PT Temorary Release Detail

[Lion06]

I've been looking for a good detail to temporarily release a PT slab from restraining wall elements that have multiple levels of wall and I'm having no luck. PTI has one detail they call a temporary release for this condition, but I see some issues with it and want something different. I specifically want to allow the slab to continue to shrink while the entire structure is going up and have the voids around the vertical wall bars grouted in at the end. The PTI detail, IMHO, only addresses the elastic shortening of the slab due to prestress and whatever shortening happens prior to the next wall lift being poured. That's not enough, to me, if you have very stiff elements at opposing ends and a pour strip is not an option.

This is specifically for shearwalls, because gravity walls can be a permanent release and then the detail gets a little easier.

With that in mind, an additional question I had is this. Is the bondbreaker critical to the release? If you get a hard trowel finish so there is no aggregate interlock, I would expect the prestress force to exceed the bond strength of the slab to wall interface and allow that movement. This may become problematic as additional floors are built and then the shrinkage may not be able to overcome all of the frictional force. My concern with a bondbreaker is that because of the cold joint, you're required to use shear friction for shear transfer across the joint for the in-plane shear forces. If you reduce the friction to essentially 0, you have no shear friction capacity. ACI 318 states that 0.6 was selected for the coefficient of friction of smooth concrete to smooth concrete to match test results, but that the primary force transfer is through dowel action. Is that reasonable to assume you can use 0.6 (or something close to it) for any interface, regardless of the coefficient of friction between the two surfaces?

I'm developing a detail, but it won't be an inexpensive detail, so I wanted to see if there is anything else out there.

 

[hokie66]

I have used corbels, similar to what dcarr proposed and then deproposed. Contractors don't much like that detail, though. In my case, I provided a slip joint on top of the corbel, and the stressing was done from a delayed pour strip, with the tie to the wall completed much later.

 

[AUCE98]

Similar to Koot, we have typically used the "beer can" method, where you place aluminum cans around the dowels. That, or move the PT away from the walls approximately 5*t, and provide additional reinforcement along the walls in that area. "Restraint Cracks and Their Mitigation in Unbonded Post-Tensioning Building Structures" by Bijan AAlami and Florian Barth is a good reference.

If you are designing a two-way PT slab, depending on the software, you should be able to see where the PT is being ineffective. the real issue with restraint is the configuration of the walls. If you have shear walls at the four corners of the building, no good. Move them to the center of the slab to allow compression from each end.

 

[Trenno]

That, or move the PT away from the walls approximately 5*t, and provide additional reinforcement along the walls in that area.

Interested in where the 5*thickness came from and how far away from the stiff element (wall) you need to go until restraint effect become negligible.

I was brainstorming a somewhat blockout detail (wall can arch around/over it) that essentially had the live end unit width greater than the width of the wall, such that the P/A would get into the slab before being sucked into the wall.