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Re: Minimum Reinforcing in Compression Members

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Dave,

First...a slight correction, even though it does not really change your
question.  The amount of required vertical reinforcment per 1910.9.1 is 1%
of Ag not 10%.  I believe you just mistyped it because I still get 15 #5s
required for a 24 in diameter pier.

As to your question, I don't think that you are missing anything.  From
everything that I would gather, that provision would apply to your piers.
Since this provision basically comes from ACI 318, I looked in the
commmentary of ACI 318 to see if that would help give some "guidance".
The minimum steel requirement is supposedly there due to the potential for
bending, even if there is no apparent bending on the member.

But, keep in mind that this provision is likely meant to apply to
compression members that are either only under compression or under both
compression and flexure but with the compression being the governing load.
Thus, I can see an arguement that if your piers are primarily flexural
members (i.e. rather small compressive load but significant flexural load,
which is certainly possible in the case of single-family dwelling) then
the minimum longitudinal reinforcement should be determined by 1910.5
instead of 1910.9.  That could be what the other engineers that you talked
to are arguing.  After all, you don't really specify what level of
compressive load or flexural load is in your case.

Don't forget that section 1809.5.2.2 would appear to place a requirement
on the amount of transverse reinforcement steel since you are in seismic
zone 4.  This refers you to section 1921.4.  But, then you may have
already gotten that.

HTH,

Scott
Ypsilanti, MI


On Thu, 31 Jul 2003, Dave Browning, P.E. wrote:

> 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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