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Re: Site Classification per IBC 2000

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I've never used different site classifications for the *same* structure, but
it could make sense for two immediately adjacent structures with an
expansion joint.  Especially for a large building where there is significant
change in grade and soil characteristics across the site.  For a single
building with a single seismic system, you have no choice but to use a
single site classification, which means the more conservative one.

However, I agree with your arguments.  Look at the final paragraph in
section 1615.1.5:
"The rock categories, Site Classes A and B, shall not be used if there is
more than 10 feet of soil between the rock surface and the bottom on the
spread footing or mat foundation."

This implies to me that site class B *CAN* be used if there is 9.5 ft. of
soil over class B rock.  Even if you have 10.5 ft. of soil over the rock,
this should not be classified as class D.  The worst case would be class C
in my far from expert opinion.

Good luck talking the geotech into raising the site class.

Jason Kilgore
Leigh & O'Kane, L.L.C.
816-444-9655 (FAX)
----- Original Message ----- 
From: "Bruce Holcomb" <bholcomb(--nospam--at)>
To: "Seaint (E-mail)" <seaint(--nospam--at)>
Sent: Friday, September 12, 2003 6:38 AM
Subject: Site Classification per IBC 2000

Has anyone used different "site classifications" in designing various
portions of one building?  I am looking at section 1615.1.1 of the 2000 IBC.

I have a geotech report which gives a site classification of "B" for a
portion of the structure bearing on limestone bedrock and "D" for a portion
of the structure bearing on shallow footings bearing on soil.  I thought
that a site classification was for a "site" and would not depend on the type
of foundation system.  This project is a single structure which we are
planning to design to allow for differential settlement because part of the
building has a level about 10' below the rest of the building and will bear
on bedrock.  The remainder of the building is to be designed to bear on
shallow spread footings.  The total building size is about 230' x 500'.

The site classification is to be based on the "average properties in the top
100 feet" according to Table 1615.1.1.  The limestone bedrock is about
8'-10' below the surface in this area, so it seems to me the site should be
"A" or "B" and I don't want to design for "D" if I can use a better soil
class.   I don't understand how a site classification could be based on an
average of 100' of soil / rock and be different for one building.

Bruce D. Holcomb, PE
Butler, Rosenbury & Partners
300 S. Jefferson, Suite 505
Springfield, MO 65806-2217

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