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RE: Site Specific Analysis Sds and Sd1 values

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I am assuming you’re referring to 2003 IBC (based on ASCE 7-02), which does not have any exceptions to calculating Sds and Sd1 per 1615.1.3. If you use 2006 IBC, which is based on ASCE 7-05 the only exception from calculating Sds and Sd1 is for simplified analysis, which is covered in section But even with that method, you don’t just substitute Ss with Sds. The only thing I can think of is that whoever prepared the structural notes didn’t have much familiarity with ASCE 7 seismic provisions.


Ben Yousefi, SE

Santa Monica, CA


From: Anantha Narayan C.K. [mailto:anant_27(--nospam--at)]
Sent: Thursday, April 12, 2007 11:05 AM
To: seaint(--nospam--at)
Subject: Site Specific Analysis Sds and Sd1 values




I have a question pertaining to computing Sds and Sd1 values from the values of a site specific analysis.


Here is the information.

Site Class = D

Ss = 0.48 g

S1 = 0.177 g


Fa = 1.42 and Fv=2.09


I used the procedures from IBC Section 1615.1.3 and used Sds = 2/3 Ss Fa and Sd1 = 2/3 S1 Fv to arrive at Sds = 0.4544 g and Sd1 = 0.2466 g.


However, in the structural notes of the job I am peer reviewing , the Ss and S1 values are reported as Sds and Sd1. Is there any place in the code that allows the use of Ss as Sds and S1 and Sd1. 


Any input on use of site specific geotech information to interpret design level seismic forces is appreciated.







Structural Engineer

Bliss and Nyitray Inc.

Miami, FL - 33134


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