Seismic Behavior of Shell-Base Connections in Large Storage Tanks

Unanchored steel tanks subjected to strong ground motion may experience rocking motion if the moment generated by the inertial mass of the content stored is greater than the resisting moment provided by the weight of the tank and its content. Rocking of the tank causes the shell-base welded connection to undergo cycles of rotation. The Eurocode provides a methodology for estimating the maximum rotation that this connection may experience. This methodology was applied to study six tanks located in Switzerland and it was found that four of these tanks did not satisfy the rotation limit set by the code. These findings motivated a more comprehensive study in which two tanks were analyzed according the Eurocode and New Zealand’s recommendations, and by means of a pushover and a time history analysis. This study showed that the Eurocode regulations can be very conservative, overestimating the shell-base rotation by a factor of 4 for one of the tanks studied.