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caisson footings

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Date: Thu, 19 Feb 1998 20:06:30
To: seaoc(--nospam--at)seaoc.org
From: Mark and Jessica Pemberton <jmpember(--nospam--at)softcom.net>
Subject: caisson footings

I have one remaining general questios regarding 94 UBC caisson footing
equations:
(Thanx to Lawrence B. Karp for answering the first question I had)
        The code allows for a developed frictional force between the walls of
        a cast in place caisson footing and the soil equal to 1/6 the allowable
        bearing stress (per table).  By using this value I find that the
frictional resistance
        is several times that derived using standard skin friction
equations.  Why is this?
        The maximum the code allows is 250 psf of frictional resistance which
        is way more than you can achieve in any standard soil.  Is this value
        perhaps only allowed for resistance in the downward direction, or can
        it be used to resist uplift as well?
Any help would be greatly appreciated.  So far I am getting widely varied 
opinion on this, and it seems different building departments are interpreting 
these provisions differently.  I have even called ICBO directly and received an
unsure at best response.