PT Slab Restraint to Shortening issue 2

[PSR_1]

The Slab am supposed to design has walls at the four corners and the restraint the walls impose is killing my effective stress in tendons and I suspect restraint cracks will be visible after construction. Since it is structural wall slip joint isn't a possibility and pour strip doesn't seem to work since the walls are at just the four corners( Correct me if it is possible to use pour strip with walls located at certain locations, I only have experience with basement walls). Is there any other way to counter the effect of restraint?

See the attached picture.

 

[Tomfh]

A lot of people use lockable dowels which allow two way movement (ie allow movement parallel and perpendicular to wall). You grout them up a month or two later.

If it’s not done perfect it locks up anyway and cracks the walls (or sometimes cracks the Slabs if the walls are strong enough). I’ve seen a number of buildings very similar to yours where the walls get pulled in and get cracks on the outer faces as a result.

 

[hokie66]

Shortening due to PT is only part of the problem. Probably 60% of the slab shortening will be due to drying shrinkage.

You are going to need a substantial amount of crack control reinforcement, so my choice for that floor plan would be to use all deformed bars, rather than PT.

 

[Tomfh]

Hokie, this slab will just snap the walls. Crack control reo won’t help you.

 

[hokie66]

Yes, I agree for the walls. But it will help for the slab. And I agree with your temporary movement joint at the walls.

A better solution is to change the architecture of the building. Those corner blades need rethinking.

 

[Tomfh]

Yeah, they’re liable to cause trouble one way or another.

 

[hokie66]

If the architect persists with this layout, I would look at casting corbels on those blades, with slide bearings, and your lockable dowels consisting of vertical bars in the corbel, with tubes around the bars in the floor plate, to be grouted later...as long as possible later.

 

[rapt]

The increased shortening from PT will be about 10% over what it would be for an equivalent RC system.

In an RC system you would not even think about it. But you have better crack control.

As Hokie said, add crack control reinforcement in the slabs to control the shrinkage cracking. Accept that there is not a lot of axial Prestress force getting into the slab and use normal reinforcing for crack control. PT will still give you ultimate strength and deflection control.

Make sure you have well spaced and detailed crack control reinforcement in the walls. The moments you are talking about will not "snap" your walls. Also make sure the slab wall connection is detailed well.

I assume this is a reasonably tall building. The problem will only be at the lower floors and the top floor or 2.

 

[Tomfh]

Rapt, by snap the walls I mean the slab shrinks and pulls the walls in towards the centre, causing flexure and shear cracking of the walls.

I’ve seen it a number of times recently on new buildings after joints failed to release. It doesn’t look very nice.

Admittedly this building isn’t very long but it’s something to be wary of. It’s common for slabs to crack due to wall restraint but it’s also common for the walls to crack.

 

[rapt]

Tomfh.

Yes cracking does happen, but you can detail reinforcing in the wall to control cracking. A single central layer will be no use, but then it is no good for earthquake either. Well detailed reinforcement properly tied into the slab will control the cracking as it will in any concrete.

I have actually seen a column "snap" due to prestressing shortening (complete separation). But that concrete member was 100m long (single pour) and the P/A was about 6MPa and there was no other axial compression on the column to provide axial compression. (don't ask why they would try it but it cost someone a couple of million British pounds!)

But prestress shortening will normally be about 1.5MPa * 1m / 23000 = .06mm per meter of slab, so for 15m it would be about 1mm.

Shrinkage shortening will be about 500 * 10-6 * 1000 = .5mm per m so about 7.5mm for 15m of slab.

Temperature change will be about the same as shrinkage.

But by the time shrinkage is affecting it there are a few more floors on top causing axial compression.

But the point is Prestress shortening at 1.5MPa axial compression is about 1mm of movement in 15m compared to about 15mm for shrinkage + temperature and you get the shrinkage and temperature effects in any slab and the columns connected to it, RC or PT.

 

[Tomfh]

Rapt,

Say there’s a 2m long x 3m high wall, being pulled in plane by say 20+ mm. Can you really control that cracking in a nice way? It’s always looked ugly when I’ve seen it. Walls with ugly diagonal cracks, opening up on the outside edge.

 

[rapt]

Tomfh,

But PT will not try to pull it 20mm. Something else is causing a lot of the shortening.

 

[Tomfh]

Yes, I was referring to the more general problem of having the slab tied to two stiff end walls, which OP appears to have regardless.

 

[Drapes]

The extent of restraint cracking in the slab due to PT shortening, drying shrinkage and temperature depends a lot on the stiffness of the supports, as highlighted below.

At one extreme, where you have flexible supports (such as columns in a moment-framed building), there is negligible restraint offered by the supports, so the cols will be more vulnerable to crack as they are pulled inwards from the shortening in the slab, leading to an imposed deformation (or forced bending) in the cols. Under this scenario, there is negligible loss in slab pre-compression from PT into the supports.

At the other extreme, where you have rigid supports (such as cores and shear walls at the ends of a building), there is high restraint offered by the supports, so the slab itself will be more prone to crack as the shortening is fully restrained by the walls and prevents the walls from pulling inwards as much. Adding crack control reinforcement in the slab is highly recommended here. Under this scenario, the pre-compression from the slab due to PT can easily get diverted into the supports if no slip joint, delayed pour strip or lockable dowels are provided, as others have already noted.

In reality, your situation will always lie somewhere in between these two extremes. But in any case, agree with rapt that the effect is most pronounced on the lower and upper few floors (and also in exposed slabs), where the 'residual' shortening effect is most apparent b/w floors. For typical successive floors of similar layout, depth and stiffness, the restraint or shortening effect between such floors will be mostly negated. It is only where you have a significant change in layout and stiffness where its of primary concern.