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Site Class E question

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I have a question pertaining to designing for site class E. My Ss = 1.71 g and S1 =0.68 g
Refer Tables a and b. Fa for site class E with Ss > 1.25 g, the Fa value is 0.9 as compared to 1.0 for site class B,C,D and E and Fv is 2.4 for S1 > 0.5
Cs for a structure whose time period is computed is given as Cs = Sds / (R/I). This I assume means a structure whose T is computed based on highest modal mass participation in either direction.
It is my understanding that the site class E is worse than D and hence, intuitively I am expecting a higher base shear. But the Cs value would be lower for site class E than for site class D. Could anyone please help me understand the difference in designing for site class E than for site class D.
Also is Cs = Sd1/ T*(R/I) only used when the Time period (T) is computed based on emperical values given in ASCE 7-02?

Structural Engineer
Bliss and Nyitray Inc.
Miami, FL - 33134

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