The Simplified Progressive Collapse Method for Analyzing Ship Structure Ultimate Longitudinal Strength

Based on Smith method, the Simplified Progressive Collapse Method for analyzing ship structure ultimate longitudinal strength is presented by applying beam-column theory, elasto-plastic assumption, plane section assumption and plastic hinge theory. The whole procedure has been coded into a FORTRAN program called SPCM. The bending moment-curvature curve is calculated by integrating the moment of each stiffened plate element to the instant elastic neutral axis of the whole section. The ultimate longitudinal strength is the peak value where the slope of moment-curvature curve equals zero or changes to minus. Its accuracy is decided by the accordance of stress-strain relationship of stiffened plate with real stress condition. The ultimate strength of stiffened plate is calculated from the compressed failures of the flange and the plate separately and the combined failures of both of them. It is shown through the comparison with the results of different methods that the compressed stress-strain relationship based on the ultimate strength of stiffened plate is quite accurate and reliable. This paper computes also the ultimate longitudinal strength of Reckling's No.23 model by using the Simplified Progressive Collapse Method. The results are quite satisfactory from the comparison of the results of this paper with the results of model test and other methods. It indicates that the computing method and program of this paper are correct and reliable and can be referenced and adopted during ship designing. At last, this paper investigates some factors which may affect the ship structure ultimate longitudinal strength heavily, including the load-shedding behavior of the stiffened plate, lateral pressure, material yield stress and steel plate thickness reduction due to corrosion. The investigation results indicate that all these factors should be paid more attentions in ship designing, operation and maintenance.