Some of the characteristics which influence design and are specific to timber are:
♦ the moisture content,
♦ the difference in strength when loads are applied parallel and perpendicular to the grain direction,
♦ the duration of the application of the load,
♦ the method adopted for strength grading of the timber.
Moisture Content• Unlike most structural materials, the
behaviour of timber is significantly influenced
by the existence and variation of its moisture
content. The moisture content, as determined
by oven drying of a test piece, is defined in
Annex H of BS 5268 as:
fibre saturation point (FSP)
• The condition in which all free water has been removed but the cell walls are still saturated is
known as the fibre saturation point (FSP).
• At levels of moisture above the FSP, most physical and mechanical properties remain constant. Variations in moisture content below the FSP cause considerable changes to properties such as weight, strength, elasticity, shrinkage and durability.
The controlled drying of timber is known as seasoning.
• Air seasoning, in which the timber is stacked and layered with air-space in opensided sheds to promote natural drying.
• Kiln drying, in which timber is dried out in a heated, ventilated and humidified oven. This requires specialist equipment and is more expensive
• The anisotropic nature of timber and
differential drying out caused by uneven
exposure to drying agents such as wind, sun
or applied heat can result in a number of
defects such as twisting, cupping, bowing and
cracking, as shown in Figure
Defects in Tim
The most common and familiar of such defects is a
knot (see Figure). Normal branch growth originates
near the pith of a tree and consequently its base
develops new layers of wood each season which
develop with the trunk.
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• A shake is produced when fibres separate along
the grain: this normally occurs between the
growth rings, as shown in Figure
• A wane can occur when part of the bark or
rounded periphery of the trunk is present in a
cut length, as shown in Figure.
Classification of Timber
• Appearance grading is frequently used by
architects to reflect the warm, attractive features
of the material such as the surface grain pattern,
the presence of knots, colour, etc.
• All structural (load-bearing) timber must be
strength-graded according to criteria which
reflect its strength and stiffness. In some cases
timber may be graded according to both
appearance and strength.
Visual Strength Grading
• As implied by the name, this method of grading is based on the physical observation of strength-reducing defects such as knots, rate of growth, cracks, wane, bowing, etc. Since the technique is based on the experience and judgment of the grader it is inherently subjective. In addition, important properties such as density, which has a significant influence on stiffness, and strength are not considered.
In the UK, visual grading is governed by
the requirements of BS 4978:1996 Specification for softwood grades
for structural use
• Visual defects considered when assessing timber strength include: location and extent of knots, slope of grain, rate of growth, fissures, wane, distortions such as bowing, springing, twisting, cupping, resin and bark pockets, and insect damage.
• Timber which contains abnormal defects such as compression wood, insect damage such as worm holes, or fungal decay (not sapstain), or which is likely to impair the serviceability of the pieces, is excluded from the grades.
Machine Strength Grading
The requirements for machine strength grading are specified in BS EN 519:1995
Structural Timber − Grading − Requirements for machine strength graded timber and
grading machines. Timber is classified into:
♦ nine classes of poplar and coniferous species ranging from the weakest grade C14 to
the highest grade C40,
♦ six classes for deciduous species ranging from the weakest grade D30 to the highest
grade D70.
In each case the number following either the ‘C’ or the ‘D’ represents the characteristic
bending strength of the timber. In BS 5268-2:2002 two additional strength classes, TR20
and TR26, are also given; this is intended for use in the design of trussed rafters.
The inherently subjective nature of visual strength grading results in a lower yield of higher strength classes than would otherwise be achieved. Machine strength grading is generally carried out by conducting bending tests on planks of timber which are fed continuously through a grading machine. The results of such tests produce a value for the modulus of elasticity. The correlation between the modulus of elasticity and strength properties such as bending, tensile and compressive strength can be used to define a particular grade/class of timber.
Material PropertiesThe strength of timber is due to certain types of
cells (called tracheids in softwoods and fibres in
hardwoods) which make up the many minute
hollow cells of which timber is composed. These
cells are roughly polygonal in cross-section and the
dimension along the grain is many times larger than
across it.
The principal constituents of the cells are cellulose and lignin. Individual cell walls
comprise four layers, one of which is more significant with respect to strength than the
others. This layer contains chains of cellulose which run nearly parallel to the main axis of
the cell. The structure of the cell enhances the strength of the timber in the grain
direction.
Material Properties
Density, which is expressed as mass per unit volume, is one of the principal properties
affecting strength. The heaviest species, i.e. those with most wood substance, have
thick cell walls and small cell cavities. They also have the highest densities and
consequently are the strongest species. Numerous properties in addition to strength,
e.g. shrinkage, stiffness and hardness, increase with increasing density.
The slope of the grain can have an important effect on the strength of a timber member.
Typically a reduction of 4% in strength can result from a slope of 1 in 25, increasing to an
11% loss for slopes of 1 in 15.
The strength of timber is also affected by the rate of growth as indicated by the width
of the annual growth rings. For most timbers the number of growth rings to produce the
op?mum strength is approximately in the range of 6−15 per 25 mm measured radially.
Timber which has grown either much more quickly or much more slowly than that
required for the optimum growth rate is likely to be weaker.
Material Properties
Like many materials, e.g. concrete, the stress−strain
relationship demonstrated by timber under load is
linear for low stress values. For all species the strains
for a given load increase with moisture content. A
consequence of this is that the strain in a beam under
constant load will increase in a damp environment
and decrease as it dries out again.
Timber demonstrates viscoelastic behaviour (creep) as high stress levels induce increasing
strains with increasing time. The magnitude of long-term strains increases with higher moisture
content.
Material Properties
The fire resistance of timber generally compares
favourably with other structural materials and is often
better than most. Steel is subject to loss of strength,
distortion, expansion and collapse, whilst concrete may
spall and crack.
The charcoal produced during the fire is a poor conductor
and will eventually provide an insulating layer between
the flame and the unburned timber.
Material Properties
Fire authorities usually consider that a normal
timber door will prevent the spread of fire to
an adjoining room for about 30 minutes.
Permissible Stress Design
The laws of structural mechanics referred to are those well established in recognised
‘elastic theory’, as follows.
The material is homogeneous,
The material is isotropic, which implies that the elastic properties are the same in all directions.
The material obeys Hooke’s Law
The material is elastic,
The modulus of elasticity is the same in tension and compression.
Plane sections remain plane during deformation
Modification FactorsThe inherently variable nature of timber and its
effects on structural material properties such as
stress−strain characteris?cs, elas?city and creep has
resulted in more than eighty different modification
factors which are used in converting grade stresses to
permissible stresses for design purposes.
• The applied stresses are calculated using elastic theory, and the permissible stresses are
determined from the code using the appropriate values relating to the strength classification
multiplied by the modification factors which are relevant to the stress condition being
considered. Symbols are defined relating to stresses and other variables in Clause 1.4 of BS
5268-2:2002 as follows:
Modification Factors
In many instances subscripts are also used to identify
various types of force, stress or geometry; these are
as follows:
Modification Factors
Modification Factors
As mentioned previously, the permissible stress is evaluated by multiplying the
grade stress for a particular strength class by the appropriate modification factors,
e.g.
Modification Factors
Modification Factors
Flexural Members
Beams are the most commonly used structural elements, for example as floor joists,
and as trimmer joists around openings, rafters, etc. The cross-section of a timber
beam may be one of a number of frequently used sections such as those indicated
in Figure
The principal considerations in the design of all beams are:
♦ shear,
♦ bending,
♦ deflection,
♦ bearing, and
♦ lateral stability.
The size of timber beams may be governed by the requirements of:
♦ the elastic section modulus (Z), to limit the bending stresses and ensure that
neither lateral torsional buckling of the compression flange nor fracture of the
tension flange induces failure,
♦ the cross-section, to ensure that the vertical and/or horizontal shear stresses do
not induce failure,
♦ the second moment of area, to limit the deflection induced by bending and/or
shear action to acceptable limits.
Flexural Members
Effective SpanMost timber beams are designed as simply supported and the effective span
which should be used is defined in Clause 2.10.3 of BS 5268-2:2002, as
illustrated in Figure
Since the required bearing length on most beams is relatively small when compared with
the actual span it is common practice to assume an effective span equal to:
♦ the clear distance between the supports + 50 mm for solid beams, and
♦ the clear distance between the supports + 100 mm for ply-web beams.
In the case of long span beams (e.g. in excess of 10.0 m), or heavily loaded beams with
consequently larger end reactions, the validity of this assumption should be checked.
Solid Rectangular BeamsThe modification factors, which are pertinent when designing solid timber
beams, are summarized in Table.
Shear
Solid Rectangular Beams
The grade and hence permissible stresses given in the BS relate to the maximum shear
stress parallel to the grain for a particular species or strength class. In solid beams of
rectangular cross-section the maximum horizontal shear stress occurs at the level of the
neutral axis, and is equal to 1.5 × the average value:
Bending
Solid Rectangular Beams
the applied bending stress is determined using simple elastic bending theory:
K2 , K3, K6 , K7 and K8 are modification factors used when appropriate.
Note: K6 = 1.0 for rectangular cross-sections.
Deflection
Solid Rectangular Beams
In the absence of any special requirements for deflection in buildings, it is
customary to adopt an arbitrary limiting value based on experience and good
practice. The recommended value adopted in BS 5268 : Part 2 is (0.003 × span)
when fully loaded. In the case of domestic floor joists there is an additional
recommendation of limiting deflection to less than or equal to 14 mm.
The calculated deflection for solid beams is usually based on the bending action of
the beam ignoring the effects of shear deflection
Bearing (Clause 2.10.2)
The behaviour of timber under the action of concentrated loads, e.g. at positions of
support, is complex and influenced by both the length and location of the bearing, as
shown in Figures
Solid Rectangular Beams
Note: In case (b), an additional modification factor K4 for bearing stress has been
included.
Solid Rectangular Beams
The actual bearing area is the net area of the
contact surface and allowance must be
made for any reduction in the width of bearing
due to wane, as shown in Figure
Lateral Stability
Solid Rectangular Beams
A beam in which the depth and length are large in comparison to the width (i.e. a
slender cross-section) may fail at a lower bending stress value due to lateral torsional
buckling, as shown in Figure
The critical value of bending moment which induces this type of failure is dependent
on several parameters, such as: the relative cross-section dimensions (i.e. aspect ratio),
shape, modulus of elasticity (E), shear modulus (G), span, degree of lateral restraint to the
compression flange, and the type of loading.
Notched Beams (Clause 2.10.9)
Solid Rectangular Beams
It is often necessary to create notches or holes in beams to accommodate fixing details
such as gutters, reduced fascias and connections with other members. In such
circumstances high stress concentrations occur at the locations of the notches or holes.
Whilst notches and holes should be kept to a minimum, when they are necessary cuts with
square re-entrant corners should be avoided. This can be achieved by providing a fillet or
taper or cutting the notch to a pre-drilled hole, typically of 8 mm diameter.
Effect on Shear Strength (Clause 2.10.4)
The projection of a notch beyond the inside edge of the bearing line at the point of
support reduces the shear capacity of a beam. There are two situations to consider,
as shown in Figure
The projection of a notch beyond the inside edge of the bearing line at the point of
support reduces the shear capacity of a beam. There are two situations to consider,
as shown in Figure
Solid Rectangular BeamsEffect on Shear Strength (Clause 2.10.4)
The reduction in shear capacity is reflected in the use of the net area and a
reduction factor K5, as indicated.
Effect on Bending Strength
The calculated bending strength of
notched beams is based on the net
cross-section, as
shown in Figure
Solid Rectangular Beams
When considering simply supported floor and roof joists which are not more than 250
mm deep and which satisfy the restrictions indicated in Figures (a) and (b), the effects
of notches and holes can be neglected.
Solid Rectangular Beams
Example 7.1: Suspended Timber Floor System
Solid Rectangular Beams
Consider the design of a suspended timber floor system in a domestic building in which
the joists at 500 mm centres are simply supported by timber beams on load-bearing
brickwork, as shown in Figure (a)
The support beams are notched at the location of the wall, as shown in Figure (b).
♦ Determine a suitable section size for the tongue and groove floor boards.
♦ Determine a suitable section size for the joists.
♦ Check the suitability of the main support beams.
Design data:
Centre of timber joists 500 mm
Distance between the centre-lines of the brickwork wall 4.5 m
Strength class of timber for joists and tongue and groove boarding and beams C22
Imposed loading (long-term) 3.0 kN/m2
Exposure condition Service Class 1