Non-structural materials
Since the invention of durable weatherproof adhesives in the
last 100 years there has been continuous development of a
variety of wood-based products, either as panels or as
structural sections that can be used instead of solid wood.
Fig 1 Non-structural panels can be used if strength is not critical
However, well before the invention of these modern adhesives,
the adhesive properties of the cellulose occurring naturally in
wood was known. Products were developed using pulped waste
material such as sawdust and wood fibres without the addition
of adhesives, held together by bonding in the same way as the
fibres are held together in a sheet of paper. In this case, large
volumes of wood cells are compressed into panel materials
such as hardboard and fibreboard.
A similar process can be employed to form panels from waste
material left over from processing logs or solid wood, such as
off-cuts, chips or sawdust but with the addition of
adhesives/resins to bind the wood particles together. Many of
these panels have limited structural strength but can be used
for purposes where this is not critical such as for linings,
thermal or acoustic insulation or cabinet work.
Structural panels
This use of otherwise waste material to produce commercially
viable products has been the motivation for the development of
many modern structural panel and “engineered” timber
products.
Figs 2a and b Structural panels and engineered timber products
Plywood
The limited size of some species of tree means that the trunks
cannot economically be cut into structural sections. One
solution, where stems of small trees are straight and of
sufficient consistency, is to rotary peel or slice thin veneers from
the trunk, that can be laminated together with adhesive to form
flat panels. In plywood construction each layer of wood or
veneer is arranged (or laid up) in a perpendicular direction to
the one below. This overcomes the tendency of wood to split
along the grain and any movement of individual veneers on
exposure to different humidities is retarded by those above and
below running in the opposite plane.
It was a technique even known to the early Egyptians, who
bonded successive layers of wood together with organic glues.
This arrangement of the veneers gives considerable strength in
both directions. Plywood has long been made in this way but
the development of water-resistant adhesives has vastly
improved both its ability to remain intact when exposed to
wetting and it structural performance.
Fig 3a Rotary cutting of veneers
Fig 3b In plywood, alternate peeled veneers are laid up
perpindicular to each other to form a more stable sheet with
strength in two directions
Laminated veneer lumber (LVL)
More recently structural sections, as well as thicker panels,
have been made in which the majority of the veneers run
parallel to the length of the section. This material is known as
“laminated veneer lumber” and because of the common
direction of the veneers it has greatly improved directional
strength over plywood.
Fig 5 Oriented strand board or OSB
Glue laminated timber (or “glulam”)
The technique of using readily available small rectangular
sections of wood to build larger structural sections is glue
lamination, generally referred to as “glulam”. This basic idea
was known to the Victorian engineers who, prior to the
development of suitable glues bolted together (mechanically
fixed) small sections of timber to form large curved beams.
Fig 8a Cross-laminated timber is formed with layered sections of
solid wood
Fig 8b A cross-laminated timber panel is craned into position on
site
Stacked planks (Brettstapel)
Large solid wood structural panels are manufactured without
the use of any adhesive. Rectangular sections of low grade
softwood are stacked together. Long hardwood dowels are then
driven diagonally into the stacks to apply the necessary
compression. Openings can be cut into the solid panels where
required, in the same way as with cross-laminated timber
panels.
Fig 9 Stacked planks at Acharacle primary school
Wood-based composite components
In addition to boards or structural sections fabricated from
veneers, strands, chips or small sections, there are now wood-
based components available in which different materials are
combined, either wood-based or steel, to form efficient
structural members.
Fig 11a I-beam compared with solid wood
Besides their use as beams, they are now frequently used for
wall framing where their depth provides space for thick
insulation. The low conductivity of the thin webs also enables
improved thermal insulation in walls.
Fig 11b I-beams are available in a variety of sizes
The advantages of “engineered” timber
Engineered timber panels have many beneficial uses in
construction:
Building up panels or structural sections from peeled
veneers, strands, chips or small sections of solid wood
makes the most of the wood resource and minimises
waste.
Larger, wider and longer components can be
manufactured than can be obtained from even the largest
trees. Curved sections can also be formed.
The structural properties of solid timber depend on the
absence of flaws (strength reducing defects) such as
knots, splits or irregular grain. These flaws do not have a
significant effect if they occur in veneers or thin sections
which are laminated into thicker sections. When solid
sections of timber are laminated into larger sections
excessive knots, or other flaws, can be cut out and the
ends finger jointed into continuous lengths, providing
superior strength properties. Small knots can be included
and will not have an effect on the strength of the
laminated sections providing that they are not coincident
between the laminates.
Fig 12 Solid timber features affecting strength of wood
Modern adhesive bonds are stronger than the solid wood
components so that glued joints do not diminish the
overall strength of the section. In fact, incorporating glass
or carbon fibre in the adhesive layers has been used to
substantially improve the overall structural strength of the
sections.
Structural calculations for “engineered” timber
components of this sort do not have to allow for the
natural variation in strength that must be factored in when
designing with solid timber components because the
material is consistent in quality throughout the full section.
Large solid wood sections will always initially contain a
high proportion of moisture since sections greater than
100mm thick cannot be fully kiln dried, consequently
residual moisture may remain in the section, which can
result in subsequent shrinkage in use. This may reduce
TRADA Academic resources / Timber as a material / Wood-based materials – version 1.0
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section sizes but can also lead to distortion depending on
the part of the log from which the section is cut. The
smaller sections involved in glue laminated timber
construction can be effectively dried but also assembled
in such a way that each laminate tends to resist the
natural distortion that would occur in the adjacent piece of
solid wood. While some allowance must be made in
design for the overall changes in size that can occur due
to moisture variation in a glulam section, this is far less
than must be allowed when using a solid section of a
similar size and any risk of distortion is largely eliminated.
The high proportion of resin in adhesive/fibre based or
laminated veneer sections means that there is virtually no
absorption or loss of moisture in the core of these
sections and therefore even less change in the overall
dimensions of the section in use. However, even in
engineered timber products where the end grain of
veneers or solid wood are left exposed to external
conditions they can absorb moisture which can lead to
delamination (separation into individual wood
components), swelling or end splits. The end grain should
therefore always be sealed if components are to be
exposed externally, just as with solid wood sections.
Note: this unit has only covered wood-based products as a
material. The structural performance of these various products
is covered in more detail in the structural modules.
References
Introduction to wood-based panel products WIS 2/3 23, TRADA
Technology 2003
Glue-laminated timber WIS 1-6, TRADA Technology 2003
Cross-laminated timber – structural principles WIS 2/3 62
TRADA Technology 2009
Ross, Downes and Lawrence, Timber in contemporary
architecture, SBN 978 1 9005106600, TRADA Technology
2009
Reusable and adaptable wood structures – sustainable
solutions for a changing world, SBN 978 1 9005106088,
TRADA Technology/woodforgood 2009
Wide span sports structures, TRADA Technology/woodforgood
2007
Panel Guide Version 3, Wood Panel Industries Federation/
TRADA Technology/ BRE/ Timber Trades Federation, 2009
Acharacle Primary School, Argyll, TRADA Timber Solutions
case study, www.trada.co.uk