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Re: 2000 IBC seismic questions

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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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