Deflections of Statically Determinate Trusses

In Art. 3.23, the basic concepts of virtual work and specifically the unit-load method are presented. Employing these concepts, this method may be adapted readily to computing the deflection at any panel point (joint) in a truss.
Specifically, Eq. (3.113), which equates external virtual work done by a virtual unit load to the corresponding internal virtual work, may be written for a truss as


where alpha=  displacement component to be calculated (also the displacement at and in the direction of an applied unit load)
n  total number of members
ƒi  axial force in member i due to unit load applied at and in the direction of the
desired —horizontal or vertical unit load for horizontal or vertical displacement,
moment for rotation
Pi  axial force in member i due to the given loads
Li  length of member i
Ei  modulus of elasticity for member i
Ai  cross-sectional area of member i