PT Slab - Intermediate Stressing Pocket questions

[structuresguy]

Hi all. We are doing a design of a large PT slab building where several walls butt up against adjacent walls. The slab spans are long enough to require stressing from both ends, but because of the walls, we don't have access on one side to stress the tendons. So our first thought was to stress the slab at construction joints with the tendons continuous. But it sounds like the contractor wants to pour more slab at each pour than our joint spacing would allow. So now we are thinking to use intermediate stressing pockets on the band lines, so that the entire slab can be poured in one shot, but still allow us to stress it in reasonable lengths.

The problem is, none of us have experience with using intermediate stress pockets. So we are looking for any tips, tricks, or best practices that you all might recommend. It would seem that additional mild rebar is needed parallel to the banded tendons between the pockets, to resist cracking due to differential prestressing levels in the slab. Also, we would plan to locate the pockets such that the tendons are at the neutral axis at the stressing blocks. But otherwise, are there other things we should consider?

Thanks very much.

Andrew

 

[Structcon]

Andrew,

A couple thoughts:

1. Since there is a contractor on board, then they have a p/t sub (the company that details and provides the tendons, and they also do the loss calculations). I would find out who that sub is and chat with their engineer on this subject.

2. Interior blockouts for stressing are not uncommon, but the only extra mild steel I would use is for anchor zone (local and general) forces per ACI or PTI. Once the tendons are stressed (the 2nd pull), then no differential should exist.

Rick

 

[structuresguy]

Hi Rick

Thanks for your thoughts. I don't think they have a PT sub yet, as they are CM providing pre-construction services, but we will talk with them when they do.

We agree that once the 2nd pull happens, that overall stresses should equalize. But in the time period between 1st pull and 2nd, there should be a stress differential. It is during this phase that we are concerned about cracking, particularly if there is a significant time delay (say 2st pull on friday, 2nd pull on monday). We're just interested if the general concensus is that it is not an issue.

Thanks

 

[Ingenuity]

What is the max overall tendon length - slab edge to slab edge?

Also, are the layout of the walls going to provide lots of restraint?

 

[rapt]

I agree with Strutcon and Ingenuity. This is too general a question to answer.

In most countries PT companies will have design departments experienced in this sort of decision making. Find one locally who will be tendering on this and get their advice.

Or hire a PT expert to advise and give them a lot more detail on the overall situation.

 

[BUGGAR]

From a construction contract administrators point of view:
"The Contractor wants................". Then the Contractor submits a proposal for a change order, including all engineering required to accomplish the change, all at no additional cost to the owner.
Unless the design is proving impossible to build. Then the owner initiates the change order to "correct" the design, including costs related thereto.

Bob

 

[rapt]

Buggar, If you read it all, it is not a contractors request for change!

If you are worried, you could always check the design with only the first stage of the prestressing completed!

Or specify that alternate tendons are stressed, so that all of the tendons from one side are not stressed at once. Or if you want to really complicate it, require simultaneous stressing of adjoining tendons (they will hate you for this one!).

Or provide a pour strip rather than overlapping tendons and reinforce the connection between the 2 areas.