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RE: Column Reinforcing

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AHA!!!  I knew there had to be a way.  I stopped at 22.2.2, where it says
"The use of structural plain concrete columns shall not be permitted."

Thanks for your help,

Jason

________________________________________
From: Wesley Werner [mailto:wwerner(--nospam--at)conewago.com] 
Sent: Thursday, March 03, 2005 4:38 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Column Reinforcing

Jason,
 
    If there is no bending in the piers, can they be made to work on paper
as plain concrete? You could design them as pedestals with an unsupported
length of up to 12' (3x the least horizontal dimension (ACI 318-02 22.8.2).
I calculated phi*Pn as being 4,145 kips (Eq 22-5 with phi=.55). Even if you
do reinforce the piers as described you wouldn't have to meet the min.
steel limits for reinforced concrete. ACI defines plain concrete as concrete
with no reinforcing or less than the minimum specified for reinforced
concrete. 
 
 
Wesley C. Werner, EIT
-----Original Message-----
From: jwknospam [mailto:jwknospam(--nospam--at)comcast.net] 
Sent: Thursday, March 03, 2005 4:48 PM
To: seaint(--nospam--at)seaint.org
Subject: Column Reinforcing
I?ve got a building where the client wants to add a partial floor below
grade.  He?s demolishing the first floor slab, removing the soil (soft clay)
to bedrock, and re-building a structural slab where the first floor used to
be.  The end result will be a below-grade parking level.

The structure is a 4-story concrete building with about 31? bays.  The
reinforced concrete columns (32? octagons) stop at grade and are supported
by 48? sq. unreinforced piers to bedrock.  These piers are confined by the
soft clay soil.  The re-built first floor will be at original grade, so I?m
not worried about the hinge.

One of the uses of this structure in the past 60 years was storage for a
greeting card company.  Anecdotal evidence has every floor covered in huge
pallets of paper stacked from floor to ceiling, i.e., MASSIVE live loads. 
The new use for the structure will be residential (condos), so the live load
is reduced to 40 psf in most places.

What I?m worried about is the newly exposed unreinforced pier.  Containment
reinforcing steel must be added, but how much?  Bending isn?t an issue ?
we?re talking about 4 or 5 columns out of about 50, and there are new
reinforced concrete shear walls around the perimeter of the excavated area. 
The maximum factored vertical load is 860 kips, and the existing pier
strength is 5500psi (per several tests).  We?re in a low to moderate seismic
zone (12% - 6%).  Any axial creep occurred decades ago.

Per ACI 318 10.8.4 and 10.9.1, the minimum reinforcing required is 0.5% of
the gross area.  Adding long bars and wrapping in new concrete raises the
area to 56?x56? which requires As=15.68, or (12) #11 bars.  This seems quite
excessive, and the owner almost fainted when I told him.

My question is, how can I reduce the number of longitudinal bars and still
comply with the code?  Can I just use the area of the new concrete wrap (4?
on a side)?  This brings the area to (56?x56? ? 48?x48?)*1% =  8.32, or (12)
#8 bars.  This is much more reasonable, but does it technically meet code?

Any other ideas?

The ties will be (2) #4 hooked stirrups (?U? shapes with hooks on the ends)
installed on opposite sides of the column, alternating directions each
level.

Thanks in advance for any ideas,

Jason W. Kilgore




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