Post tension slab tension serviceability stress exceeds the limits 1

[RamiHabchi93]

Hi,I am new in post tension design and I am facing a problem.
I had modeled a post tension slab with banded tendons in one direction and distributed in the other.
I have a span,in the direction of banded tendons,where at service load the tension in the top fibers at middle support is almost at the limits required by ACI and at transfer the tension in bottom fibers at the same support exceeds the limits required by the ACI.If I reduce the prestress force(or the balanced load)the tension in bottom fibers at transfer becomes in the limits,but that in top fibers at service exceeds the limit.The only way I have seen to solve this is by increasing strength of concrete.But I am already using 8000PSI concrete and I don't want to increase anymore.so what should I do in this case?
Thank you.

 

[cliff234]

If you are inexperienced with post-tensioned concrete design, you should be working under the supervision of someone with experience. PT design is not as straightforward as steel design - as you are discovering. There could be several things you could do to reduce the high stresses - however it's difficult to diagnose your situation via this forum. Also, 8,000 psi concrete in your slab seems unusually high. Has the firm you worked for used 8,000 psi concrete in slabs on other projects? (We've never used more than 6,000 psi concrete in slabs.)

 

[JAE]

If you are running into stress limitations then that is many times an indication that you need more slab depth in your design.

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[rapt]

Agree with JAE. Especially if you have problems at both transfer and service, your slab is too thin.

 

[Ingenuity]

I am curious, what is the slab P/A, and what is the slab thickness and span?

 

[Settingsun]

How hard have you pushed the calcs? Is it a continuous slab where you can play with parasitic/secondary moments, and round off the peak moment since the real support isn't a knife edge?

Can you use conventional reinforcement to provide the shortfall?

Is there a higher tier assessment that will allow larger stress limits at the expense of more detailed calcs? (I don't know the ACI rqmts.)

But there should be a supervising engineer who can help as has been suggested earlier.

 

[rapt]

steveh49

ACI code using their "average moment" logic in flat slabs with banded/distributed tendon systems has a maximum allowed stress level over the supports that cannot be violated. Adding reinforcement to increase the slab capacity is not an option, basically because the method is not using real stresses (it is using average over the full width) so a crack with and crack control calculation is not possible. The actual stress in the concrete in the area around the support will be about 50% higher than he is calculation using the average method.

Partial prestressing is not an option when you do not know what the actual stresses are.

 

[Ingenuity]

ACI code using their "average moment" logic in flat slabs with banded/distributed tendon systems has a maximum allowed stress level over the supports that cannot be violated.

Just add a small 'shear capital' around the column and all is good...

 

[Settingsun]

Do I sense you're not a fan of the ACI method, Rapt?

 

[rapt]

Not sure what would make you think that steveh49! Everyone else is sick of me going on about it, so I will not repeat my ravings here again!

 

[hokie66]

I'm not sick of it. I think the US way of doing PT is an abomination. I am from the US, but living in Australia has shown me the way.

 

[rapt]

Hokie,

Unfortunately some idiots here are going the other way! And they know not what they do in thinking the method is accepted in AS3600 with normal AS3600 flat slab rules developed for column/middle strip logic.

 

[Ingenuity]

Unfortunately some idiots here are going the other way!

Just to clarify - when you say "here" you refer to AU - and the "the other way", AU is NOT going unbonded, right?

Do uni's in AU teach prestressed concrete at undergraduate levels these days?

 

[rapt]

No, unbonded is not allowed in buildings in AS3600.

I meant banded/distributed using bonded PT. So then they do average moment. But without a code defining the limitations, so they treat it as a normal flat slab. Happens throughout Asia, but they even do it with Drop panels (probably because some idiot's book in USA says you can, as well as the PTI manual (at least it used to do it), you know the one I mean, one of the idiots who caused a lot of these problems in the first place). Incompetent!

Many do teach PT at undergraduate level. Not sure how much they actually teach them. Even 40 years ago, we did a half year at 2hrs per week (so about 30 hours), so really only learned about losses and how to do a Magnel diagram. Never used one since then in buildings, but if you really understand how to do it, you can teach yourself the rest. As with everything else these days as it gets more complicated, software will do it for them so they do not have to know what they are doing, including reading and understanding a design code.