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.

Experimental Study on the Seismic Behavior of Shell-Base Connections in Large Storage Tanks

Gustavo Cortes, Alain Nussbaumer

When an unanchored tank is subjected to strong motion, the impulsive mass of the contained liquid generates a moment which if strong enough could cause partial uplifting of the base plate. As the tank undergoes uplifting, the shell-base welded connection is subjected to cycles of rotation which could lead to its failure. To address this issue, the Eurocode and the New Zealand’s recommendations limit the rotation amplitude that the shell-base connection of a tank may undergo to 0.2 radians. This limit is obtained from the following two assumptions: (1) a plastic hinge with a length equal to twice the thickness of the base plate forms at the base plate, next to the connecting weld, and (2) the maximum strain that can be sustained by the base plate is 5%. While these two assumptions are reasonable and have been the state of practice for many years, no research work is believed to exist backing these two assumptions and therefore the 0.2 radians limit. For this reason, an experimental research was conducted to determine the real rotational capacity of shell-base welded connections found in tanks. A total of 24 shell-base connections were tested considering the bending and membrane stresses that develop at the base plate when uplift occurs. Constant amplitude tests were carried at different amplitudes of rotation in order to create curves of rotation versus number of cycles to failure. The main finding from this investigation is that the current limit of 0.2 radians is very conservative and that a limit of 0.4 radians would be more realistic.

2011

On the low-cycle fatigue capacity of unanchored steel liquid storage tank shell-to-base connections

Alain Nussbaumer, Gary Scott Prinz

Under strong earthquake motions, the liquid stored within large unanchored steel tanks can cause rocking and uplift of the tank base from the supporting foundation. Repeated rocking can lead to low-cycle fatigue fractures in the tank base-plate, and ultimately spillage of tank contents. Due to limited research on tank connection capacity, current European and New Zealand tank standards (EuroCode8 and NZSEE) impose unjustified rotation limits on tank shell-to-base connections (current limit being 0.2rad). These rotation limits often govern the design of new tanks, and many existing tanks require retrofit to meet compliance. This paper experimentally investigates the fatigue capacity of tank shell-to-base connections, with a focus on the effects from material ductility and applied strain range (rotation range). Twenty-seven tank connections representing two steel material grades are fatigue tested under constant range rotation cycles to generate fatigue-life curves. Tensile loads applied to the base-plate simulate membrane action present in tanks during uplift. Results indicate that the strain capacity of the base-plate base material strongly influences connection fatigue capacity as fatigue fractures originate in the base material away from the weld toe and weld HAZ. Increasing base-plate ductility drastically increases connection fatigue life. All connection specimens sustained multiple uplift cycles at rotations greater than the existing EuroCode8 and NZSEE limits.

Springer Verlag2012