Introduction to Aircraft Structural Analysis (Elsevier Aerospace Engineering)

4.3 Applications of the Principle of Virtual Work 101

Fig.4.9

Determination of the internal force in a member of a truss using virtual work.

Hence,

(^) C=
(^4) v,B
3
(i)
Equatingtheexternalvirtualworkdonebythe30kNloadtotheinternalvirtualworkdonebythe
force,FBA,inthemember,AB,wehave(seeEq.(4.23)andFig.4.6)
(^30) C=FBA (^) v,B (ii)
Substitutingfor (^) CfromEq.(i)inEq.(ii),
30 ×

4

3

(^) v,B=FBA (^) v,B
Hence,
FBA=+40kN (i.e.,FBAistensile)
Intheprecedingweare,ineffect,assigningapositive(i.e.,tensile)signtoFBAbyimposingavirtual
extensiononthememberAB.