Seismic Retrofitting of R/C Shaft Support of Elevated Tanks

The circular, reinforced concrete (R/C) shaft-type support for elevated tanks lacks redundancy, damping and additional strength typically present in building framing systems and, therefore, should be designed for larger seismic resistance. However, the Indian seismic code IS:1893-1984 prescribes the same basic seismic force as that for the most ductile building framing system for which the design force is the least. Furthermore, the code-specified one-mass idealization of elevated water tanks is not appropriate for large (large width to depth ratio) and partially filled tanks. The low design forces lead to a weak and slender support—a very unfavorable feature in high seismic areas, as evidenced in the failure of two water tanks in the 1997 Jabalpur earthquake and a great many in the 2001 Bhuj earthquake. It is rather difficult to enhance the ductility and energy dissipation capacity of thin-walled, R/C shaft supports. Concrete jacketing is used as a retrofit measure to enhance the lateral strength and ductility by changing the failure mode of concrete crushing to a more ductile tension yielding. This scheme requires substantial strengthening of the existing foundation.