PT Beams in Middle of Floor

[TomB_SCPA]

We are designing a concrete framed building with unbonded PT beams in one direction that run end to end and conventionally reinforced one-way slabs spanning to the PT beams. This has been a successful strategy for us on many projects. In previous projects, we have been able to keep the PT in one direction only and any required beams in the perpendicular direction would be conventionally reinforced.

In this current project, I have a few conditions where the architect has eliminated columns for various reasons creating long spans in the direction perpendicular to our typical PT direction. This requires PT girders that do not need to run to the end of the slab in either direction. Therefore, I end up with portions of the floor in one direction that are stressed and portions that are not, as opposed to a more typical, uniform condition.

Obviously, there would need to be blockouts to stress the tendons. My larger concern is if the fact that I am stressing the middle of the slab only will result in excessive cracking where the PT ends and the conventional slab begins, or if conventional reinforcing will be enough to keep the cracks to a minimum. Since this is a conventionally reinforced one-way slab, it is relatively heavily reinforced (compared with a PT slab). We have never had to do this before, so there are various thoughts within the office as to the ultimate effect.

Any thoughts would be helpful.

 

[slickdeals]

Don't think you would need to provide blockouts to stress the PT in the beams running left-right. Depending on the depth of the beam, you may find that the CG of the section is below the slab and tendons can be stressed at the slab soffit. As far as cracking goes, don't think that should be an issue as long as you've provide adequate top reinforcement developed beyond where the left-right beam terminates. I have typically extended beam top reinforcement into the slab and added additional reinforcement in the area that would be the flange of a T-Beam.

 

[KootK]

1) I don't see any major issues with this. It's not dissimilar from the relatively common case where you have a heavily prestressed beam within a lightly prestressed slab.

2) With respect to cracks parallel to the beam you're prestressed in that direction so good to go.

3) With respect to cracks perpendicular to the beam near the ends of the beam lines, you might be well served by introducing delay strips between the ends of the beam lines an the stair shafts.

4) Another thing to consider is that your beam precompression is going to spread out laterally a fair bit over that long length of beam prestressing. I wouldn't say that's a deal breaker, by any means, but perhaps something worthy of some additional consideration.

Will you be analyzing this with 1D or 2D modelling?