Seismic Induced Forces on Rigid Water Storage Tanks

Samir H. Helou

Abstract


Ubiquitous water storage tanks constructed of reinforced concrete are one popular and widely used component in any major water distribution network particularly for domestic water use. Structural engineers rely, at best, on finite element method program packages to solve for the structural parameters necessary for the design undertaking. In predominant instances a two dimensional formulation is still prevalent when seismic loadings are considered. Computer programs unless supplied by an adequate numerical model would give erroneous results especially when seismic forces are of due consideration. The following discourse presents a 3D structural modeling technique that accounts for the prescribed hydrodynamic pressure, indispensible for accurate evaluation of the induced forces on the outer shell; this is in line with ACI code requirement albeit these requirements are not clearly tailored for a 3D analysis. Furthermore, detailed study of the seismic parameters involved is conducted; this includes but is not limited to the kind and the number of modal modes involved and the associated aspect ratio of the tank. The discourse is limited to the common upright reinforced concrete circular cylindrical water storage tanks anchored and built on grade. This is tantamount to considering one mode shape of vibration leading to only one component of convective added mass. For quantifying the effect of support conditions the same tank model is modified to include select soil conditions modeled as area springs. The overall impact of support conditions on the overall behavior of the structure is of a magnitude to be reckoned with.


Keywords


Storage Tanks, Impulsive Pressure, Convective Pressure, Structural Modeling, Hydrodynamic Forces

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References


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