Dear Sir/Ms:
I am a mechanical engineer,i do the design of pressure vessels by PV-ELITE software (version2005).Recently i have found a problem in-which i kindly need your help.
As you know as per UBC-97 section 1634.3 "Rigid structures" if period T is less than 0.06 then Earthquake shear force is calculated by equation (34-1) which is V=0.7*Ca*I*W, But if Period T is more than 0.06 then earthquake shear force should be calculated as per equation (30-4) which is V=(Cv*I*W)/(Rw*T),of course calculated value shall not be mor than value which is obtained from equation (30-5) which is V=(2.5*Ca*I*W)/Rw.
Now my problem:
I have a horizontal Drum with Dia.=3000mm and Tangent-Tangent line=35000mm,which is supported on two saddles.The distance between saddles is 25000mm and saddles are mounted on concret slabs with 3000mm height, however PV-Elite assumes that this Drum is Rigid and calculates earthquake shear force by equation (34-1,above mentioned),i don't understand why the software doesn't consider the height of slabs,i want to know why the software does not calculate Period T in first step and then calculate earthquake shear force (please be informed that it calculate period T for vertival vessels but for horizontal no),because as i calculate the Period T, i find that this Drum is not rigid.
(please be informed that i calculate the Period T as per procedure 3-3 of "Pressure vessel design manual written by Dennis Moss)

Please refer to this post,

http://www.coade.com/cgi-local/ultimatebb.cgi?ubb=get_topic;f=3;t=000242