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RE: Approx. moments by ACI code

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I think all of us who too indeterminate structures in college remember
that for a textbook example with supports perfectly pinned, the moment
would be exactly wl^2/8.  As a practical matter, a concrete beam
monolithic with the wall would not be pinned, although it might rotate
quite a ways toward the idealized deflected shape.  I believe this
restraint at the exterior supports is the reason for the reduction in
the moment from the ideally pinned value.

>From: 	Dan Vines[SMTP:dvines(--nospam--at)]
>Sent: 	Friday, July 25, 1997 12:01 PM
>Subject: 	Approx. moments by ACI code
>Another engineer in my office has found a discrepancy in the ACI code
>for estimating moments in continuous span concrete beams.  Section 8.3.3
>allows approximate moments in lieu of a frame analysis for beams and one
>way slabs meeting certain conditions.  In particular, the code allows a
>value of wl^2/9 for the negative moment at the exterior face of the
>first interior support for a two span continuous beam.  After using this
>equation and checking using frame analysis, the actual moment appears to
>be much closer to wl^2/8.
>Has anyone else noticed this?  I would be happy to provide a design
>example for anyone who would like to see it.
>Dan Vines, EIT
>Starzer & Ritchie, Inc.