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RE: Seismic Base Shear

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I concur with Mike Valley regarding Ss and S1 but Scott, there is one other
statement in your previous email which I think needs clarification. You
wrote: "In the 1997 UBC the EPA was Ca and that was multiplied by a 2.5
fault amplification factor to get a spectral pseudo acceleration." The 2.5
factor is not a "fault factor" but rather is the maximum spectral
amplification that a building in resonance with the earthquake would be
expected to experience based on a single degree of freedom system; i.e. it
represents the peak value of the response spectrum curve as shown in Figure
16-3 of the 1997 UBC. In essence, the formulas presented in the 1997 UBC
(similarly in the IBC) are the mathematical representation of this curve.
The soil factor, and fault factor if applicable, are included in the Ca
value in the 1997 UBC. 

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