Walls and Building Structure

ARC 625: ARCHITECTURAL ENGINEERING

A TERM PAPER
ON

WALLS AND BUILDING SKELETON

BY OJEBOLA AYOBAMI OYEBAMIJI M.TECH/SET/2010/2702

MENTOR DR. R.E. OLAGUNJU

22ND AUGUST 2011

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WALLS AND BUILDING STRUCTURE
Building walls frequently become works of art externally and internally, such as when featuring mosaic work or when murals are painted on them; or as design foci when they exhibit textures or painted finishes for effect. On a ship, the walls separating compartments are termed "bulkheads", whilst the thinner walls separating cabins are termed "partitions". In architecture and civil engineering, the term curtain wall refers to the facade of a building which is not load-bearing but functions as decoration, finish, front, face, or history preservation. A partition wall is a wall for the purpose of separating rooms, or dividing a room. Partition walls are usually not load-bearing. Partition walls may be constructed with bricks or blocks from clay, terra-cotta or concrete, reinforced, or hollow. Glass blocks may also be used. They may also be constructed from sheet glass. Glass partition walls are a series of individual toughened glass panels, which are suspended from or slide along a robust aluminium ceiling track. The system does not require the use of a floor guide, which allows easy operation and an uninterrupted threshold. Timber may be used. This type of partition consists of a wooden framework either supported on the floor below or by side walls. Metal lath and plaster, properly laid, forms a reinforced partition wall. Partition walls constructed from fibre cement sheeting are popular as bases for tiling in kitchens or in wet areas like bathrooms. Galvanized sheet fixed to wooden or steel members are mostly adopted in works of temporary character. Plain or reinforced partition walls may also be constructed from concrete, including pre-cast concrete blocks. Metal framed partitioning is also available. This partition consists of track (used primarily at the base and head of the partition) and stud (vertical sections fixed at 600mm centres).Internal wall partitions also known as office partitioning are made using plasterboard (drywall), or varieties of glass. Toughened glass is a common option as it is feasible however there is also low iron glass better known as optiwhite glass which increases light and solar heat transmission. Wall partitions are constructed using beads and tracking which are either hung from the ceiling or fixed into the ground. The panels are inserted into the tracking and fixed. There are variations of wall partitions which include the level of fire resistance they have, and their acoustic performance rating

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The external walls of a structure are vital to the integrity safety and aesthetics of the building. They are more often then not the load bearing walls and must also face the harsh conditions that the weather can throw at a building. External walls normally consist of two different designs in conventional style buildings cavity and solid. CAVITY WALLS Cavity walls are normally made from brickwork and were introduced to the UK in the 19th century, becoming a commonly accepted building practice by the 1920s. Commonly a cavity wall is constructed with a 102mm half brick outer skin with a 100mm dense brick inner skin. The size of a cavity will vary. Traditionally sizes of between 50mm and 100mm were used but this is now being increased in modern builds to allow for extra insulation. The two walls are joined in places by wall ties to help spread lateral loads. BRICK Brick can also be clad onto a modern eco timber structure. Some more attractive bricks such as older London Reds, used to match other street brickwork, are recycled material and so attractive to an eco builder; but the motive to use them is to create a uniform blandness of appearance. The air in a cavity wall acts as a poor conductor of heat. This has the effect of insulating a building, as the heat does not easily pass into the air of the cavity. The air in a cavity wall will also heat up during the day releasing heat at a slow rate helping to keep a building warm. A damp proof course laid through the cavity will prevent moisture from entering the building through the walls. Whilst the air in a cavity provides greater insulation than a comparable solid wall would there are ways in which it can be further enhanced. The most common of these is to fill the cavity with insulating foam. This traps in even more heat and makes it easier to regulate the temperature inside a building. Cavity wall insulation can be injected into any building with a cavity wall;

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SOLID WALLS Solid load bearing external walls can be made using a variety of different techniques. The most common is to use masonry but other alternative construction methods are available such as rammed earth and cob building. Solid walls lose heat much more rapidly then cavity walls but they can be insulated from both the inside and outside. INSULATION Outside insulation is called rendering and is often done decoratively. This type of insulation is not cheap but it does have the added bonus of helping to weatherproof a wall. Insulating a solid wall internally is a cheaper option. INSULATING CONCRETE FRAMEWORK Insulating Concrete Framework (ICF) walling is a construction method beginning to get a lot of popularity for its ease of construction and insulating properties. In an ICF system a framework made from insulating material (usually expanded polystyrene) designed to lock into each other similar to LEGO bricks allowing the build to not require binder materials such as mortar. Once the framework is in place concrete is poured in. When this sets the structure is strong and has a high insulation value. The method of which ICF forms are created allow them to be moulded into various shapes and designs as well as requiring less labour and time to build. The use of thick concrete also provides exceptional sound insulation and prevents air leakage into a property.

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CURTAIN WALL
A curtain wall is an outer covering of a building in which the outer walls are non-structural, but merely keep out the weather. As the curtain wall is non-structural it can be made of a lightweight material reducing construction costs. When glass is used as the curtain wall, a great advantage is that natural light can penetrate deeper within the building. The curtain wall façade does not carry any dead load weight from the building other than its own dead load weight. The wall transfers horizontal wind loads that are incident upon it to the main building structure through connections at floors or columns of the building. A curtain wall is designed to resist air and water infiltration, sway induced by wind and seismic forces acting on the building and its own dead load weight forces. Curtain walls are typically designed with extruded aluminum members, although the first curtain walls were made of steel. The aluminium frame is typically infilled with glass, which provides an architecturally pleasing building, as well as benefits such as daylighting. However, parameters related to solar gain control such as thermal comfort and visual comfort are more difficult to control when using highly-glazed curtain walls. Other common infills include: stone veneer, metal panels, louvers, and operable windows or vents. Curtain walls differ from store-front systems in that they are designed to span multiple floors, and take into consideration design requirements such as: thermal expansion and contraction; building sway and movement; water diversion; and thermal efficiency for cost-effective heating, cooling, and lighting.
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HISTORY OF CURTAIN WALL

Oriel Chambers, Liverpool, England,1864. The world's first glass curtain walled building. The stone mullions are decorative.

16 Cook Street, Liverpool, England,1866. Extensive use of floor to ceiling glass is used, enabling light penetration deeper into the building maximizing floor space.

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A building project in Wuhan China, the difference in progress between the two towers illustrates the relationship between the inner load bearing structure and the exterior glass curtain. Prior to the middle of the nineteenth century, buildings were constructed with the exterior walls of the building (bearing walls, typically masonry) supporting the load of the entire structure. The development and widespread use of structural steel and later reinforced concrete allowed relatively small columns to support large loads and the exterior walls of buildings were no longer required for structural support. The exterior walls could be non-load bearing and thus much lighter and more open than the masonry load bearing walls of the past. This gave way to increased use of glass as an exterior façade and the modern day curtain wall was born. Oriel Chambers in Liverpool, England, was the world's first metal framed glass curtain walled building in 1864, followed by 16 Cook Street, Liverpool, in 1866. Both buildings were designed and built by local architect Peter Ellis. The extensive glass walls allowed light to penetrate further into the building utilising more floor space and reducing lighting costs in short winter months. Oriel Chambers comprises 43,000 sq ft (4,000 m2) set over a maximum of five floors as the elevator had not been invented. Some of the first curtain walls were made with steel mullions and the plate glass was attached to the mullions with asbestos or fibreglass modified glazing compound. Eventually silicone sealants or glazing tape were substituted. Some designs included an outer cap to hold the glass in place and to protect the integrity of the seals. The first curtain wall installed in New York City was this type of construction. Earlier modernist examples are the Bauhaus in Dessau and the Hallidie Building in San Francisco. The 1970s began the widespread use of aluminum extrusions for mullions. Aluminum offers the unique advantage of being able to be easily extruded into nearly any shape required for design and aesthetic purposes. Today, the design complexity and shapes available are nearly limitless. Custom shapes can be designed and manufactured with relative ease.Similarly, sealing methods and types have evolved over the years, and as a result, today’s curtain walls are high performance systems which require little maintenance.

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SYSTEMS AND PRINCIPLES OF CURTAIN WALLS STICK SYSTEMS The vast majority of curtain walls are installed long pieces (referred to as sticks) between floors vertically and between vertical members horizontally. Framing members may be fabricated in a shop, but all installation and glazing is typically performed at the jobsite. UNITIZED SYSTEMS Unitized curtain walls entail factory fabrication and assembly of panels and may include factory glazing. These completed units are hung on the building structure to form the building enclosure. Unitized curtain wall has the advantages of: speed; lower field installation costs; and quality control within an interior climate controlled environment. The economic benefits are typically realized on large projects or in areas of high field labour rates. RAINSCREEN PRINCIPLE A common feature in curtain wall technology, the rainscreen principle theorizes that equilibrium of air pressure between the outside and inside of the "rainscreen" prevents water penetration into the building itself. For example the glass is captured between an inner and an outer gasket in a space called the glazing rebate. The glazing rebate is ventilated to the exterior so that the pressure on the inner and outer sides of the exterior gasket is the same. When the pressure is equal across this gasket water cannot be drawn through joints or defects in the gasket. DUAL AIRLOOP SYSTEM/ TINGWALL The dual airloop system improves on the performance of earlier unitized curtain walls employing the rainscreen principle of separating rain from wind to lessen reliance on "perfect" seals. The joints around glass and metal panels contain both outer (wet, drained) and inner (dry, protecting the panel seals) air loops, to prevent water leakage no matter where the inevitable seal imperfections occur.

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DESIGN OF CURTAIN WALLS
Curtain wall systems must be designed to handle all loads imposed on it as well as keep air and water from penetrating the building envelope. LOADS The loads imposed on the curtain wall are transferred to the building structure through the anchors which attach the mullions to the building. The building structure design must account for these loads. DEAD LOAD Dead load is defined as the weight of structural elements and the permanent features on the structure. In the case of curtain walls, this load is made up of the weight of the mullions, anchors and other structural components of the curtain wall, as well as the weight of the infill material. Additional dead loads imposed on the curtain wall, such as sunshades, must be accounted for in the design of the curtain wall components and anchors. WIND LOAD Wind load acting on the building is the result of wind blowing on the building. This wind pressure must be resisted by the curtain wall system since it envelops and protects the building. Wind loads vary greatly throughout the world, with the largest wind loads being near the coast in hurricane-prone regions. For each project location, building codes specify the required design wind loads. Often, a wind tunnel study is performed on large or unusually shaped buildings. A scale model of the building and the surrounding vicinity is built and placed in a wind tunnel to determine the wind pressures acting on the structure in question. These studies take into account vortex shedding around corners and the effects of surrounding area

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SEISMIC LOAD Seismic loads need to be addressed in the design of curtain wall components and anchors. In most situations, the curtain wall is able to naturally withstand seismic and wind induced building sway because of the space provided between the glazing infill and the mullion. In tests, standard curtain wall systems are able to withstand three inches (75 mm) of relative floor movement without glass breakage or water leakage. Anchor design needs to be reviewed, however, since a large floor-to-floor displacement can place high forces on anchors. (Additional structure must be provided within the primary structure of the building to resist seismic forces from the building itself.) SNOW LOAD Snow loads and live loads are not typically an issue in curtain walls, since curtain walls are designed to be vertical or slightly inclined. If the slope of a wall exceeds 20 degrees or so, these loads may need to be considered. THERMAL LOAD Thermal loads are induced in a curtain wall system because aluminium has a relatively high coefficient of thermal expansion. This means that over the span of a couple of floors, the curtain wall will expand and contract some distance, relative to its length and the temperature differential. This expansion and contraction is accounted for by cutting horizontal mullions slightly short and allowing a space between the horizontal and vertical mullions. In unitized curtain wall, a gap is left between units, which are sealed from air and water penetration by wiper gaskets. Vertically, anchors carrying wind load only (not dead load) are slotted to account for movement. Incidentally, this slot also accounts for live load deflection and creep in the floor slabs of the building structure.

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BLAST LOAD Accidental explosions and terrorist threats have brought on increased concern for the fragility of a curtain wall system in relation to blast loads. The bombing of the Alfred P. Murrah Federal Building in Oklahoma City, Oklahoma, has spawned much of the current research and mandates in regards to building response to blast loads. Currently, all new federal buildings in the U.S. and all U.S. embassies built on foreign soil, must have some provision for resistance to bomb blasts. Since the curtain wall is at the exterior of the building, it becomes the first line of defense in a bomb attack. As such, blast resistant curtain walls must be designed to withstand such forces without compromising the interior of the building to protect its occupants. Since blast loads are very high loads with short durations, the curtain wall response should be analyzed in a dynamic load analysis, with full-scale mock-up testing performed prior to design completion and installation. Blast resistant glazing consists of laminated glass, which is meant to break but not separate from the mullions. Similar technology is used in hurricane-prone areas for the protection from wind-borne debris. INFILTRATION Air infiltration is the air which passes through the curtain wall from the exterior to the interior of the building. The air is infiltrated through the gaskets, through imperfect joinery between the horizontal and vertical mullions, through weep holes, and through imperfect sealing. The American Architectural Manufacturers Association (AAMA) is an industry trade group in the U.S. that has developed voluntary specifications regarding acceptable levels of air infiltration through a curtain wall . This limit is expressed (in the USA) in cubic feet per minute per square foot of wall area at a given test pressure. (Currently, most standards cite less than 0.6 CFM/sq ft as acceptable.) Testing is typically conducted by an independent third party agency using the ASTM E-783 standard. Water penetration is defined as any water passing from the exterior of the building through to the interior of the curtain wall system. Sometimes, depending on the building specifications, a small amount of controlled water on the interior is deemed acceptable. AAMA Voluntary

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Specifications allow for water on the interior, while the underlying ASTM E - 1105 test standard would disqualify a test subject if any water is seen inside. To test the ability of a curtain wall to withstand water penetration, a water rack is placed in front a mock-up of the wall with a positive air pressure applied to the wall. This represents a wind driven heavy rain on the wall. Field quality control checks are also performed on installed curtain walls, in which a calibrated spray nozzle is used to spray water on the curtain wall for a specified time in order to investigate known leaks or leading up to a validation test like the ASTM E-1105. DEFLECTION One of the disadvantages of using aluminum for mullions is that its modulus of elasticity is about one-third that of steel. This translates to three times more deflection in an aluminum mullion compared to the same steel section under a given a load. Building specifications set deflection limits for perpendicular (wind-induced) and in-plane (dead load-induced) deflections. It is important to note that these deflection limits are not imposed due to strength capacities of the mullions. Rather, they are designed to limit deflection of the glass (which may break under excessive deflection), and to ensure that the glass does not come out of its pocket in the mullion. Deflection limits are also necessary to control movement at the interior of the curtain wall. Building construction may be such that there is a wall located near the mullion, and excessive deflection can cause the mullion to contact the wall and cause damage. Also, if deflection of a wall is quite noticeable, public perception may raise undue concern that the wall is not strong enough. Deflection limits are typically expressed as the distance between anchor points divided by a constant number. A deflection limit of L/175 is common in curtain wall specifications, based on experience with deflection limits that are unlikely to cause damage to the glass held by the mullion. Say a given curtain wall is anchored at 12 foot (144 in) floor heights. The allowable deflection would then be 144/175 = 0.823 inches, which means the wall is allowed to deflect inward or outward a maximum of 0.823 inches at the maximum wind pressure. HOWEVER, some panels require stricter movement restrictions, or certainly those that prohibit a torquelike motion. Deflection in mullions is controlled by different shapes and depths of curtain wall members. The depth of a given curtain wall system is usually controlled by the area moment of inertia required to keep deflection limits under the specification. Another way to limit deflections in
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a given section is to add steel reinforcement to the inside tube of the mullion. Since steel deflects at 1/3 the rate of aluminium, the steel will resist much of the load at a lower cost or smaller depth. STRENGTH Strength (or maximum usable stress) available to a particular material is not related to its material stiffness (the material property governing deflection); it is a separate criterion in curtain wall design and analysis. This often affects the selection of materials and sizes for design of the system. For instance, a particular shape in aluminum will deflect almost three times as much as the same steel shape for an equivalent load (see above), though its strength (i.e. the maximum load it can sustain) may be equivalent or even slightly higher, depending on the grade of aluminium. Because aluminum is often the material of choice, given its lower unit weight and better weathering capability as compared with steel, deflection is usually the governing criteria in curtain wall design. THERMAL CRITERIA Relative to other building components, aluminum has a high heat transfer coefficient, meaning that aluminum is a very good conductor of heat. This translates into high heat loss through aluminum curtain wall mullions. There are several ways to compensate for this heat loss, the most common way being the addition of thermal breaks. Thermal breaks are barriers between exterior metal and interior metal, usually made of polyvinyl chloride (PVC). These breaks provide a significant decrease in the thermal conductivity of the curtain wall. However, since the thermal break interrupts the aluminium mullion, the overall moment of inertia of the mullion is reduced and must be accounted for in the structural analysis of the system. Thermal conductivity of the curtain wall system is important because of heat loss through the wall, which affects the heating and cooling costs of the building. On a poorly performing curtain wall, condensation may form on the interior of the mullions. This could cause damage to adjacent interior trim and walls. Rigid insulation is provided in spandrel areas to provide a higher R-value at these locations.

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INFILLS Infill refers to the large panels that are inserted into the curtain wall between mullions. Infills are typically glass but may be made up of nearly any exterior building element. Regardless of the material, infills are typically referred to as glazing, and the installer of the infill is referred to as a glazier. More commonly this trade is now known as Fenestration. Glass

The Mexican hothouse at the Jardin des Plantes, built by Charles Rohault de Fleury from 1834 to 1836, is an early example of metal and glass curtain wall architecture. By far the most common glazing type, glass can be of an almost infinite combination of color, thickness, and opacity. For commercial construction, the two most common thicknesses are 1/4 inch (6 mm) monolithic and 1 inch (25 mm) insulating glass. Presently, 1/4 inch glass is typically used only in spandrel areas, while insulating glass is used for the rest of the building (sometimes spandrel glass is specified as insulating glass as well). The 1 inch insulation glass is typically made up of two 1/4-inch lites of glass with a 1/2 inch (12 mm) airspace. The air inside is usually atmospheric air, but some inert gases, such as argon or krypton may be used to offer better thermal transmittance values. In residential construction, thicknesses commonly used are 1/8 inch (3 mm) monolithic and 5/8 inch (16 mm) insulating glass. Larger thicknesses are typically employed for buildings or areas with higher thermal, relative humidity, or sound transmission requirements, such as laboratory areas or recording studios. Glass may be used which is transparent, translucent, or opaque, or in varying degrees thereof. Transparent glass usually refers to vision glass in a curtain wall. Spandrel or vision glass may
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also contain translucent glass, which could be for security or aesthetic purposes. Opaque glass is used in areas to hide a column or spandrel beam or shear wall behind the curtain wall. Another method of hiding spandrel areas is through shadow box construction (providing a dark enclosed space behind the transparent or translucent glass). Shadow box construction creates a perception of depth behind the glass that is sometimes desired. Fabric veneer Fabric is another type of material which is common for curtain walls. Fabric is often much less expensive and serves as a less permanent solution. Unlike glass or stone, fabric is much faster to install, less expensive, and often much easier to modify after it is installed. Stone veneer Thin blocks (3 to 4 inches (75–100 mm)) of stone can be inset within a curtain wall system to provide architectural flavour. The type of stone used is limited only by the strength of the stone and the ability to manufacture it in the proper shape and size. Common stone types used are: Arriscraft(calcium silicate);granite; marble; travertine; and limestone. To reduce weight and improve strength, the natural stone may be attached to an aluminium honeycomb backing as with the StonePly system. Panels Metal panels can take various forms including aluminum plate; thin composite panels consisting of two thin aluminum sheets sandwiching a thin plastic interlayer; and panels consisting of metal sheets bonded to rigid insulation, with or without an inner metal sheet to create a sandwich panel. Other opaque panel materials include fiber-reinforced plastic (FRP), stainless steel, and terracotta. Terracotta curtain wall panels were first used in Europe, but only a few manufacturers produce high quality modern terracotta curtain wall panels. Louvers A louver is provided in an area where mechanical equipment located inside the building requires ventilation or fresh air to operate. They can also serve as a means of allowing outside air to filter into the building to take advantage of favourable climatic conditions and minimize the usage of energy-consuming HVAC systems. Curtain wall systems can be adapted to
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accept most types of louver systems to maintain the same architectural sightlines and style while providing the necessary functionality. Windows and vents Most curtain wall glazing is fixed, meaning there is no access to the exterior of the building except through doors. However, windows or vents can be glazed into the curtain wall system as well, to provide required ventilation or operable windows. Nearly any window type can be made to fit into a curtain wall system. Fire safety Combustible Polystyrene insulation in point contact with sheet metal backban. Incomplete firestop in the perimeter slab edge, made of Rockwool without top caulking. Fire stopping at the "perimeter slab edge", which is a gap between the floor and the backpan of the curtain wall is essential to slow the passage of fire and combustion gases between floors. Spandrel areas must have non-combustible insulation at the interior face of the curtain wall. Some building codes require the mullion to be wrapped in heat-retarding insulation near the ceiling to prevent the mullions from melting and spreading the fire to the floor above. It is important to note that the fire stop at the perimeter slab edge is considered a continuation of the fire-resistance rating of the floor slab. The curtain wall itself, however, is not ordinarily required to have a rating. This causes a quandary as Compartmentalization (fire protection) is typically based upon closed compartments to avoid fire and smoke migrations beyond each engaged compartment. A curtain wall by its very nature prevents the completion of the compartment (or envelope). The use of fire sprinklers has been shown to mitigate this matter. As such, unless the building is sprinklered, fire may still travel up the curtain wall, if the glass on the exposed floor is shattered due to fire influence, causing flames to lick up the outside of the building. Falling glass can endanger pedestrians, fire-fighters and firehoses below. An example of this is the First Interstate Tower fire in Los Angeles, California. The fire here leapfrogged up the tower by shattering the glass and then consuming the aluminium skeleton holding the glass. Aluminium's melting temperature is 660°C, whereas building fires can reach 1,100°C. The melting point of aluminium is typically reached within minutes of the start of a fire. Firestops for such building joints can be qualified to UL 2079 -- Tests for Fire Resistance of Building Joint Systems. Sprinklering of each floor has a profoundly positive
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effect on the fire safety of buildings with curtain walls. In the case of the aforementioned fire, it was specifically the activation of the newly installed sprinkler system, which halted the advance of the fire and allowed effective suppression. Had this not occurred, the tower would have collapsed onto fire crews and into an adjacent building, while on fire. Exceptionally sound cementitious spray fireproofing also helped to delay and ultimately to avoid the possible collapse of the building, due to having the structural steel skeleton of the building reach the critical temperature, as the post-mortem fire investigation report indicated. This fire proved the positive collective effect of both active fire protection (sprinklers) and passive fire protection (fireproofing). Fireman knock-out glazing panels are often required for venting and emergency access from the exterior. Knock-out panels are generally fully tempered glass to allow full fracturing of the panel into small pieces and relatively safe removal from the opening. Maintenance and repair Curtain walls and perimeter sealants require maintenance to maximize service life. Perimeter sealants, properly designed and installed, have a typical service life of 10 to 15 years. Removal and replacement of perimeter sealants require meticulous surface preparation and proper detailing. Aluminum frames are generally painted or anodized. Factory applied fluoropolymer thermoset coatings have good resistance to environmental degradation and require only periodic cleaning. Recoating with an air-dry fluoropolymer coating is possible but requires special surface preparation and is not as durable as the baked-on original coating. Anodized aluminum frames cannot be "re-anodized" in place, but can be cleaned and protected by proprietary clear coatings to improve appearance and durability. Exposed glazing seals and gaskets require inspection and maintenance to minimize water penetration, and to limit exposure of frame seals and insulating glass seals to wetting

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FRAME CONSTRUCTION
Framed construction methods allow different materials to be used for external walls. In a framed construction the frame bears the load of the building instead of the walls. Frames can be made from various materials including wood and steel. Timber framed A wood framed building can be made from sustainably forested materials such as FSC sourced timber instead of energy intensive man-made materials such as concrete. Wood is not as good as an insulator as masonry and concrete. Wood's low density also stops it from acting as a heat store, requiring faster acting heating systems to maintain a constant temperature. Timber framed buildings can be premade in sections in a factory. Design drawings are engineered to take account of load bearing sections etc, then the kit of parts is made and transported to site in a series of staged deliveries. The timber builders, not always from the manufacturing company, then assemble the building according to the instructions. Another method is so-called stick building, where the timber builders make up the building from the engineered drawings, but the wood is measured and cut on site. This is a cheaper method. Concrete frame construction Above is a high rise under construction in London. This is using a 1960s method which is called RC. This is reinforced concrete framing. The concrete is set in situ, using ply boards with the metal reinforcing elements pre-positioned. Concrete is then poured into the mould. It sets to give the large panels and blocks. Anything else is made of reinforced concrete beams. Steel framed construction Steel-framed construction is often coupled with a heavy use of glass. This is common in high rise buildings and in such a construction the frame bears all the weight. These structures can be coupled with Passive Solar Design (PSD) allowing a lot of heat to enter the building through solar radiation.

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The external walls of a structure are vital to the integrity safety and aesthetics of the building. They are more often then not the load bearing walls and must also face the harsh conditions that the weather can throw at a building. External walls normally consist of two different designs in conventional style buildings cavity and solid

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The internal walls of a building often do not support any load. This allows for a greater range of materials to be used with design and sound insulation becoming more important than overall strength. Timber stud walls are entirely of timber simply nailed together, with only the head and sole plate being screwed or nailed to the ceiling and floor. The studs (verticals) need to be accurately cut to length, and horizontal noggins need to be fitted between studs for rigidity and for the fixing of heavy wall mounted items such as basins or cupboards. Where services like water pipes or electric cables need to be run inside the framework, the studs must be drilled out to accommodate them. When using timber in any way it is essential to first try to source recycled timber and if none is available always ensure it comes from a sustainable forest ideally one that has been FSC certified. Insulation can be added to a timber stud wall. However care should be taken when choosing which type, whilst foams are more effective, they create problems in disposal. Ideally natural wool should be used providing thermal and acoustic insulation. Earth based internal walls can be constructed by using a fine wire mesh. Clay plaster with a higher aggregate mixture than normal can then be placed onto the mesh and, when dry, will form a breathable durable wall. Plasterboard walls, or dry walls, as they are sometimes known consist of a core of gypsum plaster wrapped in a liner of paper. Dry walls are supplied in panel lengths and as such cannot be made on site. This means that they will require transportation, which will add to the carbon footprint of the build. Dry wall manufacture is also a very heat intensive process with over 25% of the total cost of a dry wall going on natural gas used in its construction. Dry walls cut to size often lead to a large amount of offcuts. These cannot be disposed of in normal landfills and specialist disposal will be required. Solid wood walls are one of the few carbon negative building materials. When sourced from a sustainable forest the wood used can be of a benefit to the environment having taken CO2 out of the air before being turned into a building material. A solid wood wall is built from
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wood slab boards and is often created off site, being bought in as one unit. This has the advantage of speeding up construction.

When in place a solid wood wall will provide a durable wall which can take heavy mountings. The breathability of wood allows it to store some of the moisture from a building without being damaged, reducing condensation. Internal walls are also a major part of controlling noise within a build. To effectively control noise, it is important to understand the difference between sound absorption and sound insulation, as the two are often confused. Sound absorption relates to the amount of reverberation within a room and its effect on sound quality and intelligibility. Sound insulation, on the other hand, relates to the actual reduction in sound travelling from one area to another through a wall, floor or ceiling. Sound transmission in buildings is a result of both airborne and impact noise.

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Solid wooden walls will provide a good form of sound insulation as does plaster board due to there thick natures, thin skinned walls such as stud based timber walls and thin earth walls will require more insulation in order to cut noise to a lower level. The easiest way is to add insulation materials to a wall, these can be in the form of fibre boards or foam insulation which provide a trap for heat and sound (although sound specific insulation does not block much heat). Insulation materials used on internal walls are similar to those used around other areas of the home. Mineral wool can be used although its use should be kept to a minimum if possible as the small fibres involved can have a negative effect on human health. Natural wool is an ideal choice as it not irritating to human health however it does require more thickness to meet the same specifications of man made insulation. Foam insulation will work the best however this can cause some problems if a wall is required to be remodelled adding to waste that cannot be easily recycled. The choice of an internal wall is a difficult one for a green builder and there is no one solution.
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Solid wood walls provide a good degree of heat and sound insulation. Stud timber framed walls allow for lighter and fewer materials to be used. Earth walls allow the use of nature's most available and sustainable resource, but have difficulties in modern building practices.

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Dry walls require an intensive construction and transportation regime. Brickwork and blockwork is traditionally costly, heavy and non-recyclable. Insulated concrete formwork (ICF) is concrete but has green credentials as very insulating, and quick and simple to build.

Suitable for larger and more repetitive builds where more esoteric materials like cob and earth are not useful.

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STEEL FRAMED It usually refers to a building technique with a "skeleton frame" of vertical steel columns and horizontal I-beams, constructed in a rectangular grid to support the floors, roof and walls of a building which are all attached to the frame. The development of this technique made the construction of the skyscraper possible. The rolled steel "profile" or cross section of steel columns takes the shape of the letter "H". The two wide flanges of a column are thicker and wider than the flanges on a beam, to better withstand compressive stress in the structure. Square and round tubular sections of steel can also be used, often filled with concrete. Steel beams are connected to the columns with bolts and threaded fasteners, and historically connected by rivets. The central "web" of the steel "I "-beam is often wider than a column web to resist the higher bending moments that occur in beams. Wide sheets of steel deck can be used to cover the top of the steel frame as a "form" or corrugated mold, below a thick layer of concrete and steel reinforcing bars. Another popular alternative is a floor of precast concrete flooring units with some form of concrete topping. Often in office buildings the final floor surface is provided by some form of raised flooring system with the void between the walking surface and the structural floor being used for cables and air handling ducts. The frame needs to be protected from fire because steel softens at high temperature and this can cause the building to partially collapse. In the case of the columns this is usually done by encasing it in some form of fire resistant structure such as masonry, concrete or plasterboard. The beams may be cased in concrete, plasterboard or sprayed with a coating to insulate it from the heat of the fire or it can be protected by a fire resistant ceiling construction. The exterior "skin" of the building is anchored to the frame using a variety of construction techniques and following a huge variety of architectural styles. Bricks, stone, reinforced concrete, architectural glass, sheet metal and simply paint have been used to cover the frame to protect the steel from the weather.
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STEEL FRAME CONSTRUCTION Thin sheets of galvanized steel can be formed into steel studs for use as a building material for rough-framing in commercial or residential construction (pictured), and many other applications. The dimension of the room is established with horizontal track that is anchored to the floor and ceiling to outline each room. The vertical studs are arranged in the tracks, usually spaced 16" apart, and fastened at the top and bottom. The primary shapes used in residential construction are the C-shape stud and the U-shaped track, and a variety of other shapes. Framing members are generally produced in a thickness of 12 to 25 gauges. The wall finish is anchored to the two flange sides of the stud, which varies from 1-1/4" to 3" thick, and the width of web ranges from 1-5/8" to 14". Rectangular sections are removed from the web to provide access for electrical wiring. Steel mills produce galvanized sheet steel, the base material for light-gauge steel. Sheet steel is then roll-formed into the final profiles used for framing. The sheets are zinc coated (galvanized) to prevent oxidation and corrosion. Steel framing provides excellent design flexibility due to the inherent strength of steel, which allows it to span over a longer distance than wood, and also resist wind and earthquake loads. Light Steel Framing has been extensively used in cold climate countries due to its good thermal and structural behaviour. Heat loss reduction and tenement thermal comfort have been the main driving forces defining the design of these frames. The main issue to be addressed is how striving for thermal efficiency can lead to structural weakening and poor fire performance.

Interior wall studs made with light-gauge steel
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Steel framed housing development The spacing between studs is 16 inches on center for homes exterior and interior walls. In office suites the spacing is 24 inches on center for all walls except for elevator and stair wells

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DRY STONE WALLS
A dry stone wall, also known as a dry stone dyke, drystane dyke, dry stone hedge, or rock fence is a wall that is constructed from stones without any mortar to bind them together. As with other dry stone structures, the wall is held up by the interlocking of the stones. Such walls are used in building construction, as field boundaries, and on steep slopes as retaining walls for terracing. LOCATION AND TERMINOLOGY Terminology varies regionally. When used as field boundaries, dry stone structures often are known as dykes, particularly in Scotland. Dry stone walls are characteristic of upland areas of Britain and Ireland where rock outcrops naturally or large stones exist in quantity in the soil. They are especially abundant in the West of Ireland, particularly Connemara. They also may be found throughout the Mediterranean, as in Majorca, Catalonia, Languedoc, Provence, Liguria, the Apulia region of Italy, Cyprus, and in the Canary Islands, including retaining walls used for terracing. Such constructions are common where large stones are plentiful (for example, in The Burren) or conditions are too harsh for hedges capable of retaining livestock to be grown as reliable field boundaries. Many thousands of miles of such walls exist, most of them centuries old. In the United States they are common in areas with rocky soils, such as New England, New York, New Jersey, and Pennsylvania and are a notable characteristic of the bluegrass region of central Kentucky, where they are usually referred to as rock fences, and the Napa Valley in north central California. The technique of construction was brought to America primarily by Scots-Irish immigrants. The technique was also taken to Australia (principally western Victoria and some parts of Tasmania and New South Wales) and New Zealand (especially Otago).

Mosaic embedded in a dry stone wall in Italian Switzerland Similar walls also are found in the Swiss-Italian border region, where they are often used to enclose the open space under large natural boulders or outcrops.

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The higher-lying rock-rich fields and pastures in Bohemia's South-Western border range of Šumava (e.g. around the mountain river of Vydra) are often lined by dry stone walls built of field-stones removed from the arable or cultural land, serving both as cattle/sheep fences and the lot's borders; sometimes also the dry stone terracing is apparent, often combined with parts of stone masonry (house foundations and shed walls) held together by a clay-cumneedles "composite" mortar. Dry stone wall construction was known to Bantu tribes in south-eastern Africa as early at 1350 to 1500 AD. When some of the Zulu migrated west into the Waterberg region of present day South Africa, they imparted their building skills to Iron Age Bantu peoples who used dry stone walls to improve their fortifications. In Peru in the 15th century AD, the Inca made use of otherwise unusable slopes by building dry stone walls to create terraces. They also employed this mode of construction for freestanding walls. Their ashlar type construction in Machu Picchu uses the classic Inca architectural style of polished dry-stone walls of regular shape. The Incas were masters of this technique, in which blocks of stone are cut to fit together tightly without mortar. Many junctions are so perfect that not even a knife fits between the stones. The structures have persisted in the high earthquake region because of the flexibility of the walls and that in their double wall architecture, the two portions of the walls incline into each other. Construction

Using a batter-frame and guidelines to rebuild a dry stone wall in South Wales UK

Newly rebuilt dry stone wall in South Wales UK
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There are several methods of constructing dry stone walls, depending on the quantity and type of stones available. Older walls are constructed from stones and boulders cleared from the fields during preparation for agriculture (field stones) but many also from stone quarried nearby. For modern walls, quarried stone is almost always used. The type of wall built will depend on the nature of the stones available. One type of wall is called a ―Double‖ wall and is constructed by placing two rows of stones along the boundary to be walled. The rows are composed of large flattish stones. Smaller stones may be used as chocks in areas where the natural stone shape is more rounded. The walls are built up to the desired height layer-by-layer (course by course), and at intervals, large tie-stones or through stones are placed which span both faces of the wall. These have the effect of bonding what would otherwise be two thin walls leaning against each other, greatly increasing the strength of the wall. The final layer on the top of the wall also consists of large stones, called capstones, coping stones or copes. As with the tie stones, the cap stones span the entire width of the wall and prevent it breaking apart. In addition to gates a wall may contain smaller purposely built gaps for the passage or control of wildlife and livestock such as sheep. The smaller holes usually no more than 8 inches in height are called 'Bolt Holes' or 'Smoots'. Larger ones may be between eighteen and 24 inches in height, these are called a 'Cripple Hole'. Boulder walls are a type of single wall in which the wall consists primarily of large boulders, around which smaller stones are placed. Single walls work best with large, flatter stones. Ideally, the largest stones are being placed at the bottom and the whole wall tapers toward the top. Sometimes a row of capstones completes the top of a wall, with the long rectangular side of each capstone perpendicular to the wall alignment. Another variation is the ―Cornish hedge‖ or Welsh clawdd, which is a stone-clad earth bank topped by turf, scrub, or trees and characterised by a strict inward-curved batter (the slope of the ―hedge‖). As with many other varieties of wall, the height is the same as the width of the base, and the top is half the base width. Different regions have made minor modifications to the general method of construction — sometimes because of limitations of building material available, but also to create a look that is distinctive for that area. Whichever method is used to build a dry stone wall, considerable skill is required. Selection of the correct stone for every position in the wall makes an enormous difference to the lifetime of the finished product, and a skilled waller will take time making the selection. As with many older crafts, skilled wallers, today, are few in number. With the advent of modern wire fencing, fields can be fenced with much less time and expense using wire than using stone walls; however, the initial expense of building dykes is offset by their sturdiness and consequent long, low-maintenance lifetimes. As a result of the increasing appreciation of
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the landscape and heritage value of dry stone walls, wallers remain in demand, as do the walls themselves. A nationally recognised certification scheme is operated in the UK by the Dry Stone Walling Association, with four grades from Initial to Master Craftsman. NOTABLE DRY STONE WALLS
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Mourne Wall - twenty-two mile long wall in the Mourne Mountains location in County Down, Northern Ireland Ottenby Nature Preserve, built by Charles X Gustav in mid 16th century, Öland, Sweden

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DRY STONE BUILDINGS

Intihuatana ritual buildings of dry stone at Machu Picchu, Peru While the dry-stone technique is generally used for field enclosures, it also was used for buildings. The traditional turf-roofed Highland Black house was constructed using the double wall dry stone method. When buildings are constructed using this method, the middle of the wall is generally filled with earth or sand in order to eliminate draughts. During the Iron Age, and perhaps earlier, the technique also was used to build fortifications such as the walls of Eketorp Castle (Oland,Sweden), Maiden Castle, North Yorkshire, Reeth, Dunlough Castle in southwest Ireland and the rampart of the Long Scar Dyke. Many of the dry-stone walls that exist today in Scotland can be dated to the 14th century or earlier when they were built to divide fields and retain livestock. Some extremely well built examples are found on the lands of Muchalls Castle.

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Dry stone bridges

Medieval dry stone bridge in Alby, Sweden Since at least the Middle Ages some bridges capable of carrying horse or carriage traffic have been constructed using drystone techniques. An example of a well preserved bridge of this type is a double arched limestone bridge in Alby, Sweden on the island of Öland. Dry-stone markings

Dry stone marking In the UK and Switzerland, it is possible to find dry stone constructions without any obvious function. The largest and oldest of them, such as Stonehenge, are likely related to ancient pagan rituals. However, the smaller structures may be built just as signs, marking the mountain paths or boundaries of owned land. (Some stand on the boundary between Italy and Switzerland; see photo). In many countries, cairns are used as road and mountain top markers.

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Ancient walls

The Lion Gate of the Mycenae acropolis is dry stone Some dry-stone wall constructions in north-west Europe have been dated back to the Neolithic Age. Some Cornish hedges are believed by the Guild of Cornish Hedgers to date from 5000 BC,[2]although there appears to be little dating evidence. In County Mayo, Ireland, an entire field system made from dry-stone walls, since covered in peat, have been carbon-dated to 3800 BC. The cyclopean walls of the acropolis of Mycenae have been dated to 1350 BC and those of Tiryns slightly earlier. In Belize, the Mayan ruins at Lubaantun illustrate use of dry stone construction in architecture of the 8th and 9th century AD.

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RETAINING WALL

A gravity-type stone retaining wall Retaining walls are built in order to hold back earth which would otherwise move downwards. Their purpose is to stabilise slopes and provide useful areas at different elevations, e.g. terraces for agriculture, buildings, roads and railways. A retaining wall is a structure designed and constructed to resist the lateral pressure of soil when there is a desired change in ground elevation that exceeds the angle of repose of the soil. Every retaining wall supports a ―wedge‖ of soil. The wedge is defined as the soil which extends beyond the failure plane of the soil type present at the wall site, and can be calculated once the soil friction angle is known. As the setback of the wall increases, the size of the sliding wedge is reduced. This reduction lowers the pressure on the retaining wall. The basement wall is thus one form of retaining wall. However, the term is most often used to refer to a cantilever retaining wall, which is a freestanding structure without lateral support at its top.

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Typically retaining walls are cantilevered from a footing extending up beyond the grade on one side and retaining a higher level grade on the opposite side. The walls must resist the lateral pressures generated by loose soils or, in some cases, water pressures. The most important consideration in proper design and installation of retaining walls is to recognize and counteract the fact that the retained material is attempting to move forward and downslope due to gravity. This creates lateral earth pressure behind the wall which depends on the angle of internal friction (phi) and the cohesive strength (c) of the retained material, as well as the direction and magnitude of movement the retaining structure undergoes. Lateral earth pressures are zero at the top of the wall and - in homogenous ground - increase proportionally to a maximum value at the lowest depth. Earth pressures will push the wall forward or overturn it if not properly addressed. Also, any groundwater behind the wall that is not dissipated by a drainage system causes hydrostatic pressure on the wall. The total pressure or thrust may be assumed to act at one-third from the lowest depth for lengthwise stretches of uniform height. Unless the wall is designed to retain water, It is important to have proper drainage behind the wall in order to limit the pressure to the wall's design value. Drainage materials will reduce or eliminate the hydrostatic pressure and improve the stability of the material behind the wall. Drystone retaining walls are normally self-draining. As an example, the International Building Code requires retaining walls to be designed to ensure stability against overturning, sliding, excessive foundation pressure and water uplift; and that they be designed for a safety factor of 1.5 against lateral sliding and overturning. Types

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VARIOUS TYPES OF RETAINING WALLS GRAVITY

Construction types of gravity retaining walls Gravity walls depend on the weight of their mass (stone, concrete or other heavy material) to resist pressures from behind and will often have a slight 'batter' setback, to improve stability by leaning back into the retained soil. For short landscaping walls, they are often made from mortarless stone or segmental concrete units (masonry units). Dry-stacked gravity walls are somewhat flexible and do not require a rigid footing in frost areas. Home owners who build larger gravity walls that do require a rigid concrete footing can make use of the services of a professional excavator, which will make digging a trench for the base of the gravity wall much easier.

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Earlier in the 20th century, taller retaining walls were often gravity walls made from large masses of concrete or stone. Today, taller retaining walls are increasingly built as composite gravity walls such as: geosynthetic or with precast facing; gabions (stacked steel wire baskets filled with rocks); crib walls (cells built up log cabin style from precast concrete or timber and filled with soil); or soil-nailed walls (soil reinforced in place with steel and concrete rods). CANTILEVERED

Conterfort/Buttress on Cantilevered Wall Cantilevered retaining walls are made from an internal stem of steel-reinforced, cast-in-place concrete or mortared masonry (often in the shape of an inverted T). These walls cantilever loads (like a beam) to a large, structural footing, converting horizontal pressures from behind the wall to vertical pressures on the ground below. Sometimes cantilevered walls are butressed on the front, or include a counterfort on the back, to improve their strength resisting high loads. Buttresses are short wing walls at right angles to the main trend of the wall. These walls require rigid concrete footings below seasonal frost depth. This type of wall uses much less material than a traditional gravity wall.

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SHEET PILING

Sheet pile wall Sheet pile retaining walls are usually used in soft soils and tight spaces. Sheet pile walls are made out of steel, vinyl or wood planks which are driven into the ground. For a quick estimate the material is usually driven 1/3 above ground, 2/3 below ground, but this may be altered depending on the environment. Taller sheet pile walls will need a tie-back anchor, or "dead-man" placed in the soil a distance behind the face of the wall, that is tied to the wall, usually by a cable or a rod. Anchors are placed behind the potential failure plane in the soil. ANCHORED An anchored retaining wall can be constructed in any of the aforementioned styles but also includes additional strength using cables or other stays anchored in the rock or soil behind it. Usually driven into the material with boring, anchors are then expanded at the end of the cable, either by mechanical means or often by injecting pressurized concrete, which expands to form a bulb in the soil. Technically complex, this method is very useful where high loads are expected, or where the wall itself has to be slender and would otherwise be too weak.

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ALTERNATIVE RETAINING TECHNIQUES SOIL NAILING Soil nailing is a technique in which soil slopes, excavations or retaining walls are reinforced by the insertion of relatively slender elements - normally steel reinforcing bars. The bars are usually installed into a pre-drilled hole and then grouted into place or drilled and grouted simultaneously. They are usually installed untensioned at a slight downward inclination. A rigid or flexible facing (often sprayed concrete) or isolated soil nail heads may be used at the surface. SOIL-STRENGTHENED A number of systems exist that do not simply consist of the wall itself, but reduce the earth pressure acting on the wall itself. These are usually used in combination with one of the other wall types, though some may only use it as facing (i.e. for visual purposes). GABION MESHES This type of soil strengthening, often also used without an outside wall, consists of wire mesh 'boxes' into which roughly cut stone or other material is filled. The mesh cages reduce some internal movement/forces, and also reduce erosive forces. MECHANICAL STABILIZATION Mechanically stabilized earth, also called MSE, is soil constructed with artificial reinforcing via layered horizontal mats (geosynthetics) fixed at their ends. These mats provide added internal shear resistance beyond that of simple gravity wall structures. Other options include steel straps, also layered. This type of soil strengthening usually needs outer facing walls (S.R.W.'s - Segmental Retaining Walls) to affix the layers to and vice versa. The wall face is often of precast concrete units that can tolerate some differential movement. The reinforced soil's mass, along with the facing, then acts as an improved gravity wall. The reinforced mass must be built large enough to retain the pressures from the soil behind it. Gravity walls usually must be a minimum of 50 to 60 percent as deep or thick as the height of the wall, and may have to be larger if there is a slope or surcharge on the wall.

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REFERENCES
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Patrick McAfee, Irish Stone Walls: History, Building, Conservation, The O'Brien Press, 2011 Alan Brooks and Sean Adcock, Walling, a practical handbook. BTCV. 1999 The Dry Stone Walling Association, Dry Stone Walling, Techniques and Traditions. 2004 Murray-Wooley, Carolyn and Karl Raitz, Rock Fences of the Bluegrass, University Press of Kentucky. 1992. Francis Pryor, Britain BC, Harper Perennial. 2003. Colonel F. Rainsford-Hannay, Dry Stone Walling, Faber & Faber. 1957 Louis Cagin & Laetitia Nicolas, Construire en pierre sèche, editions Eyrolles, 2008 http://www.era.lib.ed.ac.uk/handle/1842/1409 Ruggiero, S. S., and Myers, J. C. 1991. ―Design and Construction of Watertight Exterior Building Walls.‖ Water in Exterior Building Walls:

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