PT Restraint Question

[Lion06]

I've been reading a lot on PT design and how if you have walls at the ends of the building you should really provide a slip detail (e.g. felt paper) between the bottom of slab and top of wall to allow the slab to shrink without being restrained by the wall to prevent cracking in either the slab or wall (most likely the slab).

This makes a lot of sense............ for gravity support walls. What if the walls are shearwalls? You can't slip the slab on a shearwall or you have no way of getting the lateral shears into the wall.

So how do you prevent restraint cracking in a slab for this condition?

 

[keenaseng]

The slip joint between PT slab and shear walls is also to account for the 'shortening' of the slab in the stressing process and to ensure the pre-compression gets into the slab (as well as initial shrinkage you noted).

I normally use a oversized grout tube over the starter bars from the wall into the slab that will accomodate movement and grout fill the tube at a nominated date after final stressing

 

[Lion06]

keenaseng-

I didn't mean to imply that all of the "shrinking" of the slab was purely from shrinkage.

Even if you provide an oversized grout tube, how is the slab slipping with respect to the wall unless you provide a slip plane between the top of the wall and the bottom of the slab? I think the oversized tube is a good idea to not bend dowels sticking up for the next wall pour, but it still requires a horizontal slip plane between the slab and the wall or it's not doing what it's intended to do, no?

 

[ron9876]

I think the answer is to not lay the building out where this condition is a problem. Masonry walls are infilled and the shearwalls are placed and oriented such that they don't present a problem.

 

[Lion06]

ron,
In a perfect world, I agree. However, we all know this isn't a perfect world, and at the end of the day we need to take shearwalls where we can get them.

I can't imagine that there haven't been PT jobs with shearwalls at the ends of the building.

 

[ron9876]

If the walls are at the ends of a building with the strong axis aligned with long dimension of the building, for instance, you may well have a problem.

Saw a three-four story condo type building with walls near both ends of a long slab oriented with the long dimension of the slab. When they stressed the cables it pulled the walls toward the center of the slab and cracked the walls. These walls were relatively lightly reinforced due to the height of the building. I am sure if the walls had been more substantial there would have been a problem with the slabs.

Normally the travel distance for fire safety will result in stair towers on opposite ends of the building with an elevator near the center. If you can orient the walls at the stairs such that they are in weak axis bending when you stress the cables there shouldn't be a problem.

Maybe I have been lucky and have been able to convince the architects to give me a good shearwall layout. I guess if that isn't possible then you have to do some type of detail to let the slab move but I don't think a slip joint is a good idea because the contractor can't pour the walls above until the cables are stressed. A pour strip in the slab is another possibilty but the contractors hate that also.

 

[Lion06]

I don't think a slip detail is an option for a shearwall. If you slip the slab on the shearwall, how do you get lateral shears into the wall?

 

[slickdeals]

ACI SP-113 Control of cracking in Prestressed Concrete, although dated, is a good reference. Have you gone through it?

 

[Teguci]

Prior to tensioning, provide a slip plane for the slab to shorten along the wall. Provide the oversized grout tubes between the wall and the slab as keenaseng mentioned. Once the slab has been tensioned, grout the grout tubes to provide a rigid connection between the shear wall and the slab (may want to wait until the next floor is poured depending on conditions). Now you have a tensioned slab locked in with a shear wall.

As for walls (and columns) being pulled out of plumb, provide a sandpocket at the base to allow rotation.

 

[Lion06]

slick-
I have not gone through that. Thanks for the reference.

Teguci-
If you provide the slip plane and oversized grout tubes, let's say 4" of movement and the wall rebar is in the center of the tube. So you slip the slab on the wall, and the slab moves 1" such that the bars are no longer centered in the holes. Now the slab has moved with no distress to it or the wall - all is good so far. Now we go ahead and grout the tubes solid. This is where I see the problem. Now all you have working for you from a shear standpoing (in a shearwall, mind you) is a 4" diameter grouted tube at some interval, let's say 18". I just don't see that being a reasonable mechanism to transfer large, accumulated shears across that plane.
Maybe I'm missing something, but I don't see it.

 

[hokie66]

This whole issue is not just for post-tensioned slabs, as normal drying shrinkage is in the order of 3 to 4 times the combined elastic shortening and creep induced by the prestress. Variations on the temporary slip joint detail have been used successfully, particularly in parking structures with exit cores at the ends. Wall corbels allow these joints to be left ungrouted for a sufficient time to make a big difference. In the same vein and more commonly used, delayed pour strips, centrally located between the restraining cores, and also with the longest waiting time possible, can make a big difference. These allow the movement to take place toward the shear cores rather than away. Sometime, both methods are required. There is always a battle and some negotiation between the structural engineer on one side and all other members of the project team on the other side in regard to the waiting time.

As to developing the shears into walls which have temporary slip joints, you have to rely on dowels and grouting. I don't depend on felt paper for the slippage, but rather teflon slide bearings, so the shear has to be taken along the vertical joint.

The other related issue is adequate slab crack control reinforcement, which is a very big subject in itself.