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# RE: Earthquake force considered in load combination (IBC2000)

• To: <seaint(--nospam--at)seaint.org>
• Subject: RE: Earthquake force considered in load combination (IBC2000)
• From: "Wesley Werner" <wwerner(--nospam--at)conewago.com>
• Date: Wed, 22 Dec 2004 10:09:18 -0500

Title: RE: Earthquake force considered in load combination (IBC2000)

E should be calculated using the full dead weight and possibly a percentage of the live load or snow load as well. I am not an expert on the codes, but I believe Equation 16-11 is essentially a load combination for overturning or uplift. Instead of calculating a factor of safety for overturning, you make sure that the building stays in equilibrium if this equation is used. BOCA used to have a note that said that no more than 2/3 (.67) of the dead load could be used to resist uplift or overturning. IBC just turned that statement into an equation and made it a little more conservative in the process. I think that the code writers want to be sure that engineers don't use a factor of safety for overturning, etc. less than 1.667 (1/.6). Also, when you calculate E the vertical portion (.2*SDS*D) of the seismic force should be opposing gravity loads.

Wesley C. Werner, EIT

-----Original Message-----
Sent:   Wednesday, December 22, 2004 9:33 AM
To:     SEAINT
Subject:        Earthquake force considered in load combination (IBC2000)

In the IBC 1605.3.1 equation 16-11 we have load combination of 0.6D+0.7E in which the dead load factor reduced significantly w.r.t. UBC97. Now the question is that whether E (in this load combination) shall be calculated based on the 0.6 dead weight of structure or E should be based on the full weight and if it is based on full weight then it seems that this load combination is over conservative and I can not understand the logic behind it. Please clarify.

Regards,