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RE: FW: Vertical component of EQ in UBC97 and IBC2000

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The vertical component issue is in a state of flux. But the seismic code developers are making progress. Previously, we only looked at what I will call inadvertent vertical effects. These were vertical elemental forces that were caused by the structural response due to lateral ground motions. They occurred in roughly the same time domain as the lateral ground motion. The ground motions were historically fairly easy to quantify.

The next considerations were direct vertical ground motion effects. This is where the ground moves in a vertical direction. The most notable US example was the 1994 Northridge event and Kobe. Previously, the concern and concentration was the lateral faults that generated the big lateral ground motions. After Northridge, we started paying more attention to the thrust faults. The other type of faulting that can generate large vertical ground motions are the subduction zone faults.

The big difference is the arrival time of the vertical ground motions. The vertical ground motions arrive with the P waves and before the large lateral ground motions (mostly S and Love waves).

Another complicating issue was the mapping. There are currently no maps that show the vertical ground motions. You are required to develop a site specific response spectra for the lateral and the vertical ground motions in order to start the procedure. This is not too much of a problem for nuclear plants. But good news is on the way. There is an effort to develop maps and / or algorithms to quantify vertical ground motions without having to develop site-specific spectra. There are elements in the DOD and USGS that are working on this and will have it fairly soon. NEHRP, ASCE 7, and IBC will follow soon. There are many of the "usual suspects" involved in all of these efforts.

Generally if you do NOT have a thrust fault or subduction zone fault within 10 km of your site, you can ROUGHLY assume about 2/3 of the lateral ground motion will be a vertical ground motion. If you are within that magic 10 km, you may have vertical ground motions that may be larger than the lateral ground motions.

If you perceive that your structure is sensitive to vertical ground motion (i.e. cantilevered structures or elements, liquid containing tanks or vessels, etc.) and you are within the magic 10 km, you may want to get a vertical response spectra.

Once you have the input, you then need to decide what to do with it. There are no R values for vertical structural responses. You may need to consider amplifying vertical forces or performing a vertical dynamic analysis. I have done both on various projects.

The IBC simply disclosed the level of our ignorance that was contained in the UBC.

Stay tuned, when this is developed further, I will notify the list. Expect it in early 2005.

Harold Sprague

From: "Ali Karimzadegan" <karimzadegan(--nospam--at)>
Reply-To: <seaint(--nospam--at)>
To: <seaint(--nospam--at)>
Subject: FW: Vertical component of EQ in UBC97 and IBC2000
Date: Sun, 7 Nov 2004 16:03:05 +0330

> 	The vertical component of earthquake load as per UBC97 (1630.1.1)
> could be ignored for WSD method. What about the vertical load component as
> per IBC? I didn't see any item that permit to ignore this component in
> IBC. Does anybody know that could we neglect it or if it should be
> considered in all cases what is the reason that makes this difference in
> IBC w.r.t. UBC97?
> Regards,
> Ali Karimzadegan
> M.S. C.E.
> Shiraz - Iran
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