trusses how they work

7/30/2019 Trusses How They Work

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GANG-NAIL Truss System

TRUSSES - HOW THEY WORK

In the evolution of building there have been twogreat developments since man first used timber or

stone to provide himself with shelter. Thesematerials were first used as simple beams. TheRomans are credited with the invention of thearch, and the truss was developed in Europeduring the middle ages.

A beam supports loads due to its bendingstrength. This is the way simple members such asrafters, battens, purlins, lintels and bressummerswork. The top edge of a beam is normally incompression and the bottom edge in tension.These stresses reach a maximum near the middleof the beams span and for every doubling of spanthe strength of the beam must increase fourtimes. Beams also tend to sag when loaded andsag is even more sensitive to increases in spanthan the requirement for increased strength.

Timber Arch

Gang-Nail trusses are based on these simplestructures. All the truss members are timber, andthe joints between the members are formed usingGang-Nail connector plates.The Romans found that if they leant stones

against one-another in the shape of an arch, theycould span greater distances than by using thestone as simple lintels or beams. In an arch thestones are in compression. The arch will performas long as the supports or buttresses at each endof the arch provide restraint, and do not spreadapart. Timber beams can also be propped againstone-another to form arches. The timber memberswill be in compression and will also act as simplebeams.

Single Truss with Arched Rafter and Tie

The characteristic appearance of a truss is aframework formed by many small triangles. Atriangle is a naturally stable shape, compared withsay a rectangular framework which can bedeformed unless its joints are rigid or it is braced

from corner to corner. Such a brace would, ofcourse, convert a rectangle into two connectedtriangles on a truss. The members forming theperimeter of a truss the chords usually act asbeams as well as ties or struts. The shorter thedistance between truss joints, the smaller thechord section required.

Roman Arch Bridge

To turn the arch into a truss, all that is required isto provide a tie between the two buttresses to stopthem from being pushed apart by the arch. Thearch, beam, tie combinations is self-supporting we call this structure a truss.

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Common A Type Gang-Nail Truss

However, the more joints there are in the truss,the more expensive it is to fabricate. The designerof a truss can choose the arrangement of thechords and webs and must balance structuralefficiency against manufacturing efficiency insupporting the applied loads.

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BASIC TRUSS MECHANICS

All trusses in a roof structure are designed for theworst possible combination of dead, live and wind

loads. The individual truss members are designedto restrain the corresponding forces i.e.., tensionor compression, or a combination of bending witheither the tension or compression force.

Tension (pulling). With this type of force themember being pulled or subjected to a tensionforce is said to be in tension. The ability of amember to restrain tension forces depends on thematerial strength of the member and its cross-sectional area.

However, if we rigidly support the 2400 mm longcolumn in the previous example at the centre, itwould then be capable of withstanding the onetonne force.

The example shows that if the cross-sectionalarea of a member is doubled, the ability of thatmember to restrain the tension forces is alsodoubled.

Compression (pushing). When a structuralmember is subjected to this type of force it issometimes referred to as a column. Unlike atension member, the ability of a column to restraincompression forces is not simply a function of thecross-sectional area, but a combination of thematerial strength, the column length and thecross-sectional shape of the column.

If one tonne is the maximum compression forcethat can be supported by a piece of 100 x 38 mmtimber, 1200 mm long without buckling, then thesame force applied to a piece of 100 x 38 mmtimber, but twice as long, would certainly cause itto buckle and possibly collapse.

Where this rigid support is applied to a webmember, it is called a web tie, which is used inconjunction with bracing. (See Figure 5A)

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Bending force, or more correctly bending moment,is the result of a force applied to a cantilever, forexample: a diving board, or to a simple beam.

Battens with bracing from the rigid supports areneeded to restrain the truss chords from bucklingsideways. (See Figure 5b).

he strength of a column is also dependent on thecross-sectional shape of a column. The squarer or

example of 100 x 25 member having across sectional area of 2500 mm is not as strong

The load carrying capacity of a beam is dependent

upon the strength of the material and also thecross-sectional shape of the beam. In the case ofthe beam, unlike the column, the deeper sectionhaving the same cross-sectional area will be thestronger member in bending. Beams subject tobending moments also require lateral restraints, aswith columns.

The deeper the beam the greater number ofrestraints required.

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more symmetrical the shape, the stronger the

column, given that the cross-sectional area is thesame.

In the

in compression as a 50 x 50 member, providedthat the other factors of length and materialstrength are equal.

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Forces in Members. Figure 10a

350 x 75 OREGON BEAM

In many common types of trusses it is possible toidentify the type of force which is in any particularmember without undertaking any calculations.

The example in figure 9 is a common A typegable truss with a uniformly distributed load alongthe top and bottom chords. This is due to thetransfer of the load of the tiles through the tilebattens and the ceiling load through the ceilingbattens.

This means that the chords are subjected tobending forces as well as compression andtension forces. This loading arrangement wouldresult in the top chord restraining compressionplus bending forces. The short web is in

compression and the long web is in tension. Thegeometry of both A &B type gable trusses isarranged so that under normal conditions, thelonger webs are in tension and the shorter webs incompression. This is done to economise on thesize of the timber required for the compressionwebs.

If the same load is applied to a steel universalbeam (see Figure 10b), the spontaneousdeflection is approximately 1 mm. The long termdeflection will also be 1 mm.

Figure 9Figure 10b

Instantaneous deflection = 1mm

Long term deflection = 1mm

310 x 165mm UNIVERSAL STEEL BEAM

Deflection.

Wherever a member is subjected to a tension,compression or bending force (bending moment),the member is deformed by the force, irrespectiveof how strong the material is or how large the

section. The amount of deformation does,however, depend on material strength and thesize and shape of the section.

The timber truss (See Figure 10c) will also deflectunder the same load, but because it is braced byits triangular web layout, it is much stiffer than theheavier Oregon beam, and is nearly as stiff as alarge steel beam which would weigh approximatelythree times more, and would probably cost fivetimes as much as the timber truss.

In Figure 10a it can be seen that the Oregon beamwould deflect 32 mm soon after the one tonnepoint load is applied at a mid-span. If this load ismaintained, the deflection may gradually increaseto three times the initial deflection after a period of20 to 24 months. This increase in deflection, withtime, without increase in load, is called creep.This characteristic is significant with timber, butcan be ignored in other structural materials like

steel.

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From these examples, it can be readilyappreciated that timber trusses are very effectivestructural components.

igure 12

russ Analysis and Member Design

and a trussshape has been chosen, the truss can be

s are subjected to combinations ofbending, shear and compression or tension. The

F

Camber

To compensate for deflection which occurs whenloaded, trusses are manufactured with an upwardbow which is called camber. Some deflectionoccurs as the truss is erected, more deflection willoccur as the roof and ceiling loads are applied tothe truss, and further deflection will occur over aperiod of time due to the creep.

Because the chords are subjected to a distributedload, they will also deflect in between panel points,in addition to the truss as a unit deflectingdownwards.

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This deflection of the chords is called paneldeflection and cannot be compensated for duringmanufacture, as can be for truss deflection(camber). All standard truss layouts, are designedto keep panel deflection within acceptable limits.

When the design loads are known

analysed to find the forces that will occur in eachof its individual members. This process is done bycomputer using well-established methods ofstructural mechanics. The computer uses aprocess of analysis that is integrated with theselection of members of suitable size and stressgrade and the calculation of expected deflectionwhen loaded.

Truss member

combinations can vary during the life of thestructure as different loading conditions occur andevery foreseeable situation has to be considered.Timber members are chosen so that they meet thestrength and serviceability requirements of AS1720.1 Timber Structures Part 1 - DesignMethods for each load case.

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GANG-NAIL CONNECTORS -HOW THEY WORK

Performance criteria for Gang-Nailconnectors

It is not economical to have a single connector thatgives optimum performance under all loadingconditions, for all of Australias wide range ofcommercial timbers. MiTek Australia Ltd. hasdeveloped a complementary range of connectorplates of varying plate thickness (gauge), toothlayout and tooth profile. These are:

A Gang-Nail connector is a steel plate with acollection of spikes or nails projecting from oneface. The spikes, or teeth, are formed bypunching slots in steel but leaving one end of theplug connected to the sheet. The teeth are thenformed so they project at right angles to the plate.During this process the teeth are shaped toproduce a rigid projection. When the teeth of aconnector plate are pressed into timber laid end-to-end, the plate welds them together by forminga Gang-Nail joint. Connectors are always used inpairs with identical plates pressed into both facesof the joint.

GQ 20 gauge (1.0 mm thick) galvanised steel.General purpose connector. Many short, sharpteeth - 128 teeth in a 100 mm x 100 mm area.

GE - 18 gauge (1.2mm thick) galvanized steel.Similar to GQ. For use when additional steelstrength is required.

G8S 18 gauge (1.2 mm thick) stainless steel.This connector is only used when the environmentis highly corrosive. 70 teeth in a 100 mm x 100mm area.

GS 16 gauge (1.6 mm thick) galvanised steel.Heavy duty connector. 144 teeth in a 100 mm x190 mm area.

Engineering Data

Gang-Nail connector properties have beenestablished in accordance with AustralianStandard AS1649 Timber - methods of test formechanical fasteners and connectors - Basicworking loads and characteristic strengths. As wellas testing new plate designs, MiTek Australia Ltd.conducts regular tests on their existing connectorrange and monitors the long term behaviour ofjoints subjected to constant loading. The CSIRODivision of Forest Products and the NSW ForestryCommission Division of Wood Technology havealso done considerable research work on toothedmetal plate connectors.

The concept is simple but the design of efficientGang-Nail connectors requires careful balancingof tooth shape and density, connector platethickness and ductility. An ongoing commitment toresearch and development ensures that MiTekslicensed truss fabricators have the most efficienttruss system at their disposal.

Full scale truss testing programs have also beencarried out at the Universities of Western Australiaand Adelaide, Australian National University andthe Cyclone Testing Station at Capricornia Instituteof Advanced Education.

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trusses how they work

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