Seismic design Of elevated tanks

Abstract

In this study, Wind Force and Seismic forces acting on an Elevated water tank e.g. Intze Tank are studied. Seismic forces acting on the tank are also calculated changing the Seismic Response Reduction Factor(R). IS: 1893-1984/2002 for seismic design and IS: 875-1987(Part III) for wind load has been referred. Then checked the Design of Intze Tank by using the software STAAD PRO. An Earthquake is a phenomenon that results from and is powered by the sudden release of stored energy in the crust that propagates Seismic waves. At the Earth's surface, earthquakes may manifest themselves by a shaking or displacement of the ground and sometimes tsunamis, which may lead to loss of life and destruction of property. Seismic safety of liquid tanks is of considerable importance. Water storage tanks should remain functional in the post earthquake period to ensure potable water supply to earthquake-affected regions and to cater the need for fire fighting demand. Industrial liquid containing tanks may contain highly toxic and inflammable liquids and these tanks should not loose their contents during the earthquake. The current design of supporting structures of elevated water tanks are extremely vulnerable under lateral forces due to an earthquake as it is designed only for the wind forces but not the seismic forces. The strength analysis of a few damaged shaft types of stagings clearly shows that all of them either met or exceeded the strength requirement of IS: 1893-1984 however they were all found deficient when compared with requirements of International Building Codes. Frame type stagings are generally regarded superior to shaft type of stagings for lateral resistance because of their large redundancy and greater capacity to absorb seismic energy through inelastic actions. Various Codes have been considered and the maximum value of the ratio of base shear coefficient of tank to building, (BSCtank / BSCbldg) is about 3 to 4 in all the codes, as against a value of 6 to 7 for low ductility tanks. This implies that design base shear for a low ductility tank is double that of a high ductility tank. Indian Standard IS: 1893-1984 provides guidelines for earthquake resistant design of several types of structures including liquid storage tanks. This standard is under revision and in the revised form it has been divided into five parts. First part, IS 1893 (Part 1): 2002; which deals with general guidelines and provisions for buildings has already been published. Second part, yet to be published, will deal with the provisions for liquid storage tanks. In this section, provisions of IS: 1893-1984 for buildings and tanks are reviewed briefly followed by an outline of the changes made in IS 1893 (Part 1): 2002. Any design of water tanks is subjected to Dead Load + Live Load and Wind Load or Earthquake load as per I S Code of Practice. Most of the times tanks are designed for Wind Load and not even checked for Earthquake load assuming that the tanks will be safe under Earthquake Loads once designed for Wind Loads. However present observation on the earthquake at Bhuj has shown that this tanks must have been designed for Wind Loads but did not stand Earthquake Load. Keeping this in view two Intze Tanks are designed with different specifications are studied by taking into account the provisions of 1893:2002 and for Elevated Tank 1893:1984 as well as NICEE suggestions and the results are presented. We have concluded that there is no uniformity in type of tanks described in various documents. All documents suggest consideration of Convective and Impulsive Components in seismic analysis of tanks. Ratio of Base Shear of tank and building is 6 to 7 for low ductility tanks and 3 to 4 for high ductility tanks. Suitable provisions for lower bound limit on spectral values for tanks are necessary. Indian Code needs to include provisions on lower bound limit on spectral values of buildings and tanks and also Convective Mode of vibration in the seismic analysis of tanks. Based on the review of various International Codes, it is recommended that IS 1893 should have values of R in range of 1.1 to 2.25 for different types of tanks. R Value taken in IS 1893:1984 is nowhere in the range corresponding to that value in different international codes. Base Shear and Base Moment increases from Zone 3 to Zone 4 to Zone 5. With the increase in R value Base Shear and Base Moment decreases. Considering the design aspect, the seismic forces remain constant in a particular Zone provided the soil properties remain same whereas the Wind force is predominant in coastal region, but in interior region earthquake forces are more predominant. For R= 2.25 and 1.8, column size (450 mm) and reinforcements (8,25 Φ bar) remain same but for R= 1.5, column size increases to 500 mm and reinforcements change to 8, 20 Φ bar. Using STAAD PRO also we got the same values.