Uncertainties in Material and Geometric Strength and Load Variables

Abstract

ABSTRACTUncertainty in the basic load and strength variables of a ship structure can significantly affect structural performance and safety. Variations in strength, load and load effects greatly impact the reliability of a structural system. Understanding and including this variation, or uncertainty, in the design and analysis of ship structures requires the use of structural reliability‐based design and assessment methodologies.For example, the design strength is based on nominal values for variables such as yield stress of the material, plate thickness, modulus of elasticity, etc. The actual values of these variables are often different from the nominal, or design, values. These actual values tend to behave in a random manner, causing random behavior of the actual structural strength. Understanding the randomness of the basic strength variables allows the designer to account for this variability in the design strength of the structure.The moment methods for calculating reliability‐based, partial safety factors (Ang and Tang 1984 and Ayyub and White 1987) require probabilistic characteristics of both strength and load variables in the limit state equation. Relevant strength variables for ship plates are the material's yield strength (stress)(Fy), modulus of elasticity (E), Poisson's ratio (v), thickness (t), and length (a) and width (b) of a plate. The relevant load variables are the external pressures due to stillwater bending moment, wave bending moment, and dynamic loads. Uncertainty, reliability, and risk measures are vital to the analysis and design of an engineering system. The reliability of the system can be stated in reference to some performance criteria. The need for reliability analysis stems from the fact that there is the presence of uncertainty in the definition, understanding, modeling, and behavior prediction of the model or models describing the system.The objective herein is to compile statistical information and data based on literature review on both strength and load random variables relevant to ship structures for quantifying the probabilistic characteristics of these variables.