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Re: Flat plate punching shear question

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Cliff,

Like you I have always considered punching shear to be a brittle (non-ductile) failure mode and therefore have not allowed for moment redistribution in calculating the moments and reactions that have to be designed for. The code does not allow moment redistribution for non-ductile members. Of the Concrete Design Professors I have questioned on this (Australian, but concrete tends to behave the same even if it is upside down), only 1 has considered that moment distribution could be allowed for, several others have been of the same opinion as me.

Interestingly, do you find moment redistribution worthwhile after doing the crack control calculations and ductility calculations. Because moment redistribution cannot be used for the service moments, 20% moment redistribution would result in 20% higher stress in the reinforcement at the face you have distributed away from making crack control provisions more critical. Also, the ductility provisions become much more stringent and the members have to be much more ductile to allow redistribution. Deflections also can increase depending on the situation.

I find normally that after redistribution these other effects tend to dominate and force me to add extra reinforcement to compensate.

At 08:57 AM 29/10/2003 -0800, you wrote:
When you have a 2-way flat plate slab, ACI 318-02
section   8.4 says that you're allowed to redistribute
(and therefore reduce) some of the negative moments at
the supports (columns).

ACI 318-02 sections 13.5.3.2 and 11.12.6 say that you
have to account for a portion of the unbalanced moment
at supports being transferred between the slab and
column by eccentricity of shear about the centroid of
the critical (punching shear) section.

My question is this:  I've always assumed that the
moment you use when you look at punching shear is
unbalanced moment WITHOUT and moment redistribution
(conservative interpretation).  I've always assumed
(conservatively) that the provisions that allow some
negative moment redistribution was for computation of
flexural reinforcing only.  Redistribution of negative
moments relies on connection ductility. Flexural
reinforcing has such ductility. I've always treated
punching shear as a phenomenon that has VERY LITTLE
ductility.  ACI 318-02 is somewhat vague on this
point. It states that you can redistribute the
negative moments but does not specifically state if
there are any limitations as to what you can use the
reduced negative moments for (i.e., use them for
"everything" or use them for flexural reinforcing
calculations only.)

I know that I'm taking the conservative path (in not
using the reduced negative moments for my punching
shear calculations), but I am being questioned about
whether I am being too conservative.

Any thoughts or opinions?

TIA,

Cliff




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