Reliability-based design of stiffened plates in ship structures subject to wheel patch loading

Reliability-based design methods are among the engineering tools for design of structures considering uncertainties in the design variables such as loads, material properties, manufacturing tolerances, etc. In design of stiffened plates of ship structures, there are some sources for uncertainties regarding the loads due to the highly nonlinear environment of the ocean. In addition to the environmental uncertainties, internal forces decrease the reliability of design due to the vagueness of items, which are included in the deadweight. Cargos with patch loading pattern like wheel and steel coil are common type of loading in some ships. These cargos are usually transferred by ferries, general cargo ships and bulk carriers. The application of reliability method in design of such structures against this local loading pattern has not been studied so far. Therefore, in this paper, the stochastic method and plastic formulas have been explained in designing steel panels subject to the patch loading. As a case study, stiffened plate structure of the ships subject to the truck wheel was investigated considering all design parameters as random variables. Bound theorem model for the patch loads is used to calculate the plastic load carrying capacity of the plate. First Order Second Moment (FOSM), First Order Reliability Method (FORM) and Importance Sampling method have been employed in the analyses. Both probability of failure and reliability index are calculated for different axel load distribution functions. The total failure probability was determined based on the rule of total probability. Sensitivity analyses for all parameters were carried out and the effects of coefficient of the variation of dominant variables were studied.