Stresses that remain in structural members after rolling or fabrication are known as residual stresses. The magnitude of the stresses is usually determined by removing longitudinal sections and measuring the strain that results. Only the longitudinal stresses are usually measured.
To meet equilibrium conditions, the axial force and moment obtained by integrating these residual stresses over any cross section of the member must be zero.
In a hot-rolled structural shape, the residual stresses result from unequal cooling rates after rolling. For example, in a wide-flange beam, the center of the flange cools more slowly and develops tensile residual stresses that are balanced by compressive stresses elsewhere on the cross section (Fig. 1.13a). In a welded member, tensile residual stresses develop near the weld and compressive stresses elsewhere provide equilibrium, as shown for the welded box section in Fig. 1.13b.
For plates with rolled edges (UM plates), the plate edges have compressive residual stresses (Fig. 1.13c). However, the edges of flame-cut plates have tensile residual stresses (Fig. 1.13d). In a welded I-shaped member, the stress condition in the edges of flanges before welding is reflected in the final residual stresses (Fig. 1.13e). Although not shown in Fig. 1.13, the residual stresses at the edges of sheared-edge plates vary through the plate
thickness. Tensile stresses are present on one surface and compressive stresses on the opposite surface.
The residual-stress distributions mentioned above are usually relatively constant along the length of the member. However, residual stresses also may occur at particular locations in a member, because of localized plastic flow from fabrication operations, such as cold straightening or heat straightening.
When loads are applied to structural members, the presence of residual stresses usually causes some premature inelastic action; that is, yielding occurs in localized portions before the nominal stress reaches the yield point. Because of the ductility of steel, the effect on strength of tension members is not usually significant, but excessive tensile residual stresses, in combination with other conditions, can cause fracture. In compression members, residual stresses decrease the buckling load from that of an ideal or perfect member. However, current design criteria in general use for compression members account for the influence of residual stress.
In bending members that have residual stresses, a small inelastic deflection of insignificant magnitude may occur with the first application of load. However, under subsequent loads of the same magnitude, the behavior is elastic. Furthermore, in ‘‘compact’’ bending members, the presence of residual stresses has no effect on the ultimate moment (plastic moment).
Consequently, in the design of statically loaded members, it is not usually necessary to consider residual stresses.