PT Restraint Question
PT Restraint Question
(OP)
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?
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?






RE: PT Restraint Question
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
RE: PT Restraint Question
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?
RE: PT Restraint Question
RE: PT Restraint Question
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.
RE: PT Restraint Question
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.
RE: PT Restraint Question
RE: PT Restraint Question
RE: PT Restraint Question
As for walls (and columns) being pulled out of plumb, provide a sandpocket at the base to allow rotation.
RE: PT Restraint Question
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.
RE: PT Restraint Question
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.