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16 Timber Design

F H Potter BSc Tech, CEng, MICE, FIWSc,AMCTSenior Tutor, Imperial College of Scienceand Technology

Contents

16.1 Introduction 16/3

16.2 Design by specification 16/316.2.1 Species and use 16/316.2.2 Availability and sectional properties 16/416.2.3 The movement of timber 16/416.2.4 Moisture content and end use 16/416.2.5 Working properties 16/516.2.6 Natural resistance to attack 16/516.2.7 Preservation treatment 16/616.2.8 Fire resistance 16/6

16.3 Stress grading and permissible stresses 16/616.3.1 Visual stress grading 16/616.3.2 Mechanical stress grading 16/616.3.3 Glued-laminated timber grades 16/616.3.4 Strength classes of timber 16/616.3.5 Permissible stresses 16/7

16.4 Design – general 16/7

16.5 Design in solid timber 16/7

16.6 Glued-laminated timber assemblies 16/7

16.7 Plywood and tempered hardboard assemblies 16/8

16.8 Timber fastenings 16/8

16.9 Timber-framed construction 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

16.1 Introduction

Timber is one of the finest structural materials: it has a highspecific strength, can be easily worked and jointed and does notinhibit design. Like most other structural materials it suffersattack causing deterioration (corrosion, weathering and bio-deterioration) but once the material is known and the causesunderstood, effective preventative measures can be taken easilyand economically.

Design is thus a confluence of specification, structural analy-sis, detailing and protection, each of which is of equal import-ance if an effective design is to be achieved.

Nowadays, structural design covers a wider range of compo-nents than ever before, for the intense wind loadings in high-risebuilding coupled with large glazed areas has meant that muchwindow joinery is now subject to structural design. In addition,the effects of wind loadings together with the requirements ofthe Building Regulations and the newer building shapes hasmeant that even in low-rise buildings, components which oncewere 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 solidbeams, joists and purlins or can be used in the more recent formsof glued-laminated construction or plywood panel construc-tion. These latter forms allow the design of exciting andeconomic structural shapes, the variety of which may be judgedfrom Tables 16.1 and 16.2.

Table 16.1 Roof selection

Some of the characteristics of timber may be found in Table16.3, whilst general properties are given in other publications.1"3

16.2 Design by specif ication

Essentially, this is a prescription of fitness for use under serviceconditions and requires the choice not only of an appropriatematerial but also of its condition, use and protection.

The success of the specification will depend upon its interpre-tation; standard glossaries are available for timber and wood-work,4 nomenclature of timber5 and preservative treatment.6

Functional and user needs will dictate the choice of materialbased on the following factors.

16.2.1 Species and use

Very many timbers are structurally useful, whereas usefulnessfor joinery purposes is often more restrictive. Where timber isused for structural joinery, the combination of requirementsmay be even more restrictive.

Table 16.3 lists most of the timbers and their characteristicsfor which working stresses are given in BS 5268: Part 2,7 whilstBS 11868 indicates the joinery use of specific species. A compre-hensive guide to West African species and their uses, bothstructural and joinery, is given in pamphlets issued by theUnited Africa Company.9

Flooring, particularly industrial flooring, has particular re-quirements and recommendations for suitable timbers for these

Construction

Solid timberLaminated, either

vertically orhorizontally,depending on size

I or box sections:flanges solid orlaminated. Websplywood ordiagonally boarded

Laminatedhorizontally

I or box sections:flanges laminatedhorizontally. Websdiagonally boarded

Laminatedhorizontally

I or box sections:flanges solid orlaminated. Websplywood ordiagonally boarded

Solid timber

Solid timber

Laminated chords.Solid webs

Minimum supportconditions

Vertical support at ends

Vertical and horizontalsupport at ends

Vertical and horizontalsupport at ends

Vertical support at ends

Maximumeconomicspans(m)

624

30

4646

2446

1224123046

Fastenings

NoneGlue

Glue and/or nails

Glue and/or nails forlaminating

Connectors for sitejoints

Glue and/or nails forlaminating

Connectors for sitejoints

Nails and/or glueConnectorsNails and/or glueConnectorsGlue and/or nails for

laminatingConnectors at joints

Division Subdivision

Beams

Arches

Portals

Trusses Belgian

Warren

Bowstring

(After: L. G. Booth, Engineering, 25 March 1960)

(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 theavailability of the structural timbers from the viewpoints ofsupply and length. The geometric properties of sawn andprecision timber to be used in design are also given in BS 5268:Part 2. Guidance on the usefulness of worldwide species may befound,11 whilst the available sizes for hardwoods are given in BS5450.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 fluctuat-ing dimensional changes in the timber, known as 'movement'.The variation can be designed-for quite simply but some know-ledge of the degree of possible movement is helpful. Someindication can be obtained from Table 16.3, whilst furtherinformation can be obtained from the publications of thePRL.231314

16.2.4 Moisture content and end use

Every species of timber will achieve a fairly steady moisturecontent for a particular environment - the equilibrium moisturecontent. The PRL has established moisture contents for variousenvironments.12 Greater reliability can be achieved by dryingtimbers to these moisture contents before construction.

Construction

Membrane formedfrom plywood orlayers of diagonalboarding

A single-skinstructure may havestiffening ribs

A double-skinstructure will havespacing ribs

Edge beams and enddiaphragmsrequired

Membrane formedwith layers ofdiagonal boarding.May havestiffening ribs

Edge beams requiredEnd diaphragms

requiredBoarded membrane

with or withoutlaminated ribs

Laminated ring beamBoarded membrane

with laminatededge beams

Boarded membranewith laminated tiedarches along edges

Boarded membranewith laminated tiedarches on ends

Edge beams required

Minimum supportconditions

Vertical supportat corners

Vertical supportat corners

Vertical supportat corners

Ring beam to besupported atintervals

Vertical supportonly at lowpoints, ifcolumns tiedtogether.

Otherwisebuttresses atlow points

Vertical supportat corners

Vertical supportat corners

Maximumeconomicsizes(m)

12x12

18x9

30x12

30 dia.

21x21

24x24

30x9

Fastenings

Nails and/or glue

Membrane with nailsand/or glue

Diaphragms with nailsor connectors

Membrane with nailsand/or glue

Beams (see Table 16.1)Diaphragms with nails

or connectors

Membrane with nailsand/or glue

Ribs and ring beamglued

Membrane with nailsand/or glue

Edge beams glued

Membrane with nailsand/or glue

Tied arches with glueand connectors

membrane with nailsand/or glue

Beams (see Table 16.1)Tied arches with glue

and connectors

Division

Plates

Singlycurvedshells

Doublycurvedshells

Subdivision

Flat

Folded

Circular cylindrical

Spherical dome

Hyperbolic paraboloid

Elliptical paraboloid

Conoid

Table 16.2 Roof selection

16.2.5 Working properties

Ease of fabrication is indicated in Table 16.3, although moredetailed information may be found in PRL publications.2-3

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 normallyattack timber having a moisture content lower than 20% but atimber's ability to resist fungal attack is classified according toTable 16.4.

The natural durability of some structural timbers is given inTable 16.3. Information on further timbers will be found in PRL

Standard name

SOFTWOODS(IMPORTED)Douglas fir-larchHem-firParana pinePitch pineE. redwoodE. whitewoodCanadian

spruce-pine-firW. red cedar

SOFTWOODS (HOMEGROWN)Douglas firLarch (E- Japan)Scots pineEuropean spruceSitka spruceCorsican pine

HARDWOODS(IMPORTED)AburaAfrican mahoganyAfrormosiaGreenheartGurjun/KeruingIrokoJarrahKarriOpepeRed merantiSapeleTeak

HARDWOODS (HOMEGROWN)European ashEuropean beechEuropean oak

Approx.density atM/C 18%

(kg/m3)

590530560720540510450

380

560560540380400510

590590720

1060720690910930780540690720

720720720

Naturaldurability

ModeratelyNotNotDurableNotNotNot

Durable

ModeratelyModeratelyNotNotNotNot

PerishableModeratelyVeryVeryModeratelyVeryVeryDurableVeryModeratelyModeratelyVery

PerishablePerishableDurable

Resistance topreservativetreatment

ResistantResistantModeratelyResistantModeratelyResistantVery

Resistant

ResistantResistantModeratelyResistantResistantModerately

ModeratelyExtremelyExtremelyExtremelyResistantExtremelyExtremelyImpermeableModeratelyResistantResistantExtremely

ModeratelyPermeableExtremely

Moisturemovement

SmallMediumMediumMediumMediumSmallMedium

Small

SmallMediumMediumSmallSmallSmall

SmallSmallSmallMediumLargeSmallMediumLargeSmallSmallMediumSmall

MediumLargeMedium

Workingquality

GoodGoodGoodGoodGoodGoodGood

Good

GoodGoodGoodGoodGoodGood

GoodMediumMediumDifficultMediumMediumDifficultDifficultMediumGoodGoodMedium

GoodGoodMedium

Supply

GoodGoodGoodGoodGoodGoodGood

Good

FairGoodGoodGoodGoodFair

GoodGoodGoodGoodGoodGoodGoodGoodGoodGoodGoodGood

FairGoodGood

Availability

Normallength(m)

4.20-4.804.20-4.503.60-3.904.50-9.001.50-7.001.50-7.002.40-5.10

2.40-7.30

1.80-4.501.80-3.601.80-3.601.80-3.601.80-3.601.80-3.60

.80-6.00

.80-7.302.40-7.304.80-9.00.80-7.30.80-6.00.80-8.40.80 up.80-6.00.80-7.30.80 up.80 up

1.80 up1.80 up1.80 up

Relative price

MediumLowLowMediumLowLowLow

Low

LowMediumLowLowMediumMedium

LowMediumMed highHighLowMediumMed highMed highMediumLowMediumHigh

LowMediumMedium

Table 16.3 Characteristics and availability of some structural timbers

Table 16.4 Durability classification of the heartwood of untreatedtimbers

Grade of durability

Very durableDurableModerately durableNondurablePerishable

Approximate life in groundcontact(y)

More than 2515-2510-155-10

Less than 10

Technical Note No. 4015 and the Handbooks on softwood andhardwoods2'3 whilst further advice on the control of decay willbe 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 generallyresistant: iroko, opepe, Californian redwood and teak. Othertimbers are given in the Handbooks on softwood and hard-woods.2'3

Marine borers are a hazard in the sea or brackish waters andPRL Leaflet No. 4620 gives advice on the protection of timbersagainst this attack. Highly resistant timbers suitable for marineworks 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 andinsects but it is often possible to obtain a more attack-resistantstructure by pressure-impregnating nondurable or perishabletimbers than by using durable species. Indeed, it is sometimesmore economic also.

The amenability of timbers to preservative treatment is givenin Table 16.3 and is related to the following classification:

Permeable: Easily treated by either pressure oropen tank.

Moderately resistant: Fairly easy to treat by pressure,penetration 6 to 20 mm in 2 to 3 h.

Resistant: Difficult to impregnate, incisingoften used. Penetration often littlemore than 6 mm.

Extremely resistant: Very little penetration can beachieved even after prolongedtreatment.

Further information and guidance on satisfactory types andmethods of treatment may be found in publications of theBritish Standards Institute (BSI)2'-23 and the Timber Researchand Development Association (TRADA). The economics oftimber preservation is discussed in Timberlab 77.24

16.2.8 Fire resistance

Although timber ignites spontaneously at about 25O0C, ignitionis a function of the external surface area to the total volume oftimber and the rate of charring does not significantly increasewith a rise of temperature. The rate of charring is generallytaken as about 0.5 mm/min (western red cedar 0.85 mm/min,dense hardwood 0.42 mm/min), but perhaps the most importantfactor is that the structural properties of uncharred materialremain 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 sacrificialmaterial; (2) chemical impregnation; and (3) protective cover-ing.2"7

16.3 Stress grading and permissiblestresses

Timber is a natural organic material and therefore is subject towide variability because of environmental, species and geneticeffects. This variability affects both visible quality and strength.

If, for any particular property and species only one designstress were specified, this would have to be set so low (to allowfor variability) that the material would have a very limited

structural application. In consequence, a number of stressgrades have been adopted, leading not only to a more economicuse of the material but also to a higher yield of structurallyuseful material.

There are two main methods of stress grading for solidtimber: (1) visual grading; and (2) mechanical grading. Eachrequires a different procedure.

16.3.1 Visual stress grading

In visual grading, data obtained from clear material (straightgrained and free from knots and fissures) are analysed statisti-cally for each species and basic stresses for each property aredevised. These basic stresses are then reduced by factors whichtake account of the strength-reducing effects of the permissiblegrowth characteristics for each stress grade.

At the present time, there are two sets of quality requirementsfor visual stress grading in this country, one for softwoods andone for hardwoods.

The first set is given in BS 4978.28 Besides setting the require-ments for two grades for solid softwood timber construction (SSand GS), this standard restates the requirements for laminatingtimber grades.

The second set is given in BS 575629 for tropical hardwoods(HS grade). In the current edition of BS 5268:Part 2 home-grown hardwoods have been deleted, but it is probable thatthese will be reintroduced.

16.3.2 Mechanical stress grading

Mechanical stress grading30 is a method of non-destructivetesting each piece to be graded. The piece is bent under aconstant central load over a constant short span. The strengthof the material can then be calculated accurately from theresultant deflection. Four grades are presented (M75, M50,MSS and MGS) and the grade stresses for the dry conditiononly are tabulated in BS 5268:Part 2. At present, machine-gradestresses are limited to six softwood species for which controlinformation is available. However, it will be possible tomachine-stress-grade other timbers in accordance with BS4978.28

16.3.3 Glued-laminated timber grades

In glued-laminated members, the presence of strength-reducingcharacteristics will have a smaller effect than in solid timber,since the probability of identical structural defects appearing inidentical positions in adjacent laminations is very small. BritishStandard 5268:Part 2, therefore allows higher grade stresses forglued-laminated timbers, these being obtained by applyingtabulated modification factors to the grade stresses for eachspecies.

16.3.4 Strength classes of timber

For the first time, BS 5268:Part 2 introduces the concept ofstrength classes for timber. Softwood species-grade combina-tions for strength classes, graded to BS 4978 are tabulated inTables 3, 4 and 5 of that standard, whilst species groupings ofhardwoods graded to BS 5756 are tabulated in Table 7 of thatstandard for the higher strength classes.

The concept, similar to the older species groupings, is that astrength class rather than a species may be specified. However,sometimes there are advantages in specifying a particular speciesand grade where the grade stress is higher than the strength classstress.

16.3.5 Permissible stresses

Permissible design stresses for both solid and laminated timbercomponents are governed by the type of component, the condi-tions of service and the type of loading. They are obtained fromgrade stresses by applying the appropriate modification factors.

16.4 Design-general

Design in timber is similar to that in any other structuralmaterial 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. Moststrength properties, of both timber and joint fasteners, varyaccording to these directions and the variation has been foundto follow the Hankinson relationship:

,v- PQP sin2 0 + Q cos2 0

where 6 is the angle between directions of load and grain, N thestress at 0 to the grain, P the stress parallel to the grain, and Qthe stress perpendicular to the grain

from which intermediate stress or strength values can be calcu-lated. This is not normally required for solid beams, joists andcolumns where only the major directions are used, but is oftenmet where members intersect at joints. The stresses given in BS5268:Part 2 are for permanent loading and increased values areallowed for short- and medium-term loads. This Code ofPractice governs general design, but additional information isavailable.3141

In the past, design has been inhibited by the relatively shortlengths of timber available (Table 16.3 indicates availability).However, the production of durable resin adhesives has led tonew construction techniques and structural forms being de-veloped. Glued-laminated timber in which thin laminae areglued together to form structural components of almost anyshape or length is a common reality, whilst structural plywoodcan be combined with either solid or glued-laminated timber toproduce composite components which are lightweight, reliableand pleasing. The design in these forms is more complex than insolid timber but information on a wide variety of structuralforms can be found,42"70 whilst advice on the selection of aparticular form is given in Tables 16.1 and 16.2. General designadvice is provided by TRADA.71

16.5 Design in solid timber

Since permissible stresses are maxima there may be someadvantage in using structural hardwoods or the higher-grade-stress softwoods whenever stress governs design. However, ifdeflection governs, there will be no advantage in using thesemore expensive materials unless the moduli of elasticity aresufficiently high. A possible exception is keruing (dipterocarpusspp.) 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 oftimber available, this has led to the development of many typesof girder framework: however, where there is sufficient head-room, trussed beams can give an economic solution for heavyloads 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 2are based on those minimum sizes.

Further reductions in section should be made for notches,mortices and bolt, screw and connector holes. Modificationfactors may also be required for the length and position ofbearing, the shape of a beam and its depth if greater than300 mm, whilst for compression members, combined factors aregiven for both slenderness and loading.

Lateral stability is important both for deep beams and forcompression members, and in built-up members web stiffenersshould be provided wherever concentrated loads occur.

General design data are available7173 applicable to particularstructural forms45-5^61 -6^67 whilst design aids have been publishedfor solid beams, portal frames and trussed rafters.

16.6 Glued-laminated timberassemblies

Glued-laminated timber is essentially a built-up section of twoor more pieces of timber whose grains are approximatelyparallel and which are fastened together with glue throughouttheir length. This enables the properties of timber to be regu-lated to some degree and provides structural sizes and shapeswhich would not be possible in solid timber. Variation in sectionis possible, whilst high-grade material can be placed in zones ofhigh stress and low-grade material in zones of low stress. Allsoftwoods glue well and are generally preferred in the UK,although occasionally there can be some advantage in usingwholly hardwood laminae.

Construction may use either vertical or horizontal lamina-tions.

With vertical laminations, the zones of equal stress are sharedbetween the laminations so that the strength of a beam can besaid to be the sum of the individual laminations. This load-sharing concept has led to the grade-stress modification factorstabled in BS 5268:Part 2 which give higher permissible stressesfor the laminated beam.

Horizontally laminated beams have been permitted since1967 but the philosophy for behaviour is entirely different fromthat for vertical laminations. A beam will consist of materialcontaining knots whose presence will affect the strength ratio ofthe beam. Since the knot effect will vary according to the sizes ofknots and the number of laminations, BS 5268:Part 2 tablesbasic stress modification factors according to these variables.

Since curved laminated beams are fabricated by bending theindividual laminations, fabrication stresses are induced whichdepend upon the degree of curvature, the thickness of thelamination and the species of timber. Therefore, modificationfactors to be applied to the grade bending stresses for differentvalues of t/R are specified in BS 5268:Part 2.

The production of long laminations depends upon the use ofefficient methods of end jointing. Where the efficiency of an endjoint is known, the laminations containing them can be includedwhen calculating the section properties, but where efficienciesare not known, the laminations containing the end joints mustbe omitted when calculating section properties. Efficiency rat-ings for plain scarf joints and for finger joints are given in BS5268:Part 2: these are used to modify the basic stresses to givethe maximum stresses to which any particular lamination maybe subjected. British Standard 529174 governs finger joints instructural softwood. Butt joints do not transmit load and shouldonly 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 designaids are noted for glulam beams.

16.7 Plywood and tempered hardboardassemblies

Plywood is a type of glued-laminated construction in which thelaminae are formed from thin flat veneers of timber. Theseveneers are produced by the rotary cutting of logs and are laidalternately at right angles in an odd number of layers. Sinceboth the shrinkage and strength of timber differ according to thegrain direction, the type of construction gives greater dimen-sional stability and tends to equalize the strength properties inboth major directions of the plywood sheet.

There are two distinct design philosophies: (1) the NorthAmerican approach which only considers the 'parallel plies', i.e.those plies whose grain lies in the direction of the load (thisapproach is based on the basic stresses and moduli for solidtimber); and (2) the Finnish and British approach, known as the'full cross-section' approach, in which grade stresses and modulifor the sheet materials have been determined from tests. In BS5268:Part 2 all the grade stresses and moduli are for full cross-section, but it is well to remember that many North Americandesign manuals will be based on the 'parallel-plies' approach.Plywood is a strong, durable and lightweight structural materialwhich 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 andportal frames.

Perhaps plywood's most useful property is that of providingexcellent shear resistance for a small cross-section, although it iswell to remember that lateral stability constraints may berequired.

Tempered hardboard is a durable compressed fibreboard forwhich BS 5268:Part 2 now gives grade stresses for use instructural components instead of plywood.

16.8 Timber fastenings

Available methods of jointing are perhaps the most importantcriteria for the design of structural components. This is particu-

Table 16.5 The relative strengths of timber joints

larly true in timber for which highly efficient methods oftransferring tensile loads have been developed only during thepast 50 yr. Split-ring and tooth-plate connectors are now avail-able which have load capacities much greater than those fornailed and bolted joints. A comparative indicator of fastenerefficiency and the required member sizes is given in Table 16.5.

The strength of mechanical fasteners depends upon membersize and thickness and the spacing of the fasteners. BritishStandard 5268:Part 2 tabulates permissible values of a widerange of variables, whereas some manuals prefer a presentationas a series of design curves.31 >34-3540

However, the major advance in fastening techniques has beenin glued joints. Early glues were unreliable, deterioratingquickly, but the present phenolic and resorcinol resins are sodurable that the risk of delamination has been almost entirelyeliminated, even under extreme exposure. Unfortunately, gluingstill requires controlled conditions and its application to sitework is still limited.

Since the shear strength of adhesives is usually higher thanthat of timber, a fastener efficiency of 100% can be achieved.Nevertheless, it is important to remember that glues seldomhave a good tensile strength, so that they should be stressed inshear as much as possible.

Information is available on gluing,80 the requirements foradhesives,81 and the compatibility between glues and preserva-tives.82 The permissible stresses for glued joints are the shearstresses for the timber;7 however, regard must be paid not onlyto the variation of that shear strength but also to the possibilityof differential shrinkage and stress concentrations in the joint.

The type of fastener chosen will depend upon the skills andequipment available, possible fabrication conditions, relativecosts and whether or not it is necessary to take down andreassemble the structural components.

Assumptions: (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/mm2. Using Table 9 value, required timber area = 5479 mm2

Comparison - axial compression in GS/M50 (SC3) European redwood

Type & dia,(mm)

NAILS3.758.00

SCREWS8.43

BOLTSM8M12

TOOTH PLATE2/5 1 mm dia.+ M12bolt2/64 mm dia.+ M12bolt

SPLIT RING2/64 mm dia.+ M12bolt

FASTENER

No. Capacity(kN)

63 40.321 42.7

16 42.2

30 41.313 40.3

5 40.5

5 43.5

3 49.7

Size(mm)

47 x 14572x145

60 x 145

44x16960 x 145

60x145

60x169

60 x 145 or72 x 120

TIMBER

Area(mm2)

55818712

6677

56766540

6540

7980

68006740

EffectiveCapacity(kN)

40.763.6

48.7

41.447.7

47.7

58.3

49.649.2

EXAMPLES OF THE DESIGN OF A SIMPLE TENSION SPLICEJOINT

LOAD CAPACITY: 25 kN DURATION: MEDIUMTERM

TIMBER. EUROPEAN REDWOOD GRADE: M50

EXPOSURE CONDITION: DRY

(1) REQ UIRED TIMBER SECTION

Dry exposure condition grade stresses, Tension //g

SC3 (TABLE 8): 3.2 N/mm2 x 1.25 = 4.0 N/mm2

European redwood (TABLE 9): 4.0 N/mm2* x 1.25 = 5.0 N/mm2

Maximum permissible timber stress = 5.0 N/mm2

Section = ?^P = 5000 mm2

allow 10% reduction in effective section at joint

SAY 41 x 145 mm planed = 5950 mm2 (TABLE 99)

(2) NAILED JOINT

CHOICE OF NAIL DIAMETER

Possible joint thickness = 3 x 41 mm = 123 mm

Maximum available stock lengths:

4 and 4.5 mm 0: 100 mm 5 mm <j>: 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 <f>: 0.7 x 57 = 40 mm (splice 41 x 145)

CLAUSE 41.4.2 TABLE 57

Required nail lengths:

4 m m <f> : 2 x 35 + 41 = 111 mm

4.5 mm <f> : 2 x 35 + 41 = 111 mm

lengths not available

5 mm <j> : 2 x 41 + 41 + = 123 mm*

DESIGN OF JOINT (5 mm nails)

Basic single shear lateral load capacity, dry exposure:

5 mm, SC3: 635 N (TABLE 57)

Multiple shear factor (CLAUSE 41.42)

0.9 x number of shear planes provided each member is thicker than0.7 x standard point size penetration

Permissible joint load (CLAUSE 41.8)

= basic x Jf48 x AT49 x AT50

duration of load moisture content number of nails(medium term: 1.12) (dry: 1.0) (< 10 'line': 1.0)

= 635 x (2 x 0.9) x 1.12 x 1.0 x 1.0 - (assumed)

= 1280.16 N.

Number of 5 mm nails required = -r-=? = 20

TABLE 56 SPACING, modified by 0.8 (CLAUSE 41.3)

TRY 4 x 5 pattern (20 nails)

25 x 40 x 40 x 40 x 25 undrilled width ^ 170 mmX X X Xx x x x joint length 4 x 40 + 2 x 40 = 240 mmX X X 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)

= 5125 mm2

Effective timber load capacity = 25.625 kN

Therefore the nailing pattern is acceptable, but requires predrill-ing

Alternatively, try 3 x 7 pattern (21 nails, but easier to control), toavoid cost of pre-drilling

x x x25 x 40 x 40 x 25 undrilled width ^ 130 mm*

X X Xx x x joint length 5 x 40 + 2 x 40 = 320 mmX X XX X XX 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 timber capacityeffective area x permissible stress

M10 bolt, n = 2.3 lines

For any larger diameter bolts, only one line of bolts would bepossible.

DESIGNOFJOINT

Basic single shear lateral load parallel to grain, (TABLE 67),member 41 mm thick.

MlO, 1.28 kN: M12, 1.84 kN: M16, 3.15 kN (interpolated)

Double shear factor (CLAUSE 43.4.2) 2.0

Permissible joint load (double shear)

= 2 basic x K55 x K56 x K51

medium terrfi= 1.25 N. number'in line'

m/cdry = 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)

. . , r, .. original numberrevised number of bolts = —-—p

A57

MlO, 8.6: M12, 6.4: M16, 3.5:

Effective timber capacity = effective area x permissible stress

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 inthe housing field.

In high-rise construction, timber will play a supplementaryrole to the heavy material, being used for partitions, infill panelsand floor and roofing systems. In this role, timber's readyadaptability to prefabrication is a great benefit.

In low-rise construction, on the other hand, timber is increas-ingly being used to provide the structural skeleton for thebuilding; indeed, at the present time, timber-frame constructionconstitutes some 20% of all house construction. The method ofconstruction is a simplification and refinement of that which hasbeen used successfully for many centuries, but which is equallywell applicable to many other uses besides housing.

The structural form is that of a free-standing skeleton forwhich standard details have been produced.50"52'56-57-59 The basisof the skeleton is the stud-framed panel for which designs aredescribed.53-55-60

Of especial importance is the structure's ability to withstand

lateral loading and, particularly, that resistance to planar defor-mation of a wall panel known as its racking resistance. Figure16.1 shows the deformation of an idealized frame structureunder lateral loading. Figure 16.1 (a) shows the uniform transla-tion of the walls which occurs when there is complete symmetryof both structure and loading. This hardly ever occurs and thislack of symmetry causes a rotation in addition to the translation(Figure 16.2(b)). The calculation of racking resistance is des-cribed by TRADA58 and will also be dealt with by BS 5268:Part6 (currently being written).

16.10 Repair and restoration

An increasing amount of work is being carried out on the repairand restoration of old timber-framed buildings. The work isspecialized and requires not only sound structural assessment ofthe existing building but also a good understanding of themethods used in its construction and of acceptable methods ofrepair and restoration.

The methods of repair may be either by replacement of thedamaged joints or members in the traditional manner or by theuse of resin or stainless steel rods and resin fillers.

Brunskill72 describes the traditional methods of timber-frameconstruction whilst Charles and Charles83 give restoration casestudies. Conservation and restoration are reported on by Fiel-den,84 Gifford,85 Ministry of Public Buildings and Works,86 andrepair 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 intropical and sub-tropical areas. The subterranean termite (whiteant or wet-wood termite) has the larger distribution, builds hugenests, farms fungi and needs to maintain contact with the dampearth. Attack is from the ground.

There are many thousands of species of subterranean termitesand a species of timber providing good termite resistance in onearea may not be resistant in another. Resistive constructiondepends upon correct detailing, ground poisoning and preserva-tive treatment.

The dry-wood termite, on the other hand, is less widelydistributed, has fewer species, and flies, mates and deposits itseggs in timber to continue the life cycle.

Fly screens and preservative treatment give the best protec-tion for new structures, whilst fumigation may be required wheninfestation is found in existing buildings. The recognition,control and detailing required are given by Harris91 and Sper-ling.92

16.12 Storm-resistant construction

Wind loading is one of the commonest and most variable typesof loading that can occur and ranges from normal wind loadingto tornadoes.

Tropical storms have wind velocities in the range 55 to117km/h and damage is usually caused by flooding, includingthat produced by waterborne detritus.

Hurricanes are more severe and damage is caused by windaction, flooding and flying debris. Air circulation is counter-clockwise with a damage area having a dimeter of 50 to 160 km.Wind speeds are commonly 120 to 190 km/h with gusting up to320km/h. Rainfall, normally 125 to 250mm, occasionallyreaches 750 mm.

Tornadoes are unquestionably the most devastating of windstorms. The vortex is much smaller than that of a hurricane,

Figure 16.1 Deformation of idealized house structure underlateral loading, (a) Uniform translation of top wall parallel to thelateral load; (b) rotation after lateral translation, caused by lack ofsymmetry

End-walldiaphragm

Roof diaphragm

Side-wallelement

Lateral load

Only end walls racked

Side-wall diaphragmDirection of rotation

Centre ofrotation

Majority of walls are racked:some are twisted and racked

causing intense damane from wind action and large pieces offlying debris.

Generally, storm-resistant structures require strength linkedto rigidity with interconnected members and components se-curely fastened together. The Southern Pine Association givesvaluable advice.93

16.13 Earthquake-resistantconstruction

Horizontal and vertical movement of the Earth's surface,caused by earthquakes, results in forces being generated by theinertia of the mass of the structure. The magnitude of theseinertial forces varies directly as the mass of the structure and inconsequence heavy structures are more severely loaded than arelightweight ones. Indeed, for timber structures, the forces maybe little higher than those produced by normal wind loading.However, unlike storm-resistant structures, those for earth-quake-resistant construction require strength linked to flexibi-lity. Several publications give outlines of sound practice.94"96

16.14 Design aids

There are three areas in which design aids can make a valuablecontribution to the design process. These are: (1) rapid pre-liminary design either for comparison or estimation of cost;(2) routine elementary design; and (3) complex design processesfor which the design time can be reduced drastically.

Nomograms and load-span tables have been used for manyyears, but the application of computer programming hasextended considerably the use of design aids.

The bibliography to this chapter indicates some of the designaids which are now available for structural design in timber.

References

General properties of timber

1 Dinwoodie, J. M. (1981) Timber, its nature and behaviour. VonNostrand 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 totimber and woodwork, BS 565. BSI, Milton Keynes.

5 British Standards Institution (1974) Nomenclature of commercialtimbers, BS 881 and 589. BSI, Milton Keynes.

6 British Standards Institution (1968) Glossary of terms relating totimber preservatives, BS 4261. BSI, Milton Keynes.

7 British Standards Institution (1984) Code of practice forpermissible 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 1and 2. UAC, London.

10 Princes Risborough Laboratory (1971) Hardwoods for industrialflooring, Technical Note No. 49, PRL, Building ResearchEstablishment, 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 andmethods 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 innew buildings. How to minimize dimensional changes. TechnicalNote No. 12. Building Research Establishment, Garston.

Natural durability and the protection of timber

15 Princes Risborough Laboratory (1975) The natural durabilityclassification of timber. Technical Note No. 40. BuildingResearch Establishment, Garston.

16 Princes Risborough Laboratory (1968) Ensuring good service lifefor window joinery. Technical Note No. 29. Building ResearchEstablishment, 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 itscontrol. Technical Note No. 57. Building ResearchEstablishment, Garston.

19 Princes Risborough Laboratory (1965) Termites and theprotection of timber. Leaflet No. 38. Building ResearchEstablishment, Garston.

20 Princes Risborough Laboratory (1950) Marine borers andmethods 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 andapplication of wood preservatives. BS 1282. BSI, Milton Keynes.

22 British Standards Institution (1977) Preservative treatments forconstruction timbers. BS 5268; Part 5. BSI, Milton Keynes.

23 British Standards Institution (1978) Code of practice forpreservation of timbers. Section 7 timber for use in prefabricatedbuilding in termite-infested areas. BS 5589. BSI, Milton Keynes.

24 Tack, C. H. (1969) 'The economics of timber preservation.Timberlab 17, Princes Risborough Laboratory, BuildingResearch Establishment, Garston.

Fire resistance

25 British Standards Institution (1978) Fire resistance of timberstructures. BS 5268: Part 4. BSI, Milton Keynes.

26 Fire Research Station (1970) Fire and the structural use of timberin buildings. BSI, Milton Keynes.

27 Wardle, T. M. (1966) Notes on the fire resistance of heavyconstruction. New Zealand Forestry Service Information Series53.

Stress grading

28 British Standards Institution (1978) Timber grades for structuraluse. BS 4978. BSI, Milton Keynes.

29 British Standards Institution (1980) Specification for tropicalhardwoods graded for structural use. BS 5756. BSI, MiltonKeynes.

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 timberconstruction manual. Wiley, New York.

32 Baird, J. A. and Ozelton, E. C. (1986) Timber designer's manual.Granada, London.

33 Breyer, D. E. and Ank, J. A. (1980) Design of wood structures.McGraw-Hill, New York.

34 Hansen, H. J. (1962) Modern timber design. Wiley, New York.35 Karlsen, G. G. (1967) Wooden structures. Mir, Moscow.36 Laminated Timber Institute of Canada (1980) Timber design

manual (metric edn). LTIC, Ottawa.37 Leicester et al. (1974) Fundamentals of timber engineering, Parts 1

and 2: 24 lectures given by officers of the Division of BuildingResearch, CSIRO, Victoria, Australia.

38 Mettem, C. J. (1986) Structural timber design and technology.Longmans.

39 Oberg, F. R. (1963) Heavy timber construction. The TechnicalPress, London.

40 Pearson, R. G., Kloot, N. H. and Boyd, J. D. (1967) Timberengineering design handbook. Jacaranda, Melbourne.

41 US Department of Agriculture (1974) Wood handbook.Handbook No. 72, US Printer's Office, Washington, DC.

Arches and portal frames

42 Burgess, H. J. (1970) Exploiting geometrical symmetry in timberstructures. Timber Research and Development Association, HighWycombe.

43 Council of Forest Industries of British Columbia (1972) Portalframe manual. COFI, London.

44 Kharna, J. and Hooley, R. F., (1965) Design of fir plywood panelarches. Council of Forest Industries of British Columbia ReportNo. TDD^t3. COFI, London.

45 Timber Research and Development Association (1969) Ridgedportals in solid timber. TRADA E/IB/18, London.

46 Wilson, T. R. C. (1939) The glued laminated wood arch. UnitedStates Department of Agriculture Technical Bulletin No. 691.

Barrel vaults

47 Kharna, J. (1964) Design of fir plywood barrel vaults. Council ofForest Industries of British Columbia Report No. TDD-40.COFI, London.

Folded plates

48 Council of Forest Industries of British Columbia (1969) Firplywood folded plate design. COFI, London.

Formwork

49 Council of Forest Industries of British Columbia (1967) Firplywood concrete form manual. COFI, London.

Housing

50 Anderson, L. O. (1970) Wood frame house construction. USDepartment of Agriculture Handbook No. 73. US GovernmentPrinting Office, Washington, DC.

51 Council of Forest Industries of British Columbia (1978) Timberframe construction - a guide to platform frame construction.COFI, London.

52 Council of Forest Industries of British Columbia (1977) Load-bearing timber-framed walls. Construction data. COFI, London.

53 Council of Forest Industries of British Columbia (n.d.) Windloading calculation for a typical timber-frame house. COFI,London.

54 Canadian Wood Council (1977) Canadian wood construction datafiles. CWC, Ottawa.

55 Swedish/Finnish Timber Council (1976) Timber stud walls ofSwedish redwood and whitewood. SFTC, Retford.

56 Swedish/Finnish Timber Council (1981) Principles of timber-framed construction. SFTC, Retford.

57 Timber Research and Development Association (1980) Timber-frame housing manual. Construction Press, TRADA, London.

58 Timber Research and Development Association (1980)Calculating the racking resistance of timber-framed walls. WoodInformation Sheet No. 1-18. TRADA, High Wycombe.

59 Timber Research and Development Association (1981)Introduction to timber-frame housing. Wood Information SheetNo. 0-3, TRADA, High Wycombe.

60 Timber Research and Development Association (1981) Timber-frame 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) Nailedfir plywood web beams. COFI, London.

63 Council of Forest Industries of British Columbia (1970) Firplywood web beam design. COFI, London.

Shells

64 Keresztcsy, L. O. (1966) 'Interconnected, prefabricated laminatedtimber diamond type shell', Proceedings, International Conferencefor Space Structures. Surrey University, Guildford.

65 Tottenham, J. (1958) The analysis of hyperbolic paraboloid shells.Timber Research and Development Association Note No. E/RR/5. TRADA, London.

66 Tottenham, H. (1959) 'Analysis of orthotropic cylindrical shells.'Civ. Engrg.

67 Council of Forest Industries of British Columbia (1971) Firplywood stressed skin panels. COFI, London.

68 Finnish Plywood Development Association (1970) Design datafor stressed skin panels in Finnish birch plywood. TechnicalBulletin No. 11 (M). FPDA, Welwyn Garden City.

69 Wardle, T. M. and Peek, J. D. (1970) Plywood stressed skinpanels: Geometric properties and selected designs. TimberResearch and Development Association Report No. E/IB/22.TRADA, High Wycombe.

Trussed rafters

70 British Standards Institute (1985) Code of Practice for trussedrafter roofs. BS 5268: Part 3. BSI, Milton Keynes.

General design data

71 Timber Research and Development Association (1967) Design oftimber members. TRADA, High Wycombe.

72 Brunskill, R. W. (1985) Timber building in Britain. Gollancz,London.

73 Timber Research and Development Association (1986) Designexamples to BS 5268: Part 2, 1984. DAl, TRADA, HighWycombe.

74 British Standards Institution (1984) Specification for finger jointsin structural softwood. BS 5291. BSI, Milton Keynes.

75 Curry, W. T. (1955) Laminated beams from two species of timber.Theory of design. Princes Risborough Laboratory Special ReportNo. 10. HMSO, London.

76 Freas, A. D. and Selbo, M. L. (1954) Fabrication and design ofglued laminated wood structural members. US Department ofAgriculture Technical Bulletin No. 1069. US GovernmentPrinting Office, Washington, DC.

77 Council of Forest Industries of British Columbia (1972)Canadian fir plywood data for designers. COFI, London.

78 Council of Forest Industries of British Columbia (1971) Plywoodconstruction manual. COFI, London.

79 Finnish Plywood Development Association (1964) Finnish birchplywood handbook, FPDA, Welwyn Garden City.

Glues for structural components

80 Princes Risborough Laboratory (1967) The gluing of woodencomponents. Technical Note No. 4. Building ResearchEstablishment, Garston.

81 Knight, R. A. G. and Newall, K. J. (1971) Requirements andproperties of adhesives for wood. Bulletin No. 20, BuildingResearch Establishment, Garston. HMSO, London.

82 Princes Risborough Laboratory (1968) Gluing preservative-treatedwood. 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 oftimber buildings. Hutchinson, London.

84 Feilden, B. M. (1982) Conservation of historic buildings.Butterworth, London.

85 Gifford, E. W. H. and Taylor, P. (1964) 'Restoring oldstructures'. Struct. Engr, 42, 10, 332-334.

86 Ministry of Public Buildings and Works (1965) Notes on therepair 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) 'Structuralrepair of heavy timber with epoxy'. For. Prod. J. 29, 3, 15-18.

89 Oates, D. W. and Richards, M. (1984) Timber engineering insitu.' Record of British Wood Preserving Association AnnualConvention 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. OverseasBuilding Note No. 170. Building Research Establishment,Garston.

Storm-resistant construction

93 Southern Pine Association (1970) How to build storm-resistantstructures. The Association, New Orleans.

Earthquake-resistant construction

94 Building Research Establishment (n.d.) Building in earthquakeareas. Overseas Building Note No. 143, BRE, Garston.

95 Architectural Institute of Japan (1970) Design essentials inearthquake-resistant structures, Chapter 4, 'Wooden structures'.Elsevier, Amsterdam.

96 Takayame, K., Hisadat and Ohsaki, Y. (1960) 'Behaviour anddesign of wooden buildings subject to earthquakes'. Proceedings,2nd World Conference on Earthquake Engineering, Tokyo.

Bibliography

GeneralBurgess, 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 aidsby Timber Research and Development Association.)

Finnish Plywood Development Association (1972) Design for roofstructures 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 structuraluse of hardwoods. Wood Information Sheet 01-17. TRADA,High Wycombe. (Span tables for 65-grade Keruing for floor androof joists, purlins, ply-box beams and two-hinged portals.)

United Africa Company (1972) Guide to the use of West Africanhardwoods. (Load-span charts and tables for beams, joists,purlins, studs and ridged portal frame members.) (Universal spancharts for any timber, grade and load duration together withsimplified tables.)

Solid timber beams and joistsBurgess, H. J. and Masters, M. A. (1976) 'Span charts for solid timber

beams'. Timber Research and Development AssociationPublication 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. TimberResearch and Development Association Publication No. DA3.84. TRADA, High Wycombe.

Burgess, H. J. (1985) Joist span tables for domestic floors and roofs toBS 5268. Timber Research and Development AssociationPublication No. DA 6. TRADA, High Wycombe.

Burgess, H. J., Collins, J. E. and Masters, M. A. (1972) Use of theTRADA universal span chart for a range of load cases, TimberResearch and Development Association Publication No. TBL 47.TRADA, High Wycombe.

Timber Research and Development Association (1984) Span tables forfloor joists to BS 5268: Part 2, Processed timber sizes to DA 3.TRADA, London.

Timber Research and Development Association (1985) Joist span tablesfor domestic floors and roofs ~ processed timber sizes to DA 6.TRADA, London.

Hearmon, R. F. S. and Rixon, B. E. (1970) Limiting spans for machinestress-graded European redwood and whitewood. PrincesRisborough Laboratory, Timberlab 30. HMSO, London. (Spantables.)

Council of Forest Industries of British Columbia (1973) Hem-fir.(Load-span tables for floor, roof and ceiling joists for a widevariety of distributed and concentrated loads.) COFI, London.

Glued-laminated timber beams

British Woodworking Federation (1967) Span-load tables for glued-laminated softwood beams. BWF, London.

Plywood box and web beams

Council of Forest Industries of British Columbia (1968) Computeranalysis of plywood web beams. (A Fortran IV program for theanalysis of both symmetrical and unsymmetrical beams.) COFI,London.

Council of Forest Industries of British Columbia (1968) Fir plywoodweb-beam selection manual. (Tabulates the properties of 4000standard glued beams.) COFI, London.

Council of Forest Industries of British Columbia (1971) Nailed firplywood web-beams. (Load span-deflection tables for twenty-fourstandard beams.) COFI, London.

Timber Research and Development Association (1984) Load tables fornailed plv box beams to BS 5268: Part 2. TRADA PublicationNo. DA 4, TRADA, High Wycombe.

Timber Research and Development Association (1984) Load tables forglued ply box beams to BS 5268: Part 2. TRADA PublicationNo. 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 fivedifferent timbers.)

Council of Forest Industries of British Columbia (1972) Portal framemanual. (Design and selection manual.) COFI, London.

Stressed-skin panels

Finnish Plywood Development Association (1970) Design data forstressed 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 American Institute of Timber Construction, 1100,17th Street NW, Washington DC 20036, USA.

BRE Building Research Establishment, Garston,Watford, Hertfordshire.

BSI British Standards Institution, 2 Park Street,London, WlA 2BS.

BWF British Woodworking Federation, 82 NewCavendish Street, London, WlM 8AD.

BWPA British Wood Preserving Association, 62 OxfordStreet, London, WIN 9WD.

CITC Canadian Institute of Timber Construction, 100Bronfon Avenue, Ontario, Canada.

CPA Chipboard Promotion Association, 50 StationRoad, 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 AvenueWest, Ottawa, Canada KlP SV5

FIDOR Fibreboard Development Association, 1 HanworthRoad, 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., UnitedAfrica House, Blackfriars Road, London, SEl.