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IBC soil profile

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RE: 2000 IBC Section 1615.1.5 and Table 1615.1.1

Per Table 1615.1.1, Site Class is determined based on the "average
properties in top 100 ft". If I have 40-ft of loose sand with an average
N-value equal to 10 and 60-ft of clay shale ("soft rock") with a minimum
N-value of 30, I calculate an average N-value for the top 100-ft (based on
Equation 16-23) of:

(40+60)/(40/10+30/60) = 22

This would result in Site Class D.

Is this correct? Some Geotechnical Engineers have proposed neglecting the
rock and only considering the soil properties above the rock, which would
put this in Site Class E. This certainly would be conservative (50% higher
base shears!), but is it required by the code? Design was completed assuming
Site Class D prior to receiving final Geotech data, so it is desirable to
keep Site Class D if that meets the code requirements.

William C. Sherman, PE
Camp Dresser & McKee, Inc.
Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)

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