THE EFFECTS OF VERTICAL COMPONENT OF EARTHQUAKE GROUND MOTION ON ELEVATED SHAFT-SUPPORTED CYLINDRICAL TANKS

Past earthquakes have proven seismic vulnerability of elevated water tank, and some of the observed damages seem to be due to the effects of near-field excitations. Thisis while most of seismic design codes are still based on far-field earthquake excitations. Near-field earthquakes, in addition to the long period strong pulse, whichcan cause extensive sloshing, may have strong vertical component, which in combination with sloshing, can cause large eccentric vertical loading on the tanksstructure, and this may create some additional moments resulting in damages to boththe tank supporting structure and its foundation. These damages can be more crucial in case of tank with a shaft-type structure. To investigate this problem, in this studytwo existing cylindrical tanks, designed based on the conventional code requirements, each in two relatively tall states, with h/D more than 0.5, and tworelatively short states, with h/D less than 0.5 (h is the height of the water in that tank and D is the tank radius) were considered. Two series of earthquakes, one includingfar-field events, and the other including near-field events, with a specific frequency range and high energy in the long period range, to excite the sloshing modes of the tanks, and with PGA values varying in a wide range of 0.2g to 0.7g were consideredfor time history analyses (THA). The THA were performed by using a powerful finite element analysis program, in which the flexibility of the tank wall and theresulting water-tanks interaction can be taken into consideration. The results show that in some cases, particularly for relatively short states, the combined effects ofsloshing and vertical ground acceleration result in large values of overturning moments, which are beyond the values obtained by the code requirements. On this basis it can be said that the codes requirements for seismic design of shaft-supported elevated tanks need some modifications to take into account the near filed effects.

کلیدواژه ها:

Seismic Design Codes ، Time History Analysis ، Finite Element Analysis ، Sloshing