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RE: Site classification - IBC

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From: "John C. Jones" <john(--nospam--at)struct-engr.com>
To: <seaint(--nospam--at)seaint.org>

Maybe that's really what my question was.  When you look at the method =
used to get the average it has "N" or blow count and depth.  Does rock =
really have a blow count?  If not, the formula blows up.

John C. Jones, PE
Barnett Associates
Pell City, AL
205-884-5334
205-884-0099 (fax)

I have been out of the office for a few days - sorry for the late response.  Section 1615.1.5 of the 2000 IBC spells out exactly how to obtain the average blow count.  In this section you will note that N cannot exceed 100.  Therefore, the N value for rock is 100.  According to the geotechs in my area this really limits the site class to C or worse unless you bear directly on rock.  In fact, due to the averaging formula, with about 25 feet or more of soil above rock you can do no better than site class D.  Keep in mind that geotechs do not regularly do borings to 100 feet.  You have to specify a 100 foot boring or they will assume soil below the normal boring depths (about 20 to 25 feet).

Adam Vakiener, PE
Southern A&E, LLC
Austell, GA


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