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RE: Steel Moment Frames in Seismic Zone 0

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Bill,

The answer is yes.  It is true that many engineers in places that
traditionally have not needed to be too concerned with seismic issues will
need to pay MUCH more attention to it now.

If you have used the UBC or are familiar with the UBC, then you should
know that traditionally the UBC has used "zones".  The zones in the UBC
have basically served two functions: 1) to determine the equivalent
lateral force; and 2) to determine the detailing requirements for the
structural system (i.e. how much ductility is required).  Under the UBCs
of the past, the zone was strictly a function of the potential seismicity
of the building site.  If I recall correctly, there was also an importance
factor that was used in the equivalent lateral force calculcations, but
DID NOT effect the zones.  Thus, the detailing requirements for the
structural system had traditionally been strictly a function of the
potential seismicity of the building location (i.e. the size or magnitude
of the "design" quake).

The next interation in this process was to include the importanct of the
building in the "equation".  If you are familiar with any of the model
codes based upon the older NEHRP provisions (such as 1993 BOCA, 1996 BOCA,
and some SBCs), then you have likely encountered this.  These codes
introduced the concept of the Seismic Performance Catergory.  The Seismic
Performance Catergory was a function of both the potential seismicity of
the location (by using Av) and the importance of the building.  In these
codes, the Seismic Performance Catergory would not affect the equivalent
lateral force, but WOULD be used to determine the detailing requirements
(and which type of seismic analysis could be used...i.e. whether you could
use the equivalent lateral force procedure).  Thus, the equivalent lateral
force WAS NOT a function of the building importance (but soil type DID
affect it) in these codes, while the detailing requirements were a
function of building importance (is it a hospital or a normal office
building?) _AND_ the potential seismicity of the location (i.e. how severe
is the potential/design quake).  The result is that you can have a
hospital that is in a low to moderate seismic zone (zone 1 or 2) that
could have the same detailing requirements (ductility) as an office
building in a traditional high seismic zone (zone 3 or 4).  While this did
have an effect on the detailing requirements, it usually did not "change"
things too drastically from the "old" way in the UBC.  It might "push" the
detailing requirements up one level, but likely that would be it.  As an
example, we are basically a zone 0/zone 1 here in Southeastern Michigan
under the "old" UBC method.  When using BOCA, the "worst" detailing
requirement that I have encounter was for a Seismic Performance Catergory
'C' which is roughly like a zone 2.

Finally, the IBC has take one more step.  It is based upon the latest
NEHRP provisions.  It has changed from using Seismic Performance
Catergories to Seismic Design Catergories, but they both serve the exact
same purpuse.  While the SPC in the older NEHRP based codes where a
function of the potential seismicity and importance of the building, the
Seismic Design Catergory of the IBC is a function of potential seismicity,
importance of the building, _AND_ the soil type at the site.  Thus, since
the detailing requirements are based upon the SDC, the required ductility
is a function of these items too.  So, if you have a building that is an
"essential" facility (i.e. hospital) that is in a low to moderate area of
seismicity _AND_ is also on a site with really crappy soils, then the
detailing requirements could get similar to a zone 3 or 4 requirements of
the "old" UBC method.  The end reult is that now the type of soil at a
site can dramatically effect the detailing requirements for the structure.
Thus, there are a lot of locations in the U.S. that traditionally have not
had a second thought about seismic design that will be a little shocked by
the level of detailing requirements that may be required for their
buildings.

As in the older NERHP based codes, the equivalent lateral force is still
_NOT_ a function of the importance of the building, but is still a
function of the seismicity _AND_ the site class/soil type.  If you think
about this, it makes sense.  A building that is in a higher are of
seismicity that is on "crappy" soil (such as deep clays) that amplifies
the seismic waves (as what happened in the Mexico City quake) should have
a higher design force than a building that is in an area of low seismicity
and sitting on rock.  And if you have two buildings that are in the same
location sitting on the same soils (i.e. they are right next to each
other), but one is a hospital and the other is the office buildign that
you or I work in, the desire would be to insure that the hospital would
perform better even though in reality it would see the same seismic "input
forces" as the office building.  Thus, the higher detailing requirements
but same lateral forces (assuming that the buildings were the same size
and configuration...that is same mass).  The "older" UBC methods (not
even today's or other recent UBCs but back further), again if I recall
correctly, achieved the similar result of requiring the hospital to
perform better by "making" it resist higher lateral loads.

BTW, there was a reasonably good article by S.K. Ghosh on this whole issue
in one of the more recent editions of Concrete International.  It would
have been about a year ago.  He went through some examples in various
cities showing how the detailing requirements could change as a result of
the IBC using the SDC.  The article is titled "New Model Building Code and
Seismic Design" and was in Volume 23, Issue 7 of CI (it appears this was
the July 2001 issue).  You can order a electronic copy of it from ACI's
website for $15 if you are a member ($20 otherwise), but I would suggest
finding someone who has it in print to borrow it from to read since $15 is
a little too steep for me.

HTH,

Scott
Ypsilanti, MI


On Sat, 20 Jul 2002, Bill Polhemus wrote:

> Okay, dumb question time (I seem to be the one that brings them up, so other
> people don't have to. C'mon, you know who you are!)
> 
> Anyway, I haven't had time to catch up with the IBC as yet, though I do have
> a copy sitting here looking pretty. I remember hearing that seismic design
> is going to HAVE to be considered even in places were it never was before
> (i.e. Zone 0/Zone 1). Is this true?
> 
> It appears from Charlie's response to Mark that this is the case. I know I
> could just crack open the UBC and start reading (and I've even purchased
> several of the supplemental publications such as the "Structural/Seismic
> Design Manual for IBC Vol. 1," so I could go there as well).
> 
> But I like the capsule descriptions that folks like Charlie and Harold
> always give.
> 
> Thanks.
> 
> 
> William L. Polhemus, Jr. P.E.
> Polhemus Engineering Company
> Katy, TX, USA
> Phone (281) 492-2251
> FAX (281) 492-8203
> email bill(--nospam--at)polhemus.cc
> 
> -----Original Message-----
> From: Carter, Charlie [mailto:carter(--nospam--at)aisc.org]
> Sent: Saturday, July 20, 2002 9:17 AM
> To: 'seaint(--nospam--at)seaint.org'
> Subject: RE: Steel Moment Frames in Seismic Zone 0
> 
> >I am preparing to design a 4 story moment
> >frame office building on the east coast
> >(seismic zone 0) and was wondering what
> >other folks have been doing.
> >What are some other engineers in this area
> >requiring?  What is standard practice?
> 
> It all depends upon the R factor you select (or are permitted to select in
> the applicable building code)....
> 
> 
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