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Re: Post Tensioning and Distributed Loads

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-----Original Message-----
From: T. Eric Gillham PE <gk2(--nospam--at)kuentos.guam.net>
To: seaoc list <seaint(--nospam--at)seaint.org>
Date: Thursday, September 16, 1999 3:17 PM
Subject: Post Tensioning and Distributed Loads


>I am currently designing a 12 condominium structure which will utilize a 2
>way post tensioned slab bearing on walls,

Do not allow the bearing walls to restrain the shortening of the slab

>>>--> Should I be accounting for these loads and their effects on the
>distribution of loads to the various vertical elements?

You answered your question ; the tendon loads are in equilibrium.
BUT  secondary moments will significantly affect column moments at the
exterior. Since column moments from flat plates are usually nominal the
change in moment value may have no practical effect. Likewise, secondary
moments slightly change vertical reactions, but seldom to any significant
degree worth including

>The net result was a 20% increase in loading on the interior columns when I
>account for the PT loads as opposed to when I don't account for them.

This sounds in error. you should be able to do a simple hand analysis of the
effects of prestressing. For typical building structures net bending load is
decreased thus reaction differences should decrease. You say "interior
columns" If there are several equal spans there is little or no difference
in support moment thus no difference in reaction is possible.

>increase is causing some concern for me, because when I check the structure
>for maximum expected displacements (using a displacement spectra) these
>columns WILL be yielding at the base, and a heavy axial load is not going
to
>help the performance of these columns.

But what about the shear in the slab due to the unbalanced moment at the TOP
of the column?
Won't the slab fail before the column in any case??? Besides, is the
foundation so rigid that a nominal difference in ultimate vertical load
will actually occur???