Influence of imperfections on the flexural resistance of steel delta girders

Residual stresses and geometrical imperfections are important factors that affect the inelastic lateral-torsional buckling (LTB) capacity of flexural members. In this paper, the influence of the magnitudes of residual stresses and initial geometrical imperfections in the form of member out-of-straightness on the flexural resistance of steel delta girders (SDG) is investigated. Based on test data reported for welded plates and monosymmetric welded I-sections, a residual stress pattern for SDG is proposed. Six different combinations of residual stress and geometrical imperfection magnitudes are then used in a finite element simulation study of a series of SDG under uniform bending and simply-supported boundary conditions. The flexural resistance curves computed for these SDG are compared with one another to demonstrate that both residual stresses and initial member out-of-straightness have a noticeable influence on the moment capacity of SDG, especially in the inelastic LTB region. These curves are also compared against the flexural strength equations provided in the current AISC specifications. The comparison reveals that the AISC equations often over-predict the flexural strength of SDG. An SDG flexural strength reduction factor is then proposed to allow for the design of these SDG using the AISC design equations.