Dear Sirs,

I have a problem while designing a tank for Seismic Analysis as per API 650 11th edition Nov 2008.

The tank is located in India; hence the applicable code for Seismic is IS-1893 & Zone given is V. For zone V the value of z is 0.36.

My concern is how to calculate the values of Ai & Ac, where Ai is Impulsive design response spectrum acceleration coefficient & Ac is Convective design response spectrum acceleration coefficient?

Does I use the equation for Ai = z/2*I/R*Sa/g [IS 1893]?

Or do I have to use the equation which is given in Appendix E of API 650?

The value of Ai calculated as per IS 1893 becomes much less than the calculated value of Appendix E, which in turn affects on the seismic moment. Because of the increased seismic moment (where Ai is calculated as per Appendix-E of API 650) mechanical anchorage is required & also the maximum longitudinal shell compression stress [E.6.2.2.2 of API 650] at the bottom of shell exceeds the allowable stress [E.6.2.2.3 of API 650] which in turn needs to increase the thicknesses of shell courses.

Kindly correct me regarding the equation which should have to consider.

Does the calculated value of Ai be less than z/2?

I'm not familiar with IS-1893, so I can't comment on the correlation between API-650 and IS-1893. However, make sure you familiarize yourself with the differences between API 11th Edition (June 2007) and API 11th Edition Addendum 1 (November 2008). If you're running TANK Version 3.10, its computations are per the 11th Editon (June 2007).