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