Abstract Summary
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 finite element model. We conducted ambient vibration measurements in a 50-year-old elevated water tank located in Kathmandu, Nepal. The tank was constructed using smooth bars before the codal provisions were introduced. 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) was performed to estimate the modal properties of a 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 developed finite element models 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. On the other hand, while considering soil flexibility, the fundamental vibration frequencies were estimated as 1.08 Hz, 0.91 Hz, and 0.78 Hz, respectively for empty, half, and full reservoir conditions. 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.
