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UBC 1808.2.2

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Mark,
I can think of several reasons for the relatively low
allowable stresses in piling.

First of all, they can't be inspected.  You don't know
that all of the concrete made it all the way to the
bottom, or that your rebar cage has concrete around it
to protect it.

Second, you don't know what's in the soil.  If it's
acidic, salt-laden, or full of sulfates, to name a few
problems, you might be glad to have extra material in
the ground 50 years from now.

Third, they're foundations.  They may (or may not) be
among the cheaper structural elements in your
building, but they're certainly the most expensive to
replace.

Fourth, the stresses may have derived from driven
piles, where experience taught that piles designed
with low in-service stresses tended to survive driving
better.

I guess we can quit counting, but there may be more
reasons.  For displacement piles, minimizing area (and
thus perimeter) only leads to very long thin piles.

Also, look at the old working stress codes.  ACI
allowed 0.45 fc' in compression;  AASHTO (and I think,
AREMA) allow 0.40 fc'.  That's for beams; actual
allowable bearing stresses (e.g. in beam seats or
under column base plates) is 0.30 fc', probably out of
concern for incidental eccentricity.

And of course, as was stated yesterday, who knows what
the unbraced column length or base fixity is of a
piling driven through 30 feet of loonshit (technical
term).

Oh, and one last question:  are these piles point
bearing on something in which you have an unwavering
faith?  I'd be very reluctant to increase loads
significantly on most point bearing piles without
tests, and would pretty much refuse to do it with
friction piles due to concerns about long-term creep.

Long answer to a short question.  Hope it helps a
little.

Mike Hemstad
TKDA
St. Paul, Minnesota

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