Draft

5.2Trusses 93

5.2 Trusses

5.2.1 Assumptions

30 Cablesandtrussesare2Dor 3Dstructurescomposedof anassemblageof simpleonedimen-

sionalcomponents which transferonlyaxialforcesalongtheiraxis.

31 Trussesareextensivelyusedforbridges,longspanroofs,electrictower,spacestructures.

32 For trusses,it is assumedthat

1. Barsarepin-connected


2. Joints arefrictionlesshinges

4

.

3. Loadsareappliedat thejoints only.

33 A trusswouldtypicallybe composedof triangularelements with thebarson theupper

chordundercompressionandthosealongthelower chordundertension.Dependingonthe

orientationof thediagonals, theycanbe undereithertensionor compression.

34 In a trussanalysisor design,we seekto determinetheinternalforcealongeach member,

Fig.5.5

Figure5.5:BridgeTruss

5.2.2 BasicRelations

SignConvention:Tensionpositive, compressionnegative. Ona trusstheaxialforcesare

indicatedas forcesactingonthejoints.

Stress-Force:=

P

A

Stress-Strain:=E"

Force-Displacement: "=

L

L

4

In practicethebarsareriveted,bolted,or weldeddirectlyto each otheror to gussetplates,thus thebars

arenotfreeto rotateandso-calledsecondarybendingmomentsaredevelopedat thebars.Anothersource

of secondarymoments is thedeadweight of theelement.