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Follow Up on Vertical Component of EQ in UBC97 and IBC2000

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I was reviewing this excerpt I clipped last year.
Any follow ups that we missed?

From: "Harold Sprague" <spraguehope(--nospam--at)>
To: seaint(--nospam--at)
Subject: RE: FW: Vertical component of EQ in UBC97 and IBC2000

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