Shear resistance of stiffened steel plate girders: Influence of eigensystems on inelastic response

Steel I-shaped plate girders typically have slender webs that are panelized by the flanges and transverse stiffeners to resist shear loads. Most numerical studies of web shear response assume that the first eigenmode shape as an initial imperfection produces the lower bound (controlling) shear capacity solution; however, recent research has shown that the second eigenmode shape controls in some cases. This study examines the relationship between the inelastic shear capacity (V_w) and the “eigensystem” (i.e., eigenmode shape and the eigenvalue, which together produce the theoretical elastic shear buckling load, V_cr), thus producing insights into shear mechanics and best numerical modeling practices. The study uses experimentally validated finite element models of 929 plate girders, based on recently fabricated geometries. Elastic eigenmode analyses indicate that the shear response of the web tends to emulate that of a plate with rotationally clamped (fixed) boundary conditions. The (elastic) lowest bound eigensystem (the first eigenmode) of a particular girder configuration does not necessarily result in the (inelastic) lowest bound solution for V_w, since the deformed shapes requiring minimum energy in the elastic versus inelastic states are not always the same. Thus, the second eigenmode imperfection shape can sometimes govern V_w. The parameters that most influence the shear capacity are the panel aspect ratio (a/h), web slenderness (h/t_w), panel slenderness (a/t_w), and flange-to-web thickness ratio (t_f/t_w). The results of this study advance the understanding of web shear behavior and can lead to formulations that more accurately predict shear capacity while considering the shear mechanics.