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Re: Elevated post-tensioned slab cracks

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At 03:14 PM 9/01/03 -1000, you wrote:
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.

I will even partly disagree with this statement Mark. It will exhibit 2way action at service if the slab is uncracked, but at overload/ultimate strength conditions it will be a 1way slab and it will fail as a 1way slab. Other cracks will appear that will make it look like a 2way cracking pattern but, as no reinforcement has been placed across these cracks, they cannot hold any moment capacity and the actual failure strength pattern will be 1way. Though some of the PT experts in USA seem to think differently, a crack can only be controlled by reinforcement across the actual crack so reinforcement 15' away from a high stress point will not do anything to contain the crack width.

And remember, the determination of the stresses in the slab to decide if it is cracked or not MUST be based on the ELASTIC moment pattern, not the average moment over the total width of the slab as is common practice in USA design. Until cracking and redistribution of actions occurs the moment pattern is 2way and there area peak moments near the columns and higher moments in the column zone than the middle zone (I will not call them column strips and middle strips because someone will then tell me, incorrectly, that they do not exist in PT flat slabs because ACI318 says so, it does not say this if you read the code correctly).

Slabs designed as banded/distributed must be designed as uncracked at service to perform adequately at service (no cracking) because as soon as they crack many of the cracks will not be reinforced and will be unrestrained (and could end up being 1/4" wide or more Oshin) and this determination of the uncracked state must be based on the actual moments.

Your problems with this slab come from a combination of the restraint effects plus the tension stress in the slab from flexure which have been ignored in the design. It would then appear that there are either detailing or construction problems adding to this as any area of a slab that can develop a 1/4" wide crack should have sufficient reinforcement across it to control the crack. This has not happened in this case and any bonded reinforcement would have at least yielded and may have failed and also may have rusted away.

Depending on the span arrangement and the location of the 1/4" crack etc, I would be very doubtful that you could justify the structural adequacy of this area of this slab. Catenery action cannot be used to justify the slabs long-term capacity. It may stop it from collapsing in the short term but cannot be relied upon long term. I would prop it immediately.


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.

I agree that this section of the code is stupid. With unbonded tendons the whole logic is wrong. There is no crack control at all unless you can guarantee that all possible stresses have been included in the calculations including restraint, temperature differentials etc. To then allow the use of average moments in this section to determine if the slab is cracked is absurd. The peak moments and stresses will be at least 50% higher than calculated if based on a column/middle strip approach and will be about 5 times higher is you actually look at the stress near the column. In other words, the slab is cracked even if the code tells you it is not.

Also, the 2way limitation in this section should only apply to banded/distributed tendon arrangements. It should not apply to proper 2way tendon arrangements like weaved tendon patterns with a 70/30 distribution of tendons in each direction.


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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