Abstract
Elevated water tanks are categorized as strategic components of water supply systems in modern urban management. Past earthquake events have revealed the high vulnerability of these structures. This paper investigates the development of rocking isolation (RI) to these structures as a response mitigation technique. Using an analytical approach, a dynamic model is developed for two isolation cases: (1) at the pedestal base and (2) under the tank. The model incorporates a simplified analogy for simulating the liquid-tank system which is modified for a tank under rocking motions. Based on the dynamics of rocking structures, the equations of motion, impact, and uplift transitions are derived. Then, free vibration and seismic response history analyses are carried out on a sample structure. Discussions are made on the effect of RI on the dynamic and seismic responses of the pedestal and components of the liquid-tank system. Effects of various RI cases, pedestal heights, and tank filling levels are studied for a group of structures excited by an ensemble of ground motions. Considering that the system may be vulnerable to other lateral loadings, the combined effects of seismic and wind hazards are also studied. The wind loads are assumed to act statically and simultaneously with the seismic excitations. Results show that the first case of RI decreases the acceleration demands of mid-rise and tall structures, thus lowering the structural demands to 50% of the fixed-base system. However, the second case of RI has almost no effect on the performance of the system, upgrading only the response of mid-rise structures. Both RI cases also aggravate the wave oscillations and increase the freeboard requirements. Finally, while the combined seismic and wind hazards have almost no effect on the operational performances, the force demands of the structures are increased by 10%.
Highlights
Rocking mechanism is a low-damage technology that reduces the spread of nonlinear damage to structural components and mitigates the downtime and repair cost of structures after an earthquake event [1]
When the excitation is over, the rocking structure will return to its original position and unlike fixed-base structures, no residual deformations are observed. is provides resilient behavior against cascading hazards such as multiple aftershocks. ese aspects of rocking mechanism, in addition to the ancient structures with unintentional rocking behavior or modern ones equipped with rocking isolation (RI), which performed well during past earthquakes [2, 3], have drawn the attention of many researchers. e first analytical model for a rigid-like structure rocking on a rigid base was proposed by Housner [2] in an effort to investigate the behavior of tall slender structures such as elevated water tanks that survived during the 1960 Chilean earthquakes
Fixed-base structure which is analyzed with an initial displacement of θ0HS assigned at the end of its shaft
Summary
Rocking mechanism is a low-damage technology that reduces the spread of nonlinear damage to structural components and mitigates the downtime and repair cost of structures after an earthquake event [1]. It was shown that despite the appearance of instability for these structures, there is a scale effect making the larger structures more stable than smaller ones with similar geometry. Hereafter, application of this technique to various structural systems like buildings and bridge piers has been the objective of many theoretical [3,4,5,6,7,8,9,10,11] and experimental studies [5, 8, 12, 13]. There are some studies conducted on the intentional application or development of rocking mechanism for nonbuilding structures such as industrial equipment [18], bridge piers [19], and architectural or art objects [20]
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