structural deflections[′strək·chə·rəl di′flek·shənz]
The deformations or movements of a structure and its components, such as beams and trusses, from their original positions. It is as important for the designer to determine deflections and strains as it is to know the stresses caused by loads. See Stress and strain
Deflections may be computed by any of several methods. Generally the computation is based on the assumption that stress is proportional to strain. As a result, deflection equations involve the modulus of elasticity E, which is a measure of the stiffness of a material.
The relation between deflections at different parts of a structure is indicated by Maxwell's law of reciprocal deflections. This states that if a load P is applied at any point A in any direction a and causes a shift of another point B in direction b, the same load applied at B in direction b will cause an equal shift of A in direction a (see illustration). The law is used in a number of ways such as in simplifying deflection calculations, checking the accuracy of computations, and producing influence lines. See Structural analysis
Beam and truss deflections usually are computed by similar methods, except that integration is used for equations and summation for trusses. Beam deflection equations involve bending moments and moments of inertia. Truss deflection equations are based on the stresses and cross-sectional areas of chords and web members. Deflections may also be determined graphically. See Beam, Truss