the design of large, modern timber structures (paper

82 VICTORIAN INSTITUTE OF ENGINEERS.

PAPER THE DESIGN OF LARGE, MODERN TIMBER

STRUCTURES. By Ian Langlands, M.Mech.E., B.E.E.

Introduction.—It is well known that steel production is one of the limiting factors in the war effort of all the combatant nations, and every country is making all possible efforts to con-serve steel in order to ensure that sufficient is available for essential uses such as munitions.

It is, therefore, not surprising that the use of steel in build-ings has had to be drastically curtailed, with the result that timber is now the standard material of construction for muni-tions factories, aerodromes, stores, etc., some of which are of very considerable size.

The development in the use of timber in large, heavily trussed buildings has been greatly facilitated by the advent of timber connectors, which were originally developed in Germany dur-ing the last war. These timber connectors are essentially metal dowels which permit high efficiency joints to be designed, thus enabling the sizes of the members to be kept to a minimum. Other recent developments have been the use of glued laminated arches and beams in large factories, and the much greater util-isation of nailed joints in the construction of warehouses, arched hangars, and stores, etc. In fact, the impact of war has caused a revolution in timber design and construction, the effects of which will endure.

Some typical examples of the types of structures constructed recently are shown in Plates.

General.—The layout and design of timber structures are essen-tially similar to that of similar steel structures ; however, the fact must be faced that the design of timber structures is defi-nitely more difficult than the design of steel structures. The properties of timber are widely different from those of steel, and it is only by a knowledge of the peculiar properties of timber that it can be used to best advantage as a structural material. For example, in the great majority of cases, green timber only is available for structural purposes, and the designer must always take into account the inevitable shrinkage that will occur in drying. (It should be noted, however, that for all practical purposes shrinkage parallel to the grain can be neglected.)

Types of Large Timber Structures. (a) Post and Beam Buildings.—This is the simplest and

cheapest form of construction, and is widely used for such pur-

Type Beam and Post Construction.

DESIGN OF LARGE, MODERN TIMBER STRUCTURES 83

Typical 40-Feet Span Truss.


poses as warehouses, where the presence of posts is not detri-mental to the utility of the building. The usual spacing of the posts is 16 feet x 16 feet or 20 feet x 20 feet, the roofing being supported by purlins resting on simple beams supported by the posts. The slope of the gable roof is usually about 10°, and par-ticular attention has to be paid to ensuring that the roof is weatherproof. The posts are usually of sawn timber, but in some buildings round poles have been used with advantage.

The main points to be watched in the design of this type of building are : (a) wind bracing, and (b) sagging of the purlins and beams under dead loads. The eventual deflection is about three times the initial deflection, and it is often found that stiff-ness rather than strength is the criterion. For temporary war-time buildings, sagging in itself is not detrimental except when fibro-cernent roofing is used, in which case excessive sagging may ,result in cracking of the fibro-cement, thus allowing rain to enter.

(b) Frame Buildings with Gabled Roofs.—This is a very com-mon type of construction at the present time. The roof trusses, which may be from 20 feet to 130 feet span, are supported by posts in the plane of the walls, lateral wind loads being taken either by knee-bracing or by flying buttresses. Longitudinal wind loads are usually taken by horizontal wind girders in the plane of the lower chords of the roof trusses, and by diagonal bracing between the columns The ratio of the rise to span of the roof trusses varies from 1 in 8 to 1 in 4. The steeper pitch is recom-mended except where stiffness is unimportant. Timber trusses are initially much less stiff than steel trusses of the same overall proportions, and, furthermore, due to creep of the timber, the deflection of timber trusses keeps on increasing (at a decreasing rate) for several years, so that the final deflection may be from six to twelve times the deflection of a steel truss of the same strength and dimensions. To allow for the high deflection, ade-quate camber should always be allowed in timber trusses. This should be at least equal to the ultimate deflection under dead loads (about three times the initial deflection), . which can be calculated with a fair degree of accuracy by the usual graphical or analytical methods: (In such calculations, the slip of the joints should always be allowed for, as it is responsible for the major proportion of the total deflection.)

For trusses of usual proportions, it has been found that a camber at mid-span of 1/240th of the span gives fairly satis-factory results.

Sufficient adjustment should be provided in door fittings to prevent jamming due to the gradually increasing deflection. This


is particularly important in structures such as hangars, where it is essential that doors should operate smoothly. Fitting of doors should be one of the last jobs in construction, thus allowing as much of the sag as possible to occur beforehand.

Small 20-Feet Span Truss, showing Simple Roof Truss and Glue-laminated Beam.

Timber connectors are used almost universally in trusses over about 40 feet and such cases, and also where timber connectors are used in smaller trusses, Pratt, Fink or Belgium bracing systems are used. Howe trusses are used only in the older type of composite steel and timber truss.


In a type of truss that has been successfully used in hangars, the slope of the top chord decreases in steps as the ridge is approached, the angles being so determined that the panel points of the top chord fall approximately on the outline of a parabola. This causes the loads in the web members . due to vertical dis-tributed loads to be greatly reduced, and this type of truss is characterised by very light web bracing.

For maximum economy the number of panels in the trusses should not be too great. It does not pay to reduce the distance between the panel points unduly with the purpose of reducing the size of the members, as the cost of a joint is large compared with the cost of timber.

The usual spacing of trusses varies from 10 ft. for a span of 20 ft. to 20 ft. at 100 ft. span.

Where the lateral wind loads are taken by the bending of the columns, very high bending moments are often encountered. For high buildings (20 ft. or more to the bottom chord of the truss) the use of steel columns is recommended, but where this is not practicable, if the required width of the column is greater than, say, 12 in., it is advisable to built it up in two pieces, using tim-ber connectors to take up the shear loads between the com-ponents.

The bending moments on the column can be reduced consid-erably by attaching the columns to the foundation by a semi-rigid joint. However, this has the effect of inducing a bending moment in the foundation, which must be specially designed to resist this moment. This considerably increases the cost of the foundation, and it has been found that it is usually more econo-pical to make a non-rigid column to foundation joint. The foundation can then be made simple and cheap, merely having to take vertical loads and a comparatively small horizontal load. In most cases the reduction in cost of the foundation more than compensates for the larger column required.

Where cranes are to be installed, the runways should be sup-ported on separate posts fastened to the columns supporting the roof in such a way that shrinkage will not change the distance between the crane rails. This is best done by the use of steel gusset plates and shear plates. However, it is better to make all columns of steel, as the racking action of a travelling crane tends to cause undue movement of the columns after shrinkage takes place. The longitudinal traction and braking loads of the crane should be taken up by knee braces connecting the crane runways and the supporting posts.


Large All-timber Hangar.

130-Feet Span Hangar.

88 VICTORIAN INSTITUTE OF ENGINEERS:

(c) Buildings with Saw-tooth Roofs.—For factories and workshops, saw-tooth roofs are often preferred to gable roofs because , of the better natural lighting. However, they are more expensive than gable roofs.

When timber connectors are employed, Pratt trusses varying in span from 40 ft. to 75 ft. are used to support the saw-tooth trusses. Composite steel and timber Howe trusses are also some-times used for the smaller spans, but they are now falling out of favour. The ratio of height to span should be about one in six, but one in eight is not uncommon. As with gable roofs, the trusses should be provided with ample camber to permit gradu-' ally increasing deflection without unsightly sagging.

Ample slope should be given to the valley gutters, so that drainage troubles will not occur as the deflection increases.

(d) Arched Buildings.—Recently "igloo" type buildings have become increasingly popular. In these the roofs are supported by arches springing from above ground level. The arch ribs, which are spaced from. 10 ft. to 20 ft. apart, depending on the span, may be of two main types: (a) framed two-pin or three-pin and (b) glued laminated three-pin.

The framed arches are usually of box section, consisting of four corner members, with diagonal bracing on all four sides. In the more heavily loaded arches, bolts or timber connectors are used to attach the bracing to the chords, but for light, war-time buildings (e.g., warehouses and hangars) sufficient strength can be obtained, even with spans up to 175 ft., by using nails only at the joints between the bracing and the chords. One type of nailed box 3-pin arch rib now being widely built has a span of 175 ft., a rise of 33 ft., and a maximum cross sectional area of 4 ft. x 3 ft. The four chords each consist of two 3 in. x 1i in. nailed together, and the bracing of 5 in. x 1 in. and 3 in. x 1 in. nailed to the chords with 22 in. x 10 gauge nails. (The load carrying capacity of nails is rather indefinite, but the safe load in shear on a 21 in. x 10 gauge nail is about 80 lb. However, for temporary structures, loads as high as 300 lbs. per nail have been used.)

Glued laminated arches may be of any span, shape or cross section, but the type now being widely constructed has a span of about 100 ft., a cross section of 24 in. x 4 in. and is of parabolic shape. The arches are usually built up from dry dressed boards with a finished thickness of about 13/16 in. These are glued to-gether with a water resistant casein cement pressure being applied by clamps, which are preferably pneumatically operated, but, if necessary, hand operated cramps may b' used. If no


Typical Saw-tooth Roof.

96-Feet Span Truss for Hangar.

90 VICTORIAN INSTITUTE OF ENGINEERS,

equipment is available, pressure may be applied by nailing the laminations together.

The individual laminations can be bent cold to a radius of about 140 times the thickness without danger of failure.

The individual laminations are, of course, much shorter than the completed arch, and in Europe and America the lamina-tions are end-jointed by glued scarf joints having a slope of about 1 in 12, but butt joints are usually used in Australia. This means that so far as strength is concerned, the net cross-section obtained by neglecting one lamination should be used in design. In calculating stiffness, however, the full cross-section may be used. The joints should be staggered so that there is not more than one joint in any cross-section perpendicular to the axis, and joints in adjacent laminations should be at least 12 in. apart.

Since dry timber is used, and since any defects that are pre-sent in individual laminations have little effect on the strength of the member as a whole, working stresses may be 50% higher than those ordinarily used for green timber. Also creep is less in glued laminated members than in green timber, the ultimate deflection at ordinary working stresses being about twice the initial deflection.

Theoretically there is no limit to the size of arches or beams that can be constructed in this way, but the cost is considerably higher than that of ordinary timber construction.

(e) Misceblaneous Structures.—Practically any form of con-struction can be carried out in timber, and considerable numbers of towers (up to 250 ft. high in Australia), bridges, wharves, derricks, etc., have been or are being built.

Working 4tresses.

Working stresses usually recommended for timber (see "Handbook of Structural Timber Design") are intended for use in the design of permanent structures. However, the major-ity of timber structures being constructed at the present time are of a temporary nature only, also, in war-time, the utmost economy is of vital importance, and greater risks may be taken than would normally be justifiable. Therefore, an increase in the permissible stresses is warranted, and it is recommended that for war-time construction, all working stresses (except for tension joints under dead loads) should be increased by 33%. In certain cases where a lower margin of safety is justifiable, or where failure of one part would not endanger the safety of the structure as a whole, the stresses may be increased by 50%.


105-Feet Span Nailed 3-Pin Arches.

95-Feet Span 3-Pin Glued-laminated Arches.

92 VICTORIAN INSTITUTE OF ENGINEERS

It should be noted that when these higher working stresses are used, the deflection of the structure will be considerably in-creased, but this mainly affects appearance, which is unimport-ant in most war-time buildings. However, if stiffness is import-ant, discretion should be used in the use of higher working stresses.

Design of Spaced Column Spaced columns (i.e., columns consisting of two or more parts

held together with spacing blocks) are commonly used in modern timber structures. It is sometimes thought that the packing blocks are equivalent to batten plates in steel columns, and it is therefore often argued that the design of spaced columns should be similar to that of steel columns. However, unless they are glued in place, the packing blocks in timber columns are not sufficiently rigid to enable the column to act as a unit. Even if they are tightly bolted up when the structure is first erected, shrinkage will cause subsequent shrinkage, with great reduction in strength. No rational method of design of such columns has been developed, and empirical methods have to be used. (See "Handbook of Structural Timber Design.") It will be seen from these formulae that the strength of a spaced column is about double the sum of the strengths of the components acting as individuals.

Species and Quality of Timber. It is not generally advisable to specify individual species by

name, as in many cases there are alternative species which are equally satisfactory, and which are more readily available. Thus, in Victoria, designs should be based on the use of any of the timbers listed in the "Handbook of Structural Timber Design" as belonging to Strength Group C. The higher strength timbers which fall in Group B are not readily available in this State, and they should only be specified when absolutely necessary.

The quality of the timber should be defined by reference to S.A.A. Emergency Standard No. (E) 0.54, which sets out clearly the permissible defects which can be permitted in Structural Timber. Due to the generally low quality of structural timber in Victoria, Standard Grade only should be specified, as only very limited quantities of Select Grade are available.

High durability is of no advantage in timbers to be used in structures, or parts of structures, completely protected from the weather, and not in contact with the ground. Therefore, Class 4 durability timbers (see "Handbook of Structural Timber Design") may be used except where the member is exposed to moisture or in contact with the ground, for which conditions


Class 3 durability timbers will give a life of, say, 5 to 10 years; Class 2 timbers a life of 15 to 20 years, and Class 1 timbers a life of 20 years and upwards. In dry localities, these periods may be greatly exceeded. For outdoor structures (except for those members in contact with the ground), Class 4 durability timbers

Latest N.S.W. Fire Lookout, 100 Feet High. Ring Connected, Assembled from Centre Derrick in Sections.

should have a life of, say, 8 to 10 years; Class 3, 10 to 15 years; Class 2, 25 to 30 years, and Class 1, 40 years and upwards.

In Victoria, the common building timbers are mountain ash, alpine ash (woollybutt or red ash) , messmate stringybark (mess-mate) and assorted stringybarks. Most of these fall in Strength Group C, but whereas mountain ash and alpine ash are in Durability Class 4, and, therefore, should not be used where they are exposed to moisture, messmate stringybark and the other stringybarks are in Class 3, and, therefore, will give a life of 5 to 10 years when in contact with the ground.


If longer life is required in members in contact with the ground, red gum or jarrah should be used. These are Class 2 durability timbers, and even under very bad conditions will last 15 to 20 years, but under usual conditions they give much longer life.

The life of members in contact with the ground can be greatly increased by applying two coats of creosote to the timber and puddling the earth around the timber with creosote.

Painting of Timber. Unless for some special reason, such as camouflage, it is pre-

ferable not to paint green timber for about twelve months to two years after erection. Painting green timber tends to retain moisture and so increases the chances of decay. However, there is no objection to painting with a wood preservative such as creosote.

Protection Against Termites. With the exception of structures in Tasmania and southern

Victoria, all timberwork near the ground should be protected from the attack of termites by means of ant-caps (See Division of Forest Products Trade Circular No. 44—" Termite (White Ant) Proof Construction"). However, in districts where the termite hazard is known not to be severe, and where regular inspections can be made, it is not necessary to use ant-caps on concrete footings. Cases have been encountered of termites gain-ing access to otherwise termite-proof timber buildings through construction joints in the concrete floors. This can be prevented by pouring creosote oil down the joints before any timbering is erected over the concrete. The creosote should form a continuous line along the joint.

Size of Timbers. If possible, the length of individual pieces of timber should

be kept below 20 ft. Lengths between 20 ft. and 30 ft. are harder to obtain, and the lengths over 30 ft. should be specified only where essential. It should be noted, however, that the objec-tion to long lengths is based on availability only ; if it can be obtained it is usually cheaper to use a long length rather than to splice two shorter pieces.

If possible, the maximum cross section should not exceed 50 sq. ins., and if larger sections than about 80 sq. ins. are required, they should be built up from smaller pieces.

Types of Timber Connector Joints. Two main types of timber connectors are available in Aus-

tralia : (a) split-rings, which fit in matching pre-cut grooves


cut in adjacent timbers, and (b) shear-plates, which are inserted in pre-cut grooves and which are flush with the surface timber. With split-rings, the ring itself carries the whole shear load, the

Trial Erection of 125-Feet Typhoon Type Radio Tower.

purpose of the bolt being to hold the joint together; but with shear-plates the bolt carries the shear load, the purpose of the connector being merely to transmit the load from the bolt to the wood.

The following types of connector joints may be used : (a) Overlapping members using split-rings. (b) Overlapping members using shear-plates in each member. (c) Plywood gusset plates with split-rings. (d) Plywood gusset plates with shear-plates. (e) Steel gusset plates or straps with shear-plates. Type (a) is the cheapest and most commonly adopted. Type

(b) is more expensive than type (a) because, (1) a minimum


of two shear plates per bolt is required, as compared with a minimum of one split-ring per bolt, (2) larger bolts are required than for split-rings, (3) the strength per connector is lower than for split-rings, (4) the shear plates themselves are dearer than split-rings, and (5) the cost of cutting the grooves is greater than for split-rings. However, it has the following advantages: (1) the shear plate can be inserted and tacked in their grooves immediately after grooving, thus enabling the inspector to be sure that no connectors are omitted from any of the joints, (2) assembly is easier and quicker, (3) provided that there is only one row of connectors per member, shrinkage of the timber does not interfere with ease of assembly, and (4) the strength of the joint is not appreciably affected by shrinkage, therefore, main-tenance is not so important as with split-ring connector joints.

Types (c) and (d) have the same relative advantages and dis-advantages as Types (a) and (b). They are more expensive than Types (a) and (b), but the use of gusset plates is of assistance in the design of the more difficult joints.

Type (e) is similar in cost, and has the same advantages and disadvantages as Type (d), but involves the use of compara-tively large quantities of steel. However, the use of shear plates with steel straps or built-up sections is the best method of of fastening timber to concrete.

Summing up, Type (a) should be considered as the standard; Type (b) should be used if the structure is to be demountable, if proper maintenance cannot be given, or if there is likelihood of appreciable shrinkage occurring between fabrication and assembly; Types (e), (d) and (e) should be used with discre-tion because of the increased cost, and in the case of Type (e), because of the large amount of steel required.

Design of Timber Connector Joints. The working loads and the appropriate design data are given

in the second edition of the "Handbook of Structural Timber Design." However, recent experience has indicated that where green hardwood is used, an "equivalent load" equal to (2 x dead load + 0.8 x live load + 0.67 x wind load) should be used when designing joints near the ends of tension members. The necessary number of connectors is then obtained by dividing this "equivalent load" by the appropriate working loads as given in the Handbook. Also for tension joints the minimum end distances should be increased 50%. Cross bolts at least $ in. diameter should be provided about 2 in. from the ends of tension members in order to limit splitting, or alternatively, corrugated strip bent in the form of an S may be driven into the end grain.


Radial Drill for Cutting Ring Connector Grooves.

Cutting Templates for 130-Feet Span Truss.


Length of Bolts. The length of the bolts required should always be shown on

the drawing. In this regard, it should be noted that Australian timbers are usually cut slightly oversize, and the bolt should be long enough to allow for oversize timber. In calculating the length of the bolt, the oversize may be allowed at about 1/32 in. per inch thickness of timber.

Design of Purling. In many timber roofs, it will be found that as much or more

timber is used in the purlins as in the trusses. Particular atten-tion should, therefore, be paid to the design of the purlins, and it is recommended that the sizes given in C.S I.R. Pamphlet, No. 112, "Building Frames—Timbers and Sizes," should not be exceeded. When galvanised iron or fibro-cement roofing are used, purlins need not normally be designed to take the com-ponent of load parallel to the plane of the roof. However, if tiles are used, the purlins should be prevented from deflecting laterally by ties attached to the ridge.

Fabrication of Trusses. The following methods of fabrication of trusses are used (1) Where there are only a few repetitions, the members are

assembled on the ground, and tacked together with nails. The bolt holes are then drilled, preferably with portable electric drills, but if necessary by hand, the members carefully marked and disassembled, the connector grooves (if any) are then cut, using portable electric drills and special grooving tools (if power is not available, hand drills can be used) after which the truss is then reassembled and lifted into place.

(2) If there is a large number of repetitions, it is much more economical to use templates. These are usually 1 in. thick thoroughly seasoned boards of the same width and length as the members they represent. The templates are assembled on the ground, and the bolt holes drilled. The members are then marked off from the templates, using close-fitting centre punches, and drilled and grooved on tables consisting of electric drills mounted over roller benches.

By complete mechanisation, the costs of fabrication can be greatly reduced. The success of the process, however, depends entirely on the accuracy employed in cutting the templates, and marking off the members.

(3) If there is a limited number of repetitions, methods (1) and (2) are sometimes combined, using the members of the first trusses as the templates.

Stack of Members with Shear-Plate Connectors in Place.

inated entirely in heavily stressed structures are compression failures and brittle heart (see D.F.P. Trade Circular, No. 32—"Causes and Detection of Brittleness in Wood"). These defects are often difficult to detect, and can most be readily eliminated at the mill.

The inspector should supervise the lay-out and cutting of templates, as any fault in the templates may cause very serious trouble in assembly, and in a job extending over several months it is advisable to reassemble the templates occasionally to ensure


Inspection. Now that the Standards Association of Australia has issued

emergency Standard No. (E) 0.54—` `Australian Standard Grading Rules for Sawn and Hewn Structural Timbers.," the inspector's job is lightened in that he has definite rules to work to. He should take care not to reject any timber that contains some imperfection, which, although permitted in the specifica-tion, affects the appearance of the piece. On the other hand, par-ticular attention should be paid to ensure that the extent of the following defects is limited to the amount permitted in the speci-fication—sapwood (only in timbers susceptible to borer (Lyctus) attack, or in timbers subjected to decay hazards), knots, sloping grain and decay. Two very weakening defects that must be dim-


that they have not changed dimensions. The marking of the timber from the templates should also be supervised.

The inspector should be sure that all connectors are in their correct position before and during assembly.

He should also insist that the structure is properly braced and guyed during erection. It is often claimed that this . is the sole responsibility of the builder, but any accident caused by insufficient bracing may seriously delay' the job, and the inspec-tor is entitled to take such steps as he thinks warranted to avoid such accidents. In this regard, there have recently been several accidents, some of them fatal, due to insufficient bracing and guying during erection of timber structures.

The inspector should not be unduly perturbed by the com-paratively large deflection of timber structures, and he should bear in mind that the deflection will increase for several years. However, any abnormal or irregular deflection should be investigated.

Maintenance. All timber structures require maintenance This is especially

true of timber trusses, the bolts of which require tightening at periodical intervals up to two or three years after erection (usually three or four tightenings in all are required), and the owners of the structure should be warned that such maintenance -is essential to safety and efficiency.

Library Digitised Collections

Title:

The design of large, modern timber structures (Paper)

Date:

1943

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http://hdl.handle.net/11343/24894

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The design of large, modern timber structures (Paper)

the design of large, modern timber structures (paper

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