Finite Element Modelling of Stiffened Steel Box Girders with Imperfections

Abstract

<p>An elasto-plastic large displacement finite element model has been developed to analyse stiffened plated box girders. It is capable of predicting inelastic buckling behaviour and ultimate strength under stated loads. Both geometric imperfections and residual stresses are included to account for inherent flaws due to fabrication processes. The model is formulated through a three-dimensional assemblage of rectangular plate elements for the webs, flanges and diaphragms, and eccentric beam elements for stiffeners. Plate and beam elements employed in the model incorporate both bending and in-plane actions. The non-conforming rectangular plate elements used to utilize special provisions for displacement continuity along junctions between components of the box. To reduce computational time and half band width requirements, diaphragms are treated as substances and then coupled with the rest of the box. For material nonlinearity, yielding is described by the Bon Mises criterion and the associated Prandtl-Reuse equations of plasticity. Subsequent yielding is governed by the isotropic hardening rule. Finite element formulation for large deflection is based upon a total Lagrangian description. In the solution to problems, the analysis involves the Newton-Raphson iterative method for the nonlinear analysis using an incremental load procedure. The formulation is first verified using a variety of beams, plates and three-dimensional plate assemblage problems. The present predictions compare favourably with other theoretical and experimental results already published in the technical literature. The proposed model is then applied to a symmetrical overhanging stiffened steel box girder recently tested to stimulate a typical pier girder of a continuous bridge. A series of numerical simulations were made both for the perfect box and its imperfect equivalent, in which imperfections are confined to the compression flange. Attention was focused on initial plate panel imperfections, longitudinal stiffener out-of-straightness, and idealized residual stresses. Analytical results to be presented include ultimate load, failure mode, state of longitudinal stress distribution, and spread of plasticity. A comparison with results from the physical model shows excellent agreement. The imperfections postulated for the theoretical model are shown to somewhat reduce the predicted strength of box girders. Any analysis neglecting this effect overestimates the buckling strength.</p>