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RE: Different concrete strengths for columns and floor system

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No. In this case I do not believe it is reasonable or justifiable. While the Code is not very specific in the definition of a floor system, I believe that this would fall outside the Code intent of a typical floor system and I would definitely treat the beam segment as part of a column, including ties through the beam depth.

Do you really need 5000 psi concrete in the columns? The problem might just go away if 4000 psi will work at the perimeter columns.

Be sure to check for slenderness, and use clear floor to slab soffit height, not floor to beam height. A 9" column won't be able to go very high before slenderness effects become important.

Byron Benson


From: MAM <mam(--nospam--at)cyber.net.pk>
Reply-To: <seaint(--nospam--at)seaint.org>
To: seaint(--nospam--at)seaint.org
Subject: Different concrete strengths for columns and floor system
Date: Fri, 07 May 2004 22:29:21 +0500

Refer to ACI Code Section 10.15 for "Transmission of column loads
through floor system" ,
which says that where the column concrete strength does not exceed the
floor concrete
strength by more than 40 percent, no special precautions need be taken.
Now, consider a case of  a mutistoried building periphery  column 9"x36"
of  5000 psi
concrete framed by  9"x48" deep spandrel beam of  4000 psi  concrete,
along the column length.
The column upto the bottom of the beam is cast with 5000 psi concrete
whereas the column portion cosisting of the 48" deep beam is cast with
4000 psi concrete, as is cutomary. Now my question is:
Is it reasonable
to apply  the above ACI clause to this situation and do nothing about
the low concrete strength for the 48" portion of column, since
the concrete strength
ratio is  less than 1.4 ? Any comments would be welcome.
MAM



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