Effects of residual stresses on the ductile strength of plane welded grillages and of ring stiffened cylinders

Abstract

Structural engineers have for a long time accepted the importance of residual stresses when considering fracture modes of failure. It is only in the last quarter-century that attention has been focused on their adverse effect on ductile modes of failure and, in particular, on buckling collapse. The last decade has seen increased research activity, spurred on by recent failures in box-girder bridges. The nature and distributions of locked-in-stresses during hot and cold forming and during welding are outlined. The effect these have on inelastic buckling collapse of flat plated grillages and ring-stiffened cylinders is examined for all likely modes of failure. Experimental data are scarce, but they confirm that both forming and welding stresses can reduce the stiffness and strength of both types of structure very significantly. This is caused by a direct reduction in buckling stresses (as usually supposed), but equally important is the loss in stiffness due to early yield and other effects. In particular the cold-bending of ring-frames can have very serious adverse effects, as shown by recent finite-element programmes. Cold-cambering, and certain other corrective measures adopted by fabricators, seem more likely to weaken the structure than to improve it.