Reliability‐Based Load and Resistance Factor Design (LRFD) of Hull Girders for Surface Ships

Abstract

ABSTRACTThe main objective of structural design is to insure the safety, functional, and performance requirements of a structural system for target reliability levels, for a specified period of time, and for a specified environment. As this must be accomplished under conditions of uncertainty, probabilistic analyses are necessary in the development of such probability‐based design criteria of hull girders for surface ships. A methodology for developing load and resistance factor design (LRFD) rules for ship structures was developed in this paper, and demonstrated for surface ship hull girders.The methodology used in this paper for developing LRFD rules for ship hull girders consists of several steps as described herein. The probabilistic characteristics of strength and load random variables that are used in hull‐girder structural design were analyzed, and values for these characteristics were recommended for reliability‐based design purposes. Different load combinations for hull girders were established and presented with combinations and correlation factors that included the still water bending, wave‐induced bending, and wave dynamic bending moments.In this paper, the reliability methods for developing the partial safety factors (PSFs) for the ship hull girder in bending are described. These factors were determined to account for the uncertainties in strength and load effects. The First‐Order Reliability Method (FORM) was used to determine these factors based on prescribed probabilistic characteristic of strength and load effects. Also, strength factors were computed for a set of load factors to meet selected target reliability levels for demonstration purposes. The resulting LRFD rules are demonstrated in this paper using an example.