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RE: ASD - Seismic

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Wind, LL, snow, DL are all loads that are intended to yield a linear response.  The pseudo static forces resulting from seismic ground motion are uniquely intended to yield a nonlinear response.  As such, E goes into the limit state load combinations as unfactored for limit state design. 
When ASD is employed, the response service forces are derived from the response limit state forces.  Therefore the implicit load factor (1.4 or 1/1.4=0.7) must be backed out to yield ASD response forces. 

Since the intended seismic performance is nonlinear, the limit state design methodologies more readily lend themselves to seismic design.  However, there are many structures that retain ASD methodologies (like steel tank design).  Recognizing this, necessitated the accomodation of ASD load combinations in the ASCE 7. 

Harold Sprague (Invoice:  That will be 3 beers.)

Date: Mon, 10 Mar 2008 06:19:42 -0700
From: engrfrancis05(--nospam--at)
Subject: ASD - Seismic
To: seaint(--nospam--at)

Dear Friends,
Why do we need to divide the seismic on ASD by 1.4(e.g. D + E/1.4)? What's the reason behind that?
Thank you.
David Francis C. Caballero
Structural Design Engineer
License No. 0103970
Professional Regulation Commision
Manila, Philippines
email: engrfrancis05(--nospam--at)
CP#: +63917-981-8588

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