5/16/2017. Timber Design

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1 Timber Design Wood is a very versatile raw material and is still widely used in construction, especially in countries such as Canada, Sweden, Finland, Norway and Poland, where there is an abundance of goodqualitytimber. Timber can be used in a range of structural applications including marine works: construction of wharves, piers, cofferdams; heavy civil works: bridges, piles, shoring, pylons; domestic housing: roofs, floors, partitions; shuttering for precast and in situ concrete; falseworkfor brick or stone construction 1

2 Timber is naturally occurring. This makes it a very difficult material to characterise and partly accounts for the wide variation in the strength of timber, not only between different species but also between timber of the same species and even from the same log. Quite naturally, this led to uneconomical use of timber which was costly for individuals and the nation as a whole. However, this problem has now been largely overcome by specifying stress graded timber. There is an enormous variety of timber species. They are divided into softwoods and hardwoods, a botanical distinction, not on the basis of mechanical strength. Softwoods are derived from trees with needleshaped leaves and are usually evergreen, e.g. fir, larch, spruce, hemlock, pine. 2

3 Hardwoods are derived from trees with broad leaves and are usually deciduous, e.g. ash, elm, oak, teak, iroko, ekki, greeheart. elm EKKi OAK Teak The task of the structural engineer has been simplified, however, by grouping timber species into sixteen strength classes for which typical design parameters, e.g. grade stresses and moduli of elasticity, have been produced Some of the characteristics which influence design and are specific to timber are: the moisture content, the difference in strength when loads are applied paralleland perpendicularto the grain direction, the duration of the application of the load, the methodadopted for strength grading of the timber. 3

4 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: Fiber saturation point (FSP) The condition in which all free water has been removed but the cell walls are still saturated is known as the fiber saturation point (FSP). At levels of moisture above the FSP, most physical and mechanical properties remain constant. Variations in moisture content below the FSPcause considerable changes to properties such as weight, strength, elasticity, shrinkage and durability. 4

5 The controlled drying of timber is known as seasoning. Air seasoning, in which the timber is stacked and layered with air-space in open sided 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 5

6 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. ادر طر ود,, A shake is produced when fibresseparate 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. 6

7 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. 7

8 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 8

9 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. 9

10 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 :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. 10

11 Material Properties The 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. Material Properties 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 thecell.thestructureofthecellenhances thestrengthofthetimberinthegraindirection. 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. 11

12 Material Properties 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 optimum 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. 12

13 Material Properties The fire resistance of timber generally compares favourablywith 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. 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 13

14 Modification Factors The inherently variable nature of timber and its effects on structural material properties such as stress strain characterisfcs, elasfcity and creep has resulted in more than eighty different modification factors which are used in converting grade stresses to permissible stresses for design purposes. Modification Factors 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 :2002 as follows: 14

15 Modification Factors In many instances subscripts are also used to identify various types of force, stress or geometry; these are as follows: 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. 15

16 Modification Factors Modification Factors 16

17 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. Flexural Members 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. 17

18 Effective Span Most timber beams are designed as simply supported and the effective span which should be used is defined in Clause of BS :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 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 Beams The modification factors, which are pertinent when designing solid timber beams, are summarized in Table. 18

19 Solid Rectangular Beams Shear 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. 19

20 Solid Rectangular Beams Deflection 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 ) 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. 20

21 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. 21

22 Solid Rectangular Beams Notched Beams (Clause ) 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 ) 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 Beams Effect on Shear Strength (Clause ) 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 22

23 Solid Rectangular Beams 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. 23

24 Solid Rectangular Beams Example 7.1: Suspended Timber Floor System 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 24

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