Study of the Effect of Added Soil Mass on Earthquake Response of Subsea Structures on Closed Caisson Foundations
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Earthquake related design activities of subsea closed caisson foundations require analyses of intricate soil-structure interaction effects. The complexity give rise to both advanced and simplified analytical tools. Today numerical approaches such as the finite element method are most commonly used. Although many analytical and simplified methods have been developed, no standardized approach is established for caisson foundations. At the ISFOG-conference held in Oslo 2015, the Simplified Modal Non-linear Analysis was introduced. The method use springs to represent the subgrade in a two degree of freedom system. Based on force-displacement and moment-rotation relations from external soil-caisson analyses, secant spring stiffness are used. An iterative modal analysis procedure is then conducted, giving non-linear properties to the springs. Relevant accelerations are retrieved from an acceleration response spectrum. Dynamic force and overturning moment are calculated with corresponding lateral displacement and rocking motion. Foundation mass and soil added mass have been neglected in previous studies of the SMNA-method. This work is a parametric study to further develop the method studying inertial effects. Numerical analyses are first conducted in PLAXIS 3D, which provide indicative results. The simplified model is then calibrated based on consistency with the numerical model. The initial SMNA-results show very low dynamic forces and excessively large moments, when soil added mass are introduced. The divergent tendencies indicate too high ratio of rotation to translation stiffness. In order to achieve compliance in the results a reduction in mass, mass moment of inertia and soil stiffness are needed.