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RE: 2000 IBC seismic questions

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Rick, thank you for the response. The definition of "building height" in
Chapter 5 is helpful - even though it is not in the same chapter, at least
it is a code definition. 

I have been looking thru the NEHRP documents (FEMA 302/303) for
clarifications. Although the section you refer to is not explicitly stated
to be what is intended by the code wording, it is probably the closest we
can get to what is intended. 

By the way, for those that are not aware, FEMA 302/303 (1997 NEHRP Seismic
Provisions and Commentary) are included in the documentation provided on the
CD-ROM "Code Central" which came with the 2000 IBC. I do use that as a
useful commentary on the seismic provisions in the IBC and ASCE 7. 


William C. Sherman, PE
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com


> -----Original Message-----
> From: Rick Burch [mailto:rburch(--nospam--at)conterra.com]
> Sent: Tuesday, May 27, 2003 7:29 PM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: 2000 IBC seismic questions
> 
> 
> Bill, 
> Here is what I came up with when I had the same questions:
> 
> 1. Since there is no definition of building height in the seismic part
> of chapter 16, I assume that the definition in chapter 5 (p. 79)
> applies:  "The vertical distance from grade plane to the 
> average height
> of the highest roof surface."  
> 
> 2. I don't think there is anything in the IBC code or commentary that
> clarifies what is meant by "dynamic characteristics included in the
> analytical model".  I asked a professor about this recently 
> and he gave
> me some good advice. He said that since the IBC Seismic provisions are
> very similar (not identical) to the FEMA 302 provisions, FEMA 
> 303 (which
> is the commentary to 302) is a often good source to explain the IBC.
> 
> FEMA 302 uses the same "dynamic characteristics included in the
> analytical model" language, and although 303 doesn't come 
> right out and
> explain how to do this (that would be too easy), it appears to me on
> pages 77 and 78 that it describes a method to accomplish 
> this. Basically
> the procedure is this: 
> 
> 1. compute the story shears using the equivalent lateral force (ELF)
> procedure.
> 2. on this basis, approximately size the structural members and then
> compute lateral displacements at floors.
> 3. replace h in the formula for vertical distibution of seismic forces
> [Cvx= Wxhx/(sum wihi)]  with the story displacements and recompute the
> story shears.
> 4. if any story shear differs from the ELF story shear by 
> more than 30%,
> analyze the buiding using modal analysis. If the differences are less
> than 30% (this is what you are hoping for), design for these forces.  
> 
> I think that this is what they mean by "dynamic 
> characteristics included
> in the analytical model", since the commentary goes on to say 
> that this
> method is equivalent to a single cycle of Newmark's method for
> calculation of the fundamental mode of vibration.
> 
> Since this method would only adjust the vertical distribution 
> of seismic
> forces, I interpret it to mean that you don't have to worry about it
> with a one-story structure since all the shear is at the roof level no
> matter how you look at it.
> 
> I paraphrased some stuff above, so I suggest you get FEMA 302 and 303
> (both free) and read them yourself. This is all I have been 
> able to come
> up with - if anyone knows any other way to include "dynamic
> characteristics in the analytical model", I would like to 
> hear about it.
> 
> Rick Burch
> Columbia, SC
> 
> 
> "Sherman, William" wrote:
> > 
> > Two questions regarding 2000 IBC seismic provisions:
> > 
> > 1. Is "building height" as used in Table 1617.6 defined 
> anywhere? Is this
> > the height at the maximum point on the structure relative to grade?
> > 
> > 2. Table 1616.6.3 permits the use of the "equivalent 
> lateral-force procedure
> > with dynamic characteristics included in the analytical model" for
> > structures falling under item 4 in the table. How must "dynamic
> > characteristics" be included in the analytical model? What is this
> > specifically referring to? Does it simply mean accounting 
> for actual center
> > of mass and center of rigidity of the various building components?
> > 
> > William C. Sherman, PE
> > CDM, Denver, CO
> > Phone: 303-298-1311
> > Fax: 303-293-8236
> > email: shermanwc(--nospam--at)cdm.com

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