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(OP)
Section 1605.3 of the 2003 IBC defines load combinations for allowable stress design, one of which is equa 16-9 which requires combination of D (deadload) + 0.7E (earthquake) + L (liveload) + (Lr or S (snow) or R (rain))...section 1605.3.1.1 allows the combined loads to be multiplied by a 0.75 reduction factor but disallows the use of the 0.7 earthquake multiplier, so, the question is this...
In determining seismic lateral loads for allowable stress design, does the code require a combination of deadload + liveload + snow or rain loads when calculating the weight, W, as used in determining the  earthquake lateral load, E.

We can reduce this combination by using the 0.75 factor but cannot apply the 0.7 factor...obviously, the combined loads with the 0.75 facor create the dominate load. Even with the 25% reduction, the probability of full liveload & max snow with a design earthquake seems improbable. Please advise.

Regards,
The Island Engineer
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If I understand your question, I would respond that the W used to determine the seismic load effect E is independent of the load combination used, and is the same value for all load combinations.

Section 2.4.1 of ASCE 7-02 allows you to use the 0.7E with the 0.75.

There is a 2004 supplement to the 2003 IBC that allows the use of the 0.75 in conjunction with the 0.7 factor.  The supplement can be ordered from the ICC website.

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