System identification based appraisal of soil structure interaction in elevated water tank for various reservoir conditions

Abstract

Abstract Due to the lack of information regarding structural details and construction technology, finite element analysis based results may be dubious for aging structures. The results will not confirm the effect of soil structure interaction either since back analysis based assumptions may not exactly represent the behavior of soil and structure accurately. To this end, realistic material properties as well as dynamic characteristics are required for a reliable numerical model. This study reports the dynamic characteristics of a 50-year-old pre-code overhead water tank constructed with smooth bars. Ambient vibration records were taken for empty, half, and full reservoir conditions. Parametric identification using the numerical algorithm for subspace state space system identification (N4SID) is performed to estimate the modal properties of the 14.43 m tall water tank. The fundamental vibration frequencies of the tank for empty, half, and full reservoir conditions are estimated as 1.1 Hz, 0.93 Hz, and 0.77 Hz, respectively. We also created the numerical model of the tank with and without soil structure interaction. The fixed base analysis resulted in the fundamental vibration frequencies of 1.21 Hz, 1.02 Hz, and 0.86 Hz, respectively for empty, half, and full reservoir conditions. Similarly, the fundamental vibration frequencies are estimated as 1.08 Hz, 0.91 Hz, and 0.78 Hz, respectively for empty, half, and full reservoir conditions when soil flexibility is considered. The results highlight that system identification-based fundamental vibration frequencies are better captured by the model created incorporating soil flexibility. We conclude that soil structure interaction is vital to be considered for special structures such as water tanks in active seismic regions with loose soil deposits.