Draft

5.2Trusses 95

Figure5.6:A StaticallyIndeterminateTruss

3. Sketch a freebodydiagramshowingalljoint loads(includingreactions)


4. Foreach joint, andstartingwith theloadedones,apply theappropriateequationsof

equilibrium(FxandFyin 2D;Fx, FyandFzin 3D).

5. Becausetrusselements canonlycarryaxialforces,theresultant force(

~

F=

~

Fx+

~

Fy) must

bealongthemember,Fig.5.7.

F

l

=

Fx

lx

=

Fy

ly

(5.4)

44 Always keeptrack of thexandycomponents of a member force(Fx,Fy), as thosemight be

neededlateronwhenconsideringtheforceequilibriumat anotherjoint to which themember

is connected.

Figure5.7:X andY Components of TrussForces

45 Thismethod shouldbe usedwhenallmember forcesshouldbe determined.

46 In trussanalysis,thereisnosignconvention. A member isassumedto be undertension

(orcompression).If afteranalysis,theforceis foundto be negative, thenthiswouldimplythat

thewrongassumptionwas made,andthatthemember shouldhave beenundercompression

(ortension).