Seismic Retrofitting of R/C Shaft Support of Elevated Tanks

Abstract

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.

Seismic response factors of reinforced concrete pedestal in elevated water tanks

“…Rai [3] studied the performance of elevated tanks in the 1997 Jabalpur and Bhuj earthquake of 2001. It was concluded that with low axial load, small longitudinal steel ratio, and a thick wall, acceptable ductility is gained for RC pedestals.…”

“…Elevated water tanks have not performed up to expectations in many earthquakes in the past. The poor performance of these structures in the past earthquakes such as Chile 1960 [1] (due to design deficiency), Manjil-Roudbar 1990 (out of date design standards) [2], Jabalpur 1997 and Gujarat 2001 [3] has been reported in the literature. Extent of damages has been ranging from minor cracks in the pedestal up to complete collapse of the entire structure.…”

Simplified seismic analysis procedures for elevated tanks considering fluid–structure–soil interaction

“…Therefore, elevated tanks should remain functional in the post-earthquake period to ensure water supply is available in earthquake-affected regions. Nevertheless, several elevated tanks were damaged or collapsed during past earthquakes (Haroun and Ellaithy, 1985;Rai, 2002). Therefore, the seismic behavior of elevated tanks should be known and understood, and they should be designed to be earthquake-resistant.…”

Use ofIn-SituDynamic Measurements to Calibrate Analytical Models of RC-Elevated Water Tanks

“…Rai [9,20,21] has studied RC shaft-elevated water tanks affected by strong events, proposing techniques for their rehabilitation and recommending alterations on world codes to significantly reduce the ductility of these types of structures. These recommendations follow the same design philosophy of EC-8 [18], ACI [16], IBC [17] and IITK-GSDMA [19], among other codes.…”