Cyclic demand at the shell-bottom connection of unanchored steel tanks

Gasoline and other oil byproducts are commonly stored in steel tanks that rest on ground without any anchorage. Under strong motion, the impulsive mass of the contained liquid generates a moment about the base of the tank capable of causing partial uplift of the tank. The uplifting of the tank is accompanied by a distortion of the shell-base welded connection which may induce low-cycle fatigue failure. Failure of this connection is critical since the liquid contained is usually hazardous to the environment. Current codes of standard practice address the capacity of the shell-base connection. In the case of the Eurocode [1], it establishes that the maximum rotation that this connection may undertake is 0.2 radians. This limit came from a series of reasonable assumptions; however, no experimental studies existed to back it up. This paper shows a mathematical model, performed in the analysis software OpenSees [2], of a tank from which the expected number of cycles at the shell-base connection was estimated for the expected maximum seismic hazard in Switzerland. The mathematical model consists of a multiple-degree of freedom (MDOF) system which accounts for the initial (at rest) tank stiffness and mass, attached to a rigid beam (simulating the tank circular base) with a system of springs at the ends of the rigid beam. The springs account for the stiffness of the soil and, if there is uplift, for the weight of the liquid being lifted up by the base. This model was calibrated by using the method of the New Zealand’s recommendations [3] which is a modified version of the work by Clough [4]. A dynamic time history analysis was then performed and the number of cycles of uplift determined from this model. The number of expected cycles (cyclic demand) was then combined with capacity curves obtained from experiments of typical shell-base connections [5] found in steel tanks. The combination of the cyclic demand and the capacity curves allowed for the determination of the rotational capacity of the tank. The results of this research have revealed that the Eurocode limit of 0.2 radians is overly conservative and that a limit of at least 0.4 radians would be more realistic.