Thanks to all for the helpful discussions.

I have two other questions regarding the response spectrum option when using the B31E approach. I would appreciate any input on this.

In section 3.4, there is an equation that includes the terms PD/(4t) and 0.75i x (Msustained+Mseismic)/Z, etc...

1st question: Is this equation to be used at allowable stress levels? I believe I read in an earlier forum that this was the case.

2nd question: If that is the case (i.e. allowable stress), is that true for both the response spectrum approach and the equivalent static approach? Typically we have a site specific spectrum from our geotechnical engineer, but following structural engineering practice, the results from the spectrum are usually either used at ultimate strength design or they are scaled down before using working stress.

Any suggestions on how I can get a definite clarification on this?

Thanks.

Marco Italia