16
Timber Design F H Potter BSc Tech, CEng, MICE, FIWSc, AMCT Senior Tutor, Imperial College of Science and Technology
Contents 16.1
Introduction
16/3
16.6
Glued-laminated timber assemblies
16/7
16.2
Design by specification 16.2.1 Species and use 16.2.2 Availability and sectional properties 16.2.3 The movement of timber 16.2.4 Moisture content and end use 16.2.5 Working properties 16.2.6 Natural resistance to attack 16.2.7 Preservation treatment 16.2.8 Fire resistance
16/3 16/3 16/4 16/4 16/4 16/5 16/5 16/6 16/6
16.7
Plywood and tempered hardboard assemblies
16/8
16.8
Timber fastenings
16/8
16.9
Timber-framed construction
Stress grading and permissible stresses 16.3.1 Visual stress grading 16.3.2 Mechanical stress grading 16.3.3 Glued-laminated timber grades 16.3.4 Strength classes of timber 16.3.5 Permissible stresses
16/6 16/6 16/6 16/6 16/6 16/7
16.4
Design – general
16/7
16.5
Design in solid timber
16/7
16.3
16/10
16.10 Repair and restoration
16/10
16.11 Termite-resistant construction
16/10
16.12 Storm-resistant construction
16/10
16.13 Earthquake-resistant construction
16/11
16.14 Design aids
16/11
References
16/11
Bibliography
16/13
This page has been reformatted by Knovel to provide easier navigation.
16.1 Introduction
Some of the characteristics of timber may be found in Table 16.3, whilst general properties are given in other publications.1"3
Timber is one of the finest structural materials: it has a high specific strength, can be easily worked and jointed and does not inhibit design. Like most other structural materials it suffers attack causing deterioration (corrosion, weathering and biodeterioration) but once the material is known and the causes understood, effective preventative measures can be taken easily and economically. Design is thus a confluence of specification, structural analysis, detailing and protection, each of which is of equal importance if an effective design is to be achieved. Nowadays, structural design covers a wider range of components than ever before, for the intense wind loadings in high-rise building coupled with large glazed areas has meant that much window joinery is now subject to structural design. In addition, the effects of wind loadings together with the requirements of the Building Regulations and the newer building shapes has meant that even in low-rise buildings, components which once were built must now be designed. Timber is thus used for a wide variety of structural purposes, either on its own or in combination with one of the 'heavier' materials. It can take extremely simple forms such as solid beams, joists and purlins or can be used in the more recent forms of glued-laminated construction or plywood panel construction. These latter forms allow the design of exciting and economic structural shapes, the variety of which may be judged from Tables 16.1 and 16.2.
16.2 Design by specif ication Essentially, this is a prescription of fitness for use under service conditions and requires the choice not only of an appropriate material but also of its condition, use and protection. The success of the specification will depend upon its interpretation; standard glossaries are available for timber and woodwork,4 nomenclature of timber5 and preservative treatment.6 Functional and user needs will dictate the choice of material based on the following factors. 16.2.1 Species and use Very many timbers are structurally useful, whereas usefulness for joinery purposes is often more restrictive. Where timber is used for structural joinery, the combination of requirements may be even more restrictive. Table 16.3 lists most of the timbers and their characteristics for which working stresses are given in BS 5268: Part 2,7 whilst BS 11868 indicates the joinery use of specific species. A comprehensive guide to West African species and their uses, both structural and joinery, is given in pamphlets issued by the United Africa Company.9 Flooring, particularly industrial flooring, has particular requirements and recommendations for suitable timbers for these
Table 16.1 Roof selection Maximum economic spans (m)
Division Subdivision
Construction
Minimum support conditions
Beams
Solid timber Laminated, either vertically or horizontally, depending on size I or box sections: flanges solid or laminated. Webs plywood or diagonally boarded Laminated horizontally I or box sections: flanges laminated horizontally. Webs diagonally boarded Laminated horizontally I or box sections: flanges solid or laminated. Webs plywood or diagonally boarded Solid timber
6 Vertical support at ends 24
Arches
Portals
Trusses Belgian Warren
Solid timber
Bowstring
Laminated chords. Solid webs
(After: L. G. Booth, Engineering, 25 March 1960)
Fastenings
None Glue
30
Glue and/or nails
46 46
Glue and/or nails for laminating Connectors for site joints
24 46
Glue and/or nails for laminating Connectors for site joints
Vertical support at ends 12 24 12 30 46
Nails and/or glue Connectors Nails and/or glue Connectors Glue and/or nails for laminating Connectors at joints
Vertical and horizontal support at ends
Vertical and horizontal support at ends
Table 16.2 Roof selection Fastenings
Division
Subdivision
Construction
Minimum support Maximum economic conditions sizes (m)
Plates
Flat
Membrane formed from plywood or layers of diagonal boarding A single-skin structure may have stiffening ribs A double-skin structure will have spacing ribs Edge beams and end diaphragms required Membrane formed with layers of diagonal boarding. May have stiffening ribs Edge beams required End diaphragms required Boarded membrane with or without laminated ribs Laminated ring beam Boarded membrane with laminated edge beams
Vertical support at corners
12x12
Nails and/or glue
Vertical support at corners
18x9
Membrane with nails and/or glue Diaphragms with nails or connectors
Vertical support at corners
30x12
Membrane with nails and/or glue Beams (see Table 16.1) Diaphragms with nails or connectors
Ring beam to be supported at intervals
30 dia.
Vertical support only at low points, if columns tied together. Otherwise buttresses at low points Vertical support Boarded membrane with laminated tied at corners arches along edges
21x21
Membrane with nails and/or glue Ribs and ring beam glued Membrane with nails and/or glue Edge beams glued
Vertical support Boarded membrane with laminated tied at corners arches on ends Edge beams required
30x9
Folded
Singly Circular cylindrical curved shells
Doubly Spherical dome curved shells Hyperbolic paraboloid
Elliptical paraboloid
Conoid
24x24
Membrane with nails and/or glue Tied arches with glue and connectors membrane with nails and/or glue Beams (see Table 16.1) Tied arches with glue and connectors
(After: L. G. Booth, Engineering, 25 March 1960)
requirements are given in Princes Risborough Laboratory (PRL) Technical Note No. 49.10 16.2.2 Availability and sectional properties Availability is equally important, and Table 16.3 indicates the availability of the structural timbers from the viewpoints of supply and length. The geometric properties of sawn and precision timber to be used in design are also given in BS 5268: Part 2. Guidance on the usefulness of worldwide species may be found,11 whilst the available sizes for hardwoods are given in BS 5450.l2 16.2.3 The movement of timber Even with dried timber, changes in atmospheric conditions will
result in a varying moisture content which will induce fluctuating dimensional changes in the timber, known as 'movement'. The variation can be designed-for quite simply but some knowledge of the degree of possible movement is helpful. Some indication can be obtained from Table 16.3, whilst further information can be obtained from the publications of the PRL. 2 3 1 3 1 4
16.2.4 Moisture content and end use Every species of timber will achieve a fairly steady moisture content for a particular environment - the equilibrium moisture content. The PRL has established moisture contents for various environments.12 Greater reliability can be achieved by drying timbers to these moisture contents before construction.
Table 16.3 Characteristics and availability of some structural timbers Standard name
SOFTWOODS (IMPORTED) Douglas fir-larch Hem-fir Parana pine Pitch pine E. redwood E. whitewood Canadian spruce-pine-fir W. red cedar
Approx. density at M/C 18% (kg/m3)
Natural durability
Resistance to Moisture preservative movement treatment
Working quality
Availability Supply
Normal length (m)
Relative price
590 530 560 720 540 510 450
Moderately Not Not Durable Not Not Not
Resistant Resistant Moderately Resistant Moderately Resistant Very
Small Medium Medium Medium Medium Small Medium
Good Good Good Good Good Good Good
Good Good Good Good Good Good Good
4.20-4.80 4.20-4.50 3.60-3.90 4.50-9.00 1.50-7.00 1.50-7.00 2.40-5.10
Medium Low Low Medium Low Low Low
380
Durable
Resistant
Small
Good
Good
2.40-7.30
Low
SOFTWOODS (HOME GROWN) Douglas fir Larch (E- Japan) Scots pine European spruce Sitka spruce Corsican pine
560 560 540 380 400 510
Moderately Moderately Not Not Not Not
Resistant Resistant Moderately Resistant Resistant Moderately
Small Medium Medium Small Small Small
Good Good Good Good Good Good
Fair Good Good Good Good Fair
1.80-4.50 1.80-3.60 1.80-3.60 1.80-3.60 1.80-3.60 1.80-3.60
Low Medium Low Low Medium Medium
HARDWOODS (IMPORTED) Abura African mahogany Afrormosia Greenheart Gurjun/Keruing Iroko Jarrah Karri Opepe Red meranti Sapele Teak
590 590 720 1060 720 690 910 930 780 540 690 720
Perishable Moderately Very Very Moderately Very Very Durable Very Moderately Moderately Very
Moderately Extremely Extremely Extremely Resistant Extremely Extremely Impermeable Moderately Resistant Resistant Extremely
Small Small Small Medium Large Small Medium Large Small Small Medium Small
Good Medium Medium Difficult Medium Medium Difficult Difficult Medium Good Good Medium
Good Good Good Good Good Good Good Good Good Good Good Good
.80-6.00 .80-7.30 2.40-7.30 4.80-9.00 .80-7.30 .80-6.00 .80-8.40 .80 up .80-6.00 .80-7.30 .80 up .80 up
Low Medium Med high High Low Medium Med high Med high Medium Low Medium High
HARDWOODS (HOME GROWN) European ash European beech European oak
720 720 720
Perishable Perishable Durable
Moderately Permeable Extremely
Medium Large Medium
Good Good Medium
Fair Good Good
1.80 up 1.80 up 1.80 up
Low Medium Medium
16.2.5 Working properties Ease of fabrication is indicated in Table 16.3, although more detailed information may be found in PRL publications.2-3
Table 16.4 Durability classification of the heartwood of untreated timbers Grade of durability
16.2.6 Natural resistance to attack Timber has a widely varying resistance to attack by fungi, insects, marine borers and termites. Fungi will not normally attack timber having a moisture content lower than 20% but a timber's ability to resist fungal attack is classified according to Table 16.4. The natural durability of some structural timbers is given in Table 16.3. Information on further timbers will be found in PRL
Approximate life in ground contact
(y) Very durable Durable Moderately durable Nondurable Perishable
More than 25 15-25 10-15 5-10 Less than 10
Technical Note No. 4015 and the Handbooks on softwood and hardwoods2'3 whilst further advice on the control of decay will be found in PRL Technical Notes 29, 44 and 57.1^18 Termite attack and its prevention are dealt with by the PRL19 in which the following timbers are mentioned as being generally resistant: iroko, opepe, Californian redwood and teak. Other timbers are given in the Handbooks on softwood and hardwoods.2'3 Marine borers are a hazard in the sea or brackish waters and PRL Leaflet No. 4620 gives advice on the protection of timbers against this attack. Highly resistant timbers suitable for marine works are: greenheart, pyinkado, turpentine, totara, jarrah, basralocus and manbarklak. 16.2.7 Preservative treatment The sapwood of all timbers is liable to attack by fungi and insects but it is often possible to obtain a more attack-resistant structure by pressure-impregnating nondurable or perishable timbers than by using durable species. Indeed, it is sometimes more economic also. The amenability of timbers to preservative treatment is given in Table 16.3 and is related to the following classification: Permeable: Moderately resistant: Resistant: Extremely resistant:
Easily treated by either pressure or open tank. Fairly easy to treat by pressure, penetration 6 to 20 mm in 2 to 3 h. Difficult to impregnate, incising often used. Penetration often little more than 6 mm. Very little penetration can be achieved even after prolonged treatment.
Further information and guidance on satisfactory types and methods of treatment may be found in publications of the British Standards Institute (BSI)2'-23 and the Timber Research and Development Association (TRADA). The economics of timber preservation is discussed in Timberlab 77.24 16.2.8 Fire resistance Although timber ignites spontaneously at about 25O0C, ignition is a function of the external surface area to the total volume of timber and the rate of charring does not significantly increase with a rise of temperature. The rate of charring is generally taken as about 0.5 mm/min (western red cedar 0.85 mm/min, dense hardwood 0.42 mm/min), but perhaps the most important factor is that the structural properties of uncharred material remain virtually unchanged. Thus, if adequate protection against combustion is provided, timber is one of the safest structural materials in a severe fire. These measures are usually: (1) the provision of sacrificial material; (2) chemical impregnation; and (3) protective covering.2"7
16.3 Stress grading and permissible stresses Timber is a natural organic material and therefore is subject to wide variability because of environmental, species and genetic effects. This variability affects both visible quality and strength. If, for any particular property and species only one design stress were specified, this would have to be set so low (to allow for variability) that the material would have a very limited
structural application. In consequence, a number of stress grades have been adopted, leading not only to a more economic use of the material but also to a higher yield of structurally useful material. There are two main methods of stress grading for solid timber: (1) visual grading; and (2) mechanical grading. Each requires a different procedure. 16.3.1 Visual stress grading In visual grading, data obtained from clear material (straight grained and free from knots and fissures) are analysed statistically for each species and basic stresses for each property are devised. These basic stresses are then reduced by factors which take account of the strength-reducing effects of the permissible growth characteristics for each stress grade. At the present time, there are two sets of quality requirements for visual stress grading in this country, one for softwoods and one for hardwoods. The first set is given in BS 4978.28 Besides setting the requirements for two grades for solid softwood timber construction (SS and GS), this standard restates the requirements for laminating timber grades. The second set is given in BS 575629 for tropical hardwoods (HS grade). In the current edition of BS 5268:Part 2 homegrown hardwoods have been deleted, but it is probable that these will be reintroduced. 16.3.2 Mechanical stress grading Mechanical stress grading30 is a method of non-destructive testing each piece to be graded. The piece is bent under a constant central load over a constant short span. The strength of the material can then be calculated accurately from the resultant deflection. Four grades are presented (M75, M50, MSS and MGS) and the grade stresses for the dry condition only are tabulated in BS 5268:Part 2. At present, machine-grade stresses are limited to six softwood species for which control information is available. However, it will be possible to machine-stress-grade other timbers in accordance with BS 4978.28 16.3.3 Glued-laminated timber grades In glued-laminated members, the presence of strength-reducing characteristics will have a smaller effect than in solid timber, since the probability of identical structural defects appearing in identical positions in adjacent laminations is very small. British Standard 5268:Part 2, therefore allows higher grade stresses for glued-laminated timbers, these being obtained by applying tabulated modification factors to the grade stresses for each species. 16.3.4 Strength classes of timber For the first time, BS 5268:Part 2 introduces the concept of strength classes for timber. Softwood species-grade combinations for strength classes, graded to BS 4978 are tabulated in Tables 3, 4 and 5 of that standard, whilst species groupings of hardwoods graded to BS 5756 are tabulated in Table 7 of that standard for the higher strength classes. The concept, similar to the older species groupings, is that a strength class rather than a species may be specified. However, sometimes there are advantages in specifying a particular species and grade where the grade stress is higher than the strength class stress.
16.3.5 Permissible stresses Permissible design stresses for both solid and laminated timber components are governed by the type of component, the conditions of service and the type of loading. They are obtained from grade stresses by applying the appropriate modification factors.
16.4 Design-general Design in timber is similar to that in any other structural material as long as timber's peculiar qualities are acknowledged; indeed, these qualities can be exploited by resourceful designers. Timber is idealized as an orthotropic material, but in practice, only two directions need be considered: that parallel to the grain (along the trunk) and that perpendicular to the grain. Most strength properties, of both timber and joint fasteners, vary according to these directions and the variation has been found to follow the Hankinson relationship:
,v-
PQ
P sin2 0 + Q cos2 0
where 6 is the angle between directions of load and grain, N the stress at 0 to the grain, P the stress parallel to the grain, and Q the stress perpendicular to the grain from which intermediate stress or strength values can be calculated. This is not normally required for solid beams, joists and columns where only the major directions are used, but is often met where members intersect at joints. The stresses given in BS 5268:Part 2 are for permanent loading and increased values are allowed for short- and medium-term loads. This Code of Practice governs general design, but additional information is available. 3141 In the past, design has been inhibited by the relatively short lengths of timber available (Table 16.3 indicates availability). However, the production of durable resin adhesives has led to new construction techniques and structural forms being developed. Glued-laminated timber in which thin laminae are glued together to form structural components of almost any shape or length is a common reality, whilst structural plywood can be combined with either solid or glued-laminated timber to produce composite components which are lightweight, reliable and pleasing. The design in these forms is more complex than in solid timber but information on a wide variety of structural forms can be found,42"70 whilst advice on the selection of a particular form is given in Tables 16.1 and 16.2. General design advice is provided by TRADA.71
16.5 Design in solid timber Since permissible stresses are maxima there may be some advantage in using structural hardwoods or the higher-gradestress softwoods whenever stress governs design. However, if deflection governs, there will be no advantage in using these more expensive materials unless the moduli of elasticity are sufficiently high. A possible exception is keruing (dipterocarpus spp.) whose current cost is roughly similar to that of softwoods. Some indication of price is given in Table 16.3. As design in solid timber is limited by the maximum size of timber available, this has led to the development of many types of girder framework: however, where there is sufficient headroom, trussed beams can give an economic solution for heavy loads and long spans.31-3541 In BS 5268:Part 2, minimum sizes are specified and the
geometric properties tabulated in Appendix D of BS 5268:Part 2 are based on those minimum sizes. Further reductions in section should be made for notches, mortices and bolt, screw and connector holes. Modification factors may also be required for the length and position of bearing, the shape of a beam and its depth if greater than 300 mm, whilst for compression members, combined factors are given for both slenderness and loading. Lateral stability is important both for deep beams and for compression members, and in built-up members web stiffeners should be provided wherever concentrated loads occur. General design data are available 7173 applicable to particular structural forms45-5^61 -6^67 whilst design aids have been published for solid beams, portal frames and trussed rafters.
16.6 Glued-laminated timber assemblies Glued-laminated timber is essentially a built-up section of two or more pieces of timber whose grains are approximately parallel and which are fastened together with glue throughout their length. This enables the properties of timber to be regulated to some degree and provides structural sizes and shapes which would not be possible in solid timber. Variation in section is possible, whilst high-grade material can be placed in zones of high stress and low-grade material in zones of low stress. All softwoods glue well and are generally preferred in the UK, although occasionally there can be some advantage in using wholly hardwood laminae. Construction may use either vertical or horizontal laminations. With vertical laminations, the zones of equal stress are shared between the laminations so that the strength of a beam can be said to be the sum of the individual laminations. This loadsharing concept has led to the grade-stress modification factors tabled in BS 5268:Part 2 which give higher permissible stresses for the laminated beam. Horizontally laminated beams have been permitted since 1967 but the philosophy for behaviour is entirely different from that for vertical laminations. A beam will consist of material containing knots whose presence will affect the strength ratio of the beam. Since the knot effect will vary according to the sizes of knots and the number of laminations, BS 5268:Part 2 tables basic stress modification factors according to these variables. Since curved laminated beams are fabricated by bending the individual laminations, fabrication stresses are induced which depend upon the degree of curvature, the thickness of the lamination and the species of timber. Therefore, modification factors to be applied to the grade bending stresses for different values of t/R are specified in BS 5268:Part 2. The production of long laminations depends upon the use of efficient methods of end jointing. Where the efficiency of an end joint is known, the laminations containing them can be included when calculating the section properties, but where efficiencies are not known, the laminations containing the end joints must be omitted when calculating section properties. Efficiency ratings for plain scarf joints and for finger joints are given in BS 5268:Part 2: these are used to modify the basic stresses to give the maximum stresses to which any particular lamination may be subjected. British Standard 529174 governs finger joints in structural softwood. Butt joints do not transmit load and should only be used in zones of zero or very low stress. Apart from the consideration of end joints and curvature, design is similar to that for solid timber,46'64-75'76 whilst design aids are noted for glulam beams.
16.7 Plywood and tempered hardboard assemblies Plywood is a type of glued-laminated construction in which the laminae are formed from thin flat veneers of timber. These veneers are produced by the rotary cutting of logs and are laid alternately at right angles in an odd number of layers. Since both the shrinkage and strength of timber differ according to the grain direction, the type of construction gives greater dimensional stability and tends to equalize the strength properties in both major directions of the plywood sheet. There are two distinct design philosophies: (1) the North American approach which only considers the 'parallel plies', i.e. those plies whose grain lies in the direction of the load (this approach is based on the basic stresses and moduli for solid timber); and (2) the Finnish and British approach, known as the 'full cross-section' approach, in which grade stresses and moduli for the sheet materials have been determined from tests. In BS 5268:Part 2 all the grade stresses and moduli are for full crosssection, but it is well to remember that many North American design manuals will be based on the 'parallel-plies' approach. Plywood is a strong, durable and lightweight structural material which can be used to produce exciting structural shapes.44-47^9 61~63 Design data are available for a variety of constructions77'79 whilst design aids are available for stressed skin panels and portal frames. Perhaps plywood's most useful property is that of providing excellent shear resistance for a small cross-section, although it is well to remember that lateral stability constraints may be required. Tempered hardboard is a durable compressed fibreboard for which BS 5268:Part 2 now gives grade stresses for use in structural components instead of plywood.
larly true in timber for which highly efficient methods of transferring tensile loads have been developed only during the past 50 yr. Split-ring and tooth-plate connectors are now available which have load capacities much greater than those for nailed and bolted joints. A comparative indicator of fastener efficiency and the required member sizes is given in Table 16.5. The strength of mechanical fasteners depends upon member size and thickness and the spacing of the fasteners. British Standard 5268:Part 2 tabulates permissible values of a wide range of variables, whereas some manuals prefer a presentation as a series of design curves.31 >34 - 3540 However, the major advance in fastening techniques has been in glued joints. Early glues were unreliable, deteriorating quickly, but the present phenolic and resorcinol resins are so durable that the risk of delamination has been almost entirely eliminated, even under extreme exposure. Unfortunately, gluing still requires controlled conditions and its application to site work is still limited. Since the shear strength of adhesives is usually higher than that of timber, a fastener efficiency of 100% can be achieved. Nevertheless, it is important to remember that glues seldom have a good tensile strength, so that they should be stressed in shear as much as possible. Information is available on gluing,80 the requirements for adhesives,81 and the compatibility between glues and preservatives.82 The permissible stresses for glued joints are the shear stresses for the timber;7 however, regard must be paid not only to the variation of that shear strength but also to the possibility of differential shrinkage and stress concentrations in the joint. The type of fastener chosen will depend upon the skills and equipment available, possible fabrication conditions, relative costs and whether or not it is necessary to take down and reassemble the structural components.
16.8 Timber fastenings Available methods of jointing are perhaps the most important criteria for the design of structural components. This is particu-
Table 16.5 The relative strengths of timber joints Comparison - axial compression in GS/M50 (SC3) European redwood FASTENER
TIMBER Effective
Type & dia, (mm) NAILS 3.75 8.00 SCREWS 8.43 BOLTS M8 M12 TOOTH PLATE 2/5 1 mm dia. + M12bolt 2/64 mm dia. + M12bolt SPLIT RING 2/64 mm dia. + M12bolt Assumptions:
Capacity (kN)
Size (mm)
Area (mm2)
Capacity (kN)
63 21
40.3 42.7
47 x 145 72x145
5581 8712
40.7 63.6
16
42.2
60 x 145
6677
48.7
30 13
41.3 40.3
44x169 60 x 145
5676 6540
41.4 47.7
5
40.5
60x145
6540
47.7
5
43.5
60x169
7980
58.3
3
49.7
60 x 145 or 72 x 120
6800 6740
49.6 49.2
No.
(1) three member joints loaded to 4OkN in axial compression parallel to grain (2) timber to timber joints (3) GS/M50 European redwood Grade stress Table 8 SC3 6.8 N/mm. 2 Grade stress Table 9 M50 7.3 N/mm 2 . Using Table 9 value, required timber area = 5479 mm2
EXAMPLES OF THE DESIGN OF A SIMPLE TENSION SPLICE JOINT
= 635 x (2 x 0.9) x 1.12 x 1.0 x 1.0 - (assumed) = 1280.16 N.
LOAD CAPACITY: 25 kN
DURATION: MEDIUM TERM
TIMBER. EUROPEAN REDWOOD
Number of 5 mm nails required = -r-=? = 20
GRADE: M50 TABLE 56 SPACING, modified by 0.8 (CLAUSE 41.3)
EXPOSURE CONDITION: DRY TRY 4 x 5 pattern (20 nails) (1) REQ UIRED TIMBER SECTION 25 x 40 x 40 x 40 x 25 Dry exposure condition grade stresses, Tension //g
X
X
X
X
x
x
x
x
X
SC3 (TABLE 8): 3.2 N/mm2 x 1.25 = 4.0 N/mm 2 European redwood (TABLE 9): 4.0 N/mm2* x 1.25 = 5.0 N/mm2
X
X
undrilled width ^ 170 mm joint length 4 x 4 0 + 2 x 4 0 = 2 4 0 m m
X
25 x 12 x 12 x 12 x 25
drilled width ^ 86 mm*
Effective area = (145 - 4 x 5) 41 mm2 (CLAUSE 41.2)
Maximum permissible timber stress = 5.0 N/mm 2
= 5125 mm2
Section = ?^P = 5000 mm2
Effective timber load capacity = 25.625 kN
allow 10% reduction in effective section at joint
Therefore the nailing pattern is acceptable, but requires predrilling
2
SAY 41 x 145 mm planed = 5950 mm (TABLE 99) Alternatively, try 3 x 7 pattern (21 nails, but easier to control), to avoid cost of pre-drilling x x x 25 x 40 x 40 x 25 undrilled width ^ 130 mm*
(2) NAILED JOINT CHOICE OF NAIL DIAMETER Possible joint thickness = 3 x 41 mm = 123 mm
5 mm : 125 mm
Standard thicknesses for members in double shear: 4 mm
0: 0.7 x 44 = 31 mm
(splice 35 x 145)
4.5 mm
0: 0.7 x 51 = 36 mm
(splice 35 x 145)
5 mm
: 0.7 x 57 = 40 mm
(splice 41 x 145)
CLAUSE 41.4.2
TABLE 57
X
x
x
X X X
joint length 5 x 40 + 2 x 40 = 320 mm
X X X
Effective area = (145 - 3 x 5) 41 = 5330 mm2 (CLAUSE 41.2) Effective timber load capacity
= 26.65 kN
Fastener capacity = 21 x (2 x 0.9) x 1.12 x 1.0 x 635 kN = 26.9 kN (3) BOLTED JOINT CHOICE OF BOLT DIAMETER Number of lines of bolts («) possible in a 145 mm wide member
Required nail lengths: 4mm
X
x
X X X
Maximum available stock lengths: 4 and 4.5 mm 0: 100 mm
X
—
: 2 x 35 + 41 = 111 mm
required timber capacity effective area x permissible stress
4.5 mm : 2 x 35 + 41 = 111 mm
M10 bolt, n = 2.3 lines
lengths not available
For any larger diameter bolts, only one line of bolts would be possible.
5 mm : 2 x 41 + 41 + = 123 mm*
DESIGNOFJOINT DESIGN OF JOINT (5 mm nails) Basic single shear lateral load capacity, dry exposure:
Basic single shear lateral load parallel to grain, (TABLE 67), member 41 mm thick.
5 mm, SC3: 635 N (TABLE 57)
MlO,
Multiple shear factor (CLAUSE 41.42)
Double shear factor (CLAUSE 43.4.2) 2.0
0.9 x number of shear planes provided each member is thicker than 0.7 x standard point size penetration
Permissible joint load (double shear)
1.28 kN: M12, 1.84 kN: M16, 3.15 kN (interpolated)
= 2 basic x K55 x K56 x K51 Permissible joint load (CLAUSE 41.8) medium terrfi= 1.25
N.
number'in line'
= basic x Jf48 x AT49 x AT50 m/cdry = 1.0 duration of load (medium term: 1.12)
moisture content (dry: 1.0)
number of nails (< 10 'line': 1.0)
Number of bolts required
MlO, 7.8: M12, 5.44: M16, 3.17: bolts 'in line' and (K57) = [1 - 3^1 *] for n < 10 MlO, 4 (.91): M12, 6 (.85): M16, 4 (.91) . . , , .. original number revised number ofr bolts = —-—p A57
MlO, 8.6: M12, 6.4: M16, 3.5: Effective timber capacity = effective area x permissible stress
lateral loading and, particularly, that resistance to planar deformation of a wall panel known as its racking resistance. Figure 16.1 shows the deformation of an idealized frame structure under lateral loading. Figure 16.1 (a) shows the uniform translation of the walls which occurs when there is complete symmetry of both structure and loading. This hardly ever occurs and this lack of symmetry causes a rotation in addition to the translation (Figure 16.2(b)). The calculation of racking resistance is described by TRADA58 and will also be dealt with by BS 5268:Part 6 (currently being written).
16.10 Repair and restoration
MlO, 25.63 kN: M12, 27.26 kN: M16, 26.45 kN BOLTS REQUIRED MlO, 9 (in two lines): M12, 7 (one line): M16, 4 (one line) Compare with Timber Research and Development Association (1986) Design aid DAl. p. 25.
16.9 Timber-frame construction It is estimated that the major use of structural timber will be in the housing field. In high-rise construction, timber will play a supplementary role to the heavy material, being used for partitions, infill panels and floor and roofing systems. In this role, timber's ready adaptability to prefabrication is a great benefit. In low-rise construction, on the other hand, timber is increasingly being used to provide the structural skeleton for the building; indeed, at the present time, timber-frame construction constitutes some 20% of all house construction. The method of construction is a simplification and refinement of that which has been used successfully for many centuries, but which is equally well applicable to many other uses besides housing. The structural form is that of a free-standing skeleton for which standard details have been produced.50"52'56-57-59 The basis of the skeleton is the stud-framed panel for which designs are described.53-55-60 Of especial importance is the structure's ability to withstand
Roof diaphragm
Side-wall element Lateral load
End-wall diaphragm
An increasing amount of work is being carried out on the repair and restoration of old timber-framed buildings. The work is specialized and requires not only sound structural assessment of the existing building but also a good understanding of the methods used in its construction and of acceptable methods of repair and restoration. The methods of repair may be either by replacement of the damaged joints or members in the traditional manner or by the use of resin or stainless steel rods and resin fillers. Brunskill72 describes the traditional methods of timber-frame construction whilst Charles and Charles83 give restoration case studies. Conservation and restoration are reported on by Fielden,84 Gifford,85 Ministry of Public Buildings and Works,86 and repair by Avent era/., 87~88 Gates and Richards89 and Powys.90
16.11 Termite-resistant construction There are two basic kinds of termite, which are mainly found in tropical and sub-tropical areas. The subterranean termite (white ant or wet-wood termite) has the larger distribution, builds huge nests, farms fungi and needs to maintain contact with the damp earth. Attack is from the ground. There are many thousands of species of subterranean termites and a species of timber providing good termite resistance in one area may not be resistant in another. Resistive construction depends upon correct detailing, ground poisoning and preservative treatment. The dry-wood termite, on the other hand, is less widely distributed, has fewer species, and flies, mates and deposits its eggs in timber to continue the life cycle. Fly screens and preservative treatment give the best protection for new structures, whilst fumigation may be required when infestation is found in existing buildings. The recognition, control and detailing required are given by Harris91 and Sperling.92
Only end walls racked Side-wall diaphragm Direction of rotation Centre of rotation
Majority of walls are racked: some are twisted and racked Figure 16.1 Deformation of idealized house structure under lateral loading, (a) Uniform translation of top wall parallel to the lateral load; (b) rotation after lateral translation, caused by lack of symmetry
16.12 Storm-resistant construction Wind loading is one of the commonest and most variable types of loading that can occur and ranges from normal wind loading to tornadoes. Tropical storms have wind velocities in the range 55 to 117km/h and damage is usually caused by flooding, including that produced by waterborne detritus. Hurricanes are more severe and damage is caused by wind action, flooding and flying debris. Air circulation is counterclockwise with a damage area having a dimeter of 50 to 160 km. Wind speeds are commonly 120 to 190 km/h with gusting up to 320km/h. Rainfall, normally 125 to 250mm, occasionally reaches 750 mm. Tornadoes are unquestionably the most devastating of wind storms. The vortex is much smaller than that of a hurricane,
causing intense damane from wind action and large pieces of flying debris. Generally, storm-resistant structures require strength linked to rigidity with interconnected members and components securely fastened together. The Southern Pine Association gives valuable advice.93
16.13 Earthquake-resistant construction Horizontal and vertical movement of the Earth's surface, caused by earthquakes, results in forces being generated by the inertia of the mass of the structure. The magnitude of these inertial forces varies directly as the mass of the structure and in consequence heavy structures are more severely loaded than are lightweight ones. Indeed, for timber structures, the forces may be little higher than those produced by normal wind loading. However, unlike storm-resistant structures, those for earthquake-resistant construction require strength linked to flexibility. Several publications give outlines of sound practice.94"96
16.14 Design aids There are three areas in which design aids can make a valuable contribution to the design process. These are: (1) rapid preliminary design either for comparison or estimation of cost; (2) routine elementary design; and (3) complex design processes for which the design time can be reduced drastically. Nomograms and load-span tables have been used for many years, but the application of computer programming has extended considerably the use of design aids. The bibliography to this chapter indicates some of the design aids which are now available for structural design in timber.
References General properties of timber 1
Dinwoodie, J. M. (1981) Timber, its nature and behaviour. Von Nostrand Reinhold, New York. 2 Princes Risborough Laboratory (1972) Handbook of hardwoods. Building Research Establishment, Garston. 3 Princes Risborough Laboratory (1977) Handbook of softwoods. Building Research Establishment, Garston. Glossaries 4 British Standards Institution (1972) Glossary of terms relating to timber and woodwork, BS 565. BSI, Milton Keynes. 5 British Standards Institution (1974) Nomenclature of commercial timbers, BS 881 and 589. BSI, Milton Keynes. 6 British Standards Institution (1968) Glossary of terms relating to timber preservatives, BS 4261. BSI, Milton Keynes. 7 British Standards Institution (1984) Code of practice for permissible stress design, materials and workmanship, BS 5268: Part 2. BSI, Milton Keynes. Species, use and availability 8 British Standards Institution (1971) Quality of timber, BS 1186: Part 1. BSI, Milton Keynes. 9 United Africa Company (1971) West African hardwoods, Parts 1 and 2. UAC, London. 10 Princes Risborough Laboratory (1971) Hardwoods for industrial flooring, Technical Note No. 49, PRL, Building Research Establishment, Garston.
11
Timber Research and Development Association (1979/1980) Timbers of the world, VoIs 1 and 2. Longman, London. 12 British Standards Institution (198-') Sizes of hardwoods and methods of measurement, BS 5450. SI, Milton Keynes. Moisture content, moisture movement 13 Princes Risborough Laboratory (1969) The movement of timbers, Technical Note No. 38. Building Research Establishment, Garston. 14 Princes Risborough Laboratory (1971) Flooring and joinery in new buildings. How to minimize dimensional changes. Technical Note No. 12. Building Research Establishment, Garston. Natural durability and the protection of timber 15 Princes Risborough Laboratory (1975) The natural durability classification of timber. Technical Note No. 40. Building Research Establishment, Garston. 16 Princes Risborough Laboratory (1968) Ensuring good service life for window joinery. Technical Note No. 29. Building Research Establishment, Garston. 17 Princes Risborough Laboratory (1971) Decay in buildings recognition, preservation and cure. Technical Note No. 44. Building Research Establishment, Garston. 18 Princes Risborough Laboratory (1972) Timber decay and its control. Technical Note No. 57. Building Research Establishment, Garston. 19 Princes Risborough Laboratory (1965) Termites and the protection of timber. Leaflet No. 38. Building Research Establishment, Garston. 20 Princes Risborough Laboratory (1950) Marine borers and methods of preserving timber against their attack. Leaflet No. 46. Building Research Establishment, Garston. Preservative treatment 21
British Standards Institution (1975) Guide to the choice, use and application of wood preservatives. BS 1282. BSI, Milton Keynes. 22 British Standards Institution (1977) Preservative treatments for construction timbers. BS 5268; Part 5. BSI, Milton Keynes. 23 British Standards Institution (1978) Code of practice for preservation of timbers. Section 7 timber for use in prefabricated building in termite-infested areas. BS 5589. BSI, Milton Keynes. 24 Tack, C. H. (1969) 'The economics of timber preservation. Timberlab 17, Princes Risborough Laboratory, Building Research Establishment, Garston. Fire resistance 25 British Standards Institution (1978) Fire resistance of timber structures. BS 5268: Part 4. BSI, Milton Keynes. 26 Fire Research Station (1970) Fire and the structural use of timber in buildings. BSI, Milton Keynes. 27 Wardle, T. M. (1966) Notes on the fire resistance of heavy construction. New Zealand Forestry Service Information Series 53.
Stress grading 28 British Standards Institution (1978) Timber grades for structural use. BS 4978. BSI, Milton Keynes. 29 British Standards Institution (1980) Specification for tropical hardwoods graded for structural use. BS 5756. BSI, Milton Keynes. 30 Curry, W. T. (1969) 'Mechanical stress grading of timber'. Timberlab 18, Building Research Establishment, Garston. Design Textbooks, etc. 31 American Institute of Timber Construction (1985) AITC timber construction manual. Wiley, New York.
32 33 34 35 36 37 38 39 40 41
Baird, J. A. and Ozelton, E. C. (1986) Timber designer's manual. Granada, London. Breyer, D. E. and Ank, J. A. (1980) Design of wood structures. McGraw-Hill, New York. Hansen, H. J. (1962) Modern timber design. Wiley, New York. Karlsen, G. G. (1967) Wooden structures. Mir, Moscow. Laminated Timber Institute of Canada (1980) Timber design manual (metric edn). LTIC, Ottawa. Leicester et al. (1974) Fundamentals of timber engineering, Parts 1 and 2: 24 lectures given by officers of the Division of Building Research, CSIRO, Victoria, Australia. Mettem, C. J. (1986) Structural timber design and technology. Longmans. Oberg, F. R. (1963) Heavy timber construction. The Technical Press, London. Pearson, R. G., Kloot, N. H. and Boyd, J. D. (1967) Timber engineering design handbook. Jacaranda, Melbourne. US Department of Agriculture (1974) Wood handbook. Handbook No. 72, US Printer's Office, Washington, DC.
Arches and portal frames
42 43 44 45 46
Burgess, H. J. (1970) Exploiting geometrical symmetry in timber structures. Timber Research and Development Association, High Wycombe. Council of Forest Industries of British Columbia (1972) Portal frame manual. COFI, London. Kharna, J. and Hooley, R. F., (1965) Design of fir plywood panel arches. Council of Forest Industries of British Columbia Report No. TDD^t3. COFI, London. Timber Research and Development Association (1969) Ridged portals in solid timber. TRADA E/IB/18, London. Wilson, T. R. C. (1939) The glued laminated wood arch. United States Department of Agriculture Technical Bulletin No. 691.
57 Timber Research and Development Association (1980) Timberframe housing manual. Construction Press, TRADA, London. 58 Timber Research and Development Association (1980) Calculating the racking resistance of timber-framed walls. Wood Information Sheet No. 1-18. TRADA, High Wycombe. 59 Timber Research and Development Association (1981) Introduction to timber-frame housing. Wood Information Sheet No. 0-3, TRADA, High Wycombe. 60 Timber Research and Development Association (1981) Timberframe housing, structural recommendations. Construction Press, TRADA, High Wycombe.
Plyweb beams 61
Burgess, H. J. (1970) Introduction to the design of ply-web beams. Timber Research and Development Association Note No. E/IB/ 24. TRADA, High Wycombe. 62 Council of Forest Industries of British Columbia (1963) Nailed fir plywood web beams. COFI, London. 63 Council of Forest Industries of British Columbia (1970) Fir plywood web beam design. COFI, London. Shells
64 65 66 67 68
Barrel vaults
47
Kharna, J. (1964) Design of fir plywood barrel vaults. Council of Forest Industries of British Columbia Report No. TDD-40. COFI, London.
69
Keresztcsy, L. O. (1966) 'Interconnected, prefabricated laminated timber diamond type shell', Proceedings, International Conference for Space Structures. Surrey University, Guildford. Tottenham, J. (1958) The analysis of hyperbolic paraboloid shells. Timber Research and Development Association Note No. E/RR/ 5. TRADA, London. Tottenham, H. (1959) 'Analysis of orthotropic cylindrical shells.' Civ. Engrg. Council of Forest Industries of British Columbia (1971) Fir plywood stressed skin panels. COFI, London. Finnish Plywood Development Association (1970) Design data for stressed skin panels in Finnish birch plywood. Technical Bulletin No. 11 (M). FPDA, Welwyn Garden City. Wardle, T. M. and Peek, J. D. (1970) Plywood stressed skin panels: Geometric properties and selected designs. Timber Research and Development Association Report No. E/IB/22. TRADA, High Wycombe.
Folded plates
48 Council of Forest Industries of British Columbia (1969) Fir plywood folded plate design. COFI, London.
Formwork 49 Council of Forest Industries of British Columbia (1967) Fir plywood concrete form manual. COFI, London.
Trussed rafters 70
General design data 71 72
Housing 50 Anderson, L. O. (1970) Wood frame house construction. US Department of Agriculture Handbook No. 73. US Government Printing Office, Washington, DC. 51 Council of Forest Industries of British Columbia (1978) Timber frame construction - a guide to platform frame construction. COFI, London. 52 Council of Forest Industries of British Columbia (1977) Loadbearing timber-framed walls. Construction data. COFI, London. 53 Council of Forest Industries of British Columbia (n.d.) Wind loading calculation for a typical timber-frame house. COFI, London. 54 Canadian Wood Council (1977) Canadian wood construction data files. CWC, Ottawa. 55 Swedish/Finnish Timber Council (1976) Timber stud walls of Swedish redwood and whitewood. SFTC, Retford. 56 Swedish/Finnish Timber Council (1981) Principles of timberframed construction. SFTC, Retford.
British Standards Institute (1985) Code of Practice for trussed rafter roofs. BS 5268: Part 3. BSI, Milton Keynes.
73 74 75 76
77 78 79
Timber Research and Development Association (1967) Design of timber members. TRADA, High Wycombe. Brunskill, R. W. (1985) Timber building in Britain. Gollancz, London. Timber Research and Development Association (1986) Design examples to BS 5268: Part 2, 1984. DAl, TRADA, High Wycombe. British Standards Institution (1984) Specification for finger joints in structural softwood. BS 5291. BSI, Milton Keynes. Curry, W. T. (1955) Laminated beams from two species of timber. Theory of design. Princes Risborough Laboratory Special Report No. 10. HMSO, London. Freas, A. D. and Selbo, M. L. (1954) Fabrication and design of glued laminated wood structural members. US Department of Agriculture Technical Bulletin No. 1069. US Government Printing Office, Washington, DC. Council of Forest Industries of British Columbia (1972) Canadian fir plywood data for designers. COFI, London. Council of Forest Industries of British Columbia (1971) Plywood construction manual. COFI, London. Finnish Plywood Development Association (1964) Finnish birch plywood handbook, FPDA, Welwyn Garden City.
Glues for structural components 80 Princes Risborough Laboratory (1967) The gluing of wooden components. Technical Note No. 4. Building Research Establishment, Garston. 81 Knight, R. A. G. and Newall, K. J. (1971) Requirements and properties of adhesives for wood. Bulletin No. 20, Building Research Establishment, Garston. HMSO, London. 82 Princes Risborough Laboratory (1968) Gluing preservative-treated wood. Technical Note No. 31. Building Research Establishment, Garston.
The repair and restoration of timber structures 83 Charles, F. W. B. and Charles, Mary (1984) Conservation of timber buildings. Hutchinson, London. 84 Feilden, B. M. (1982) Conservation of historic buildings. Butterworth, London. 85 Gifford, E. W. H. and Taylor, P. (1964) 'Restoring old structures'. Struct. Engr, 42, 10, 332-334. 86 Ministry of Public Buildings and Works (1965) Notes on the repair and restoration of historic buildings. HMSO, London. 87 Avent, R. R., Emkin, L. Z., Howard, R. H. and Chapman, C. L. (1970) 'Epoxy-repaired bolted timber connections'. J. Struc. Div. Am. Soc. Civ. Engrs, 102, 821-838. 88 Avent, R. R., Sanders, P. H. and Emkin, L. Z. (1979) 'Structural repair of heavy timber with epoxy'. For. Prod. J. 29, 3, 15-18. 89 Oates, D. W. and Richards, M. (1984) Timber engineering in situ.' Record of British Wood Preserving Association Annual Convention 1984, Paper 8, pp. 76-88. 90 Powys, A. R. (1981) Repair of ancient buildings. Dent and Sons, London (1929). Reprinted by Robert Maclehose, Glasgow.
Termite-resistant construction 91 Harris, W. V. (1971) Termites - their recognition and control. Longman, London. 92 Sperling, R. (1976) Termites and tropical building. Overseas Building Note No. 170. Building Research Establishment, Garston.
Storm-resistant construction 93 Southern Pine Association (1970) How to build storm-resistant structures. The Association, New Orleans.
Earthquake-resistant construction 94 Building Research Establishment (n.d.) Building in earthquake areas. Overseas Building Note No. 143, BRE, Garston. 95 Architectural Institute of Japan (1970) Design essentials in earthquake-resistant structures, Chapter 4, 'Wooden structures'. Elsevier, Amsterdam. 96 Takayame, K., Hisadat and Ohsaki, Y. (1960) 'Behaviour and design of wooden buildings subject to earthquakes'. Proceedings, 2nd World Conference on Earthquake Engineering, Tokyo.
Bibliography General Burgess, H. J. (1971) 'Design aids including computer programmes'. Paper No. WCH/71/5/8 World Work, Consultation Housing, Vancouver. (General appraisal of the development of design aids by Timber Research and Development Association.)
Finnish Plywood Development Association (1972) Design for roof structures in Finply. Technical Publication No. 17. FPDA, Welwyn Garden City. (Includes standard designs for box and I-beams, stressed-skin panels, portal frames and gussetted trusses.) Timber Research and Development Association (1984) The structural use of hardwoods. Wood Information Sheet 01-17. TRADA, High Wycombe. (Span tables for 65-grade Keruing for floor and roof joists, purlins, ply-box beams and two-hinged portals.) United Africa Company (1972) Guide to the use of West African hardwoods. (Load-span charts and tables for beams, joists, purlins, studs and ridged portal frame members.) (Universal span charts for any timber, grade and load duration together with simplified tables.)
Solid timber beams and joists Burgess, H. J. and Masters, M. A. (1976) 'Span charts for solid timber beams'. Timber Research and Development Association Publication No. TBL 34. TRADA, High Wycombe. Burgess, H. J. (1971) Further applications of TRADA span charts. Timber Research and Development Association Publication No. TBL 42. TRADA, High Wycombe. Burgess, H. J. (1984) Span tables for floor joists to BS 5268. Timber Research and Development Association Publication No. DA 3.84. TRADA, High Wycombe. Burgess, H. J. (1985) Joist span tables for domestic floors and roofs to BS 5268. Timber Research and Development Association Publication No. DA 6. TRADA, High Wycombe. Burgess, H. J., Collins, J. E. and Masters, M. A. (1972) Use of the TRADA universal span chart for a range of load cases, Timber Research and Development Association Publication No. TBL 47. TRADA, High Wycombe. Timber Research and Development Association (1984) Span tables for floor joists to BS 5268: Part 2, Processed timber sizes to DA 3. TRADA, London. Timber Research and Development Association (1985) Joist span tables for domestic floors and roofs ~ processed timber sizes to DA 6. TRADA, London. Hearmon, R. F. S. and Rixon, B. E. (1970) Limiting spans for machine stress-graded European redwood and whitewood. Princes Risborough Laboratory, Timberlab 30. HMSO, London. (Span tables.) Council of Forest Industries of British Columbia (1973) Hem-fir. (Load-span tables for floor, roof and ceiling joists for a wide variety of distributed and concentrated loads.) COFI, London.
Glued-laminated timber beams British Woodworking Federation (1967) Span-load tables for gluedlaminated softwood beams. BWF, London.
Plywood box and web beams Council of Forest Industries of British Columbia (1968) Computer analysis of plywood web beams. (A Fortran IV program for the analysis of both symmetrical and unsymmetrical beams.) COFI, London. Council of Forest Industries of British Columbia (1968) Fir plywood web-beam selection manual. (Tabulates the properties of 4000 standard glued beams.) COFI, London. Council of Forest Industries of British Columbia (1971) Nailed fir plywood web-beams. (Load span-deflection tables for twenty-four standard beams.) COFI, London. Timber Research and Development Association (1984) Load tables for nailed plv box beams to BS 5268: Part 2. TRADA Publication No. DA 4, TRADA, High Wycombe. Timber Research and Development Association (1984) Load tables for glued ply box beams to BS 5268: Part 2. TRADA Publication No. DA 5. TRADA, High Wycombe.
Portal frames Burgess, H. J. et at. (1970) Span tables for ridged portals in solid timber. Timber Research and Development Association Publication No. E/IB/17. (Selection tables for portal member sizes for five different timbers.) Council of Forest Industries of British Columbia (1972) Portal frame manual. (Design and selection manual.) COFI, London.
Stressed-skin panels Finnish Plywood Development Association (1970) Design data for stressed skin panels. Technical Publication No. 11 (M). FPDA, Welwyn Garden City. Wardle, T. M. and Peek, J. D. (1970) Plywood stressed skin panels. Timber Research and Development Association Publication No. E/IB/22. (Geometric properties and selected designs.) TRADA, High Wycombe.
Abbreviations and useful addresses AITC BRE BSI
American Institute of Timber Construction, 1100, 17th Street NW, Washington DC 20036, USA. Building Research Establishment, Garston, Watford, Hertfordshire. British Standards Institution, 2 Park Street, London, WlA 2BS.
BWF
British Woodworking Federation, 82 New Cavendish Street, London, W l M 8AD. BWPA British Wood Preserving Association, 62 Oxford Street, London, WIN 9WD. CITC Canadian Institute of Timber Construction, 100 Bronfon Avenue, Ontario, Canada. CPA Chipboard Promotion Association, 50 Station Road, Marlow, Bucks, SL7 INN COFI Council of Forest Industries of British Columbia, Tileman House, 131-133 Upper Richmond Road, Putney, London, SW15 2TR. CWC Canadian Wood Council, 701-710 Laurier Avenue West, Ottawa, Canada K l P SV5 FIDOR Fibreboard Development Association, 1 Hanworth Road, Feltham, Middlesex, TW13 5AF. FINPLY Finnish Plywood Development Association, P.O. Box 99, Welwyn Garden City, Herts, A16 OHJ. PRL Princes Risborough Laboratory (Timberlab), Building Research Establishment, Aylesbury, Bucks, and at Building Research Station, Garston, Watford, Herts. S/FTC Swedish/Finnish Timber Council, 21 Carolgate, Retford, Notts, DN22 6BZ. TRADA Timber Research and Development Association, Stocking Lane, Hughenden Valley, High Wycombe, Bucks. UAC United Africa Company (Timber) Ltd., United Africa House, Blackfriars Road, London, SEl.
