Formulation of Idealized Structural Unit Method to Perform Progressive Collapse Analyses of Welded Box Girders in Bending

Abstract The Idealized Structural Unit Method (ISUM) is a simplified but very efficient approach to perform progressive collapse analyses of ship structures. Larger structural units are defined as ISUM-plate elements with idealized material and geometrical nonlinearities, that progressive collapse analyses are performed with reduced numerical efforts compared to nonlinear finite element analyses (FEA). A summary of different generations of the ISUM-plate element formulation is proposed. The newest generation of ISUM is described in detail with respect to its applicability to determine the ultimate strength of welded steel structures. The developed concept to apply welding residual stresses to plates and stiffened plate panels is explained. A combined ISUM-plate/beam-column element model to determine the progressive collapse behavior of welded box girders in four-point bending is proposed. The numerical results are compared to experimental data. For further comparison nonlinear finite element analyses are performed. Details about the FE model and the application of welding related imperfections are given. The ISUM model is used to determine the progressive collapse behavior of a box girder specimen for different load cases of biaxial bending. The influence of lateral pressure acting on the bottom structure on the ultimate strength of the box girder specimen is investigated with ISUM.