Elevated Tanks

INTRODUCTION Many elevated water tanks suffered damage to their staging (support structure) in the Mw 7.7 Bhuj earthquake of January 26, 2001. Three elevated water tanks located in a radius of approximately 125 km from the epicenter collapsed completely, and many more were damaged. The majority of these damaged and collapsed tanks were supported on a cylindrical reinforced concrete (RC) shaft. As a result of the January 26 event, many of these RC cylindrical shafts developed circumferential flexural cracks near the base. Similar damage to support shafts has also been observed in past earthquakes, most recently in the 1997 Jabalpur earthquake (Rai et al. 1998). Reinforced concrete framed stagings are not very common for elevated tanks in this part of India. At least one RC framed tank located near the epicenter collapsed, and a few developed cracking near beam-column joint regions. The poor performance of critical facilities like water tanks requires careful scrutiny of their designs, especially of those tanks farther away from the epicentral tract and located in areas that experienced shaking of intensity IX or VIII on the MSK scale. Even in the regions of highest shaking of intensity X, these structures should have remained functional. These water tanks are part of a very large network of drinking water distribution system in which water is transported hundreds of kilometers from underground sources and river basins to towns and villages in the arid regions of western Gujarat. The water is first stored in underground sump storage tanks, usually two to three times the capacity of the elevated tank, and is chlorinated before being pumped up into tanks for distribution. Elevated tanks are used to meet demand during peak supply hours. However, at the time of the earthquake, many elevated tanks were not being used because direct pumping was sufficient to maintain the required water pressure. As a result, a large number were either empty or only partially full when the earthquake struck. These elevated tanks were constructed under various schemes of the state government. At present, the Gujarat Water Supply and Sewerage Board (GWSSB) oversees the operation and maintenance of these facilities. However, major towns, municipalities, and local governing bodies also operate their own elevated tanks and sump storage tanks for water supply.