INELASTIC SEISMIC DEMAND OF SHAFT TYPE STAGING FOR ELEVATED TANKS

Abstract

Circular shaft type supporting structures are typically used in bridges and for elevated water tanks. The present study is on those of the elevated water tanks. The popularity of the shaft type staging is because of its ease of construction and the more solid form it provides than that of the frame construction. Elevated water tanks are important public utility structures of post earthquake importance. Reports of past earthquakes have indicated that damage of elevated water tanks was due to failure of the supporting structure. Thin shells of circular shafts have performed unsatisfactorily in the past earthquakes because they possess limited ductility and also they lack redundancy, damping and secondary load paths that are generally present in framed structures. Studies have shown that the curvature ductility is appreciable only for stagings of low axial load ratio, low longitudinal steel ratio and thick shells. The present study is to estimate the inelastic seismic demand (displacement ductility, base shear and base overturning moments) imposed by earthquakes on shaft type staging of elevated water tanks using SNAP-2DX (Structural Non-linear Analysis Program for 2D Structures). Two numerical models of the tanks were considered for carrying out the analysis, which primarily differed in the manner the water mass is modeled. Housner's two-mass model accounts for sloshing action of the water, whereas the one-mass model ignores it. In_ elastic time history analyses are carried out on the models so developed for seven scaled ground motions (El Centro, Chile, Chamoli, Koyna, Kobe, Northridge, and a spectrum compatible synthetic ground motion). The ground motions were scaled according to the UBC- f 97 scaling rule with respect to the 5% damped design spectra of Draft IS:1893-2000 on soil site. Three tank structures were designed either to remain completely elastic or designed to yield with a ductility level of 2. They were also designed for loading environments implied in moderate and high seismic zones (Zones III and V). In this study, it is observed that the design forces for the tank structures designed to remain elastic (R=1) are many times larger than those specified by the current code IS:1893-1984. Moreover, inelastic time history analyses indicate that even in moderate seismic zones they will be subjected to inelastic deformations, contrary to the design expectations. Thin-shell staging is non-ductile and cannot supply the inelastic demands in the range of 2.6 to 4.86 on average. The low level of ductility demands observed for thin-shell stagings designed to remain elastic is probably acceptable in low to moderate seismic regions if the section's ultimate strength falls in the tension region of the axial load and moment interaction curve. However, high ductility demands observed for loading environments in high seismic zones (Zone V) can only be met by ductile sections. The systems designed for low ductility with thick-shell (inside to outside diameter -0.7) is more likely to meet the expected ductility demands.