An experimental study of the design and overload capacity of structural grillages subjected to ice loads

Kim, Hyunmin (2014) An experimental study of the design and overload capacity of structural grillages subjected to ice loads. Masters thesis, Memorial University of Newfoundland.

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Abstract

It is well known that the design limit states for plating and framing of Polar Class ships are based on simplified plastic collapse mechanisms which ignore the beneficial effect of membrane stress and strain hardening; therefore, real structure will have a substantial reserve capacity beyond the design point. However, it is challenging to quantitatively estimate the level of the reserve capacity. Significant research efforts have been carried out to further understand the interaction between ice and ship structures. However, most experimental studies of structural response have used steel plates or rigid indenters for ice loading rather than real ice. Similarly most ice crushing tests use rigid indenters to crush ice as opposed to compliant structures. However, while the results of previous experiments and simulations well present post-yield behavior of the grillage with rigid indenter, there was no insight in terms of the interaction between ice and ship structures. In this study, laboratory grown conical shaped ice samples (1 m diameter) were used to load structural grillages, typical of a transversely framed 10,000 ton Ice Class PC6 midbody ice belt arrangement, well beyond its yield point and design point. This allowed for investigation into structural deformation considering the failure of ice. Two large grillages (named #1 and #2 respectively) were tested with ice specimens in a quasi-static condition (0.5 mm per second). The tests on Grillage #1 were performed in two load steps at identical loading positions in the midspan of the central stiffener and aimed at studying the ultimate load-carrying capacity of the grillage. The tests on Grillage #2 were carried out in three load steps at different loading positions along the span of the central stiffener; right off-centre (330 mm away from the centre), centre, and left off-centre (330 mm away from the centre). The varying locations of Grillage #2 tests were intended to investigate the effect of damage at nearby locations on capacity of the structure. These experiments suggest that local deformations of up to 11 % of the frame span and prior deformations nearby loading locations will not compromise the overall strength of the ship. A finite element (FE) model was developed to analyze the experiments numerically. The load-deflection curves and deformation shapes measured by the MicroScribe® were used to validate the numerical results. The FE analysis results show strong agreement with the physical experiments and demonstrate that FE model can be used for analysis of an ice-strengthened ship structures subjected to extreme ice loading. This thesis describes the procedure of the large grillage tests, discusses the results and pressure-area relationships, and compares against non-linear finite element analysis in conjunction with load-deflection curves and deformed shapes of the grillage. Each chapter presents several plots of data obtained from the experiments and uses photos to support the discussion of the results.

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