P-Delta Effect in Seismic Behavior of Elevated Water Tanks Incorporating Foundation Flexibility

Abstract

P -? effect may play a very crucial role to trigger an early failure of high-rise structures under seismicground motions. This effect may further be aggravated for structures like elevated tanks which have a large mass concentrated at the top of a relatively slender structure. Though studies on P - ? effect in high-rise multistoried building is still available, the similar study on elevated tank is terse. In this context, the present study is an effort to address the issue of P -? effect on elevated tanks subjected to seismic loading. A few realistic example elevated tanks are chosen for the purpose of study. The study clearly indicates the severity of P -? effect during inelastic range seismic behavior and provides a broad idea about the increase in overturning moment to be accounted in the design due to the presence of such P - ? effect. To study such effect, firstly fixed base condition has been considered and then to make the study more realistic soil flexibility has been taken into consideration. For studying the effect of soilstructure interaction, the flexibility of soil has been incorporated through idealized springs as proposed by Gazetas in a recent literature. The increase in overturning moment due to P - ? effect is found to be more when the effect of soil flexibility is incorporated. Apart from this, the similar trend in variation of P - ? effect has been observed irrespective of whether soil flexibility is considered or not. In fact, increased inelasticity as manifested through increase in response reduction factor may cause a sharp increase in P -?effect. Further, increase in number of panels and decrease in number of columns cause a decrease in framing action resulting in a decrease in P -?effect. Moreover, the elevated tank with lower height/radius ratio for tank container has P -? effect less sensitive to hysteresis model. In these cases, the effect is primarily contributed by the convective mass, the vibration of which is marginally influenced by inelastic range hysteresis model. Furthermore, such structures in general exhibit a lesser P -?effect as compared to their counterparts with higher height/radius ratio. However, such effect is found to be marginal during linear range behavior. The results of the study are physically interpreted and presented in the paper. These may provide useful input in performance based seismic design of elevated water tanks.