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RE: IBC Seismic

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UBC (section 1630.1.1) required you to include vertical seismic forces, Ev, for USD but let you ignore it for ASD. According to SEAC's Blue Book commentary, that was because "For allowable stress design, these vertical response effects are considered accommodated in the difference between design and actual vertical loads and in special requirements related to reduced dead load..."
I don't know why this was changed for the IBC, but it's been that way since IBC2000, and I don't see any discussion of the change in the commentary.
It seems more logical, particularly in high seismic areas, to consider the loads directly rather than to rely on some stress safety factor, since things may go inelastic on you.
-----Original Message-----
From: Ed Tornberg [mailto:ed(--nospam--at)]
Sent: Monday, January 31, 2005 10:26 AM
To: seaint(--nospam--at)
Subject: IBC Seismic

As a former UBC'r, I'm being baptized into the structology of QsubE, Sds, and an Esubm that has no "subm" in ASCE 7, because confusion is apparently sacred. 


For a "structural steel system not specifically detailed for seismic resistance (R = 3), I am enlightened that vertical seismic effects can be roughly ½ of horizontal effects (ref Eq


Does that sound right?


How did the UBC ASD method survive without this?  How could they be so naïve and ASCE 7 be so righteous?


Ed Tornberg

Tornberg Consulting, LLC