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

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Steve,
In the code development world, we don't get very far without someone asking "Where are the bodies?". Academic code changes for the sake of academics are generally shot down early on.

Northridge had a very strong vertical component. There were cantilevered parking structures that collapsed. There were some tanks that had hoop stress failures. There were bridge cranes that collapsed. Kobe also had a strong vertical component, and had failures in the same types of structures. These events were a fraction of the potential in vertical earthquake ground motion, but the effected area for direct vertical ground motion is very limited (10 km from the fault).

The first level of effort had to focus on mapping where we experience large vertical ground motions. We focused on where we would exceed the normal margin for regular gravity load design.

This consideration will not be an across the board code requirement for all structures, and will be limited to particular structures that are known to be sensative to vertical ground motion. The design effort will also be a function of the the criticality of the structure.

I design unusual structures, and I have performed vertical dynamic analyses on structures. On one such structure, I noted an acceleration due to the response of the structure was an increase by a factor of 4 over the free field vertical ground motions. That would have greatly exceeded our gravity load margin.

Regards,
Harold Sprague




From: "S. Gordin" <mailbox(--nospam--at)sgeconsulting.com>
Reply-To: <seaint(--nospam--at)seaint.org>
To: <seaint(--nospam--at)seaint.org>
Subject: Re: FW: Vertical component of EQ in UBC97 and IBC2000
Date: Mon, 8 Nov 2004 10:32:45 -0800

Harold,

The big question is - are there any known instances of damage due to vertical accelerations? If so, then to what type of structures?

So far, even after the code-changing frenzy of 1994, the corresponding requirements of the code had not been changed. Actually, the seismic loads on the residential and other structures generally remained the same as they were in 1956.

According to my experience in observations and research after the Northridge earthquake, even in the areas of the highest recorded vertical PGA (Tarzana), damage was the same as elsewhere in the proximity of the epicenter.

May be, the ambiguity of the code reflects just that - we do not know about any historic damage, but can theoretically assume that damage may develop under some conditions.

"If ain't broken, don't fix it." At least for the most common structures, where the code changes may have a heavy and not quite justified yet effect.


Steve Gordin, SE
Irvine CA

  ----- Original Message -----
  From: Harold Sprague
  To: seaint(--nospam--at)seaint.org
  Sent: Monday, November 08, 2004 9:56 AM
  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.

  Regards,
  Harold Sprague




  >From: "Ali Karimzadegan" <karimzadegan(--nospam--at)pidec.com>
  >Reply-To: <seaint(--nospam--at)seaint.org>
  >To: <seaint(--nospam--at)seaint.org>
  >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.
  > > PIDEC
  > > Shiraz - Iran
  > >
  > >
  ><< winmail.dat >>

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