Ultimate Longitudinal Strength Analysis of Container Ships Considering Bottom Local Loads: Part 2: Development of Practical Method of Progressive Collapse Analysis

This paper is the second of the two companion papers dealing with ultimate longitudinal strength analysis of container ships considering the effects of bottom local loads. The major causes of reduction of ultimate hull girder strength due to local loads were discussed based on the finite element analysis of a hold model in the Part 1. The objective of this paper is to develop a practical method of progressive collapse analysis of a hull girder subjected to combined longitudinal bending and bottom local loads.Smith’s method is widely used to estimate the hull girder ultimate strength under pure bending. It however cannot consider the local deformations such as double-bottom bending because it assumes that a hull-girder cross section remains plane. A new methodology is therefore proposed, which idealizes the double bottom structures as a plane grillage consisting of longitudinal and transverse beams and extending over a hold length in the longitudinal direction. The rest part of a hull-girder cross section, such as a ship side and a bilge, is modeled as a unit beam and connected with the grillage model along the bilge parts using multi-point constraints. The calculation of the stiffness of longitudinal beam elements is based on the original Smith’s method, including the definition of plate and stiffened-panel elements and the application of a concept of average stress-average strain relationship for each element. The proposed model may be called an “extended Smith’s method”. The progressive collapse behaviors and hull girder ultimate strength predicted by the extended Smith’s method are compared with the result of nonlinear finite element analysis.