Effect of welding induced residual stresses on compressive failure load of column structures

Abstract

Structural members carrying dominantly compressive forces are present in many types of structure. These members are referred to as columns and are frequently present in for example lifting appliances and offshore structures. Whether or not a column consists of longitudinally welded subsections is relevant for its mechanical performance in compression. Steel columns consisting of longitudinally welded members contain residual stresses caused by the non uniform longitudinal expansion and shrinkage during the welding process. The distribution and magnitude of these residual stresses are dependent on the dimensions of the heat affected zone (HAZ), and varies for instance as function of the welding procedure, and whether or not post weld measures are taken to diminish these residual stresses. In this thesis the width of the region in a column's cross section where tensile residual stress is present is referred to as the HAZ width. Using practical experiments an estimate is made of realistic HAZ widths in column structures using _nite element analyses. These column structures where welded and kept free of restraints during this process, so that a resulting curvature is developed upon cooling down to ambient temperature. The established value of the width of the tension zone is on the order of the thickness of the column's cross section. Based on existing norms a trapezoidal distribution of residual stress having a width of the earlier found value is applied on a _nite element model of a single square column, both for pinned and clamped boundary conditions. Mesh independence of the obtained results are verified by convergence studies. The compressive load capacity of the considered column is accected in the intermediate range of slenderness ratios, and shows a maximum reduction of approximately 21%. The effect of welding residual stresses is also investigated on a scaled model inspired by an existing design of a tower crane. A crane section is modelled by four vertical columns connected by multiple side bars. These side bars reduce the effective slenderness ratio of the columns loaded in compression, and therefore a less severe effect of the residual stresses on the collapse load is found.