Progressive Collapse Analysis of a Ship's Hull under Longitudinal Bending

Abstract

It is very important to estimate the load carrying capacity of a ship's hull as a whole from the viewpoints of safety and economy. For this purpose, a simple method was proposed to simulate the progressive collapse behaviour of a ship's hull girder subjected to longitudinal bending. In this method, the cross section of a hull girder is divided into small elements composed of a stiffener and attatched plating. For each stiffener element, the average stress-strain relationship under axial load is derived based on the equilibrium conditions of forces and moments. The buckling and yielding in both stiffener and plate are taken into account. Then, a step-by-step increase of vertical curvature is applied to the hull girder assuming that the plane cross section remains plane. At each incremental step, the tangential flexural rigidity of the cross section is evaluated using the tangential slope of the average stress-strain curves of the elements as well as the incremental bending moment due to the curvature increment.Performing sample calculations on existing girder models tested under pure bending, the rationality of the proposed method was examined. Then, the analysis was performed on an existing bulk carrier, and the progressive collapse behaviour of the cross section under bending load was discussed. It was found that the full plastic bending moment can not be attained due to buckling of the deck plates under sagging condition and that of the bottom and inner bottom plates under hogging condition. It was also found that the maximum bending moment carried by the cross section under sagging condition is 20% lower than that under hogging condition.