Seismic vulnerability assessment of a case study anchored liquid storage tank by considering fixed and flexible base restraints

Liquid storage tanks are among vital infrastructures, and their seismic vulnerability assessment plays a pivotal role in uninterrupted operation of an industrial plant. Technically, vulnerability notion relates capacity of each tank component's resistance to failure subjected to different seismic hazard levels. The predominant source of damage to liquid-containing tanks is steel shell buckling which can be intensified by base flexibility of fully anchored tanks. This flexibility is mainly resulted from anchor bolt failure and base concrete damage. In other words, the presence of anchors does not necessarily preclude anchorage failure or loss of concrete support, especially during large earthquake events. A case study on a liquid storage tank is introduced in this paper to highlight the influence of base flexibility on the seismic performance of fully anchored tanks. The tank is initially designed in accordance with the requirements of American Petroleum Institute, API-650. Fluid-structure interaction (FSI) is simulated by means of so-called added-mass approach. Two sets of finite element models are constructed, namely; fixed base (FB) and flexible base (FLB) tank. Nonlinear time history analysis based on a suite of twenty-two multi-directional spectrally matched acceleration time histories are conducted. Simultaneous input motions of two horizontal components are of particular importance as they shift the position of failure mechanism to directions being more affected by combined accelerations. Critical intensity measures (IMs) according to incremental dynamic analysis are then determined. Fragility curves are obtained by introducing conditional probability of failure as a function of critical IM. Results reveal that modelling base flexibility may contribute to lower value of critical IMs compared to that obtained from restrained support model. More specifically, FLB model demonstrates lower value of endurable peak ground acceleration (PGA) compared to the initial value selected for the tank design as per API requirements