Reliability-Based Load and Resistance Factor Design (LRFD) Guidelines for Hull Girder Bending

Future guidelines for ship hull girder design will be developed using reliability methods, which can be expressed in a special format such as the Load and Resistance Factor Design (LRFD) format. The main objective of this paper is to summarize the development methodology and results of reliability-based guidelines (i.e., LRFD guidelines) for ship structures that were performed for the U.S. Navy and other government agencies. The methodology for developing the LRFD format for ship hull girder bending used in this paper consists of the following steps: (1) Probabilistic characteristics of basic strength and load random variables that are used in structural design were analyzed. Values for these characteristics were recommended for reliability-based design purposes. They were determined based on the statistical analysis of data collected on these design parameters, on values recommended in other studies, or sometimes based on personal judgment. (2) Different load combinations were established and presented with combination and correlation factors; these combinations included stillwater, wave-induced, and wave dynamic bending moments. The correlation between these different load components was accounted for and expressed in the form of correlation factors. (3) Limit states for these load combinations were established based on structural modes of failures. (4) A comparison of different design practices was conducted based on the determination of the nominal values of strength and load values for ship structures to recommend the format required for each design variable. Methods for determining the design (nominal) values of both strength and load variables were presented as detailed calculation procedures. (5) Target reliability levels as used in other studies were summarized and ranges of target reliability levels were selected for the limit states. (6) Partial safety factors for the ranges of target reliability levels were calculated based on level 2 reliability methods.The paper also includes a detailed description of the methodology and sample guidelines for ship hull girder design with examples demonstrating their use.