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RE: Minimum Reinforcing in Compression Members

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If you are using the UBC, at one time there was a provision that said if you
can get the pier to work with half the area then you are allowed to use as
little as 0.5% Ag for As.  Basically the provision is for when you have a
big pier for architectural reasons, not for strength reasons.  I don't know
if that is still allowed in Zone 4.  Maybe someone else knows the section I
am trying to remember.....

Jake Watson, P.E.
Salt Lake City, UT

-----Original Message-----
From: Dave Browning, P.E. [mailto:dave(--nospam--at)mcneileng.com]
Sent: Thursday, July 31, 2003 3:50 PM
To: seaint(--nospam--at)seaint.org
Subject: Minimum Reinforcing in Compression Members


I think I may be missing something.  Can anybody enlighten me?

I'm working on a single-family dwelling on a steep downhill lot in seismic
zone 4 (code is CBC '01).  The foundation is a cast-in-place pier and grade
beam system.  The piers are designed to resist gravity and seismic along
with downhill creep of the upper layer of soil.

Section 1910.9.1 requires minimum steel in compression elements of 10% Ag.
For a 24" diameter pier, this amounts to (15) #5 bars longitudinally.  It
doesn't bother me that this is about three times what would be required for
strength considerations alone, but first the builder, then the geotechnical
engineer, and now the city building department are all telling the homeowner
that this is more than they are accustomed to seeing; that usually the
engineer will provide four bars total, with perhaps a larger bar on the
uphill side to take care of the bending from the soil creepage.

So I guess I just want to know if they are correct.  I talked to the
building department and asked how other engineers get around section
1910.9.1 and they gave me an unconvincing explanation (after two days of
intra-departmental discussion).  The woman told me that these piers are not
considered "compression" elements because they are primarily intended to
resist bending from the soil creepage.  This logic escapes me.

So, will somebody tell me what I'm missing in the code?  Is there some
exception for residential construction, or for foundation elements, or what?
The commentary in ACI 318-95 explains the logic behind this provision pretty
well and I don't see why my piers are any different than any other
compression elements.

David F. Browning, P.E.
McNeil Engineering Structural, L.C.


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