Seismic Reliability Analysis Methods for Elevated Spherical Tanks

Recently, there has been increased attention focused on the seismic safety of those major industrial facilities (e.g., refineries and chemical plants) located in seismic regions; particularly on those critical structures that when failed during an earthquake can potentially endanger a large population or cause substantial economical loss. Elevated spherical tanks, a unique type of structure commonly' used in the major industrial facilities to store extremely toxic and/or flammable material under pressure, fit into this group of critical structures whose reliability against failure under seismic load is of critical concern.

The primary goal of this study is to develop different systematic approaches, from different perspectives and at different levels of complexity, for evaluating the seismic performance (reliability) of elevated spherical tanks. A discretized-mass mechanical system with masses and stiffnesses as functions of liquid fill height is constructed to model the dynamic effects of liquid sloshing. The component reliability analysis, the first of the three methods developed, computes the annual failure probabilities of the structural components at intact state using only the hazard curve of the region and the site-dependent dynamic amplification factor spectrum as the seismic load input. Using the same input as the component reliability analysis but taking progressive failure and load redistribution into account, the system reliability analysis, the second method developed, identifies the most likely component failure sequences and obtains the overall system failure probability. The third method, the random vibration analysis, uses the nonstationary ground motion in the frequency domain as the seismic load input and a hysteretic restoring force model to include the nonlinear behavior of the elevated spherical tank supporting frame in the analysis. It evaluates the maximum horizontal displacement statistics at various ductility ratio levels.

A liquid containing elevated spherical tank located in the San Francisco Bay area is analyzed using all three methods as an illustrative example. The results of the analyses show that although the formulations u' the three analyses are different, their results are satisfactorily consistent. In addition, it is found that anyone of the three methods developed can be suitably incorporated into the analysis/design or the seismic risk evaluation process for elevated spherical tanks.