The Post-critical Response of Metal Stiffened Plate Girders

Once one or more web panels of a metal stiffened plate girder have buckled, predicting its post-critical response still constitutes a hard problem to handle, both analytically and numerically. In practical applications, most codes define two limiting situations, specifically, the onset of buck-ling in the web (the serviceability limit state) and the final collapse of the girder (the ultimate limit state), for which design rules are given. However, a strong difference exists between the two cases. In the former case, in fact, the stress can be accurately determined, while in the latest one, the stress in the web is only roughly conjectured by examining the response under shear of elastic rectangular panels contoured by rigid flanges and stiffeners or very flexible flanges and rigid stiffeners [1]. In both cases, a uniform uniaxial tensile state of stress is assumed within the reacting zone. Thus, depending on the real stiffness of the boundary elements, this choice may result inadequate to represent the real state of stress in the web, which is anything but uniform. Equally arbitrary appear other elaborated models, later proposed (See [2] for a wide review). In order to deal with the above problem through a more systematic approach, in [3] we proposed a structural model where web panels were modelled as elastic membranes, able to wrinkle, partly or totally, according to the property of buckled panels of sustaining pure tensile states of stress only. In addition, yielding in flanges and stiffeners was entered by considering these ele-ments as elastic-perfectly plastic beams. By this way, the effects of the main parameters, such as the panel aspect ratio and the mechanical ratio (extensional stiffness of the panel vs. flexural stiffness of the contouring elements) on the equilibrium solution were investigated and the se-quence and location of the plastic hinges in flange and stiffeners was determined. The analysis showed how both a shear collapse mechanism or a local buckling of the com-pressed flange might cause the ultimate failure of the girder. Therefore, in order to afford such issues, here we propose an extension to the previous model by introducing a yielding criterion for the web, and so accounting for its limited tensile strength, and by defining a buckling load for flange and stiffeners. In this way, all the artificial and simplifying assumptions used in prac-tical applications are removed. The stated problem is numerically solved by an iterative algorithm implemented within an ad hoc developed F.E. code that fully exploits the formal identity existing between plasticity and wrinkling formulations. Thus, the response of typical metallic stiffened plate girders can be ana-lysed up to their collapse and the effects of the main geometrical and mechanical parameters on the load capacity of the girder are investigated.