An efficient procedure for modal seismic response analysis of axisymmetric structures surrounded by water

This paper presents a new procedure for the seismic response assessment of water-surrounded axisymmetric structures through the inclusion of earthquake-induced hydrodynamic effects into input ground motion accelerations. The resulting modified time-history and spectral ground motion accelerations can then be applied directly to the dry axisymmetric structure ( i.e. without water) through dynamic modal superposition or response spectrum analyses. Therefore, recourse to specialized software accounting for fluid-structure interaction dynamic effects can be avoided. The proposed approach evaluates the individual impact of hydrodynamic effects associated with any specific structural vibration mode on the global structural seismic response. The procedure implements hydrodynamic pressure formulation or simplified added masses as an alternative to classical nodal lumping. A hydrodynamic modification factor to efficiently estimate amplification or de-amplification of acceleration seismic demands due to earthquake-induced hydrodynamic effects is proposed. The application of the proposed methods is illustrated numerically through examples of three water- surrounded axisymmetric structures subjected to two earthquakes. The results of key response indicators, including relative displacements, base shear, and stresses are shown to be in excellent agreement with the coupled finite element solutions. The coupled effects of hydrodynamic-pressure and structural flexibility on the seismic behaviour of the studied systems are discussed.