Seismic assessment of an industrial frame-tank system: development of fragility functions

Evidence from past earthquakes has shown high levels of damage to industrial facilities, demonstrating that seismic loading can cause damage or failure, not only to the main structural elements, but also to secondary or non-structural components. In contrast to other types of building structures, failure of a secondary component could imply both huge downtime costs for low intensity/high probability events and catastrophic accidents for high intensity/low probability events. It is thus necessary to investigate the seismic vulnerability of industrial plants in a comprehensive manner, that is (1) by including both structural and non-structural components in the framework of analysis; and (2) by giving full account of multiple damage conditions, which entails identification/consideration of a wide range of seismic intensities. In light of this, the analyses presented in this paper fall within a probabilistic framework and focus on the derivation of seismic fragility functions at multiple damage states, which describe the physical vulnerability of an industrial frame supporting a cylindrical steel tank. The structure in question is a steel special concentrically braced frame, whilst the tank is a small liquid-storage item commonly used for a vast variety of industrial processes. To begin with, a fibre-based finite element model of the supporting structure was developed within an open-source platform, and then an incremental dynamic analysis was carried out by considering 47 natural accelerograms that were scaled to 9 seismic intensities. For the first run, the mass of the tank was lumped together with that of the rest of the structure, and acceleration floor spectra were computed, permitting the evaluation of the maximum response of the tank, as completely uncoupled from the structure. Additionally, a second analysis was run to explicitly account for both the dynamic interaction between the frame and the tank as well as the hydrodynamic response of the fluid-tank system. To this end, use was made of a well-known analytical model that treats the impulsive and convective components of motion as two uncoupled single degree of freedom systems. Demand resulting from both methods of analysis was convolved with capacity corresponding to the attainment of limit state conditions for both the frame and the tank. Fragility models were derived and compared so as to evaluate the vulnerability of the case study as well as the effectiveness of both analysis approaches.