Analytical Plastic Capacity Formulation for Plates Subject to Ice Loads and Similar Types of Patch Loadings

The response of plates subjected to patch loads with length/ height less than the dimensions of the plate field is of importance for the design of hulls strengthened for operation in ice covered waters and other comparable types of loading. An analytical plastic capacity formulation for plates subjected to patch loading of any rectangular geometry configuration has been derived, which from comparison with nonlinear finite element analysis is seen implicitly to limit the permanent deflection to be up to 0,40% of the frame spacing. The analytically derived formulations for the plastic bending based load capacity are based on yield line theory. Expressions for single and multiple patch loads are included, where the derivation is based on the assumption that the response for a single patch load is equal to the response from a sequence of identical patch loads located within the same plate field, and spaced a certain minimum distance apart. The expressions may serve as basis for rule requirements for plates subject to ice loads and other types of patch loads. For the considered longitudinally and transversely stiffened plate cases, nonlinear FE calculations show that the proposed plate bending based load capacity formulations generally give rise to permanent deformations in the plate up to 0.40% of the frame spacing, when the plate model analyzed is extended to include adjacent plate fields. Comparisons of nonlinear FE calculations show the pressure-deformation relationship of the single and repeated load patch to be the same within the range of permanent deformations considered. Comparison of the analytically derived plate formula with DNV Arctic Rules and Finnish-Swedish Ice Class Rules (FSICR) indicates that the plate formula of the DNV/FSICR gives similar results for the same load levels. The analytically derived formula, however, provides a more consistent utilization of the plate bending based capacity that is valid for a wider range of patch load geometries. Comparison of the IACS Polar Class (PC) Rules formulation indicates that the IACS plate formulation is increasingly non-conservative for small patch lengths, and that the application should be restricted to cover larger patch lengths only, e.g. l/s > 1,0.