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Fw: Moment re-distribution in seismic designs

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As far as I know, for UBC94 anyway, there are no explicit requirements
concerning the redistribution of beam moments EXCEPT those found in 1908.4.


On the other hand, I don't feel the UBC precludes the redistribution of
moments.  If you want to find an excellent (IMO) discussion on
redistribution of moments (and associated shears, axial loads etc) take a
look at Paulay and Priestley's book.  I have found that their limits on %
of redistributed moments is higher than those found in 1908.4.

You could also try the New Zealand Building Code.

As far as which members you can redistribute FROM, I would say that beams
would be the only ones at a joint, since assuming that a column itself
would shed moment would require it to be yielding (which in some cases, eg
topmost roof level of a structure, would be acceptable).  Now if you assume
that at a particular joint, two beams two columns, one of the beams is
going to shed some of the moment indicated by the analysis, then the
overall moment input by the beams would be less, and the moment input by
the columns would be less, so the shear on the columns may also decrease. 
In this case, reduction of the moment on the columns would be a consequence
of letting one of the beams yield.  Of course, the moment at the other end
of the beam (or some other beam along that line) would have to increase to
take up the slack and preserve equilibrium, so the columns at another joint
would presumably be taking MORE moment than the analysis indicated.  In
other words, equilibrium of forces (story shear and associated moments)
must be preserved.

Hope this helps.

T. Eric Gillham PE

----------
> From: S Avraam <saemco(--nospam--at)zenon.logos.cy.net>
> To: seaint(--nospam--at)seaint.org
> Subject: Moment re-distribution in seismic designs
> Date: Friday, September 18, 1998 8:50 AM
> 
> Any suggestions for re-distributing beams moments
> from a 3D-Frame analysis in seismic regions ?
> 
> If yes, re-distribute in beams only or all the members 
> in the joint ?  Suggestions should refer to USA and 
> European design codes.
> 
> 
> S Avraam
> 
> 
>