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Re: Elevated post-tensioned slab cracks

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I generally agree. The ACI 318 justification of PT banded tendons as a 2-way
slab only works if the slab stays UNcracked under its service loads, or at
or close to the balanced load, hence behaving as an elastic plate.

If you get any cracking, you will have moment re-distribution, then the slab
will act as it was reinforced - hence as Roger described, and unreinforced
CRACKED concrete between the banded tendons does not have too much capacity!

And it beats me why ACI 318-02 ups the permissible flexural tensile stress
to 7.5 x SQRT f'c from 6. As for having T, U and C classes is idiotic - it
is either cracked or it is NOT. Seems a very feeble attempt at partial
prestressing. But note that you can NOT design 2-way slabs as anything but
class U.

Roger, you did state that the "...banded tendons creating an over-compressed
in-slab beam" but do not forget that P/A from the banded tendons will be
dispersed over the entire panel, so it will not really be over-compressed,
if I understand your terminology correctly.


Mark Geoghegan
Honolulu, HI




> -----Original Message-----
> From: Roger Turk [mailto:73527.1356(--nospam--at)compuserve.com]
> Sent: Thursday, January 09, 2003 6:10 AM
> To: Oshin Tosounian
> Subject: Elevated post-tensioned slab cracks
>
>
> Oshin,
>
> I have contended for years that post-tensioned concrete slabs
> with banded
> tendons in one direction is really a one-way slab, spanning
> in the direction
> of the distributed tendons with the banded tendons creating an
> over-compressed in-slab beam.
>
> It is absurd to believe that banding tendons is the same as
> distributing
> the tendons.
>
> If you look at the research that supposedly shows that
> two-way action is not
> dependent on distribution of tendons, you will see that the
> slab fails as a
> one-way slab, with cracks first appearing parallel to the
> distributed tendons.
>
> A. Roger Turk, P.E.(Structural)
> Tucson, Arizona
>

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