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Re: LRFD and the "Special Seismic Load Combos"

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Dave,

Not a bonehead question.

The special load combination generally occur in collector member and connection designs. The intent of the code is to beef up these members/connections using Omega level forces to ensure that they are capable of delivering seismic loads to the vertical resisting elements which will resist these loads. If these connections/members fail, the full potential of the shearwalls/braces/ moment frames etc... may not be realized in a huge seismic event.

There is an exception to using a Phi factor of 1.0 (not sure if that applies to LRFD though because it is in reference to Allowable stress design which I don't think uses Phi factors) and if you are using ASD, 1.7 increase in the allowable capacity is permitted IN LIEU of the 1.33 normally associated with steel design.

Hope this helps
-Gerard

Gerard Madden, P.E.
Civil Engineer

Middlebrook + Louie, Structural Engineers
71 Stevenson Street, Suite 2100
San Francisco, CA 94105
Tel: 415.546.4900
Fax: 415.974.3680
Email: gmadden(--nospam--at)mplusl.com