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RE: Site Class E question

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You will need to check all 4 eqn’s of ASCE ch. 9.5.5.  Compute period T on the approx method, T = ct hn x .  The Cs coeff. should be based on, but need not be greater than


David A. Topete, SE

Structural Engineer


GFDS Engineers

543 Howard St., First Floor

San Francisco, CA 94105

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From: Anantha Narayan C.K. [mailto:anant_27(--nospam--at)]
Sent: Tuesday, January 16, 2007 7:12 AM
To: seaint(--nospam--at)
Subject: Site Class E question




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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