On the modal response of an instrumented steel water-storage tank including soil-structure interaction

Abstract The modal response of liquid-storage steel tanks and the associated prevailing frequencies in the horizontal impulsive mode of vibration are explored by means of earthquake recordings. To this end, a well-documented case study is employed, referring to an instrumented steel cylindrical tank which is a property of the HEL.PE. Oil Company in the Kalochori area, west of Thessaloniki in Northern Greece. The 28.5m-diameter tank has a self-supporting dome roof fixed to the tank walls and it is partially filled with water, corresponding to a ratio of the liquid free surface height to the tank's radius (H/R) at 0.74. Two accelerometric stations are mounted close to the center of the tank roof and on the ground surface at free-field conditions. Seventy four (74) acceleration records from low-intensity earthquake motions are processed to identify the fundamental frequencies of the tank and of the foundation soil from pertinent transfer functions in the frequency domain. The experimental findings are compared with predictions from theoretical formulae recommended by seismic design codes for fixed-base and flexibly-supported tanks, thus allowing the assessment of soil-structure interaction effects on the fundamental period of the tank. A simplified three-dimensional finite-element model of the fluid-tank system resting on flexible supports is also analyzed to provide further insight on the recorded response, as affected by critical structural features of the tank. Results highlight a dominant rigid-body mode which is associated to the lateral fundamental period of the flexibly-supported system, in agreement with relevant code provisions for tanks with H/R below unity.