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

Abstract eng:
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 uplift of the tank’s 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.

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