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Site Classification per IBC 2000

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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
ph (417) 865-6100
fax (417) 865-6102

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