Layout: The main span of a footbridge should, wherever possible, be at right angles to the road carriageway. Supports: Where a footbridge crosses a dual carriageway, preference should be given to spanning both carriageways in a single span, to avoid the need for a support in the central reserve. Supports which may be subject to collisions by errant vehicles shall be designed to resist collision loading.
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Dimensions for footbridges Width of bridges for pedestrians only: The dimensions of the clear widths of the main span, ramps and stairs of a footbridge would be derived on the basis of the information in Table 9 (below) meet the peak pedestrian traffic.
Gradient
Clear width (mm)
< 1/20
300mm per 20 persons per minute
Steps or > 1/20
300mm per 14 persons per minute
Table 9: Recommended clear widths (mm) for alternative bridge gradients Minimum widths of 1800mm for general purposes, or 2000mm stipulated for bridges enhanced for use by disabled persons. Where bridges are to be designed for the combined use of pedestrians and cyclists, further width requirements apply. These range from a 1200mm wide footpath separated by a white line from a 500mm wide cycle track, to a 1950mm wide lane for each, separated by railings. The DoT criteria for heights of parapets vary according to the types and combinations of bridge user. They are summarised in Table 10, below.
Type of footbridge
Parapet height (m)
Pedestrians only, where bridge is in area of high prevailing winds or with headroom under bridge greater than 10m
1.15 – 1.30
Pedestrians and cyclists
1.4
Table 10: Height requirements for bridge parapets Stairway requirements may be summarised as follows: Maximum number of stairs in a flight is 20 Riser dimension < 150mm Tread dimension > 300mm. It is a preference that ramps should not be steeper than 1 in 20. However if limitations of space dictate then steeper ramps may be used, up to a maximum gradient of 1 in 12. For ramps with gradients greater than 1 in 20, landings must be provided in order that the rise of any ramp section does not exceed 3.5m
Evaluation of loads using BS 5400: Part 2
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As mentioned above, timber footbridges are designed using the unfactored nominal loads of BS 5400: Part 2. Interestingly, the use of unfactored nominal loads in structural design is not only limited to timber, with the design of foundations and that for aluminium structures also being based on unfactored loads. Where adequate statistical distributions are available, the nominal loads in BS 5400: Part 2 are those appertaining to a return period of 120 years. The following types of nominal load relating to footbridges are considered by BS 5400: Part 2 1. Permanent loads • •
Dead-weight of structural elements Superimposed dead – road surfacing, etc.
2. Live loads from pedestrian traffic • •
Nominal vertical live load Nominal load on pedestrian parapets
3. Wind loads • • •
Transverse Longitudinal Vertical
4. Loads from temperature effects 5. Erection loads BS 5400: Part 2 suggests that in most cases snow loads can be ignored. This is logical since the full pedestrian design load is improbable under heavy snow falls of the duration likely to be experienced in the UK. The maximum wind gust speed is evaluated by applying gust factors to mean hourly wind speeds extracted from a map of isotachs. The magnitude of the gust factor depends on the height of the bridge, and the horizontal wind loaded length. For footbridges only, BS 5400: Part 2 allows the following reductions in wind load: 1. The mean hourly wind speed is reduced by 0.94, which is a conversion factor to obtain 50-year return period values from 120-year return period values. 2. A reduction in the gust factor when the bridge is located in urban areas or a rural environment with many windbreaks. To use BS 5268: Part 2 for the design of bridge members, the designer has to decide upon the service conditions for the bridge. This mainly involves deciding the exposure and duration clauses that are appropriate for the member concerned. Experience has shown that designers usually are on the conservative side by choosing to design members using wet exposure stresses. With BS 5268, the threshold moisture content between wet and dry exposure conditions is 18%. Hence this is often unnecessarily conservative. For a vertical imposed uniformly distributed loading which
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represents a crowded bridge, experience has shown that designers usually select medium-term duration. This is the duration class used with BS 5268 for snow loading in the UK. Horizontal loads on handrails or parapets are usually designated as shortterm. This is the same load duration class as that arising from the case of a man standing on a roof member.
10.2 Deflection limits The limited guidance given in BS 5268: Part 2 regarding deflection limit is of little relevance to the design of bridge members. Enquiries indicate that a static deflection limit of Span/200 under imposed load only is often used for beam members. The "Countryside Commission for Scotland" publication recommends a tighter limit of Span/240 under total loading. Lightly precambered glulam bridge beams are often designed using a deflection limit for live load only, which is permitted in principle by BS 5268.
10.3 Eurocode 5 As explained above, it has proven possible to design acceptable timber footbridges using BS 5268: Part 2 recommendations, supplemented by additional guidance from elsewhere. The eventual publication of EC5, Part 2 will be a great step forward and will be welcomed by everybody involved. Meanwhile, EC5, Part I is shortly to be available. Thus even at this stage, the Eurocodes will bring advantages to the more sophisticated aspects of timber engineering such as bridges. EC5 introduces limit states design to timber for the first time in the UK. It is therefore a more radical change for timber than the introduction of Eurocodes for other major structural materials. EC5 contains the essential rules for design, but unlike the British Code, BS 5268, it does not include material properties, tables of fastener loads and other such other design information. An immediately obvious change is that wherever possible, EC5 uses equations rather than tables. Also much of the nomenclature and terminology is considerable different. As in all instances of limit states design codes, EC5 will require clear thinking about the distinction between ultimate and serviceability limit states. The latter are, of course associated with deflections, deformations and vibration. EC5 treats these matters in a considerably more sophisticated manner than their coverage in BS 5268. As has already been pointed out, BS 5268 gives no adequate guidance related to deflection limits for bridges. Furthermore it is likely that the Working Group dealing with EC5, Part 2 will require to give considerable thought to the serviceability topics. EC5 offers a number of advantages over BS 5268. It provides the opportunity to design with a wide selection of materials and components. The use of characteristic values for materials, based directly on test results, means that new materials and components, which have achieved suitable technical approval can more easily be assimilated, thus facilitating development and innovation. More guidance is given on the design of built-up components than in BS 5268, and EC5 provides a unified design and safety basis for laterally loaded dowel-type joints (nails, staples, screws, bolts and steel dowels). The ENV contains no information on the design of joints using connectors such as toothed plates, shear plates and split rings, but a procedure is being developed through other sponsored research programmes. Interim guidance on the design of such joints is contained in the UK NAD.
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10.4 Overseas practice - Decks The principal function of a deck is to distribute the loads produced by the traffic to the supporting elements of the bridge. If the deck is designed as a structural diaphragm, it can be used as a major spanning element in its own right. It can also brace other main components, and transfer horizontal wind or brake loads through to other parts of the structure, and ultimately to the foundations. Dependent on its location, a secondary function of the deck may be to protect the main structure from moisture and mechanical damage from traffic. An effective and durable protection of substantial parts of the timber structure may be achieved with closed, high-level decks. Structural timber decks have been used in North America since at least the 1930s. Initially, nailed laminated construction was used. Timber-concrete composites were also introduced at quite an early stage. Later, glued laminated beams connected with shear devices started to be used extensively. Nowadays, structural timber decks exist in many designs, using half-round timbers, sawn timber, glued laminated timber, and structural timber composites of various types. In parts of Europe, particularly the German-speaking countries, structural decks of all the types mentioned above are found. A more traditional type, used on lighter bridges, including footbridges, is the two-layer herringbone boarded pattern. This does not make a major structural diaphragm contribution, in comparison with stressed laminated decks, for example, but it does contribute to the lateral bracing of the structure. A good example is in the large Main-Donau Canal 190 m long tension ribbon bridge, described in STEP Lecture E17. There is significant coverage of structural deck design in prEN Eurocode 5 Part 2. Research and practise in structural decks of various forms has also occurred in the Nordic countries and in Australia and New Zealand.
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Common Timber Deck Types In Canada: Longitudinal and transverse nail-laminated (LNL/TNL) decks: In longitudinal laminated decks, the timber laminations are orientated parallel to the direction of the traffic, whereas in transverse laminated decks the laminates are orientated perpendicular to the direction of the traffic flow.
Figure 16: Longitudinal laminated deck ( left) and transverse laminated deck (right)
Nailed laminated decks consist of planks of timber laid on edge side by side. The laminations are nailed together to form a slab. Nails are driven through the faces of the planks to fix them together laterally. 1. Timber-concrete composite (TCC) decks: A concrete topping is applied to a timber deck, traditionally a longitudinal nail laminated type, giving the slab a concrete compression zone and wearing course. Shear keys are required between the timber and the concrete layers. 2. Longitudinal or transverse stress laminated (LSL/TSL) decks: In addition to nails, post tensioning is applied using high strength steel to improve the load transfer of the deck. Although the stiffness of the timber is low perpendicular to the grain, pre-stressing allows a plate action in the deck. Edge members are often made of hardwood or steel, in order to avoid damage due to high bearing stresses perpendicular to the grain at the pre-stressing points.
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Figure 17: Tension rods 3. Floor beam ( or timber tie) decks: This type is a beam and plank deck. the planks overlay heavy transverse beams or ties. 4. Two layer plank decks: As above, but with two layers of heavy planks.
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Primary highways subjected to high volume of traffic
New bridges on secondary roads with medium volume of traffic
New bridges on secondary roads with low volume of traffic
Lightly used park and forest access roads with occasional heavy vehicles
Longitudinal nail laminated
Very low volume roads with little or no commercial traffic
Deck replacement on steel truss bridges
Economical when supported on timber pile bents
Economical alternative to replacing concrete Can give higher live load capacity Speedy replacement
Transverse nail laminated
On steel or timber girders
Timberconcrete composite
Spans up to 7m
Longitudinal stress laminated
Viable option
Will span up to 8m for 286mm deep decks and longer for larger timber sizes
Transverse stress laminated
Not yet used, but shows promise
On steel or timber girders
Floor beam
Economical
Two layer plank decks
Economical
Table 11: Summary of the principal applications for various combinations of timber decks, beams and other structures used for highway bridges in Canada
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11.0 FUTURE CHALLENGES Fresh challenges constantly arise, both for timber engineering in general, and specifically for bridges. Salient aspects of timber engineering materials and design relating to timber bridges which are still under development are outlined as follows:
11.1 High efficiency composite materials Although timber provides high specific strength and stiffness, until recently, its natural variability remained a challenge. However, through efficient strength grading procedures and associated quality assurance measures, timber and wood-based composites offer variability levels as low as those attained in metallic engineering materials, and certainly much lower than in many commonly used grades of concrete. High-strength softwood glued laminated beams, using laminations of adequate controlled quality and regulated end jointing techniques, can be produced with characteristic bending strengths up to typically 36 N/mm2. By substituting STCs for the outer laminations, composite elements of 15~25% higher bending strength may be guaranteed, without increase in weight. Such composite materials are comparable in strength to high-performance reinforced concrete, but have a mass of only onequarter of the latter. European research is also in progress in which, by utilising structural composites based on temperate hardwood veneers, bridge beams with characteristic bending strengths greater than 60 N/mm2 are feasible, with weight increases of only 15%.
11.2 New adhesive bonding technologies The classical timber engineering adhesives have been proven adequate for small and medium spans in buildings. However new technical and economical requirements have been driving towards improved adhesives and new bonding techniques. Adhesive bonding is only now becoming accepted in the field of bridge engineering in general, with strides having been made in resin bonding technologies for the refurbishment and upgrading of steel and concrete structures. Until recently, these techniques have depended upon the use of steel as the bonded-on reinforcing medium. Recently however, field trials and in situ monitoring have commenced on bridges reinforced with Fibre Reinforced Plastics (FRPs), involving such advanced materials as Carbon Fibre. In the light of such developments, the climate of acceptance has grown for the possibility of the greater use of bonding technologies in timber bridge engineering. prEN Eurocode 5 Part 2 Bridges contains an Informative Annex on the use of “Gluedin Steel Rods”. These rules have recently been supported by a major European Collaborative research Programme, known as “GIROD”. This will permit bonded-in rods to form safety critical connections in timber bridge structures, and also provide the basis for calculations concerning bonded-in reinforcements for applications such as large structural deck plates. At present, the Eurocode Informative Annex design recommendations apply only to steel rod-type reinforcements and connections, these having already been subjected
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to earlier research programmes including outdoor exposure assessments. However, this research is now being extended to embrace bonded-in materials of other classes, including FRP Pultrusions joined to timber and timber composites (Mettem, 1996). Bridge parts depending upon adhesive bonding for their manufacture and assembly entail substantial manufacturing planning and production arrangements, to ensure that the conditions for correct control are met. In large decks and other bridge structural members, sections of up to about 1m 2 of cross-sectional area have so far been produced, and this upper limit is constantly being expanded.
11.3 Steel reinforced timber As mentioned above in connection with new bonding technologies, steel reinforcements are already being used within timber elements contained in bridge structures. In prEN Eurocode 5 Part 2 Bridges, design rules can be found for reinforced members, transversely reinforced timber, and for deck plates containing such reinforcements. Most frequently, these steel reinforced timber elements serve as high duty deck plates, although local reinforcing also takes place in stress-critical zones in the main structural members. Reinforcement is especially worth considering where elemental design is restricted through the relatively low tensile strength of timber transverse to the grain direction. It is also possible to use certain techniques to improve shear resistance, in situations where the action effects tend to cause sliding of adjacent fibres (known as shear parallel to the grain). Another instance where this type of reinforcement may be considered is at notches and at other abrupt changes in section, where stress concentrations can cause Mode I or Mixed Mode I/II crack growth and fracture. Finally, it is considered in some instances where highly concentrated action effects are to be transferred between parts of the structure, and where greater strength can be attained if such forces are transferred to zones deep within the receiving members. Where timber road bridge decks are steel reinforced, in general, the bars or tendons are located in directions inclined or parallel to the major fibre directions of the parent wood. In some instances, bonded-in steel-reinforcements are pre-stressed, to compensate for shrinkage effects which may occur with the components in service
11.4 Timber-concrete composites Current timber-concrete composites are described above, and as stated, design principles and application rules are already given in the structural timber Eurocodes. Challenges which remain for such systems, include searching for ways of achieving still further efficiency in the composite interaction benefits that can be achieved with better, more durable, shear key systems. Effort is also concentrated on securing continuous improvements in the sealing and moisture protection of these types of deck.
11.5 Deck protection systems Better deck protection systems are being given a high priority in several of the research programmes outside the UK. Higher traffic speeds and intensities of wheel passage have drawn attention to the need for greater deck protection against wetting
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that is spread from the vehicles themselves. Deck systems and structural elements both need carefully designed protection to avoid cumulative damage from these effects. To some extent, the timber deck and the main structure may be treated independently in this regard.
12.0 PRIORITY WORK AREAS This Report has set out the state of the art of timber bridge design in the UK, with wider references to Europe in general. From this, it is possible to identify the priority work areas in which it is felt that effort should be concentrated, from a UK point of view.
12.1 Innovative Timber Engineering for the Countryside The above is the title of the Pilot Project in which BRE and TRADA Technology are engaged as Technology Providers. 1.
Review serviceability criteria for footbridges in all materials.
2.
Continue bridge survey work, paying special attention to serviceability performance and durability aspects.
3.
Carry out design studies leading to the construction of prototypes which are to be used for serviceability assessments and durability monitoring.
4.
Produce nationally applicable design guidance.
5.
Maximise the use of UK grown species in sizes (round and sawn) likely to be available locally for bridge construction
Within item 3 above, the initial design studies have already led to the conclusion that there are four areas where supplementary design guidance is required. These are as follows: 1.
The design and detailing of bracing systems to ensure member stability and to resist horizontal wind loads
2.
Methods to ensure satisfactory vibration performance of bridges under human footfalls
3.
Simple methods to ensure that the excitation of bridges by wind is avoided.
4.
In the case of mechanically laminated beams, a simplified method of evaluating deflections is required.
12.2 prEN Eurocode 5, Part 2 The Project Team has identified several priority areas coinciding with those felt to be necessary in the UK. The following have been suggested: 1.
The dynamic behaviour of bridges under pedestrian and wind loadings.
2.
The fatigue behaviour of connections.
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3.
Timber-concrete composite behaviour.
4.
Bonded-in steel rods.
5.
Durability requirements for timber bridges, including those for the connectors.
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13.0 CONCLUSIONS Timber engineers have the expertise to provide aesthetically exciting, well-protected, and durable bridge structures. To achieve impact, economic drivers must be harnessed, to unlock consumer and specifier indifference. Key motivators include: • • • • • • •
National cycle routes City regeneration, calling for aesthetically exciting, well-performing links. Canal and rail regeneration Marina and dockside development Housing developments, with associated bridging needs. Forest roads and infrastructure maintenance in remote regions. Linking to value-added forest products.
The use of sustainably grown and locally produced timber for bridge, foundation and sea defence engineering will increasingly be seen as favourable. In addition there are concerns and moves in Europe away from the use of timber treatments such as creosote and Copper Chrome Arsenic. Applied research and development, demonstration projects, and benchmarking involving the use of domestic grown timber are seen as vital. Above all, however, well-informed promotion is recognised as of paramount importance in unlocking demand for timber bridges as flagship projects in sustainable development, environmental protection, and improvements to the quality of life.
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REFERENCES AND BIBLIOGRAPHY Anon. (1988) Design Manual for Roads and Bridges. Volume 2 Highway Structures: Design (Substructures and Special Structures), Materials; Section 2 Special Structures; Departmental Standard BD 29/87 Design Criteria for Footbridges. HMSO, September 1988. Anon (1985) Grubenmann, Hans Ulrich; and Grubenmann, Johannes. Entry in Encyclopaedia Britannica. Web 2001 URL: http://www.britannica.com/ Anon (1985) Pre-fabricated Modular Timber Bridges; Part 1. General Description; Part 2. Manufacture of Pre-fabricated Parts and Design Selection; Part 3. Construction and Launching; Part 4. Timber Technology; Part 5. Typical Design, 15m span Four Truss Bridge. Restricted, UNIDO/IO/R.163, UNIDO, Vienna. Anon (1983) The National Timber Bridge Initiative, Report on fiscal year 1983. US Department of Agriculture Forest Service, Washington DC. Anon (2001). Timber Engineering Design – Engineering Guidance Documents. See entries in: TRADA Web 2001 URL: http://www.trada.co.uk/frames_bookshop.asp Anon (1999) Timber Footbridge A4 Calne, Wiltshire. Journal, Institution of Structural Engineers, Vol. 77, No. 16, 17 August 1999. ISBN 1466-5123. Bainbridge R. J., Harvey K., Mettem C.J., and Ansell M.P.(2000) Fatigue Performance of Bonded-In Rods in Glulam, Using Three Adhesive Types. International Council for Research and Innovation in Building and Construction, Working Commission W18 – Timber Structures, 33-7-12, Delft. Bakke, K. and Soli, K.H.(1996) Nordic Timber Bridge Project – Market Survey. Nordic Timber Council, ISBN 91 89002 01 06. Binding, John (1993). Brunel's Cornish Viaducts. Pendragon Book, Atlantic Transport Publications, Penryn, Cornwall, ISBN 0 906899 56 7. Blaser, Werner (1982) Schweizer Holzbruken - Wooden Bridges in Switzerland. Birkhauser Verlag, Basel, 1982. ISBN 3 7643 1334 X. Booth L.G., and Booth V. (1996) Timber Railway Bridges in England in the Period 1835 – 1860: their Structural Forms and Contemporary Lithographic Illustrations. Journal, Institute of Wood Science, Vol. 14, No. 1, Issue 79, Summer 1996. Booth, L.G.(1971-72) Laminated Timber Arch Railway Bridges in England and Scotland. Excerpt, Transactions of the Newcomen Society, Vol. XLIV. Booth, L.G.(1971) The Development of Laminated Timber Arch Structures In Bavaria, France and England in the Early Nineteenth Century. Journal, Institute of Wood Science, Vol. 5, No. 5, July. Booth, L.G.(1964) The Strength Testing of Timber During the 17th and 18th Centuries. Journal, Institute of Wood Science, Vol. 3, No. 13, November . Boutelje J. B. and Bravery A. F. (1968) Observations on the bacterial attack of piles supporting a Stockholm building, J. Inst. Wood Science Vol. 4, 47-57.
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British Standards Institution (1986) British Standard Code of practice for foundations. BS 8004: See BSI, London, Yearbook, for amendments and dates. British Standards Institution (1998) British Standard Specification for Manufacture of glued-laminated timber structural members. BS 4169: 1988. See BSI, London, Yearbook, for amendments and dates. British Standards Institution (1988) British Standard Specification for Softwood grades for structural use. BS4978: 1988. See BSI, London, Yearbook, for amendments and dates. British Standards Institution (1980) British Standard Specification for Tropical hardwoods graded for structural use. BS 5756: 1980. See BSI, London, Yearbook, for amendments and dates. British Standards Institution (1988) Code of practice for maritime structures - Part 2: Design of quay walls, jetties and dolphins. BS 6349-2: 1988. See BSI, London, Yearbook, for amendments and dates. British Standards Institution (various) Steel, Concrete and Composite Bridges. BS 5400 series, Parts as follows: Part 1 – General statement giving design objectives and definitions. Part 2 – Specification for loads. Part 3-5 – Codes of practice for design of steel, concrete and composite bridges. Part 6-8 – Specification for materials and workmanship for steel, concrete and composite bridges. Part 9 – Specification for bridge bearings. Part 10 – Code of practice for fatigue. See BSI, London, Yearbook, for amendments and dates. British Standards Institution (1996) Structural use of timber – Part 2: Code of practice for permissible stress design, materials and workmanship. BS 5268: Part 2: 1996. See BSI, London, Yearbook, for amendments and dates. British Standards Institution (1992) BS EN 335-1 Hazard classes of wood and woodbased products against biological attack. Classification of hazard classes, BSI London British Standards Institution (1994) BS EN 350-1 Durability of wood and wood-based products. Natural durability of solid wood. Guide to the principles of testing and classification of natural durability of wood, BSI London British Standards Institution (1994) BS EN 460 Durability of wood and wood-based products. Natural durability of solid wood. Guide to the durability requirements for wood to be used in hazard classes, BSI London British Standards Institution (1997) BS EN 599-1 Durability of wood and wood-based products. Performance of preservatives as determined by biological tests. Specification according to hazard class, BSI London
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British Standards Institution (1996) BS EN 351-1 Durability of wood and wood-based products. Preservative-treated solid wood. Classification of preservative penetration and retention, BSI London British Standards Institution (1998) DD 239 Recommendations for the preservation of timber, BSI London British Standards Institution (1989) BS 5589 Code of practice for preservation of timber, BSI London Brown D. J. (1988) Bridges – Three Thousand Years of Defying Nature. Mitchell Beazley. (Rev.) ISBN 1 85732 163 4. Building Research Establishment (1998) Digest 429 Timbers: their natural durability and resistance to preservative treatment. Canadian Wood Council (1997) Permanent Wood Foundations, CWC, Ottawa, Ontario, ISBN 0 921628 49 8. Canadian Wood Council (1991) Wood Piles, CWC, Ottawa, Ontario. ISBN 0 921628 08 0. Chugg, W.A. (1962) Report on a Visit to Switzerland to Inspect Glued Laminated Timber Structures Over Ten Years Old. E/IB/7, Timber Research And Development Association, High Wycombe. Chugg, W.A. (1958) The Structural Glued Laminated Timber Industry in North America. E/IB/4, Timber Development Association, London. Copani, P. and Funis, F. (1999) Il ponte di Palladio sul Cismone e le altre tre “invenzioni senza porre altrimenti pali nel fiume” – Palladio’s bridge on the Cismone River and the other three “invenzioni senza porre altrimenti pali nel fiume” Bollettino Ingegneri N. 12 – Collegio Ingegneri, Firenze, dicembre. Countryside Commission for Scotland (1989) Footbridges in the Countryside – Design and Construction. Reiach Hall Blyth Partnership. 2nd ed. Battleby, Perth, Scotland, ISBN 0 902226 52 5. Curry, W T. (1974) Grade stresses for structural laminated timber. Republished as Chapter 5 of: The strength properties of timber. MTP Construction, Lancaster, England. Department of Transport (1992) Design Criteria for Footbridges. BD29/87, Reprint November, London Desch H. E. and Dinwoodie, J. M. (1996) Timber - Structure, Properties, Conversion and Use, Macmillan Press Ltd, London. Farmer, R.H. (1972) Handbook of hardwoods, Rev. 2nd. Ed, HMSO London, ISBN 0 11 470541. Fischer, J.(1995) Timber Bridges. In: Chapter E17 of: Timber Engineering, STEP2 – Design – Details and Structural Systems. Ed. Blass, H.J. et al. Centrum Hout, Almere, Netherlands, ISBN 90 5645 002 6.
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Graham, P.E.(2000) Design of Timber Foundation Piling for Highway Bridges and Other Structures. National Timber Piling Council, Rye, N.Y. Grob, A. Krähenbuhl, A. and Wagner, A. (1983) Survey, Design and Construction of Trail Suspension Bridges for Remote Areas. SKAT, Swiss Center for Appropriate Technology, St. Gallen, Switzerland. Haldeman, B. (1982) The History of Timber Piles. American Wood Preservers Institute, Annual Meeting. Heinz, R. A. (1975) Protection of piles: wood, concrete, steel. Journal of Civil Engineering, A.S.C.E., Dec. pp 60-64. HMSO (1972) Handbook of hardwoods, London Institution of Civil Engineers (1947) Deterioration of structures of timber, metal and concrete exposed to the action of sea-water 19th Report of the Committee of the Institution of Civil Engineers. James, J.G.(1982) The Evolution of Wooden Bridge Trusses to 1850. Pre-print, Paper Presented to Institute of Wood Science Timber Engineering Group, Imperial College, March. Jayanetti, D.L. (1990) Timber Pole Construction. Intermediate Technology Publications, TRADA, High Wycombe. ISBN 1 85339 068 2. Levin, E. (1956) A Review of the Laminated Timber Industry in Western Europe. E/IB/2, Timber Development Association, London. Mettem, C J. (1994) Design of timber footbridges, a review of design and manufacturing criteria. TTL/BRE Report, Restricted Circulation, TRADA, High Wycombe. Mettem, C J. (1993) Design of timber footbridges, a survey of existing bridges. TTL/BRE report, restricted circulation, TRADA, High Wycombe. Mettem, C.J. Gordon, J.A. and Bedding, B. (1996) Structural Timber Composites – Design Guide. TRADA Technology TTL DG1, High Wycombe. ISBN 1 00510 01 4. Morris (1990) Earth Roads: a practical manual for the provision of access for agricultural and forestry projects in developing countries. Cranfield Press, ISBN 0 947767 93 2. Nash, J.K.T.L. (1981) The Foundations of London Bridge. Canadian Geotechnical Journal, Vol. 18 pp 331 – 356. National Museum of Science and Technology, Milan, Web 2001 URL: http://www.museoscienza.org/english/leonardo/pontegirevole.html Nelson, L.H.(1990) The Colossus of 1812: An American Engineering Superlative. American Society of Civil Engineers, New York.
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Newlands, J. (1990) The Carpenter and Joiner’s Assistant. Glasgow, Blackie, 1857. Reprinted, Studio Editions, London. Nordic Timber Council (1999) Timber Bridges – a presentation of 22 Nordic timber bridges. NTC, Stockholm. O'Connor, C. (1993) Roman Bridges. Cambridge University Press, 1993. Paolillo, G.G.M. (1999) Il ponte di Trajano sul Danubio presso Drobeta nella Dacia Inferior – Trajan’s Bridge on the Danube at Drobeta in the Dacia Inferior. Bollettino Ingegneri N. 12 – Collegio Ingegneri, Firenze, dicembre. Peek, R.D. and Willeitner, H. (1981) Behaviour of Wooden Pilings in Long Time Service. Proc. 10th Internal Conf. on Soil Mechanics and Foundation Engineering, Stockholm, AA Balkema, Vol. 3 pp 147-152. Popa, N. Bancila, R. and Florea, S. (1992) Trajan’s Bridge Over the Danube at Drobeta Turnu Severin. In: Proc. International Conference - Bridges on the Danube, Vienna – Bratislava – Budapest. Hungarian Nat. Cttee. IABSE, Sept. ISBN 963 421 5009. Reiach Hall Blyth Partnership (1981) Footbridges in the Countryside. ISBN 0 902226 52 5. Countryside Commission for Scotland, Perth, 1981. Ruddock, Ted (1979) Arch Bridges and their Builders 1735 - 1835. Cambridge, 1979. Taylor R.J., and Keenan, F.J. (1992) Wood Highway Bridges. ISBN 0-921628-12-9, Canadian Wood Council, Ottawa. The American Wood Preservers Institute (2000) Construction Guidelines for Timber Piling, AWPI Web, URL: http://www.awpi.org/ppt/home.html/ The Highways Agency (1996) The Appearance of Bridges and Other Highway Structures. HMSO, London. ISBN 0 11 551804 5. Wilson, T.R.C.(1939) The Glued Laminated Wooden Arch. USDA Tech. Bulletin 691. Yeomans, D.T. (1985) Historic Development of Timber Structures. In: Timber in Construction. Eds. Sunley, J.G. and Bedding, B. Batsford/TRADA, London, ISBN 0 7134 5053 3.
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Timber Bridges and Foundations A report produced for the Forestry Commission
PREPARED BY:
G Freedman - FCE (InTeC chairman) C Mettem, P Larsen, S Edwards - TRADA Technology T Reynolds, V Enjily - BRE November 2002
BRE Ltd, Bucknalls Lane, Garston, Watford, WD2 7JR 01923 664000 TRADA Technology Ltd, Stocking Lane, Hughenden Valley, High Wycombe, HP14 4ND 01494 563091 Forestry Civil Engineering, Greenside, Peebles, EH45 8JA 01721 720 448
© Building Research Establishment Ltd 2002 © TRADA Technology Ltd 2002 © Forestry Civil Engineering 2002
Front Cover Picture - Footbridge at Garpenburg, Sweden, with timber caisson foundations (photo BRE)
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EXECUTIVE SUMMARY Bridges are one of the highest forms of civil engineering - few other structures command the same combination of functionality and visual impact. In the United Kingdom bridge building in timber has been very limited. This is in marked contrast to the initiatives which have taken place in North America (USDA Forest Service Timber Bridge Initiative), Canada, and Northern Europe (Nordic Timber Bridge Programme). World-wide, the use of timber for bridges is experiencing a major revival. In most industrialised countries other than the UK, timber is widely and increasingly being used, for vehicular, as well as for pedestrian bridges. The strength, lightness in weight, energy absorption and environmental features of timber make it highly desirable for bridge construction. Although there is an established history, and a continued use, of timber for bridges in the United Kingdom applications tend to be limited both in span and capacity, than is merited by the virtues of this aesthetic, sustainable material. Experience elsewhere in the world is showing that with correct design, timber is also a durable material for vehicle carrying bridge structures and, additionally, piled foundations. Nevertheless, this aspect remains a significant query in the minds of many mainstream designers, both engineers and architects, who advise UK clients. Revitalised timber bridge activities elsewhere are impressing UK specialists. Nevertheless, there is a great need to disseminate awareness and knowledge to mainstream designers, commissioners of projects and the public. At present, timber bridge producers in the UK are a small, niche sector of the UK timber industry, and some firms are really only representatives of producers that are adding the main value elsewhere in Europe. Timber engineers have the expertise to provide aesthetically exciting, well-protected, and durable bridge structures. To achieve impact, economic drivers must be harnessed, to unlock consumer and specifier indifference. Key motivators include: • • • • • • •
National cycle routes City regeneration, calling for aesthetically exciting, well-performing links. Canal and rail regeneration Marina and dockside development Housing developments, with associated bridging needs. Forest roads and infrastructure maintenance in remote regions. Linking to value-added forest products.
The use of sustainably grown and locally produced timber for bridge, foundation and sea defence engineering will increasingly be seen as favourable. In addition there are concerns and moves in Europe away from the use of timber treatments such as creosote and Copper Chrome Arsenic. Applied research and development, demonstration projects, and benchmarking involving the use of domestic grown timber are seen as vital. Above all, however, well-informed promotion is recognised as of paramount importance in unlocking demand for timber bridges as flagship projects in sustainable development, environmental protection, and improvements to the quality of life.
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Contents
EXECUTIVE SUMMARY 1.0
INTRODUCTION AND PROJECT BACKGROUND
1
2.0
THE HISTORY OF TIMBER BRIDGES
5
2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10
Bridges in ancient history Mediaeval bridges The Renaissance and the growth of trade Long spans - the triumphs of bridge carpentry The dawning of industrialisation Laminated timber - from mechanical to reliable adhesive technology The railway era Protective design lessons from history Maintenance of historic timber bridges in Britain New materials
5 6 6 7 8 8 9 11 11 12
3.0
THE OVERSEAS DEVELOPMENT OF TIMBER BRIDGES
13
3.1 3.2
Relevant history The way forward to make use of international research
14 18
4.0
CURRENT UK POSITION
20
5.0
CATEGORIES OF TIMBER BRIDGES
24
5.1 5.2
Categories of use Locations
24 24
6.0
STRUCTURAL FORMS
26
6.1 6.2 6.3 6.4 6.5 6.6
General Beams, including bowed types, no arch action Arches Girder beams & trusses Lift & swing bridges Further design fundamentals
26 29 29 29 29 29
7.0
MATERIALS
31
7.1 7.2 7.3 7.4
Principal elements Decks & decking – UK current practice Parapets & handrails Connections
31 36 37 38
8
DURABILITY
40
8.1 8.2 8.3
Detailing Natural durability Preservative treatments
40 42 44
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9.0
TIMBER FOUNDATIONS
47
9.1 9.2 9.3 9.4 9.5 9.6
History and overseas Use Durability of timber piles Traditional timber species and treatments Marine structures Pile driving and design Other geotechnical uses for timber
47 48 49 49 50 52
10.0
BRIDGE DESIGN PRACTICE
55
10.1 10.2 10.3 10.4
General practice for design of bridges in the UK Deflection limits Eurocode 5 Overseas practice - Decks:
55 60 60 61
11.0
FUTURE CHALLENGES
65
11.1 11.2 11.3 11.4 11.5
High efficiency composite materials New adhesive bonding technologies Steel reinforced timber Timber concrete composites Deck protection systems
65 65 66 66 66
12.0
PRIORITY WORK AREAS
67
12.1 12.2 13.0
Innovative Timber Engineering for the Countryside prEN Eurocode 5, Part 2 CONCLUSIONS
67 67 68
REFERENCES AND BIBLIOGRAPHY
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1.0 INTRODUCTION Bridges are one of the highest forms of civil engineering - few other structures command the same combination of functionality and visual impact. In the United Kingdom bridge building in timber has been very limited. This is in marked contrast to the initiatives which have taken place in North America (USDA Forest Service Timber Bridge Initiative), Canada, and Northern Europe (Nordic Timber Bridge Programme). World-wide, the use of timber for bridges is experiencing a major revival. In most industrialised countries other than the UK, timber is widely and increasingly being used, for vehicular, as well as for pedestrian bridges. The strength, lightness in weight, energy absorption and environmental features of timber make it highly desirable for bridge construction. Although there is an established history, and a continued use, of timber for bridges in the United Kingdom applications tend to be quite limited - although some very fine short span timber footbridges are constructed. Experience elsewhere in the world is showing that with correct design, timber is also a capable material for vehicle carrying bridge structures and, additionally, piled foundations. Nevertheless, this aspect remains a significant query in the minds of many mainstream designers, both engineers and architects, who advise UK clients. Revitalised timber bridge activities elsewhere are impressing UK specialists. Nevertheless, there is a great need to disseminate awareness and knowledge to mainstream designers, commissioners of projects and the public. At present, timber bridge producers in the UK are a small, niche sector of the UK timber industry, and some firms are really only representatives of producers that are adding the main value elsewhere in Europe. To illustrate the extent of use elsewhere, the United States Department of Agriculture reports that approximately 41,700 road bridges of over 6 m span are made of timber, and improvements are continually being introduced, through the federal Highway Administration Timber Bridge Programme. Also in North America, a number of significant modern timber bridge innovations were first introduced in Canada, in the 1970’s. These included the stressed laminated deck, details of which were added to the Ontario Bridge Code at that time. Since then, use of the material has continued, and the technologies have further improved, with several additional innovations such as new types of structural deck, and prefabrication systems. North American experience has been that in situations where salts and other de-icing chemicals are extensively applied, modern timber bridges are more durable than concrete structures. In Finland, about 700 timber bridges are owned by the Finnish Road Administration, and along with other Nordic countries, (Denmark, Finland, Norway and Sweden), a development programme has been in progress since 1994, to extend relevant techniques and experience. Due to the investments into research activities applied to timber bridges in the Nordic Region, and the increase of the general interest in the use of natural materials, timber bridges have become again a real alternative in bridge engineering (Figure 1). In continental Europe, particularly but not exclusively the alpine regions, impressive modern bridge structures are also to be seen, and major contributions have been made to developing harmonised codes and guidance documents, spearheaded by the new bridges Eurocode itself.
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Figure 1: Modern timber road bridge - Evenstad, Norway; 5 spans of bowstring trusses; 180m total length; creosote treated pine glulam; internally flitched steel gusset plates, attached with stainless steel dowels. (photo CM/Trada)
The advantages of timber for bridges is also recognised by quite a large number of emerging countries, such as the West African territories, notably Ghana; countries in Central and South America, as well as a number of Asian regions. In developing countries, the revival of interest in timber bridges in the fully industrialised zones of the world encourages a futuristic view, rather than a “poor material” attitude. For those with rapidly growing populations, this is eminently appropriate, not only from an environmental viewpoint, but also in order to be able to avoid expensive imported technologies and materials. Bridge clients, engineers and architects are beginning to become aware once more that bridges using this traditional material can be designed, fabricated and constructed in exciting new ways, as well as being created in forms sensitive to past traditions. Developments such as new, efficient connection techniques, and the introduction of modern wood-based composites which can be preservatively treated in environmentally acceptable ways, are further encouraging innovations. To re-establish timber bridges in the UK, a great deal needs to be done, especially in terms of “Knowledge Re-Packaging” and technical dissemination. Architecturally
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pleasing solutions need to be backed up by the ability and confidence to provide good protective design measures and to overcome prejudices about lack of longevity. Modern timber bridges need to be seen as more than just a routine, and possibly poorer alternative to concrete or steel bridges. Timber is a renewable construction material with impeccable "green" credentials. Trees, while they grow, absorb carbon dioxide and release oxygen. 1 cubic metre of dry softwood represents around 611kg of carbon dioxide that has been removed from the atmosphere. In addition, forests also provide areas for wildlife and recreation. Timber is light to transport, easy to handle and work with on site, and has a natural empathy with the landscape. To ensure a viable future timber supply chain for engineered, exterior structures, including bridges, the industry needs to grow both the high-profile, spectacular projects, and also the bread-and-butter access structures and smaller bridges that are of great amenity and community value. Producers and advocates of timber bridges also need to establish, sustain and grow their abilities to meet exacting performance requirements, in terms of safety, serviceability, and design life, as well as providing client satisfaction through elegance, tactility, warmth and craftsmanship. Contractors, looking for rapid delivery, and even faster erection, seek standard solutions. The importance of minimising road or track closures is paramount, and competing answers, especially steel footbridges, are fully geared up to these demands. Softwood timber production in the UK has doubled in the last 10 years and is about to double again in the next 10 years but pulp, paper and board markets are so saturated that new markets need to be developed. The structural market is poorly penetrated by home grown products and timber is the UK’s second biggest import. The focus of this research is to utilise poorer quality home grown timber for the high quality structural market. The timber-housing sector is growing steadily but needs a boost and any structural developments will be welcome. Rural structures e.g. bridges, towers and crash barriers are high profile uses will help move timber into the public eye and act as a catalyst for other developments. Research is essential to support these uses in the UK. To compound an already serious situation the value of the raw material has dropped dramatically over the last 5 years and this has led to a drop in harvesting and a general weakness in the industry at a time, ironically, when Forestry has been granted ‘Industry Cluster’ status in Scotland. This means that it is one of the country’s 5 ‘core industries’ employing a large number of people and as such, requires to flourish for the sake of the economy. These factors point to the desperate need for the creation of new initiatives, in the knowledge that they will be well supported by Government agencies. Innovative Timber Engineering for the Countryside: Against this background Forestry Civil Engineering (FCE) of Forest Enterprise (FE) and the two major players in timber research, Building Research Establishment (BRE) and Timber Research and Development Association (TRADA), came together to gather ideas. It was during initial meetings that agreement was quickly reached on the focus being “Countryside and Highway” and that to utilise lower quality home grown timber in high value added products “Engineering” would be required. Innovative ideas for research projects were put forward and immediately some common factors surfaced. Timber has some very desirable properties but it is relatively low in stiffness compared with steel. In the interest of sustainability and optimal utilisation of existing forest and woodland resources, there is a desire to include the use of lower grade material. This led us to accept that composites with
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steel, high quality timber or fibre reinforced polymer composites (FRPs) would be required to develop a product in which timber could display its best value. The objective of the InTeC project is to stimulate by research and demonstration the use of timber for road bridges as well as pedestrian traffic, and the use of timber for other related civil engineering including abutments, retaining walls and foundations.
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2.0 THE HISTORY OF TIMBER BRIDGES 2.1 Bridges in ancient history Timber is a traditional bridge building material, with examples in authenticated records dating to as long ago as 600 years BC. It is suspected that even before this, ancient cultures, including those in China, Persia, the Asian subcontinent and around the Mediterranean rim, had quite sophisticated timber bridge structures. Roman bridges are recorded in works quite accessible today. Julius Caesar himself, for example, records a large timber bridge in Italy, whilst Padillio (1518 – 1580) discusses another big bridge which was used by the Romans to cross the Rhine into Germany. There is also some evidence that the Roman bridge in London was by no means a crude or simple structure (O'Connor, 1993). One of the largest and best documented of the Roman timber bridges was built over the Danube, in what is now Bulgaria, in 104 AD. This is often known as “Trajan’s Bridge” (Figure 2), because its images are recorded on Trajan’s Column, now standing in Rome. This bridge consisted of 20 piers up to 45m high, each joined by a semi-circular timber arch of about 52m span. The thrusts in the triangulated timberwork, correctly transmitted into the masonry piers according to modern engineering concepts, seemed to be fully understood by the Roman engineers, who constructed and rapidly erected this prodigious feat. Methods of timber conversion and treatment for durability were also recorded in contemporary Latin texts.
Figure 2: An arch of Trajan's bridge, modelled by architectural historians, Florence University. (photo CM/Trada)
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2.2 Mediaeval bridges The oldest timber bridges that still exist in Europe date from the late mediaeval period, that is from the 14th to the early 16th century. Many of these are covered bridges, owing their longevity to this simple structural protective device. Several examples of these ancient bridges are in Lucerne, for example the Kapell bridge and the Spreuer bridge. Other such timber bridges, which are very important from both the historical and also the technical point of view, are those built by the State of Berne during the 16th century. These include bridges at Neubrugg 1532, Gummenen 1555, Wangen 1559 and Aarberg, 1568. These are still in good condition, most of them having their original main elements, and some still carrying heavy traffic. It is to be emphasised yet again that all of these bridges follow the same structural principle; that is protection of the timber against direct wetting from rain, sleet and snow, by means of a duopitched roof with a large overhang.
2.3 The renaissance & the growth of trade A large number of timber bridges which are still on record, and sometimes still in use, were built from the 16th through to the 18th century, when increasing trade and transport needs resulted in the construction of new and better roads. As a result of the beginnings of an understanding of engineering principles, during the spread of learning after the Renaissance, more technically advanced designs began to appear, and new construction techniques were introduced. These included arches, trusses and suspension bridges. Palladio, mentioned earlier as the recorder of Roman bridges, also documented and illustrated a series of his own “Inventions”. The sites of some, such as the oftenillustrated Cismone Bridge, have been rediscovered and archeologically investigated. Less well known are some ably-conceived timber trussed arch bridges, also by this same influential architect. Leonardo da Vinci, (1452-1519) Italian painter, sculptor, architect, engineer and scientist, was one of the greatest figures of the Italian Renaissance. He was active in Florence, Milan, and, from 1516, in France. Amongst his design sketches and notes are a series of ingenious timber bridges, several of which have been modelled in recent exhibitions of his life and works (Figure 3).
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Figure 3: Bridge designs by Leonardo da Vinci (photo CM/Trada)
2.4 Long spans – the triumphs of bridge carpentry During the 18th century, very long timber bridge spans were achieved through the use of arched trusses. Typical European examples include a Rhine bridge, constructed at Schaffhausen in 1758 by Hans Ulrich Grubenmann. This had an overall span of 119m, with the construction including a redundant pier at mid-span, which this famous bridge builder was obliged to include at the behest of the dubious “Bergermeisters” of the commissioning town. The structure had laminated arched ribs, each with a depth of about 2 metres and comprising seven courses of timber, notched and banded together. This same pioneer of timber bridging constructed a number of other impressive structures, all of which had complex end-jointing details, and many other advanced features. Expansion of trade and business in North America also gave rise to some very large timber arched spans, one of the most noteworthy being the “Colossus Bridge” over the Schuylkill river at Philadelphia, USA. This was constructed in 1812 by Lewis Wernwag, and had an amazing free span of 340 feet (102 m). The laminated arch elements each comprised six 6 x 14 inch (150 x 350 mm) heart-sawn baulks of softwood, separated, but strongly linked together with iron bands and threaded rods.
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2.5 The dawning of industrialisation The next stages in the evolution of timber construction saw a gradual transition from carpentry to engineering principles. This entailed the greater use of metallic fasteners in the form of bolts, rods, spikes, straps and other devices, such as special keys. These developments also involved the greater use of side-lapped members, rather than members intersecting in a single plain through mortises, tenons and other such carpentry joints. There was also an increasing reliance upon triangulation, and in some instances standard designs accompanied by published calculations. Truss systems started to be introduced for timber bridging, particularly in North America, where the European custom of roofing timber bridges had been adopted. Entrepreneurs such as Palmer, Town, Long and Howe introduced “Patented” Truss Systems. Town and Howe trusses in particular were very successful, owing their popularity to their simplicity and ease of construction from a relatively standardised range of member sizes. Many covered bridges of these types have remained in use in North America for over a hundred years. They are now regarded as part of the historical industrial heritage, and even have “Preservation Societies” dedicated to their upkeep. A few bridges of the Howe type were also built in Europe, and some of these too remain in use. Although the records are generally difficult to obtain, it seems likely that early suspension bridges used timber walkways and support gantries, along with other natural materials as the cables and suspenders. Such bridges must have pre-dated arches and trusses, but by their nature they would have been regarded as less permanent affairs. However there do exist 19th century photographs by Forrest, the Scottish plant collector, of suspension bridges in China, using timber and other materials, which are probably directly similar to centuries old designs. It is also evident that the suspension bridge goes back long into history, from some of the forms of such bridges that are still built in remote regions of Asia and the South Pacific, without the benefit of any metal parts or cables. In the 19th Century, impressive suspension bridges created very long spans using steel cabling along with stiffening trusses and decking in timber. A good example is the footbridge in Ojuela, Mexico, which was built in 1892. This has a span of 278m, and is still in use today. Through European development aid, particularly from Switzerland, impressive modern steel and timber suspension bridges, for which a series of design manuals is available, have been constructed in Nepal. The mountainous terrain, use of pack animals, and extreme inaccessibility of some regions, makes these structures a continued necessity of life.
2.6 Laminated timber – from mechanical to reliable adhesive technology Very early applications of mechanical laminating are discussed by Newlands (1857). For example, he cites the knowledge, on the part of Col. M. Emy, in France, commencing in 1819, of the much earlier mechanical laminating system of Philibert De Lorme. Newlands also discusses a report for the Society for the Encouragement of National Industry in 1831, by Emy, publishing his laminating inventions and techniques. He illustrates a roof for a shed at Marac, near Bayonne, and a “ridinghouse” (cavalry training structure) at Libourne. Newlands then shows a Gothic church roof at Grassendale, near Liverpool, which he states followed the Emy system. It is Nov 2002
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not at present known whether this still stands. He also gives quite elaborate details of various forms of bending apparatus, manufacturing for curved roof laminations (“of the bending of timber”). The Timber Development Association (forerunner of TRADA) historic photographic archives contain several examples of mechanically laminated worsted mills, in the Bradford region of England. Booth (1964) discusses mechanically laminated railway station roofs, such as GWR, Bath (by Brunel), as well as dealing extensively with railway bridges, as indicated below. James (1982) provides densely annotated lists of potential primary sources for those able to pursue early American and other international (e.g. Russian) mechanically laminated bridges. Developments in the use of glued, as opposed to mechanically laminated timber, began surprisingly early, and in Europe it was established by the start of the 19th century. During 1807 – 1809 a Bavarian engineer named Wiebeking developed horizontally laminated timber arch bridges with spans of up to 60 m. Most of his bridges used very thick iron bolted or rod-connected laminations of oak. However in 1809, the first major glued laminated timber bridge structure was built by Wiebeking, at Altenmarkt. This had ribbed laminations fabricated in situ, working (presumably with great difficulty) from scaffolding and temporary piling. Thinner spruce boards were used for this bridge than with the mechanically laminated oak types that he had built previously, and there was an appreciation of the benefits of staggering end-joints, relative to adjacent laminations. Further evidence of the well-established nature of glulam is the mid-nineteenth century Congregational Sunday School roof in Manchester, 1864, documented by Booth (1971) and surviving until demolition in 1963. A former schoolroom, now used as the Southampton Register Office, in Southampton, 1860 is documented by the GLTA, and is also corroborated by Booth, as the earliest known use of glulam arches in a building. Yeomans cites the “German Gymnasium” in London as another stillstanding structure with more than one hundred years service. Private correspondence and photographs, courtesy of P. J. Steer, show a nineteenth century music hall in Nottingham during recent restorations, that is glulam roofed, and still in use. By the start of the twentieth century, patents were being taken out for glulam in Germany. In Switzerland, certain structures, laminated with casein adhesive, were constructed that still stand today (Chugg, 1962). In 1939, in the USA, a landmark technical publication appeared that strongly influenced subsequent North American codes. This was entitled “The glued laminated wooden arch”, by T. R. C. Wilson (1939) of the USDA. Evidently, glued laminated softwood bridges were well established by then, since a footbridge of such construction is included in this reference.
2.7 The railway era The civil engineering construction associated with the rapid 19th century development of the railways made extensive use of timber bridging. Some of the finest examples included Brunel’s designs, although there were also other successful British railway pioneers using timber for bridges and for other structures, including the Greens (John, and his son Benjamin) in the North East of England. For the Newcastle and North Shields Railway, these engineers continued the use of mechanically laminated timber for structures such as Ouseburn Viaduct.
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Returning briefly to the famous Isambard Kingdom Brunel, there is only space to say that he made extensive use of timber for many railway viaducts, which were built across the valleys of south-west England and South Wales. Surprisingly sophisticated concepts were involved, including the use of timbers that were preservatively-treated using chemicals applied under pressure. An early process of this type was Kyanising (1832, Kyan’s patent, using chloride of mercury, Newlands p. 106). Brunel planned his designs to allow maintenance to be carried out on these structures without interrupting the passage of trains. He built forty-three viaducts in Cornwall alone, spanning a total of eight kilometres. The last of Brunel’s timber railway viaducts were only dismantled in South Wales the 1930’s, and generally these structures were replaced only to construct bridges able to carry much heavier traffic, rather than because of deterioration through decay. At Barmouth estuary, in North Wales, a timber railway viaduct designed and constructed according to similar principles remains in use today (Figure 4), with pitch pine piles having been replaced by the extremely durable greenheart timber.
Figure 4: Barmouth Bridge - one half mile long timber trestle pile viaduct completed in 1867, the only large timber viaduct in Britain still in use. It spans the Mawddach estuary on 113 short spans. There are two steel girders at the north end, one of which used to swing to allow ships up river. For a wealth of further information on timber railway bridges in England in the nineteenth century, the copious and scholarly work of Booth (1996) is an essential starting point for the serious historian as it contains many secondary reference sources, including Booth’s own. These would lead to many prime source references, many of which are available in UK libraries such as the Science Museum (Imperial College) and archives such as those of the Great Western Railway. As mentioned above, the “Kyanising Process”, and similar methods, were known to Brunel’s contemporaries. Other chemical treatment processes of that era are described by Newlands. These include, for example, Sir William Burnett, 1838, chloride of zinc; Payne’s patent. 1841, sulphate of iron and muriate of lime; the early use of tars and essential oils (Newlands cites as an example a 1737 patent by one Alexander Emerson); Bethell, who gave the basis for modern creosote treatments, in 1838.
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Early patents for American bridges, during the “Palmer, Town, Long and Howe” era, often involved the co-incident publication of patents concerning timber treatments. Later, in North America, in the 1930s, the widespread industrial introduction of pressure preservation processes, using substances such as creosote, is said to have led to an expansion of the use of timber for large truss and girder bridge forms. An impressive example was constructed at Sioux Narrows, in Kenora, Ontario, where large, preservative treated Douglas fir members were arranged in a box-Howe Truss pattern, to create what was for many years the world’s longest single-span highway traffic bridge. At 64m main span, this bridge still remains in service.
2.8 Protective design lessons from history Although decay has always been one of the factors affecting the service life of timber bridges, they have more often been destroyed by war, natural disasters and fire. It is known from the durability records of ancient timber churches, cathedrals and houses, as well as roofed bridges, that preserving timber structures with adequate protective design measures considerably reduces decay risks. The importance of good protective design detailing is a lesson from history that cannot be emphasised too strongly, in the context of modern timber bridges.
Figure 5: Good protective design features - in 1,000 year old Norwegian stave church. Left; Stone sill elevates timber ground sill, sacrificial boarding protects exterior of corner posts and cladding. Right; Elevated post base, drip moulding at cladding bottoms. Note also extensive use of pitch preservative in both cases. (photo CM/Trada)
2.9 Maintenance of historic timber bridges in Britain Many historic timber bridges may still be found throughout Europe, including Britain, and fortunately, nowadays, restoration work is undertaken to preserve them. The
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continuous maintenance and replacement of timber elements, using like-for-like carpentry, in the Lucerne covered bridges has already been mentioned, as has the restoration work on Barmouth viaduct. Until quite recently, the Scottish East Coast main line railway crossed one of the few surviving timber viaducts, over a peat bog, near Inverness. The live track no longer passes over this structure, which has, however, been conserved. In 1915, John Saner, engineer to the Weaver Navigation System in North West England, designed a structure known as Dutton Horse Bridge. This has twin elliptical spans skewing across the River Weaver’s sluice channel, which leads eventually to the Mersey. The bridge is constructed from mechanically laminated greenheart, and is believed to be the oldest such structure remaining in service in the world. Greenheart planked caissons and many other impressive engineering features are to be found in and around this outstanding structure. The durability of this structure is unsurprising however, since greenheart is renowned for its longevity. In Guyana itself, its country of origin, marine jetties of over one hundred years in age stand, even in such adverse, warm sea-water and termite ridden conditions, whilst the cathedral of Georgetown is claimed to be the tallest (greenheart) nineteenth century ecclesiastical building in the world. Two examples of King post truss vehicular bridges in the river Spey region of Scotland were used as lecture examples by TRADA until quite recently, and were said to be still in service. There has not yet been opportunity to verify their current status.
2.10 New materials Following the epoch making construction of Ironbridge in Shropshire, England, (with a framework arrangement based on contemporary timber designs!) the 19th and early 20th centuries saw the rapid spread of the industrialised use of the ‘new’ materials, iron, steel and later on, reinforced and pre-stressed concrete. This completely altered the concepts of bridge construction, making the increased requirements regarding longer spans, larger roadways, and higher loads achievable with ease. However with higher frequency, heavier traffic, and the need to guarantee an all-year-round use of the roads, problems have arisen with these ‘modern’ materials. Besides faults due to inadequate design and execution, which may happen with all materials, high maintenance costs have been incurred as a result of the use of salt as a de-icing agent on roads. This has caused corrosion problems with reinforcing bars and prestressing steels in concrete bridges, as well as deterioration of paints and member surfaces in steel structures. The lesson has gradually been learnt that adequate protective measures against direct and indirect hazards of the climate are also necessary for these so-called “durable” materials, and that such measures invoke considerable penalties in terms of whole-life costs. The complete replacement of quite new bridges necessitated by poor durability has also demonstrated the high costs of dismantling concrete structures. In the face of this situation, timber engineers have recognised new opportunities, proposing their material to solve some of the problems that bridge engineers have been encountering. Timber engineering itself also has become armed with “new” materials, several of which have very high performance, low variability and excellent reliability, thus offering additional advantages over the “traditional” version of solid sawn timber.
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3.0 OVERSEAS DEVELOPMENT OF TIMBER BRIDGES The USA, Canada, Australasia and the rest of Europe are well ahead of the UK in the design and production of “modern” timber bridges: • • • •
Glulam timber and transverse decks Longitudinal glulam decks Dowel-laminated, longitudinal panel decks Stress laminated decks
A common factor between modern bridge designs is load sharing through composite action which distinguishes them from the old “stick” designs. Timber for bridges has advantages over other structural materials which have been recognised overseas but ignored in the UK. Timber is:• • • • • • • • • •
Durable and long-lasting - with modern treatments bridges are expected to last at least 50 years. Simple construction ~ Construction usually demands low skills and simple equipment available locally. Maintenance is also within the scope of local labour Prefabricated Components ~ Modern timber bridges are either entirely factory made or factory component manufactured thus assuring good quality Wood has high strength to weight ratio ~ This saves in foundations and gives confidence to reuse old foundations. Crane loadings are reduced and money is saved. Competitive ~ Small span rural bridges can be built in timber at a significantly lower cost than from steel or concrete. Aesthetics ~ Timber is natural and is appreciated by everyone and looks as good in the countryside as in an urban location. Chemically stable ~ Timber is not affected by de-icing salts as is steel and concrete Expansion ~ Timber does not expand and contract much with heat so road surfaces can be continuous over them without the need for troublesome joints. Renewable and Sustainable ~ This is important to the economy Removes Carbon from Atmosphere and locks it on the Ground ~ This is of ultimate important in today’s environmentally conscience world.
There is a lot of catching up to do and much development is needed to enable modern bridge ideas to be imported to the UK and then assimilate them to UK practice, codes and materials.
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3.1 Relevant History Scandinavia: Around 1990 the Norwegian/Nordic Timber Council took steps to plan the introduction of more timber bridges to the public road network (notable examples are shown in Figures 6 to 8). Their reasons were not in the first instance economic but more to use indigenous materials and later to assess the whole value when practice had produced the best solutions. Otto Kleppe, Chief Bridge Engineer for the Government in Oslo, travelled the world to study old timber bridges in order to gain insight into efficient design and durability. He learned lessons from 100 year old covered timber bridges as well as the latest forms of modern stress laminated decks. He returned to Norway and has engineered the development of some remarkable new timber structures, not only on the public road network, but also over motorways. Norway is fortunate in having vast reserves of very high quality timber available which makes the task of producing elegant long spans much easier. The Norwegians have worked on many fronts with a view to providing a full range of timber bridge solutions and included experimental work with preservatives. A very successful design is a combination of longitudinal glulam beams with CCA treatment subsequently stress laminated and treated with creosote. They have developed very high quality jointing systems which permit large king post and truss structures and have innovative ideas allowing timber crash barriers. The high quality structures are protected using copper sheeting on the structure and bitumen compounds on the deck. In both Norway and Sweden simple stress laminated decks are factory made for minor road and forestry road bridges. These low cost options are treated with preservative but not protected in any other way. Even with a shorter life these structures will have a very competitive whole life cost.
Figure 6: Vihantasalmi bridge, Finland - Glulam king-post trusses each spanning 42m; composite concrete-steel-glulam deck. (photo Nordic Timber Council)
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Figure 7: Evenstad bridge, Norway - glulam truss beams each spanning 36m with stress laminated timber deck. (photo Nordic Timber Council)
Figure 8: Sinettäjoki footbridge, Finland - glulam king post trusses spanning 18.8m with lumber deck. (photo Nordic Timber Council)
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USA and Canada: Timber, which was readily available in enormous quantities, played a major role as a construction material in the development of North America since the early pioneer days. Indeed, it is estimated that by 1900 half of the total forest area of the continent was felled. Timber trestles were used extensively to span gorges and rivers for the transcontinental railways. Many century old covered road bridges are still in service and are considered to be heritage items. With a main span of 64m, the Sioux Narrows bridge, built in 1936 in Kenoria, Ontario, is one of the worlds longest single span wood highway bridges (Figure 9)
Figure 9: Sioux Narrows Bridge - Howe trusses formed from solid sawn Douglas fir (photo Canadian Wood Council) Currently in the USA there are nearly 600,000 bridges, 7% of which are timber and a further 7.3% have timber decks. Recent studies have shown that 240,000 of these bridges are classified structurally deficient or functionally obsolete. This critical state of affairs prompted Congress into introducing the Timber Bridge Initiative (TBI) in 1989 and another similar programme which promotes demonstration bridges, research and information transfer. Under the programme a 50% grant in available to build a bridge which demonstrates modern technology. Timber structures declined in number from 50 years ago when large trees became scarce and concrete technology became reliable. However the modern materials, concrete and steel, have not been without their problems and since the middle 70’s much research effort has been undertaken to utilise smaller wood sections to build large structures. It was in Canada in the 1970’s the real pioneering work was carried out on the stress-laminated decks which has become important to the new bridge initiative in the USA and could become a very useful concept in the UK. Timber engineering and technology has benefited greatly through these programmes and many hundreds of bridges have been built although much remains to be done.
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There are so many avenues of help in the USA especially as these initiatives were taken by government agencies but personal contacts will be crucial to ensure the accelerated programme necessary in the UK, in order to catch up.
Australasia: In Australia and New Zealand the increased production of plantation timber has generated a modern timber engineering industry. The UK should have been shadowing these recent impressive developments. Many are the same as those in America and are necessary in the UK to catch up and create some high value markets for our new increasing production. Although there are many initiatives in transportation structures, there are also significant ideas in building, from which UK practice could benefit.
Developing World: There has been substantial experience involving the UK timber research organisations, and TRL (see website http://www.trl.co.uk/bridges.htm), in the overseas development uses of timber for bridges. These have been carried out through assistance provided via organisations such as United Nations Industrial Development Organization (reported in Anon 1985) and the UK Department for International Development (DFID, formerly ODA). For example, prefabricated modular timber road bridges have been successfully introduced into a significant number of developing countries on four continents. The first of a series of standard designs for modular timber road bridges was prototyped in Kenya, some thirty five years ago. Further development work continued in Central America the Far East and elsewhere. This development included contributions of local expertise, and associated professional training. Similar road bridges are still being produced, in accordance with well-tried design manuals and drawings, using local timbers and labour, to the great advantage of rural communities in more than two dozen countries, in all of the tropical continents. Extensions to the original designs, and substantially new types of standard design, have subsequently been added.
Figure 10: UNIDO prefabricated bridge (photo UNIDO)
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It has been found that by giving ministry departments, and their associated professionals, renewed confidence in timber in communally important, heavy-duty applications such as these, there is spin-off resulting in an enormously improved usage for smaller-scale applications. These are initiated and executed entirely by communities themselves, on their own initiative. This is another important lesson that the timber industry needs to take on board, here in our own country.
3.2 The Way Forward to Make use of International Research When specific areas of research are identified and funding is in place for individual projects, the initial literature search will be extended using references and contacts. Research partnerships will be explored and when the missing knowledge is identified work will begin. Much will be assimilation of past international work to accord with UK timber species. There are a number of codes of practice in existence which will be of use but climatic conditions, safety regimes, species differences etc will create many transfer problems. The mission of InTeC is not just to carry out the research, solve the problems and show that things can work but also to produce the codes and guidance so that the ideas are taken up. This part of InTeC’s work will be time consuming, but if the past successes of concrete and steel as construction materials are to be emulated then the information circle must be closed. Young designers will not adopt timber unless it is made easy, logical and sensible. Some Specific International Ideas likely to be Transferred to the UK: Stress laminated bridge decks are certain to become a useful solution for minor rural bridges and they are a timely product of recent ability and need. Accurate sawing is essential. Safe bacteriological treatment is demanded and high tensile stressing of steel tendons is the key to the structure. All of these are now available at low cost. The technology level is not high and their production could become a cottage industry. This idea has arrived at a time when UK softwood timber production is about to double again for the second successive decade and funds for rural bridging are low. Abutments for bridges and retaining walls have traditionally been constructed from ‘permanent materials’ like concrete and masonry but the question of ‘life cycle’ needs to be addressed. A public road bridge in the UK is designed for a 120 year life but forestry bridges are designed for 50 years as that is the economic cycle of the industry, being the growth time from plant to mature tree. We have, in the past, expected buildings to last forever given enough maintenance, but supermarket buildings are now financially appraised over 7 years, that being the predictable trading projection limit. Perhaps it is time to look at structures with a shorter life, provided that short life gives a unit cost per year less than the more permanent structure. A timber bridge deck and abutments could be constructed for a 20 or 30 year life, require no maintenance and be replaced within the unit annual cost of the ‘permanent structure’. Appraisal of Timber Structures: Overseas countries have realised that timber structures enhance the environment, local economies, society, aesthetics etc. factors which must be introduced into any appraisal. Energy values are used in appraisals, where the inputs to refine the component materials and the energy to carry out the construction and demolition are evaluated to calculate a whole life cost. Timber structures excel in all of the components of appraisal. A local timber industry creates stable rural employment which tourism can latch onto. Timber structures are popular with rural people and Nov 2002
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their visitors. The western society accepts that timber structures in the countryside make sense. An international research check has proved that we are behind the rest of the developed world in this area of applied engineering. This does, however, mean that with careful planning and study of current research we can catch up quickly and then concentrate our efforts on the most relevant areas. It means that much of our research will be to assimilate ideas developed for other species. The most valuable gain however will be in finding the ways to extend already good ideas. This is much easier when a fresh mind takes up a partly developed piece of work. InTeC researchers bring that quality and with the correct funding many exciting extensions of existing work that could bring benefits to the UK. A programme similar the Timber Bridge Initiative in the USA would be welcomed in the UK and could become the cornerstone for future development while bringing forward the many bridge replacements necessary in the UK.
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4.0 CURRENT UK POSITION Unfortunately, the official bridge design scene, viewed from the position of the average civil and structural consulting engineer in the United Kingdom, includes nothing to encourage the use of timber. There is no British Standard dealing specifically with design. The BS 5400 series only covers steel, concrete, and steel-concrete composite bridges. This absence of a British Standard is thought to have inhibited the specification of timber as the structural medium for many footbridges, as well as having resulted in a number of designs whose performance has not been entirely satisfactory. Although authorities such as Highways Agency (HA) have their own standards, recognising timber in footbridges to a small degree, the absence of a main code of practice and accompanying support standards, is a serious deterrent. Awakening awareness by some influential specialists within authorities of the development of new Eurocodes relating to bridges, is likely to provide better hope for the future, provided that this is seized as an opportunity by the timber industry itself. Timber interests in the UK were extremely impressed by the manner in which the US National Timber Bridge Initiative was launched (USDA 1983), and by its subsequent success. Their programme involves many demonstration timber bridges, together with research and technology transfer. Starting from a relatively small financial basis, it was difficult to see how anything comparable could possibly be started in the United Kingdom. However, there are now some positive signs. Work was carried out about seven years ago, with support by DETR. This led to two preliminary study reports (Mettem 1993) and (Mettem 1994). A pilot project, termed “Innovative Timber Engineering for the Countryside”, has been initiated involving BRE, TRADA Technology, and Forestry Civil Engineering, with support from the Forestry Commission, and this report relates to this particular project. The second positive step is that active work has now been started on an EN version of Eurocode 5: Design of Timber Structures Part 2: Bridges. This is scheduled for issue for public comment in 2003, with the target of a final draft for printing in 2004. The principal Eurocode 5 for timber structures, to which the bridges part refers for all of its main technology, is ahead of this, and has already been strongly promulgated and supported by design guidance, involving TRADA Technology and its various industrial and research partners. BS DD ENV 1995-1-1 was issued in 1994 and is already used in practice mainly by more experienced timber engineering designers. A BS EN version is expected to be published early in 2004, before which, training will be given to all practising designers and new students. Current Requirements: Current requirements for bridges are generally formulated independently of the materials to be used. In general terms, bridge design has to fulfil certain main requirements which can be related to timber and wood-based materials as follows: Load Capacity & Vehicle Clearances: Modern timber bridge designs for vehicular traffic are perfectly possible, and are in fact already being designed and constructed in a number of countries and regions. Appropriately designed timber and composite deck systems can provide for increasing traffic loads. Clearances, given in regulations, are taken into account wherever necessary. For road bridges, vehicle size obviously affects the design of
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both the carriageway widths, and also, in the case of covered and arched bridges, the overhead clearance. Long Spans: Earlier limitations of timber brought about by its availability only in the sawn form no longer apply. Glued laminated timber, structural timber composites - STCs (Mettem 1996) improved strength grading procedures, reliable connection techniques and the use of other materials acting compositely in conjunction with timber, all help to make long spans possible. Roadway Surface Conditions: Normally there is a requirement that there shall be no difference in the surface conditions and levels between the bridge and the connecting road pavements. With appropriate deck systems and sealed wearing surfaces, such requirements can be fulfilled using timber structures. Routing of the Bridge: Modern bridges have to be integrated into the general route-planning scheme. Consequently skew, cambered and curved deck bridges are often required. Such forms are attainable with timber bridges. Recent Developments in Timber Bridge Decks: Timber & Concrete Composite Decks Timber and concrete composite decks have existed in regions such as New Zealand and North America for decades. Early systems comprised nailed laminated decking with un-reinforced concrete and a thin asphalt surface. More recently, thicker reinforced-concrete layers and shear connectors have been added, giving greater composite action. The effective width of the concrete flange is determined as for a concrete T-Section. All of the shear force transmission between the two materials takes place via special, strength-calculated connectors, and not by natural bonds. No tensile strength is recognised within the concrete layer. Some design rules for this form of construction are given in Eurocode 5 Part 1-1, the general design document, whilst supplementary rules are contained within prEN Eurocode 5 Part 2, Bridges. Developments in the composite timber/concrete deck continue, for example: • •
•
More stable laminated timber decking, using post-tensioning systems. More efficient shear connecting systems between concrete and timber, to achieve more reliable composite action during the service life of the deck. Better systems to seal the concrete surface, and to provide protective and hard wearing road surfaces.
Laminated timber deck plates are made of individual laminations which are held together by nailing or adhesive bonding. In the case of pre-stressed plates, discussed below, there is in addition a permanent lateral pressure, which guarantees continued friction between the faces of the laminations, and according to the type of construction, between any un-bonded adjacent faces which may exist between the individual slabs.
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Pre-Stressed Timber Decks Pre-stressing in a timber bridge deck is defined as a permanent effect due to controlled forces and/or deformations imposed upon the structure. The plates are normally pre-stressed by means of steel bars or tendons. Pre-stressed decks in timber bridges first appeared in Canada, where they were introduced as a repair method for nailed timber-laminated decks. The correct choice of materials and specification of moisture contents overcame early problems with loss in pre-stressing force due to timber shrinkage. These forms of deck are now very common in North America, and their use has been spreading to other regions where modern timber bridge developments are occurring. These include Australia, and Finland, Norway and Sweden. The additional step of using glued laminated timber rather than solid sawn timber for the decks was probably taken first in Switzerland. Recently in Australia and Scandinavia, progress has been made in utilising other modern STCs such as Laminated Veneer Lumber (LVL), for which reliable pressure preservative treatment processes have now been developed. Glued laminated timber and STCs are always supplied at low, factoryconditioned moisture contents, and with these, early problems in loss of pre-stressing have been completely overcome. Decks with no re-stressing requirements are being achieved by following design recommendations such as those given in prEN Eurocode 5 Part 2, Bridges. Pre-stressing bars are also now sometimes bonded in, resulting in a high degree of corrosion resistance and good load carrying capacity. Dowel-Type Fasteners & Mechanically Laminated Bridge Structures: The term “dowel-type fastener” is used throughout the structural timber Eurocodes, and in the latest edition of BS 5268 (BSI 2002), to refer to fasteners whose crosssection is essentially of a cylindrically prismatic form, and whose function is to transmit forces in lateral shear between adjacent layers of the timber. In the context of current codes, such fasteners essentially consist of steels of adequate and defined strength, although research is now in progress on the use of non-metallic, and in particular Fibre Reinforced Plastics (FRP) dowels. Bolts, lag screws and plain steel rods acting in transverse shear are all examples of dowel-type fasteners that are commonly employed in bridge design. Over the past twenty-five years, timber engineering researchers have extensively explored the design theories associated with these types of device, and the theories are now well adapted to reflect real fastener behaviour in actual structures. Essentially, the theories depend upon a knowledge of the behaviour of the fastener as a rod-like device, which tends to embed itself elasto-plastically into the surrounding timber. The response of the latter is modelled as a yielding elastic foundation. At the same time, allowance is made for the tendency for the steel of the fastener itself to yield plastically, and to form plastic hinges at various points, whose locations depend upon the exact interface arrangements. The development and use of such theories for the design of dowel-type fasteners, along with methods enabling designers to predict changes in the effective section modulus, due to slip between adjacent layers, has enabled the accurate design of large and impressive modern mechanically laminated timber bridge designs. Mechanically laminated timber bridges are designed and constructed throughout Europe, but are particularly prevalent in the UK, Netherlands and Germany. Very dense, durable tropical hardwood timbers are normally used for these designs, with
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one particular timber , being found to be very successful. This is the hardwood named “Ekki” in the UK, and “Azobé” in Continental Europe (the same species of timber in either case, namely Lophira Alata).
5.0 CATEGORIES OF TIMBER BRIDGES Nov 2002
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5.1 Categories of use Three broad categories of use can be considered, as follows: a) Highway and adopted road bridges. b) Footbridges. c) Footbridges with occasional vehicular access (e.g. farm, golf-course and parkland bridges). Category a) in timber is extremely rare in the UK, and in the main restricted to special and historic structures. The present volumes of use in categories b) and c) are modest, but growing, with opportunities for bridge and associated timber suppliers and engineers, especially in the light of the positive factors mentioned in the introduction. Even in category b) however, there are significant obstacles to the procurement of timber, where influential authorities are involved. The Highways Agency inventory for England, for example, contains only one timber pedestrian crossing bridge over the roads for which they are responsible. It is the intention of the present project to conduct more thorough market research, costing studies and business potential investigations.
5.2 Locations Generically, locations for footbridges and light vehicular bridges can be divided into the following four types of crossing: • • • •
Over roads – general access. Over rivers, canals and other water features. Associated with the leisure industry, various crossings, including the three types above. Over railways– general access.
Bridges associated with alternative modes of transport, such as cycling, might arguably be regarded as a separate category. However, provision of routes and facilities for serious and mundane access to work, education, and other aspects of daily life by such means can hardly be argued to have reached the stage to warrant separation from the leisure category. a) Road crossings Many footbridges are used to provide safe pedestrian crossings. Timber is permitted, as well as steel and concrete. However, the Highways Agency (formerly, through a Department of Transport document, currently undergoing revision) points out the following, in its Standard BD 29/87 (DoT/HMSO 1987): "A footbridge is the least suitable form of crossing for disabled people and should only be provided when other forms of crossing – e.g. a crossing at grade or a subway are deemed to be unsuitable." Timber has only a small share of this market. Furthermore, its share is probably even smaller, as a result of some unfortunate instances of glulam bridges de-laminating during the 1980’s. These were manufactured by firms that were not members of the Glued Laminated Timber Association (GLTA). Lack of independent third party manufacturing control and certification is recognised to have been part of the
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problem. However, the failures achieved notoriety for the timber industry as a whole, causing a major setback to information dissemination and promotional exercises. b) Crossing rivers, canals and other water features This is an important market, with timber footbridges, boardwalks and piers having a sizeable portion of the total. Often the surroundings and environment are such as to suggest the choice of timber as the most sympathetic material. Timber weathers particularly well in marine environments compared with steel or reinforced concrete. c) Associated with the leisure industry. This is generally an expanding and promising market for timber bridges and other landscape features. Example applications include golf courses, theme parks, visitor centres, wildlife and animal sanctuaries and nature reserves. d) Crossing railways Timber has only a very small share of this market. Historically, the extensive facilities for iron and steelwork available to railway builders tended to facilitate the choice of metal in the first instance. There are some modern examples of timber station structures, and a few of timber footbridge crossings. Not all of the engineers concerned are opposed to this material, but railway structural engineers have a cautious approach, and need assistance to specify in performance terms, rather than by prescription. Reorganisation of the administration of the national rail network has also made it difficult, in recent years, to decide where best to focus impact.
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6.0 STRUCTURAL FORMS 6.1 General To describe the majority of footbridge and light vehicular access types, the following five categories of structural form have been devised. The forms refer to the principal structural elements of the bridge: • • • • •
Beams, including bowed types, no arch action. Arches. Girder beams and trusses. Lift and swing bridges. Cable stayed and suspension types.
These five categories of bridge based upon the form of the principal members led to the summary shown in Figure 11 and Table 1 (below). This classification also relates to the usual static system for the principal members, connected in turn to the structural analysis that will be required in the design. Also shown in Table 1 is the form, or shape alternatives for the principal members, and an indication of the materials which are commonly chosen for each of the types.
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A wide range of structural forms is available for bridge solutions in timber. Within this report, the most common and appropriate are broadly categorised into five main types, as follows: a) Beams, including bowed types, no arch action.
b) Arches.
c) Girder beams and trusses.
d) Lift & Swing Bridges.
e) Cable Stayed & Suspension Types.
Figure 11
Five principal types of timber bridge.
(Not to scale).
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Table 1: Structural forms of timber bridge a) Beams
b) Arches
c) Girder trusses
Usual static system
Single simply supported span; flat or bowed (positive precamber), but no arch action.
3-pinned, round & parabolic.
Single simply supported span; Triangulated, e.g. King post trusses. Bow-string trusses.
Additiona l forms
Multiple simply supported spans. Cantilever side spans supporting suspended central span.
Two-pinned, multiple spans.
Multiple simply Single-leaf supported spans. cantilever Multiple bow-strings.
Single or twin towers with side spans.
Form of principal member s
Straight, lightly curved or precambered.
Circular or parabolic (wide range of radii).
Parallel or nearparallel chorded (often Warren or Pratt trusses).
Deck beams straight or tapered
Structura l form
d) Lift & swing
e) Cable Stayed & Suspension
Figure 1
Two-leaf cantilever.
Main (lower) beams straight or singletapered
Timber masts; steel cables/links.
Towers, parallel masts or A-frames.
Balance (higher) beams straight or double-tapered.
Common materials
Sawn timber – softwood or hardwood. Timber poles – natural, debarked or turned/ profiled.
Glulam. Mechlam.
Sawn timber– softwood or hardwood.
Beams – sawn timber – softwood or hardwood.
Glulam. Glulam. Mechlam. Portals – glulam, mechlam.
Towers and masts – Sawn timber, glulam, timber poles – natural, debarked or turned/ profiled. Decks as in 1.
Glulam. Mechlam.
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6.2 Beams, including bowed types, no arch action Beam bridges range from a single, simply supported span, to multiple spans and cantilever arrangements. In laminated construction, pre-camber, and slightly bowed forms (without expressly designed arch action) are quite common. Span ranges for beam bridges may be from as little as 3m for a very small solid timber footbridge, to about 24m in bowed laminated construction.
6.3 Arches Site, terrain and clearance considerations may lead to choice of the arched form, which is architecturally very striking. Much larger spans are possible than with beams, in the order of 12m to 70m being feasible. Various deck arrangements and positioning levels can also be provided.
6.4 Girder beams & trusses Trussed girders provide greater load carrying capacity and stiffness than simple beams. Various trussing arrangements are possible. Girders are often formed from several lines of trusses. These require to be cross-linked with bracing, and the design may involve other lateral members, such as transoms. Deck levels may also be varied. Camber and light curvature are often applied. Well-designed timber girder bridges are architecturally pleasing. Viewers “read” the structural forms, and appropriate designs can be conceived for both urban and rural situations. Individual spans for bridges formed from girders of this type are likely to range from about 9 m to 45 m. Modern timber engineering versions of several traditional timber bridge forms have also appeared recently in the Nordic regions, for example. Both “bow string” and “King post truss” types have been given an updated treatment through the use of new connections technologies, innovative deck types, and environmentally sensitive timber treatment processes. Use of these forms has been extended into multiple spans creating some of the longest timber bridges constructed in modern times.
6.5 Lift & swing bridges There are several practical and available moveable bridge forms in timber. These include bascule bridges, which can be lifted by tilting, and swing bridges. It is of practical importance in dockland, harbour, and inland waterway situations to be able to obtain clearance for waterway traffic. Recently, in such areas, many regeneration and refurbishment schemes have been undertaken. These continue to be needed with expansion of walking and cycle routes, and linking-up of riverside districts. Modern timber design methods, materials and fabrication concepts can provide similar solutions to those used in the past for industrial duties. Either traditional or contemporary architectural styles are possible. Spans for this type of bridge tend to be fairly modest, with those in excess of about 24 m being uncommon.
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6.6 Further design fundamentals Once the principal structural form has been selected, further fundamental structural considerations are necessary. The elevation of the deck in relation to the remainder of the structure is another very important design issue. The option of roofing the bridge must also be taken into account at the early design and cost estimating stages. According to the authorities concerned, regulations exist which may affect several other key elements of the bridge structure, both at ground level and above. These include for example abutments, piers or other supports, deck width, roof clearance (if provided), stair or ramp accesses, and parapet height and design. a)Deck levels There are usually several options in choosing the elevation of the deck. For most forms of bridge shown in Table 1, these are fundamentally low-level, mid-level or highlevel decks. The choice of deck level has considerable influence on the architectural form and engineering design of the structure. It also relates to planning considerations and functional aspects. The former includes for example headroom for vehicles or vessels beneath the bridge. The latter include the measure of protection provided by the deck to the remainder of the structure. Possible elevations of the deck are interpreted in relation to the principal structural forms in Table 2. This second table also incorporates some notes on variations on the basic forms. It mentions for example roofed bridges. Although these are uncommon in the UK, they are not unknown.
a)
Beams
a)
Arches
a)
Girder trusses
Structural form
a)
Lift & swing
a)
Cable Stayed & Suspension
Figure 1
Deck elevations
Over beams. Between beams.
Additions & variations
Roof. Beams as part of parapet.
Over the crown of arch(es).
Over girders.
Mansard arch with stairs.
Roof. Roof supports as part of main girders.
Tied arch, tangent at deck or other level.
Trusses as part of parapet
Skewed plan form.
Deck itself, as for 6.
Deck itself, as for 6.
Generally keeps to classical form of Dutch drawbridge.
Single cable, central in plan, from Aframed tower(s), or similar principle using stays.
Between girders.
Parallel or pitched top chords
Vessel passage also achievable by swing-bridge (another cantilever form).
Table 2: Forms of timber bridge, deck elevations and variations
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b) Materials and durability Correct choice of materials, also understanding and applying best practice options to achieve durability, are such vital aspects of timber bridge design that these topics are discussed under major sub-headings below.
7.0 MATERIALS 7.1 Principal elements In relation to the categories of principal structural forms shown in Table 1, the materials that are commonly used as elements are shown in the table. In essence, these are as follows: a) Sawn timber. b) Timber in round and pole forms. c) Glued laminated timber (glulam). d) Other structural timber composites e) Mechanically laminated timber (“mechlam”). a) Sawn Timber: This may be used for all of the forms in Table 1 which do not involve significant curvature of the members and for which adequate lengths can be obtained to meet the main spans and to provide the other structural spanning requirements. Sawn timbers can range from small sections of softwood or hardwood, suitable for the simplest of short-span beam bridges, through larger sections, more usually hardwood, to very long lengths of specialist hardwoods that can be used for the biggest members, such as masts for cable stayed bridges and for pilings. The latter are still in some cases hewn, rather than sawn sections, although technically this has little effect to the designer. The types of hardwood used for the intermediate applications include temperate species such as oak, often British grown, and established tropical hardwoods that are available for structures in the UK. BS 5268: Part 2 lists data for oak, and also for twelve tropical hardwoods. Typical examples of the latter are Iroko (West Africa, e.g. Ghana) Strength class D40; Keruing (South East Asia, e.g. Malaysia) D50 and Ekki (West Africa e.g. Cameroon, Ghana) D60. For the largest lengths and crosssections, including big beams and masts, Greenheart (Guyana) D70 and Basralocus (Dicorynia guianensis- not listed in BS 5256) are used. Sustainability: Mention of tropical timbers clearly immediately raises the issue of sustainability. The key organisation for producers is the ITTO (International Tropical Timber Organisation) which facilitates discussion, consultation and international cooperation on issues relating to the international trade and utilisation of tropical timber and the sustainable management of its resource base. Regarding certification, it has this to say, in one of its most recent (2000) reports: “Certification is a process which has, to date, not found favour with many producer members, largely because it is seen to be discriminatory. But as countries make progress towards sustainable management, certification may become increasingly
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attractive. Indeed, Malaysia has already established its own national certification scheme, and in Indonesia the project ‘Training development on the assessment of sustainable forest management in Indonesia’ (PD01/95) is being operated jointly with the Indonesian Eco-labelling Working Group, one of the aims of which is to develop training materials for the inspection of sustainable forest management.” Principal ITTO exporter countries are listed in Table 3. Ghana is working towards a similar position to Malaysia, in having its own, nationally-based scheme. Good progress towards Certified sustainability is also being made independently by some small tropical producing countries such as Fiji, Honduras, and several African and Caribbean countries that have plantations of non-indigenous species, such as teak. This material is particularly suited to bridges and other landscaping applications.
ITTO exporters of tropical timber Malaysia Indonesia Brazil Cameroon Papua New Guinea Gabon Côte d’Ivoire Ghana Table 3: ITTO tropical hardwood producers and exporters Softwoods: Where smaller sawn cross-sections and lengths are required, there are better opportunities in bridges than in the building market generally for specially valued British grown softwoods. The great majority of British softwood production is of Sitka and Norway spruce, non-durable species that are hard to treat with preservatives, and unsuited to prolonged external exposure. Those British softwoods worthy of consideration include Scots Pine (SS grade = C22), which has good preservative retention, and Larch (three British grown species, SS grade = C24), which has a good degree of natural durability. Douglas fir grown in the UK also has a degree of natural durability, and sufficient availability to be considered in some regions. In comparison with imported Douglas fir (SS grade = C18), it seems to have suffered from an unduly cautious down-rating of its strength properties. However, this has recently been rectified, for larger cross-sections at least, by a Code amendment approved but awaiting printing. b) Timber in round and pole forms. All three of the British softwood species or groups mentioned above have a long record of accomplishment of good service in exposed conditions as power-line and telegraph poles. Suitable treatment regimes for these are well established, and these have been supplemented by further quite recent research into the topic of durability. Poles themselves are also used, as the main beams for small or medium span footbridges, and occasionally for small masts. They are usually of softwood, with a preference for species with a degree of natural durability such as Larch and Douglas
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fir. Debarked, straight poles are a simple version of this class of element. A limit on taper, such as 10mm per metre, is recommended. Strength grading rules for round timber in uses other than power lines and telegraph poles are not yet fully standardised. In the past, similar ad hoc grading rules and stresses have been derived for other projects. These have included applications in developing countries, where the use of plantation softwood poles is of great economical importance. Advantages of using round timbers include the retention of the natural strength of the tree form itself, and, in appropriate situations, a good appearance. Disadvantages include response of the round timber to drying, and difficulties in connecting such elements neatly and efficiently. Both of these have, to some extent, been overcome, as a result of applied research and developments of the technologies concerned. Treatment techniques for poles are well understood and documented. In countries such as Switzerland and Austria, where a great deal of use is made of timber for bridges, round timbers are also partially shaped. Profiles that are used include circular with one slot, to relieve radial drying stresses and eliminate splitting; circular with one or two flats; and circular with one V-shaped segment removed. Connection systems have also been developed for these more sophisticated forms of round timber. c) Glued laminated timber (glulam). Glued laminated timber (glulam) bridge elements are manufactured to BS EN 386, and other supporting standards, to which this principal document refers. Others cover strength grading of laminations; adhesives; and end-joint testing. Strength classes for glulam are contained in prEN 1194. Both softwood and hardwood laminations are used for bridges, the latter to a far greater extent than in glulam beams for buildings. The British timber code and its related standards used a system of grading laminations and performing design calculations which was peculiar to the UK, but which stood the test of time (8). Some of the procedures and requirements described in the former British Standard for glulam, BS4978 are still followed for hardwood glulam, since the European documents have been developed principally with softwoods in mind. An introduction to glulam production and strength classes is available in STEP/EUROFORTECH Volume 1, Lecture A8. Glulam bridge beams are possibly more common in one particular laminated hardwood, namely Iroko, than in any of the softwoods. This timber has found favour for its combination of good durability, the ability to be bent and glued, and its good joinery properties. Substitutes are now being considered, because Iroko is under pressure through perceived sustainability issues, and may even be coming into genuine shortage from some forest regions. Alternatives might include Dahoma (Piptadeniastrum africanum). This has been used successfully in several vehicular bridges in its country of origin, to demonstrate the concept of sustainability through choice of alternative (“Secondary”) species. Since these bridges were built in a highly termite-susceptible region, pressure preservative treatment was used in addition to selecting a durable species. Where laminated softwoods are specified, European redwood rather than whitewood is preferred for external structures, by Nordic glulam producers, due to its greater amenability to pressure preservative treatment. Douglas fir was hitherto more widely used in these situations in the UK. Also, there are no technical reasons why larch should not be chosen. Indeed, one particular specialist timber engineering manufacturer prefers this timber, finding that it bonds very well. It was the preferred
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choice for Scottish fishing boats, including laminated keels, but is now uncommon in the timber engineering industry. Unlike the case in Germany, laminations for external structures such as bridges are not restricted, in Britain, to a maximum of 33mm. In that country, this is understood to be a precaution to ensure that full clamping pressure is applied throughout the depth of the elements. However, a prescriptive standard of this nature would no longer be likely, within the harmonised European system. The matter would be left to the discretion of individual manufacturers, under third party quality assurance controls. The normally permitted maximum of 45mm for straight laminations is quite common for glulam in both softwoods and hardwoods. Permitted adhesive types are of course selected from the most rigorous exterior/high hazard exposure category, and this normally indicates a phenol/resorcinol formaldehyde type. Provided that the adhesive specification and manufacturing procedures are correct, including quality control tests in relation to the finger joints, there seems to be no reason to believe that 45mm thick laminations, including those from selected hardwoods, are unsuitable. d) Other structural timber composites. The 'family’ of structural timber composites (STCs) is growing. Glulam is really the best known, and longest established, structural timber composite, but it has been joined by other products that are manufactured from veneers, strands and flakes. These are dried, graded and reconstituted, using modern synthetic adhesives, applied under heat and pressure. The exact processes vary, but long, prismatic structural sections always ensue, as opposed to wide, flat boards. The newer STCs are still not as widely known as glulam, by generalist engineers and architects. These tend to take a long time to become aware of such changes, but publicity by major European producers, and support work on codes and standards, is beginning to take effect. STCs are manufactured using well established techniques and materials that have been developed over many years for the production of structural wood-based board materials. Indeed, each member of the ‘family’ of STCs has its ‘relatives’ in the structural board materials range, most of which have long-standing references by codes such as BS 5268. Several types of STC are suitable for bridges, and instances of their application for this purpose can already be cited. The following outlines the three main types of STC, commenting on their potential for this purpose : Laminated veneer lumber (LVL): Bonding together dried, graded, spliced and trimmed veneers that are peeled from a log, in much the same way as making plywood, produces laminated veneer lumber (LVL). Once pressed and trimmed, the resulting long panels are sliced into prismatic structural-sized sections. Unlike plywood, successive veneers are generally orientated in a common grain direction, although a hybrid product, that has every fifth veneer laid orthogonally, has also been found useful for diaphragm applications, including bridge decks. European LVL is manufactured in Finland, from Norway spruce, by Finnforest Oy under the name Kerto LVL. The standard (all veneers longitudinal) type is named Kerto S, and the special (cross-veneered) type is Kerto Q.
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LVL is also manufactured elsewhere in the world, the great majority of the manufacturing machinery emanating from Finland. In Australia, for example, LVL made from Radiata pine is applied in bridges. LVL is found to be amenable to full penetration by pressure-applied preservatives, and is thus one of the best established of the new STCs for exterior structures. Parallel strand lumber (PSL): Parallel strand lumber is manufactured from peeled veneers that are cut into long strands. These are then coated with adhesive and combined under heat and pressure, in a quasi-extrusion process, to form structuralsized sections in long lengths. PSL is manufactured in the USA, by TrusJoist MacMillan, under the name Parallam. It is made mainly from Southern yellow pine and Douglas fir. PSL is also understood to be amenable to pressure-applied preservatives, but there is less experience in its use for exterior structures in Europe, possibly due in part to pricing differentials, compared with LVL. Laminated strand lumber (LSL): Laminated strand lumber is produced by bonding together flakes of wood, again under heat and pressure, to produce structural sections. LSL is also manufactured in the USA, by Trus Joist MacMillan, under the name Intrallam. It is produced mainly from Aspen strands, a timber that is perishable (extremely non-durable), rendering this an unlikely choice for bridge structures, even if preservatives were to be introduced. Advantages of STCs: The major advantages of STCs are that large dimensions are available, with higher characteristic strength values than those of the raw material itself. This is brought about by defect dispersal within the manufacturing processes. These products are manufactured at a low timber moisture content, their dimensions are accurate, and when installed, moisture-related movements, such as shrinkage, twisting and warping, are virtually eliminated. The strength of solid timber sections depends largely on the influence of defects, such as knots and irregular or sloping grain, rather than on the inherent strength of the clear straight grained species. Clear, kiln dried timber is normally at least two and a half times stronger than average quality commercial sawn timber, at air dried moisture content. When making composites, the veneers, strands or flakes are recombined. This dispersal of defects produces a material, which has significantly more consistent structural properties than solid timber. The longer spans that can achieved often mean that fewer intermediate supports are required, and simpler structural systems are possible. Construction times can be significantly reduced, by taking advantage of these features, in combination with a number of innovative, partially prefabricated techniques for element and connection formation. Guidance and standards for STCs: The design of elements and components using STCs may, in general terms, be undertaken in accordance with the rules given in BS 5268, or with those in DD ENV 1995-1-1 Eurocode 5 Design of timber structures Part 1.1. There are at present no British or European Standards for STCs, and as a matter of principle, no materials have their design data included as part of the Eurocodes. In fact, it is considered an advantage of the newer Eurocodes that they are more “open” to the introduction of such innovative products.
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Suitable information on generically classified STCs is shortly to be provided, in intended European Standards (ENs). Examples of existing Technical Approvals that address STCs such as Kerto are the British Board of Agreement Certificates. Meanwhile, procedures are also being developed to establish European Harmonised Technical Approvals that will address materials such as these. e) Mechanically laminated timber (“mechlam”). Mechanically laminated members are termed "mechlam" as a convenient abbreviation in this review, although it should be clarified that this is not a universally recognised term. Recently encountered has been an interesting example of a mechanically laminated Greenheart bridge, which was built in Cheshire in 1915 and which, having remained in good condition, has just been refurbished. The modern manufacturing process, which was developed in Germany, and used quite extensively there and in the Netherlands, has become quite familiar in the UK. Numerous examples of bridges containing members of this type are to be found, ranging from simple short-span beam bridges, to the more ambitious types such as arches and cable stayed structures. Formerly, the timber used was almost exclusively Ekki, or Azobé, as it is known in Continental Europe. Recently, experiments and a few actual applications have occurred using oak. The design of mechlam structures involves some special considerations involving slip between the layers that leads to incomplete composite behaviour. This affects ultimate limit states, as well as serviceability design. Some of the fundamental principles are provided by Eurocode 5 Part 1-1, and STEP Lecture B11 also explains the basis of the computations, with elementary examples, based on this code. However, mechlam timber bridges are now offered in Western Europe, including UK, by some half a dozen firms, on a design, supply and erect basis. All of these types of supplier tend to guard precise details of the full basis of design from mainstream practitioners, as well as from organisations such as BRE and TRADA Technology.
7.2 Decks & decking – UK current practice Structural diaphragm decks do not at present form part of the British timber bridge designer’s vocabulary. They are an extremely significant item that needs to be brought forward for their attention, in order to improve efficiency, as outlined in the Introduction. For convenience however, the following only discusses decks that span as secondary or tertiary items between transoms or stringers, and which do not act as composite diaphragms. The latter are briefly introduced in Section 10.5 of this report, dealing with Overseas Practice – Decks. The commonest form of simple one-way spanning, non-diaphragm deck uses spaced sawn planks. These are usually laid transverse, but are sometimes placed longitudinally. The deck planks can be softwood or hardwood, with certain hardwoods preferred for maximum wear and durability. In connection with wear, designers are in either case advised to discount a proportion of the section, as newly-placed, when calculating for strength and stiffness. This point is addressed specifically by prEN Eurocode 5 Part 2. Softwood decking planks can be specified as either GS or SS grade to BS 4978. Suitable preservative treatment may be considered. This would tend to lead specifiers towards timbers such as Scots pine/European redwood, Douglas fir and larch.
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Hardwood decking planks are usually from a naturally durable species, such as Iroko, Jarrah or Ekki, and are specified as HS grade to BS 5756. Slipperiness of timber decks in general is recognised as something to avoid. It is an issue also related to maintenance and upkeep. It is an aspect of timber footbridges that would merit attention that is more specific to these structures, where the question of incline of ramps and arches also enters into the equation. Where foot grip is especially important, profiled decking planks provide a good solution. Hardwood profiled planks, in timbers such as ekki, are marketed extensively as a separate purchasable item, by timber bridge suppliers, for customers’ use in landscaping and crossing structures beyond the realm of bridges. In footbridges specifically, at higher gradients, profiled planks are sometimes used in conjunction with kick-plates, which are nailed down to the deck. Unless these are well executed however, they are a notorious source of early wear and hence maintenance cost. Recently a proprietary form of profiled, treated softwood decking board has also become available. This has embedded inserts of non-slip material, which are grooved into each castellation of the profile. It is generally felt that the gaps between simple decking in rural footbridges should not be less than 5mm, in order that dirt and debris can pass through the deck. This also allows air to circulate around the planks, thereby avoiding damp pockets where fungal decay can start. Larger gaps are sometimes used, and in remote country areas, deliberate gaps of up to 25mm have been specified. For certain bridges over roads and railways, particularly in more urban environments, gaps in the walkway are not permitted, due to concern over vandals dropping objects onto vehicles or persons below. This has led some designers to use glulam beams, which can provide the spanning medium for the bridge, as well as the deck. Such laminated decks are abutted together, to provide the walkway. An alternative solution has been to use plywood decking with additional non-slip surfaces, but this does not seem to have had a good record. Wear has been rapid and it has become evident that plywood decking requires special attention to drainage details. LVL decks, with appropriate pressure preservative treatment and added wear protection, have started to occur in a few instances, in the UK. This type of progressive solution is moving towards the concept of treating the deck both as a walking surface, and as a structural diaphragm. As mentioned above, this is discussed further in section 8.5.
7.3 Parapets & handrails The primary function of the handrails and parapets is of course the protection of bridge users. Occasionally, in very remote areas such as forests and moorland trails, bridges are built with no parapet, or with only one handrail. A pair are however the norm. Various configurations are used, with the choice primarily depending on the following: a) The type of footbridge user (for example – pedestrians only, or cyclists and pedestrians). b) The nature of the site and locality, for example whether it is a rural or urban location, and whether it passes over a main road, railway or a stream.
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The first item dictates the height and strength requirements for the parapet, as discussed in more detail below. The second affects the degree of openness that is permitted for the handrail, intermediate rails (if any), spindles and posts. Common solutions for bridges in rural locations involve cantilevered handrails. A better, and very traditional arrangement in these types of location may be for the vertical post to be triangulated, by projecting out the decking members in the vicinity of the post, and adding a diagonal raking member. This type of design is often adopted for short-span bridges, where the main beam is of limited depth. For suburban bridges, it is not uncommon for mesh infill, or even solid, surfaceprofiled metal sheet, to be fixed to the handrail, to prevent children from falling through the gaps. The cheap, but somewhat inelegant solution is mesh infill. A better alternative is light section, close centred spindles with adequate intermediate longitudinal rails for stiffness. In fully urban areas, an altogether lesser degree of openness is usually required, whilst for bridges over trunk roads, motorways and railways, there is serious concern over objects being accidentally kicked, or deliberately dropped, onto the highway, rails, or traffic. In applications such as these, authorities will invariably stipulate the required dimensions of enclosure that will normally prevent an open solution. This has resulted in a number of bridges where the deck is located near the centre, or towards the base of the main beams. These then provide the lower half or two-thirds of the parapet. This type of arrangement is common in glulam footbridges with through decks. Larger, girder truss bridges ( for example the type illustrated in Figure 11 c), also often incorporate part of the structural girder depth into the parapet. This of course has further detailing implications, but such structures tend to be offered by specialists who have evolved practical and acceptable solutions that comply with the rules and customs of the various European countries in which they operate. Both softwoods and hardwoods are used for handrails, with the latter, in a suitable species, preferred for durability and smoothness to touch. Most, if not all, of the smoothest-to-hand timbers used in joinery are of tropical origin. External weathering tends to aggravate splinter pick-up in open grained species. Hence, this is a particular issue that needs to be clarified, in relation to user-inhibitions through preference for avoiding tropical timbers, because of perceived sustainability questions. Besides sustainability, however, is the matter of toxicity to skin of the splinters of some species. Good detailing of parapets and handrails is thus undoubtedly an aspect of the furtherance of timber ridges that requires co-operation between timber engineers and wood technologists. Regarding grade and strength class specifications, softwood handrail members will again be GS or SS grade to BS4978, assigned to the appropriate strength class, and with suitable preservative treatment, if required. Hardwoods are usually from a naturally durable species, such as Iroko or Opepe, and are HS grade to BS 5756 and thus to the appropriate hardwood strength class (D Classes). Parapet and handrail detailing to achieve protective design against long-term deterioration, through weathering and decay, is addressed by several Nordic and German-language publications. Some indication of the type of guidance available can be seen in STEP Lecture E17 (Fisher 1995). This is another aspect that will be well worth further attention for UK design guidance.
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7.4 Connections Connections within modern timber bridge structures is an enormous subject, that could quite easily occupy a two year applied research and “knowledge repackaging” project, in its own right. TRADA Technology is currently engaged in a separate project to this, under DETR Partners in Innovation funding, entitled “Connections IT Toolbox”. Good introductory texts for engineers on mechanically fastened timber joints in general are also contained in STEP Volume 1, Lectures C1 to C19. The TRADA Technology Eurocode Design Guidance Documents already published also address the topic extensively. Mechanical fasteners and connectors for bridge structures are at present normally of steel, and quite often of stainless steel specifications, rather than from plain carbon steel. In virtually all instances, some form of corrosion protection is required on other that stainless items. Where flitched or spliced joints involve the use of steel plates, these are usually specified with a thickness of not less than 6mm, following steel bridge design practice. Again corrosion protection is essential and the Eurocodes 5 (both Part 1-1 and Part 2), supported by the documents described above, provide an entry point. Signposts to research on bonded-in connections, a lot of which is highly relevant to bridges, are given in the state of art reference cited above. Work has also been started on bonded-in non-metallic (Fibre Reinforced Polymer, or FRP) connections that may in future be relevant, especially in view of their potential corrosion resistance, as well as their high tensile strength (Bainbridge et al, 2000). Fatigue within timber structural connections for bridges is another aspect that researchers have started to address. Some success has already been achieved, showing this to be not a hyper-critical issue, but one that can be handled using established timber research and code formatting techniques.
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8.0 DURABILITY Timber, when suitably protected, can be remarkably durable and can outlast in certain conditions other materials such as metals, brick, stone and concrete. Timber is invulnerable to salt water, either from sea or de-icing salts, and freeze-thaw action. In a timber bridge, elements which are not covered will frequently attain moisture contents above 20% - the threshold for fungal decay. The threshold for insect attack is even lower, at 12%, although only sapwood and decayed heartwood is vulnerable. Preservative treatment will be necessary only if the natural durability of a timber is insufficient to meet the required service life.
8.1 Detailing Bridges are a particularly exacting application, and ensuring that the timber members have adequate durability is a vital consideration. Before considering this item from the perspective of material selection, it is important to note that much can be achieved in terms of increased durability by means of improved detailing. Indeed the converse is also unfortunately true, in that if poor detailing is provided then premature failure of timber components can occur. Table 4 identifies seven susceptible parts of a timber bridge in general. Most of these points apply to all types of bridge, irrespective of the precise form of the structure. The table then exemplifies poor detailing aspects and gives better alternatives. At this stage, the items in the table are regarded as pointers for guidance, and as suggestions for closer attention, rather than definitive solutions. It is anticipated that it will be necessary to pay considerable attention to detailing, and that these aspects will require discussion by timber experts and bridge manufacturers, in conjunction with the analysis of the survey results. The benefits of effective and well maintained finishes have been very apparent in the survey work which has already been performed and which continues. Modern water repellent finishes offer a considerable measure of protection to exterior timber structures such as bridges. The prevention of weathering of the timber surface itself has an important role in this respect. A high specification of finish and a good maintenance programme for the same would always be advocated in addition to the correct detailing and choice of durable species mentioned above.
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Part of the structure End grain of members in general e.g. beams
Upper edges of exposed members e.g. beams and handrails
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Examples of poor detailing Exposed end grain, leading to fissures, unattractive and ultimately a seat of decay
Flat upper edges where water lies and which trap dirt, especially when weathered/ fissured
Examples of better detailing Protected end grain eg: by attaching other timber members having side grain, or by ventilated capping/sealing Chamfered and sloped upper edges which freely drain Edges protected by ventilated capping
Joinery details e.g.: handrails, parapet to beam connections
Details which trap moisture in mortises, fixing holes, recesses etc.
Freely draining, ventilated, flush details Raise parapet above splash level with separate drained kerb
Decking and its attachments
DPC between deck and beams
Member intersection points, column bases, especially with steelwork Bearing points, supports, bank seats etc.
Deck which is tight jointed or with a sealed surface but which merely traps moisture Attachments to beams which form traps DPC between deck and beams Intersection points can easily form moisture/dirt entrapment regions. Intersection points can trap moisture and remain damp.
Poorly ventilated, susceptible to silting up, dirt and debris entrapment
Deck which freely drains, laterally and longitudinally, even when worn Drip mouldings beneath deck boards
Not easily avoided, but detail for maximum ventilation and drainage eg: by drilling/arranging gaps Design steelwork to allow drainage and ventilation. Avoid details which allow the collection of water . As well raised from surroundings, eg: by masonry and supporting steel, as possible
Table 4: Susceptible parts of timber bridge structures, with examples of detailing weaknesses and improvements
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8.2 Natural durability The biological natural durability of timber is due to the anatomy of the timber species and in some cases the presence of naturally occurring extractives within the heartwood. Each timber species has its own characteristic set of these chemicals, some of which are toxic to wood-destroying organisms. Even when the detailing is as good as possible, for an exacting, fully exposed application of timber such as a bridge, it is advisable to consider the use of a timber which falls into a natural durability category which is at least as good as "moderately durable", Table 5. It is important to note that the biological natural durability of a timber refers only to its heartwood. Such classifications are well-established in Britain for all of the better-known construction timbers, both softwoods and hardwoods, including all of those listed in BS 5268: Part 2. Table 6 shows the natural durability classifications of the twelve tropical hardwoods listed in the code, together with European redwood and Douglas fir, for comparison. These classifications are based principally but not exclusively upon traditional ground contact stake tests. It should be noted that the ratings relate to UK conditions, which do not include a termite hazard, but which represent a high risk from fungal attack. Exposure trials have been conducted using EN 330 "L-joint" type specimens, both to assess natural (untreated) durability, and to evaluate various forms of preservative treatment beneath a coating. In due course, the information from this project will be of value to bridge designers, especially when they consider the joinery items such as parapets and handrails. Certified, sustainable and naturally durable timbers, including, for example, plantation teak from various sources, may be admitted for the most exposed parts of superstructures, such as handrails and parapets, on the basis of evidence from such tests.
Durability Category
Approximate life in ground contact, 50mm x 50mm section (years)
Very durable
More than 25
Durable
15 – 25
Moderately durable
10 – 15
Non-durable
5 – 10
Perishable
Less than 5 Table 5: Natural durability categories
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Timber, Standard Name; Species
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Region of Origin
Natural durability
Balau Dense Shorea spp
SE Asia
Durable
Ekki (Azobe) Lophira alata
W Africa
Very durable
Greenheart Ocotea rodiaei
Guyana
Very durable
Iroko Milicia excelsa
W Africa
Very durable
Jarrah Eucalyptus marginata
W. Australia
Very durable
Kapur Dryobalanops spp
SE Asia
Very durable
Karri Eucalyptus disersicolor
W. Australia
Durable
Kempas Koompasia malaccensis
SE Asia
Durable
Keruing Dipterocarpus spp
SE Asia
Moderately durable
Merbau Intsia spp
SE Asia
Opepe Nauclea diderrichii
W Africa
Very durable
Teak Tectona grandis
SE Asia
Very durable
Douglas fir Pseudotsuga menziesii
N America
Moderately durable
European Larch Larix decidua (L. uropaea)
Europe, incl. UK
Moderately durable
Scots pine/European redwood Pinus sylvestris
Europe, incl. UK
Non-durable
Durable
Table 6: Natural durability classifications of the twelve tropical hardwoods listed in BS 5268: Part 2, and of Douglas fir and European redwood
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8.3 Preservative treatment In the modern philosophy of designing for durability, the use of chemicals to treat the timber, normally through pressure application, is regarded as the third line of defence, following good detailing and species selection. Increasingly, though, the most sustainable use of our principal commercial timbers (i.e. non-durable softwoods) is to extend their service lives through preservative treatment. This allows more than sufficient time for forest growth to compensate for the consumption of timber. However, evidence from around Europe suggests that traditional preservative active ingredients are going to come under increasing environmental scrutiny and legislation. In Denmark and the Netherlands, legislation has already instigated restrictions on the use of copper/chromium/arsenic (CCA) - the most widely used wood preservative. Amendment to the EC Marketing and Use Directive for arsenic will limit CCA to a few derogated uses including bridges. Creosote is due to be withdrawn for public/domestic use in the EC in 2003, but will still be available for industrial applications such as utility poles and bridges. Of the 150 bridges constructed recently under the Nordic Timber Bridge Project (Nordic Timber Council, 1999), the majority were either creosote or CCA treated. Clearly up to date guidance is needed for timber bridge designers and certain clients may demand "environmentally friendly" solutions. In the United Kingdom only formulations approved under the Control of Pesticides Regulations 1986 by the HSE Pesticides Safety Directive are used for timber treatment, and formal authorisation procedures are in place to ensure that operations comply with legislation relating to aspects such as employee health, safety, material control and waste disposal. Treated timber is, of course, far more widely used in applications such as fencing, decking and utility poles than bridges. The latter may, however, come under greater scrutiny because of their siting over sensitive watercourses. For timber bridge members the principal chemical preservative treatments applicable are either Copper Chrome Arsenic (CCA) or creosote, applied under pressure, although there are some alternatives on the market. In general, hardwoods either do not require treatment or are difficult to treat due to poor penetration, whilst most softwoods are treated. Spruce and hemlock are difficult to treat, although penetration can be improved by incising. BRE Digest 429 (1998) gives guidance on both natural durability and resistance to preservative treatment. Copper Chrome Arsenic: CCA (marketed under the trademarks of "Celcure" or "Tanalith") is a water-borne preservative with a particular application for softwood glulam bridge beams, usually in conjunction with European redwood as the timber. Both pre-treatment of individual laminations and treatment of the entire member after complete manufacture and machining are known. Pressure cylinders of up to 25m length are available. Timber is dried before application of CCA under high pressure/vacuum process and allowed to dry again for between 7 to 14 days during which fixation occurs. The advantages of CCA treatment are that the timber is free from odour and that paints and varnishes can be applied. CCA treated timbers (such as off-cuts) should not be burnt in an open fire, since this releases the preservative elements in the ash and smoke. Waste CCA treated wood should either be burnt with flue gas recovery or put into landfill.
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Creosote: Creosote is a complex mixture of over 300 substances derived from the distillation of coal tar and tends to have a poor image due to an association with odorous garden fences and weeping telegraph poles. In fact, creosote is a very long serving and effective wood preservative which has low water solubility and is biodegradable when dispersed in soil. There are many instances of creosoted timber structures and wood piles still giving good service after 100 years of ground contact, and timber currently treated by the pressure process can offer a service life of at least 40 years. Although fresh creosote will burn the skin, requiring gloves to be worn during handling of treated lumber, it is not a systemic poison. Creosote also protects wood from the development of splits. Creosoted timber should not be used for parts of the bridge which come into contact with unprotected skin, such as handrails. Freshly creosoted timbers may cause the formation of on oil sheen if in contact with water. There are two forms of creosote treatment, full cell and empty cell. In the full cell process all the available voids in the wood structure are filled as far as possible with creosote by first applying a vacuum to the timber, then flooding the pressure cylinder with preservative. After the vacuum is released atmospheric pressure forces the creosote deep into the structure. Further application of pressure after this stage achieves even greater penetration. At the end of the cycle, a second short period under vacuum is applied to withdraw a small amount of preservative from the surface of the timber leaving it dry and in a reasonable state for handling. In the empty cell process a longer period under vacuum is applied to remove a greater amount of preservative, leaving the voids in the wood only partly filled but with the internal walls of the wood cells coated. Although creosote is used undiluted by solvents, freshly treated timber is normally allowed to dry for up to 7 days to allow the more volatile components to evaporate. Alternatives: Many preservative products have been developed over the last 10 years that have aimed to provide alternatives to CCA whilst providing equivalent performance in the field. A number of these have focused on removing the arsenic compound from the preservative formulations, such as CCB systems that use boron. There are also systems available that are both chromium and arsenic free where tebunconazole and boron based preservative have replaced those ingredients. These preservatives still provide the timber with its characteristic green colouration, and are increasing used in markets such as garden decking. All these products are approved for use in Pesticides 2001. Selection system for timber bridges: Selection of an appropriate system for timber bridge component should follow the European Standards below: 1. Design bridge (timber components in contact with the water, soil and out of ground contact) 2. Assess the hazard class of the end use of the timber (EN 335-1). For example a timber in freshwater contact is Hazard Class 4. Out of ground contact timber is Hazard Class 3 and is a less challenging environment for the timber component. 3. Select wood species (EN 350-1 Natural durability) Nov 2002
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4. Is wood species natural durability sufficient for the performance required for the Hazard Class? (EN 460 Links durability to Hazard Class) 5. If species is insufficiently naturally durable then select and specify the preservative (EN 599-1 and required treatment result EN 351-1 and DD 239) Alternatively a similar philosophy is passed through in BS 5589 and BS 5268 with specifying based on preservative treatment schedules applied to particular timber species being fit for purpose.
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9.0 TIMBER FOUNDATIONS Timber piles are a highly suitable choice of foundation, given appropriate ground conditions, for many structures including bridges. Timber piles are economical, easy to transport, handle, cut to length and work with on site. They are particularly suited for locations with access difficulties or where minimal disturbance is a priority such as structures in the countryside and canal-side and shoreline sites where excavations and the delivery of concrete would pose problems. Short driven timber piles can be the solution for foundations in ground with a high water table or where firm strata exists below surface material of loose sand, soft clays, highly organic soils or fill. Timber piles are also resistant to acidic and alkaline soils, and soils with high sulphate or free carbon dioxide content. Treated or durable timber can also be used for the construction of wingwalls and bank seats, as well as for foundation pads and footings. Timber is a sustainable construction material with obvious environmental advantages over both steel and concrete. Trees, while they grow, adsorb carbon dioxide and release oxygen. Forests provide areas for wildlife and recreation. One of the suggested methods of reducing global warming has been to create carbon sinks - to lock up carbon for long periods of time. Using timber for foundations would effectively achieve this.
9.1 History and overseas use Timber has been used for piled foundations for centuries. Before 1900 nearly all piles were either untreated wood or stone. Old London Bridge was founded in 1176 on stone filled starlings constructed from elm piles, and lasted 600 years (Nash, 1981). The City of Louisiana is founded on timber piles, so too is Pont Notredame bridge in Paris, The Royal Place in Amsterdam, The National Theatre of Finland, The Dome of Utrecht and The Reichstag in Berlin. In 1902 the Campanile Tower in Venice was rebuilt on the 1000 year old piles, still in excellent condition, which supported the original structure (Haldeman, 1982). More recently, Graham (2000) reports on the use of 30 tonne capacity timber piles for the foundations of the Cargo Terminal at John F. Kennedy Airport. Timber piles were also used for the 210m diameter Louisiana Superdome supporting 130,000 m 3 of concrete and 18,000 tonnes of steel. Timber piles with 70 tonne design loads are in use on a 300m long viaduct near Winnemuca, Nevada. In Canada alone over 30,000m 3 of treated wood piles are used annually. Most of the deep foundation support for highway bridges in North America comprise of treated timber piles. The US Army Corps of Engineers used over six million timber piles to construct the locks and dams for the Inland Waterway System.
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Figure 12: Timber trestle piles supporting a concrete deck bridge (Bellingen, NSW) (photo Kardon Piling)
9.2 Durability of timber piles Timber piles, when driven in below the ground water level, are virtually immune to biological degradation and can have an almost indefinite life. Timber piles have been recovered from the remains of Roman and medieval constructions in a state of perfect preservation. The section of a pile above ground water level is, however, vulnerable to decay and one option is to terminate the pile below the water table and continue the foundations in a different material such as concrete. In the past this was accomplished with stone or masonry. The timber piles of historic buildings may decay if the local water table is lowered below the tops of the piles for long periods, either by abstraction, drainage or de-watering for nearby excavations. Both York Minster and the Mansion House in London were built in marshy ground on timber foundations which subsequently degraded due to drainage. Bouteje and Bravery(1968) report on similar problems with buildings in Stockholm. Above the water table in soil, fungi attack will lead to severe deterioration of untreated non-durable timber in a matter of months only. Under fresh water such as in rivers and lakes but also in soil below the water table the outer layer of sapwood of untreated timber will become infected by anaerobic bacteria which can degrade the strength properties over a time period of decades. In central Europe untreated species of non-durable softwoods such as Scots pine and Norway spruce were formerly used extensively. Timber piles are highly resistant to both acid and alkaline soil conditions. In Austrailia, at the Ulan coal mine, treated timber piles were chosen for a bridge carrying ore trucks because high free carbon dioxide levels and extreme acidity in the soil would have destroyed both steel and concrete piles. Timber piles were also used for the foundations of the Brambles Container Terminal in Burnie Tasmania (soil pH 11.5) and the Auburn, N.S.W., Waste Transfer Station (soil pH 2.5).
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9.3 Traditional timber species and treatments BS 8004 (1986) gives guidance on the use of timber for piles, and states that in the United Kingdom Douglas fir imported in sections up to 400mm square and 15m length is the most common softwood used for piles. Pitch pine is also available in sections up to 500mm square. Greenheart was formerly the most commonly used hardwood, imported rough-hewn in sections up to 475mm square and up to 24m long. Other suitable tropical hardwoods given by the Code include ekki, jarrah, and opepe. In the past, domestic grown hardwoods such as oak, beech, ash and sweet chestnut have been used for piles. Elm is also durable below ground, so much so that it was used for water pipes and also coffins. In Scandinavia and Central Europe Norway spruce, Scots pine and to a lesser degree fir and larch have historically been used (Peek and Willeitner, 1981). In the US and Canada southern pine is used extensively as well as larch and western red cedar. The Romans are known to have treated timber for pilings by smearing the wood with cedar oil, pitch and then charring. Pressure injection of coal-tar creosote began in England in 1838. Following the use of pressure impregnated railway sleepers (or railroad ties) in the United States the process was first applied to foundation pilings in the early 1880's. Today, pressure impregnation of creosote or copper-chromearsenic (CCA) are the two main types of chemical wood preservation applied to timber used for piles. Only softwoods are suitable for chemical impregnation, with spruce and hemlock being difficult to impregnate. Both types of preservative are applied during a high pressure/vacuum process. In the case of CCA preservative, which is water-borne, the chromium acts as an oxidising agent and the metals become highly fixed into the wood structure. Creosote, which is derived from the distillation of coal tar, may be applied as a "full cell" or "empty cell" process. In the latter case a vacuum is applied post application to remove surplus creosote which might otherwise bleed under the influence of sunlight. For softwood timber piles timber selected with a thick sapwood layer which adsorbs preservative treatment better than heartwood is beneficial since this provides a thick protective layer of well impregnated material. Spikes or hooks should not be used to handle treated timber piles since this may expose less well protected wood in the inside of the log. All cutoffs and drill holes should be liberally applied with preservative. For hardwoods the vulnerable sapwood is removed and the timber is normally supplied squared off.
9.4 Marine structures Timber is favoured for marine works because of its ability to absorb impacts, its ease of handling over water, and the poor performance, historically, of cast iron, steel and reinforced concrete. Timber is used for groynes and sea defence works as well as jetties and dolphins. In seawater and brackish estuary waters untreated timbers are liable to attack by marine borers, around the British Isles principally the mollusc Teredo (the shipworm) and crustacean Limnoria (the gribble). Teredo bores circular tunnels 15mm in diameter and up to 150mm long horizontally and vertically in timbers leading, ultimately, to severe weakening. Occasionally Teredo damage is observed in timbers which have been floated in marine waters prior to sawing, the damage being characterised by lack of bore dust and the chalky white calcareous tunnel linings (Desch and Dinwoodie, 1996). Limnoria creates shallow tunnels approximately 2.5mm in diameter and penetrating less than 15mm in depth, the extensive nature of
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which leads to erosion. Another crustacean Chelura, is associated with attack by Limnoria, but cannot by itself burrow very far into timber. The Sea Action Committee of the Institution of Civil Engineers (ICE 1947) found Limnoria and Chelura to be active in British waters, with Terodo active south of the Mersey and Humber. Greenheart, kauri and jarrah were found very resistant to marine borers, while oak and untreated softwoods were not resistant. Borer attack was found to be limited in polluted water such as in docks, although this observation may not be relevant nowadays. Greenheart was found in excellent condition after 60 years service in Liverpool and similarly Danzig fir after 52 years service in the Thames at Northfleet. Creosoted Baltic pine (i.e. slower grown Scots pine) was recommended for British ports on the grounds of its useful economic life. Other suitable softwoods include Douglas fir, Western hemlock and European larch. The Handbook of Hardwoods (HMSO 1972) lists a number of tropical hardwoods recognised as being resistant to marine borers such as basralocus, belian, okan as well as the Australian hardwoods jarrah, ironbark, southern blue gum and turpentine, the latter being particularly long favoured. Currently there are no FSC approved sources of greenheart or ekki which were traditionally used for marine piles, although possible alternatives include Acariquara and Purpleheart. In tropical waters untreated timber piles of non-durable species can have a useful life of only a few months. For softwoods, combined treatment of CCA and creosote has been found very effective. Hardwoods, such as terpentine, will also benefit greatly from the provision of an outer barrier layer of treated timber. Timber piles may also be protected by providing copper or aluminium sheaths, and there has been some development of the use of PVC (Heinz, 1975). Methods of repair of marine borer attacked piles include jacketing with concrete. The principal inspection method for marine timber piles is by diver. Abrasion resistance is an important design consideration for marine structures such as sea defences and groynes, particularly on shingle beaches. Timber can withstand wear in the marine situation better than either steel or reinforced concrete, with tropical hardwoods being particularly durable. Dense softwoods such as Douglas fir and pitch pine also perform well. Timber structures can be protected from scour simply by providing a sacrificial layer of planking.
9.5 Pile driving and design All timber piles are displacement piles, therefore suitable ground conditions must exist for their use. Conventional pile drivers are used to insert timber piles with the normal weight of a drop hammer being 1.5 times the weight of the pile. A long narrow drop hammer increases the chance that the pile is hit axially, avoiding damage to the pile and maximising the downward impulse. Diesel hammers are also suitable, including those that run on bio-diesel. Care must be taken with all hammers not to over-stress the pile or to cause splitting of the pile toe. A helmet or cap protects the pile head from fracturing or brooming, and in addition the pile may be banded to prevent splitting. Hard driving should not be continued in an attempt to meet a prescribed set, since this may result in the pile head disintegrating. Where there is a surface layer of hard fill a pre-bore may be performed. Timber piles can also be inserted using vibratory methods. Groups of timber piles inserted into soft clays may need to be loaded temporally to prevent the effect on soil pore water pressures causing buoyancy. Timber piles can be spliced and extended in length using short sections of steel tube, angle or plates to reach loadbearing strata.
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Figure 13: Timber pile driving (photo Kardon Piling)
Timber piles support loads by end bearing, shaft friction or combined end bearing and friction depending on the nature of the strata into which they are inserted. Driven thin end down, trees make natural tapered piles enhancing shaft friction. Timber has a high strength to weight ratio, and is particularly strong in compression parallel to grain. The timber selected for piles should be straight grained and free from defects and, in general, suitable material is obtained from SS grades and better. The centreline of a sawn pile should not deviate by more than 25mm throughout its length, and for round piles a deviation of up to 25mm on a 6m chord may be permitted.
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Piles are designed as columns, but consideration should be given to bracing for unsupported lengths above ground level. BS 5268 (2002) may be used to calculate the allowable compression parallel to grain. Stresses on the timber during insertion are usually far higher than those encountered in service. Similar equations exist for the loadbearing capacity of timber piles to those of steel and concrete, including those based on dynamic pile driving formulae. Appropriate factors of safety for foundation design are given in BS 8004 (1986) and BS 6349 (1988). To reduce the need for excessively hard driving the working loads are often limited to 300kN (30 tonnes) on a 300mm x 300mm pile. The American Wood Preservers Institute (2000) and Canadian Wood Council (1991) also give guidance and design examples on the use of timber piles.
9.6 Other geotechnical uses for timber Timber is attractive and its use for earth retaining structures such as bridge abutments is suitable in sensitive countryside, in particular for forest tracks. In addition, there is the obvious benefit of utilising an inexpensive, locally produced building material in situations were the delivery of heavier materials would pose problems. Timber can easily be combined with soil anchors and geotextiles in the same way as concrete or steel. Round timber and sheet piles can provide an economical wall for moderate heights of retained material. Examples of interlocking timber sheet piles are given in BS 6349-2 (1988). Further demonstration of the suitability of treated timber as foundation material is provided by the permanent wood foundation or PWF (CWC, 1997). PWF is a load bearing wood-frame system designed as a foundation for light-frame construction for residential houses, commercial premises such as hotels and factories, and agricultural buildings. Its use dates back to 1967 in Alberta, Canada. Water-based pressure treated timber is laid directly onto a granular drainage layer 300mm deep. This drainage layer prevents hydrostatic pressure building up against the foundations, and allows timber framed basements to be constructed in suitable locations. All connectors are corrosion resistant. A polythene moisture barrier extends over the outside of the walls below ground level terminating at the top of the drainage layer (Figure 14). A separate moisture barrier exists under the floor of the basement, either over-site concrete or a suspended timber floor. There is no need for a damp proof course between the timber footing plate and the bottom rails of the wall panels which, like timber frame, comprise the main support of the structure.
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Figure 14: Permanent Wood Foundation (illustration Canadian Wood Council) Stanchions and columns can be founded on footings comprising two layers of nailed treated timber running at 90 degrees to each other (Figure 15). The timber footing is laid on a thin layer of sand over undisturbed soil. A steel plate placed over the top of the footing helps to transfer the load from the column over the timbers. The principal advantages of these treated timber foundations are that they are economical, fast, require less plant, and do not need measures to protect them from freezing - this last aspect being particularly important in Canada. Timber foundations also make excellent usage of an abundant, renewable and local material. Timber foundations can also take the form of embedded poles with concrete pads and collars. Examples of these types of foundation for countryside structures, together with half round timber footings and an example of a simple wood foundation for a footbridge are given by Jayanetti (1990).
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Figure 15: Timber column base (illustration Canadian Wood Council)
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10.0 BRIDGE DESIGN PRACTICE 10.1 General practice for design of bridges in the UK Bridges constructed from steel or concrete, whether highway or footbridges, are generally designed using the BS 5400 series of Standards. This series comprises ten parts, as follows: Part 1
General statement giving design objectives and definitions.
Part 2
Specification for loads.
Part 3-5
Codes of practice for design of steel, concrete and composite bridges.
Part 6-8
Specifications for materials and workmanship for steel, concrete and composite bridges.
Part 9
Specification for bridge bearings.
Part 10
Code of practice for fatigue.
The partial factor design process for a bridge will primarily involve only two of the above parts. Firstly the loads, and the partial safety factors for the loads, are obtained from Part 2. Secondly the design (including partial safety factors for materials) is undertaken in accordance with Part 3,4 or 5 depending on which construction material is being used. In the case of road bridges or bridges over roads, the Highways Agency (formerly Department of Transport) has produced a number of Department Standards which occasionally override the requirements of the BS 5400 Series. One such Department Standard BD29/87 gives directions to engineers on how to design footbridges, including timber types.
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An Outline Procedure For The Design Of Timber Footbridges Over Roads Or In Urban Areas In outline, the procedure may be considered in three stages, as follows: 1.
Establish the general arrangements for the bridge, taking note of the requirements for layout and minimum dimensions given in DoT Departmental Standard BD29/87.
2.
Evaluate the loads acting on the bridge, using the unfactored loads of BS 5400: Part 2, unless these are made more onerous by an HA Standard.
3.
Design the members of the bridge in accordance with BS 5268: Part 2 which is a permissible stress code, used principally for timber members in buildings. BS 5268 does however contain sufficient basic materials properties, fastener design information and member design procedures for simpler types of timber bridge, as explained above in the section dealing with materials.
An Outline Procedure For Design Of Timber Footbridges In Suburban Or Rural Areas Many engineers specialising in timber bridges consider the BS 5400/HA provisions for minimum dimensions and loading too severe for lightly trafficked footbridges in suburban and rural areas. For such footbridges, typical alternative procedures are exemplified as follows: 1.
Establish the general arrangements for the bridge taking note of the minimum dimensional recommendations given in publications such as "Footbridges in the Countryside, Design and Construction" (Countryside Commission for Scotland, 1989).
2.
Evaluate the loads acting on the bridge using the unfactored loads of BS 5400: Part 2, or consider making them less onerous on the basic of recommendations given in publications such as the above.
3.
Design the bridge members in accordance with BS 5268: Part 2.
Examples of how the "Countryside Commission for Scotland" publication recommends less onerous minimum dimensions and loadings are given in Tables 7 and 8 below:
Source of data Location of bridge
DoT Standard BD29/87 Urban area
Pedestrians only 1800 People with disabilities 2000
"Countryside Commission for Scotland" publication Accessible Inaccessible rural area rural area (one(two-way way traffic) traffic) 1200 900 1700 1200
Table 7: Recommended deck widths (mm) for alternative bridge locations
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Source of data
BS 5400: Part 2
"Countryside Commission for Scotland" publication
Location of bridge
Urban Area
Rural Area
Vertical imposed uniformly distributed load on bridge deck (kN/m‘)
5.0
2.3 – 3.2
Horizontal load per metre (kN/m) perpendicular to handrail
1.4
0.74 – 1.4
Table 7: Recommended imposed loadings for alternative bridge locations
Highways Agency criteria for layout and dimensions of footbridges Layout of Footbridge The HA (former DoT) Standard BD29/87 stipulates several criteria relating to the layout of footbridges, some of which are quite fundamental to the bridge design. These may be summarised as follows: Access: Access to footbridges located adjacent to carriageways should be sited so that pedestrians walking down the access face on-coming traffic. Plain ramped access is preferred to stairs as it is more satisfactory for people in wheelchairs and pedestrians pushing prams. However wherever possible both forms of access should be provided. Layout: The main span of a footbridge should, wherever possible, be at right angles to the road carriageway. Supports: Where a footbridge crosses a dual carriageway, preference should be given to spanning both carriageways in a single span, to avoid the need for a support in the central reserve. Supports which may be subject to collisions by errant vehicles shall be designed to resist collision loading.
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Dimensions for footbridges Width of bridges for pedestrians only: The dimensions of the clear widths of the main span, ramps and stairs of a footbridge would be derived on the basis of the information in Table 9 (below) meet the peak pedestrian traffic.
Gradient
Clear width (mm)
< 1/20
300mm per 20 persons per minute
Steps or > 1/20
300mm per 14 persons per minute
Table 9: Recommended clear widths (mm) for alternative bridge gradients Minimum widths of 1800mm for general purposes, or 2000mm stipulated for bridges enhanced for use by disabled persons. Where bridges are to be designed for the combined use of pedestrians and cyclists, further width requirements apply. These range from a 1200mm wide footpath separated by a white line from a 500mm wide cycle track, to a 1950mm wide lane for each, separated by railings. The DoT criteria for heights of parapets vary according to the types and combinations of bridge user. They are summarised in Table 10, below.
Type of footbridge
Parapet height (m)
Pedestrians only, where bridge is in area of high prevailing winds or with headroom under bridge greater than 10m
1.15 – 1.30
Pedestrians and cyclists
1.4
Table 10: Height requirements for bridge parapets Stairway requirements may be summarised as follows: Maximum number of stairs in a flight is 20 Riser dimension < 150mm Tread dimension > 300mm. It is a preference that ramps should not be steeper than 1 in 20. However if limitations of space dictate then steeper ramps may be used, up to a maximum gradient of 1 in 12. For ramps with gradients greater than 1 in 20, landings must be provided in order that the rise of any ramp section does not exceed 3.5m
Evaluation of loads using BS 5400: Part 2
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As mentioned above, timber footbridges are designed using the unfactored nominal loads of BS 5400: Part 2. Interestingly, the use of unfactored nominal loads in structural design is not only limited to timber, with the design of foundations and that for aluminium structures also being based on unfactored loads. Where adequate statistical distributions are available, the nominal loads in BS 5400: Part 2 are those appertaining to a return period of 120 years. The following types of nominal load relating to footbridges are considered by BS 5400: Part 2 1. Permanent loads • •
Dead-weight of structural elements Superimposed dead – road surfacing, etc.
2. Live loads from pedestrian traffic • •
Nominal vertical live load Nominal load on pedestrian parapets
3. Wind loads • • •
Transverse Longitudinal Vertical
4. Loads from temperature effects 5. Erection loads BS 5400: Part 2 suggests that in most cases snow loads can be ignored. This is logical since the full pedestrian design load is improbable under heavy snow falls of the duration likely to be experienced in the UK. The maximum wind gust speed is evaluated by applying gust factors to mean hourly wind speeds extracted from a map of isotachs. The magnitude of the gust factor depends on the height of the bridge, and the horizontal wind loaded length. For footbridges only, BS 5400: Part 2 allows the following reductions in wind load: 1. The mean hourly wind speed is reduced by 0.94, which is a conversion factor to obtain 50-year return period values from 120-year return period values. 2. A reduction in the gust factor when the bridge is located in urban areas or a rural environment with many windbreaks. To use BS 5268: Part 2 for the design of bridge members, the designer has to decide upon the service conditions for the bridge. This mainly involves deciding the exposure and duration clauses that are appropriate for the member concerned. Experience has shown that designers usually are on the conservative side by choosing to design members using wet exposure stresses. With BS 5268, the threshold moisture content between wet and dry exposure conditions is 18%. Hence this is often unnecessarily conservative. For a vertical imposed uniformly distributed loading which
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represents a crowded bridge, experience has shown that designers usually select medium-term duration. This is the duration class used with BS 5268 for snow loading in the UK. Horizontal loads on handrails or parapets are usually designated as shortterm. This is the same load duration class as that arising from the case of a man standing on a roof member.
10.2 Deflection limits The limited guidance given in BS 5268: Part 2 regarding deflection limit is of little relevance to the design of bridge members. Enquiries indicate that a static deflection limit of Span/200 under imposed load only is often used for beam members. The "Countryside Commission for Scotland" publication recommends a tighter limit of Span/240 under total loading. Lightly precambered glulam bridge beams are often designed using a deflection limit for live load only, which is permitted in principle by BS 5268.
10.3 Eurocode 5 As explained above, it has proven possible to design acceptable timber footbridges using BS 5268: Part 2 recommendations, supplemented by additional guidance from elsewhere. The eventual publication of EC5, Part 2 will be a great step forward and will be welcomed by everybody involved. Meanwhile, EC5, Part I is shortly to be available. Thus even at this stage, the Eurocodes will bring advantages to the more sophisticated aspects of timber engineering such as bridges. EC5 introduces limit states design to timber for the first time in the UK. It is therefore a more radical change for timber than the introduction of Eurocodes for other major structural materials. EC5 contains the essential rules for design, but unlike the British Code, BS 5268, it does not include material properties, tables of fastener loads and other such other design information. An immediately obvious change is that wherever possible, EC5 uses equations rather than tables. Also much of the nomenclature and terminology is considerable different. As in all instances of limit states design codes, EC5 will require clear thinking about the distinction between ultimate and serviceability limit states. The latter are, of course associated with deflections, deformations and vibration. EC5 treats these matters in a considerably more sophisticated manner than their coverage in BS 5268. As has already been pointed out, BS 5268 gives no adequate guidance related to deflection limits for bridges. Furthermore it is likely that the Working Group dealing with EC5, Part 2 will require to give considerable thought to the serviceability topics. EC5 offers a number of advantages over BS 5268. It provides the opportunity to design with a wide selection of materials and components. The use of characteristic values for materials, based directly on test results, means that new materials and components, which have achieved suitable technical approval can more easily be assimilated, thus facilitating development and innovation. More guidance is given on the design of built-up components than in BS 5268, and EC5 provides a unified design and safety basis for laterally loaded dowel-type joints (nails, staples, screws, bolts and steel dowels). The ENV contains no information on the design of joints using connectors such as toothed plates, shear plates and split rings, but a procedure is being developed through other sponsored research programmes. Interim guidance on the design of such joints is contained in the UK NAD.
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10.4 Overseas practice - Decks The principal function of a deck is to distribute the loads produced by the traffic to the supporting elements of the bridge. If the deck is designed as a structural diaphragm, it can be used as a major spanning element in its own right. It can also brace other main components, and transfer horizontal wind or brake loads through to other parts of the structure, and ultimately to the foundations. Dependent on its location, a secondary function of the deck may be to protect the main structure from moisture and mechanical damage from traffic. An effective and durable protection of substantial parts of the timber structure may be achieved with closed, high-level decks. Structural timber decks have been used in North America since at least the 1930s. Initially, nailed laminated construction was used. Timber-concrete composites were also introduced at quite an early stage. Later, glued laminated beams connected with shear devices started to be used extensively. Nowadays, structural timber decks exist in many designs, using half-round timbers, sawn timber, glued laminated timber, and structural timber composites of various types. In parts of Europe, particularly the German-speaking countries, structural decks of all the types mentioned above are found. A more traditional type, used on lighter bridges, including footbridges, is the two-layer herringbone boarded pattern. This does not make a major structural diaphragm contribution, in comparison with stressed laminated decks, for example, but it does contribute to the lateral bracing of the structure. A good example is in the large Main-Donau Canal 190 m long tension ribbon bridge, described in STEP Lecture E17. There is significant coverage of structural deck design in prEN Eurocode 5 Part 2. Research and practise in structural decks of various forms has also occurred in the Nordic countries and in Australia and New Zealand.
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Common Timber Deck Types In Canada: Longitudinal and transverse nail-laminated (LNL/TNL) decks: In longitudinal laminated decks, the timber laminations are orientated parallel to the direction of the traffic, whereas in transverse laminated decks the laminates are orientated perpendicular to the direction of the traffic flow.
Figure 16: Longitudinal laminated deck ( left) and transverse laminated deck (right)
Nailed laminated decks consist of planks of timber laid on edge side by side. The laminations are nailed together to form a slab. Nails are driven through the faces of the planks to fix them together laterally. 1. Timber-concrete composite (TCC) decks: A concrete topping is applied to a timber deck, traditionally a longitudinal nail laminated type, giving the slab a concrete compression zone and wearing course. Shear keys are required between the timber and the concrete layers. 2. Longitudinal or transverse stress laminated (LSL/TSL) decks: In addition to nails, post tensioning is applied using high strength steel to improve the load transfer of the deck. Although the stiffness of the timber is low perpendicular to the grain, pre-stressing allows a plate action in the deck. Edge members are often made of hardwood or steel, in order to avoid damage due to high bearing stresses perpendicular to the grain at the pre-stressing points.
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Figure 17: Tension rods 3. Floor beam ( or timber tie) decks: This type is a beam and plank deck. the planks overlay heavy transverse beams or ties. 4. Two layer plank decks: As above, but with two layers of heavy planks.
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Primary highways subjected to high volume of traffic
New bridges on secondary roads with medium volume of traffic
New bridges on secondary roads with low volume of traffic
Lightly used park and forest access roads with occasional heavy vehicles
Longitudinal nail laminated
Very low volume roads with little or no commercial traffic
Deck replacement on steel truss bridges
Economical when supported on timber pile bents
Economical alternative to replacing concrete Can give higher live load capacity Speedy replacement
Transverse nail laminated
On steel or timber girders
Timberconcrete composite
Spans up to 7m
Longitudinal stress laminated
Viable option
Will span up to 8m for 286mm deep decks and longer for larger timber sizes
Transverse stress laminated
Not yet used, but shows promise
On steel or timber girders
Floor beam
Economical
Two layer plank decks
Economical
Table 11: Summary of the principal applications for various combinations of timber decks, beams and other structures used for highway bridges in Canada
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11.0 FUTURE CHALLENGES Fresh challenges constantly arise, both for timber engineering in general, and specifically for bridges. Salient aspects of timber engineering materials and design relating to timber bridges which are still under development are outlined as follows:
11.1 High efficiency composite materials Although timber provides high specific strength and stiffness, until recently, its natural variability remained a challenge. However, through efficient strength grading procedures and associated quality assurance measures, timber and wood-based composites offer variability levels as low as those attained in metallic engineering materials, and certainly much lower than in many commonly used grades of concrete. High-strength softwood glued laminated beams, using laminations of adequate controlled quality and regulated end jointing techniques, can be produced with characteristic bending strengths up to typically 36 N/mm2. By substituting STCs for the outer laminations, composite elements of 15~25% higher bending strength may be guaranteed, without increase in weight. Such composite materials are comparable in strength to high-performance reinforced concrete, but have a mass of only onequarter of the latter. European research is also in progress in which, by utilising structural composites based on temperate hardwood veneers, bridge beams with characteristic bending strengths greater than 60 N/mm2 are feasible, with weight increases of only 15%.
11.2 New adhesive bonding technologies The classical timber engineering adhesives have been proven adequate for small and medium spans in buildings. However new technical and economical requirements have been driving towards improved adhesives and new bonding techniques. Adhesive bonding is only now becoming accepted in the field of bridge engineering in general, with strides having been made in resin bonding technologies for the refurbishment and upgrading of steel and concrete structures. Until recently, these techniques have depended upon the use of steel as the bonded-on reinforcing medium. Recently however, field trials and in situ monitoring have commenced on bridges reinforced with Fibre Reinforced Plastics (FRPs), involving such advanced materials as Carbon Fibre. In the light of such developments, the climate of acceptance has grown for the possibility of the greater use of bonding technologies in timber bridge engineering. prEN Eurocode 5 Part 2 Bridges contains an Informative Annex on the use of “Gluedin Steel Rods”. These rules have recently been supported by a major European Collaborative research Programme, known as “GIROD”. This will permit bonded-in rods to form safety critical connections in timber bridge structures, and also provide the basis for calculations concerning bonded-in reinforcements for applications such as large structural deck plates. At present, the Eurocode Informative Annex design recommendations apply only to steel rod-type reinforcements and connections, these having already been subjected
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to earlier research programmes including outdoor exposure assessments. However, this research is now being extended to embrace bonded-in materials of other classes, including FRP Pultrusions joined to timber and timber composites (Mettem, 1996). Bridge parts depending upon adhesive bonding for their manufacture and assembly entail substantial manufacturing planning and production arrangements, to ensure that the conditions for correct control are met. In large decks and other bridge structural members, sections of up to about 1m 2 of cross-sectional area have so far been produced, and this upper limit is constantly being expanded.
11.3 Steel reinforced timber As mentioned above in connection with new bonding technologies, steel reinforcements are already being used within timber elements contained in bridge structures. In prEN Eurocode 5 Part 2 Bridges, design rules can be found for reinforced members, transversely reinforced timber, and for deck plates containing such reinforcements. Most frequently, these steel reinforced timber elements serve as high duty deck plates, although local reinforcing also takes place in stress-critical zones in the main structural members. Reinforcement is especially worth considering where elemental design is restricted through the relatively low tensile strength of timber transverse to the grain direction. It is also possible to use certain techniques to improve shear resistance, in situations where the action effects tend to cause sliding of adjacent fibres (known as shear parallel to the grain). Another instance where this type of reinforcement may be considered is at notches and at other abrupt changes in section, where stress concentrations can cause Mode I or Mixed Mode I/II crack growth and fracture. Finally, it is considered in some instances where highly concentrated action effects are to be transferred between parts of the structure, and where greater strength can be attained if such forces are transferred to zones deep within the receiving members. Where timber road bridge decks are steel reinforced, in general, the bars or tendons are located in directions inclined or parallel to the major fibre directions of the parent wood. In some instances, bonded-in steel-reinforcements are pre-stressed, to compensate for shrinkage effects which may occur with the components in service
11.4 Timber-concrete composites Current timber-concrete composites are described above, and as stated, design principles and application rules are already given in the structural timber Eurocodes. Challenges which remain for such systems, include searching for ways of achieving still further efficiency in the composite interaction benefits that can be achieved with better, more durable, shear key systems. Effort is also concentrated on securing continuous improvements in the sealing and moisture protection of these types of deck.
11.5 Deck protection systems Better deck protection systems are being given a high priority in several of the research programmes outside the UK. Higher traffic speeds and intensities of wheel passage have drawn attention to the need for greater deck protection against wetting
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that is spread from the vehicles themselves. Deck systems and structural elements both need carefully designed protection to avoid cumulative damage from these effects. To some extent, the timber deck and the main structure may be treated independently in this regard.
12.0 PRIORITY WORK AREAS This Report has set out the state of the art of timber bridge design in the UK, with wider references to Europe in general. From this, it is possible to identify the priority work areas in which it is felt that effort should be concentrated, from a UK point of view.
12.1 Innovative Timber Engineering for the Countryside The above is the title of the Pilot Project in which BRE and TRADA Technology are engaged as Technology Providers. 1.
Review serviceability criteria for footbridges in all materials.
2.
Continue bridge survey work, paying special attention to serviceability performance and durability aspects.
3.
Carry out design studies leading to the construction of prototypes which are to be used for serviceability assessments and durability monitoring.
4.
Produce nationally applicable design guidance.
5.
Maximise the use of UK grown species in sizes (round and sawn) likely to be available locally for bridge construction
Within item 3 above, the initial design studies have already led to the conclusion that there are four areas where supplementary design guidance is required. These are as follows: 1.
The design and detailing of bracing systems to ensure member stability and to resist horizontal wind loads
2.
Methods to ensure satisfactory vibration performance of bridges under human footfalls
3.
Simple methods to ensure that the excitation of bridges by wind is avoided.
4.
In the case of mechanically laminated beams, a simplified method of evaluating deflections is required.
12.2 prEN Eurocode 5, Part 2 The Project Team has identified several priority areas coinciding with those felt to be necessary in the UK. The following have been suggested: 1.
The dynamic behaviour of bridges under pedestrian and wind loadings.
2.
The fatigue behaviour of connections.
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3.
Timber-concrete composite behaviour.
4.
Bonded-in steel rods.
5.
Durability requirements for timber bridges, including those for the connectors.
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13.0 CONCLUSIONS Timber engineers have the expertise to provide aesthetically exciting, well-protected, and durable bridge structures. To achieve impact, economic drivers must be harnessed, to unlock consumer and specifier indifference. Key motivators include: • • • • • • •
National cycle routes City regeneration, calling for aesthetically exciting, well-performing links. Canal and rail regeneration Marina and dockside development Housing developments, with associated bridging needs. Forest roads and infrastructure maintenance in remote regions. Linking to value-added forest products.
The use of sustainably grown and locally produced timber for bridge, foundation and sea defence engineering will increasingly be seen as favourable. In addition there are concerns and moves in Europe away from the use of timber treatments such as creosote and Copper Chrome Arsenic. Applied research and development, demonstration projects, and benchmarking involving the use of domestic grown timber are seen as vital. Above all, however, well-informed promotion is recognised as of paramount importance in unlocking demand for timber bridges as flagship projects in sustainable development, environmental protection, and improvements to the quality of life.
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British Standards Institution (1996) BS EN 351-1 Durability of wood and wood-based products. Preservative-treated solid wood. Classification of preservative penetration and retention, BSI London British Standards Institution (1998) DD 239 Recommendations for the preservation of timber, BSI London British Standards Institution (1989) BS 5589 Code of practice for preservation of timber, BSI London Brown D. J. (1988) Bridges – Three Thousand Years of Defying Nature. Mitchell Beazley. (Rev.) ISBN 1 85732 163 4. Building Research Establishment (1998) Digest 429 Timbers: their natural durability and resistance to preservative treatment. Canadian Wood Council (1997) Permanent Wood Foundations, CWC, Ottawa, Ontario, ISBN 0 921628 49 8. Canadian Wood Council (1991) Wood Piles, CWC, Ottawa, Ontario. ISBN 0 921628 08 0. Chugg, W.A. (1962) Report on a Visit to Switzerland to Inspect Glued Laminated Timber Structures Over Ten Years Old. E/IB/7, Timber Research And Development Association, High Wycombe. Chugg, W.A. (1958) The Structural Glued Laminated Timber Industry in North America. E/IB/4, Timber Development Association, London. Copani, P. and Funis, F. (1999) Il ponte di Palladio sul Cismone e le altre tre “invenzioni senza porre altrimenti pali nel fiume” – Palladio’s bridge on the Cismone River and the other three “invenzioni senza porre altrimenti pali nel fiume” Bollettino Ingegneri N. 12 – Collegio Ingegneri, Firenze, dicembre. Countryside Commission for Scotland (1989) Footbridges in the Countryside – Design and Construction. Reiach Hall Blyth Partnership. 2nd ed. Battleby, Perth, Scotland, ISBN 0 902226 52 5. Curry, W T. (1974) Grade stresses for structural laminated timber. Republished as Chapter 5 of: The strength properties of timber. MTP Construction, Lancaster, England. Department of Transport (1992) Design Criteria for Footbridges. BD29/87, Reprint November, London Desch H. E. and Dinwoodie, J. M. (1996) Timber - Structure, Properties, Conversion and Use, Macmillan Press Ltd, London. Farmer, R.H. (1972) Handbook of hardwoods, Rev. 2nd. Ed, HMSO London, ISBN 0 11 470541. Fischer, J.(1995) Timber Bridges. In: Chapter E17 of: Timber Engineering, STEP2 – Design – Details and Structural Systems. Ed. Blass, H.J. et al. Centrum Hout, Almere, Netherlands, ISBN 90 5645 002 6.
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Newlands, J. (1990) The Carpenter and Joiner’s Assistant. Glasgow, Blackie, 1857. Reprinted, Studio Editions, London. Nordic Timber Council (1999) Timber Bridges – a presentation of 22 Nordic timber bridges. NTC, Stockholm. O'Connor, C. (1993) Roman Bridges. Cambridge University Press, 1993. Paolillo, G.G.M. (1999) Il ponte di Trajano sul Danubio presso Drobeta nella Dacia Inferior – Trajan’s Bridge on the Danube at Drobeta in the Dacia Inferior. Bollettino Ingegneri N. 12 – Collegio Ingegneri, Firenze, dicembre. Peek, R.D. and Willeitner, H. (1981) Behaviour of Wooden Pilings in Long Time Service. Proc. 10th Internal Conf. on Soil Mechanics and Foundation Engineering, Stockholm, AA Balkema, Vol. 3 pp 147-152. Popa, N. Bancila, R. and Florea, S. (1992) Trajan’s Bridge Over the Danube at Drobeta Turnu Severin. In: Proc. International Conference - Bridges on the Danube, Vienna – Bratislava – Budapest. Hungarian Nat. Cttee. IABSE, Sept. ISBN 963 421 5009. Reiach Hall Blyth Partnership (1981) Footbridges in the Countryside. ISBN 0 902226 52 5. Countryside Commission for Scotland, Perth, 1981. Ruddock, Ted (1979) Arch Bridges and their Builders 1735 - 1835. Cambridge, 1979. Taylor R.J., and Keenan, F.J. (1992) Wood Highway Bridges. ISBN 0-921628-12-9, Canadian Wood Council, Ottawa. The American Wood Preservers Institute (2000) Construction Guidelines for Timber Piling, AWPI Web, URL: http://www.awpi.org/ppt/home.html/ The Highways Agency (1996) The Appearance of Bridges and Other Highway Structures. HMSO, London. ISBN 0 11 551804 5. Wilson, T.R.C.(1939) The Glued Laminated Wooden Arch. USDA Tech. Bulletin 691. Yeomans, D.T. (1985) Historic Development of Timber Structures. In: Timber in Construction. Eds. Sunley, J.G. and Bedding, B. Batsford/TRADA, London, ISBN 0 7134 5053 3.
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