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I have used moment redistribution fairly often, but mainly in the design
for seismic loading, and sometimes for 2 way slab design.  I would design
the exterior support for the redistributed shear/reaction, because that is
the situation you will be looking at when the ultimate loading condition
occurs.  Similarly, the interior support will be "underdesigned" when
compared against the elastic analysis moment--> that is why you are
redistributing the moment.  As to shear/reaction at the interior support, I
would think that unless it gets you into trouble, designing for the elastic
analysis shear/reaction would be a conservative approach.

I think that this brings up a good point on elastic analyses in general,
when used for R/C design.  At the ultimate load level, the structure will
be cracked, and the members will all have an effective moment of inertia
which is less than Ig (speaking about conventional reinforcement).  So,
unless one takes the time to "accurately" model the variable Ie of the
beam, the elastic analysis moments/shears and reactions are just going to
be an approximation of the actual conditions at ultimate loading.  Besides,
strength design uses ultimate loading, so strictly speaking (IMHO), a
collapse mechanism should be used if the most efficient design is to be
obtained.  Of course, that would be a major pain in many cases, and
serviceability concerns would also need to be addressed, but my point is
that moment redistribution is a) going to happen anyway and b) is a very
good method of increasing the efficiency of a design by reducing
unnecessary steel content, which can also lead to a more constructable
design (esp. wrt seismic moment resisting frames).

T. Eric Gillham PE

> From: Chance, Acie P. <acie_chance(--nospam--at)>
> To: 'seaoc(--nospam--at)'
> Subject: Moment Redistribution
> Date: Friday, June 26, 1998 1:30 AM
>      I came across a condition new to me the other day ( I have not done
> lot of concrete work ) and would like to get some feed back from the
> When redistributing the negative moment  over an interior support (
> Sec. 8.4 ) the positive moment in the span goes up along with the shear
> reaction at the exterior support.  Is the exterior support designed for
> increased load from the redistributed plastic moment or the load from the
> elastic analysis. Also what load is the interior  support designed for
> higher elastic or the redistributed smaller plastic moment. I designed
> for the worst case however this does not set well with me as there is
> 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
> on the beam capacity. I got mixed reviews at the office on which
> to use in design.