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Re: Moment Redistribution in Concrete Frame

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In continuous structures, the span moments are smaller than the support moments. Structural collapse does not occur as soon as the the most stressed section reaches its moment capacity. A so called plastic hinge forms at that section and inelastic deformation begins at that section at "constant" bending moment as the loading is increased further. The process continues until an interior section of the span reaches its capacity. At that point the structure is said to have become a mechanism and is assumed to have failed.
 
If my memory of my college notes can be trusted, in continuous beam type structures with 3 or more spans, of equal spans with each span carrying the same magnitude of uniform loading, a plastic hinge anlysis would show that the beam can safely carry 1.25w, where 'w' is the load producing the largest support moment. The moment redistribution method takes advantage of this behaviour. Thus, a beam section can be sized for 75% of the maximum support moment. ACI, conservatively, limits the redistribution to 20%.
 
In theory, a single span frame structure does not satisfy the assumptions involved in ACI recommendation. You may perform a plastic hinge analysis to determine how much of moment redistribution is possible in the frame structure you are dealing with. The redistribution will also depend on the column base condition.
 
Rajendran 
 

richard lewis <rlewistx(--nospam--at)juno.com> wrote:
I have a single span concrete frame I am looking at. I was wondering if
moment redistribution is applicable. ACI 8.4 allows up to 20 percent of
the moment to be redistributed. What I was wondering was what the
definition of "continuous flexural member" would be. My gut feeling is
that it means the beam must have at least 3 supports so my span of 2
suppports would meet it. But, if the beam has columns at ends with
negative moment steel, it is indeterminate. Would that be considered
continuous?

Thanks.

Rich

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