The Idealized Structural Unit Method (ISUM) is a simplified matrix method for the collapse analysis of large structures. The geometrical and the material nonlinearities are included inside the newest ISUM element formulation. In this paper, possible modifications of the special shape functions especially for high plate slenderness ratios will be discussed. To embed ISUM in a commercial finite element software package the degrees of freedom have to be reduced to nodal degrees of freedom. Therefore the internal degrees of freedom are condensed out before the iterative solution of governing equations starts. The applicability and accuracy of the condensation procedure is presented.
The evaluation of the maximum load-carrying capacity of structures is an essential issue in determining their safety. Only if the prediction of the ultimate strength is ensured, their probability of survival in extreme load conditions can be estimated. Particularly for safe dimensioning of ship structures, the determination of structural strength against buckling and yielding is required by Common Structural Rules (CSR). For ultimate strength calculations of ship structures, including ultimate hull girder capacity and ultimate strength of plates and stiffeners, the Finite Element Method (FEM) is a feasible tool. Despite the enormous development in computer technology, nonlinear finite element analyses (FEA) are still very time consuming and not easily to perform as well. Therefore the Idealized Structural Unit Method (ISUM) can be applied for collapse analysis of large structures. Within the ISUM formulation proposed by Ueda and Rashed (1974) geometrical and material nonlinearities are included in an idealized manner. Due to these idealizations a large structural unit can be regarded as one element. Therefore a structural system can be discretized by fewer elements, so that the computational time with ISUM is reduced significantly in comparison to FEA. Despite further improvements of this formulation (Ueda et al., 1993) the applicability of these ISUM elements to stiffened plates is very complicated.
