Progressive collapse assessment of intact and damaged ship structures under combined bending and torsional loading

Abstract

The Simplified Progressive Collapse Method is a quick and well‐established method recommended by International Association of Classification Societies (IACS) for the evaluation of a ship’s ultimate strength due to longitudinal bending. However, in the case which the torsional rigidity of the structure is reduced i.e. containerships, longitudinal bending may not be the dominant reason for failure. Torsion and shear forces may influence the global strength and therefore the effect of combined loading needs to be investigated. The current research aims to understand hull girder failure modes under these combined loads both for intact and damage case scenarios and incorporate torsional loading effect into the existing method. A hull girder which is subjected to longitudinal bending moment and torsional loading generates compressive/tensile and shear stresses on its plates and stiffened panels. Therefore, a thorough investigation of plates under these combined loads has been carried out. The outcome of this study is interaction diagrams of compression/tension and shear for steel and aluminium plates. The ultimate strength of the structure is then estimated for an applied amount of torsion calculating the shear flow distribution of the plates in the cross section and using the results of the previous study. The same procedure is repeated for different amounts of torsion and the interaction diagram of vertical/horizontal bending moment and torsional moment is derived. For validation, the proposed method is applied to a number of different intact box girders and to a hull girder of a 10000 TEU containership. The effect of damage on the ultimate strength is examined only under bending due to the current representation of damage in the simplified method. The results are compared with the results of non‐linear finite element models which have been generated and analysed for this purpose. The results for all studies show very good correlation, however the simplified method provides a sufficient advantage of time estimation and simplicity in comparison with the existing methods. It can be used in both concept and preliminary design to provide accurate estimation of hull girder strength. This extension of the Simplified Progressive Collapse Method, which accounts for combined load effects, is particularly useful in cases which require quick estimate of ultimate strength.