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RE: Moment Redistribution

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Although not specifically precluded in the ACI, the intent was not to redistribute shear and reaction forces.  The intent of section 8.4 was only to take advantage of plastic hinges forming at the supports which reduces the negative rebar and increases the positive rebar.   Hopefully your structure is not so sensitive to not have the capacity for the small increase in reaction forces.

Few engineers use moment redistribution in the design stage unless the elements are highly repetitive.  In practice it is generally used as a back door for increasing rated capacities of elements.

Harold Sprague, PE
Krawinkler, Luth & Assoc.

-----Original Message-----
From:	Chance, Acie P. [SMTP:acie_chance(--nospam--at)]
Sent:	Thursday, June 25, 1998 9:30 AM
To:	'seaoc(--nospam--at)'
Subject:	Moment Redistribution

     I came across a condition new to me the other day ( I have not done a
lot of concrete work ) and would like to get some feed back from the list.
When redistributing the negative moment  over an interior support ( ACI-318
Sec. 8.4 ) the positive moment in the span goes up along with the shear and
reaction at the exterior support.  Is the exterior support designed for the
increased load from the redistributed plastic moment or the load from the
elastic analysis. Also what load is the interior  support designed for the
higher elastic or the redistributed smaller plastic moment. I designed both
for the worst case however this does not set well with me as there is only
one load and one load path and a unique value for the reaction should be
possible. The redistributed reactions seams like the right one to me based
on the beam capacity. I got mixed reviews at the office on which reactions
to use in design.