Experimental Study of "Elephant Foot Bulge" Instability of Thin-Walled Metal Tanks

Keywords: Seismic Loading, Base Excitation, Lifelines, Cylindrical Liquid Storage Tanks, Critical Facilities, Shell Instability, Elephant Foot Bulge, Diamond Buckling, and Shaking Table Tests

Abstract: In almost all strong earthquakes, ground supported liquid storage tanks experience considerable damage. The failure mechanism is one of instability of the tank shells, and the most significant type is known as `elephant foot bulge.' The true cause of this type of failure is still not fully understood. There exists a question as to whether or not the current design criteria are conservative, reasonably accurate, or unsafe. One of the most important reasons for these uncertainties is the lack of experimental data associated with this type of failure mechanism. This study was primarily intended to identify and quantify the 'elephant foot bulge' failure mechanism. Shaking table studies were carried out to investigate the buckling behavior of cylindrical liquid storage tanks under base excitations. Two specially designed aluminum tank models were fabricated, and they were subjected to various seismic loadings. The intensity of the shaping was gradually increased until the `bulge' occurred. Other dynamic response behaviors of cylindrical tanks also were studied as part of the overall program. During the tests, the phenomena of `elephant foot bulge' was clearly observed in the field. Test results of this study indicate that current seismic design criteria do not necessarily safeguard against this type of failure. New design criteria and design alternatives are needed.