Post tensioned slab on spandrel beam

[containerwall]

What's the current thinking on the detail of a post tensioned slab supported on all sides by spandrel beams. The beams can be considered as a top bond beam of five inch width cast in place within a wall cavity. Will the shortening of the slab cause adverse effect to the beam? Should one let one side slip, adjacent sides, both sides? Should there be any rebar from the spandrel beam into the post tensioned slab? Thanks.

 

[BAretired]

A similar problem occurs where a post tensioned slab rests on concrete foundation walls. The slab will shorten in both directions. One method I have used is to allow the slab to slip for a short distance at each corner. The length of slip is an engineering judgment call, but it depends on the dimensions of the slab.

BA

 

[containerwall]

Thank you. What would your opinion be of sleeving the beam with L rebars in the slab, tensioning the slab so the vertical L s could move in, then grouting the sleeve after a while?

 

[BAretired]

That idea seems okay to me.

BA

 

[rapt]

This question is too general. What are the dimensions of the slab? And what is the shape of the beam and connections to walls?

In many cases there is no need for separation of any kind.

 

[containerwall]

I'm seeking general design guidelines. Basic specs 21 feet spans, 6" thick slab.

Slab supported by 2" thick stay in place pavers for 2" cover for fire protection.

 

[rapt]

So it is a single slab panel, about 35' by 21'?

Shortening due to prestress in the 21' direction will be about .02" total (depending on the P/A level), half from each end. Shortening from shrinkage will be about .15", About 7 times as much. Shortening from temperature change will be a similar amount to shrinkage.

Yes, you would need unstressed reinforcement in the slab to control cracking but you would have that anyway with an unbonded PT slab (I hope).

What effect do you think it is going to have on the beams? That seems to be what you are worried about.

I would be more worried about a 6" wide beam and the 4.5" connection to the slab. And about a 6" slab.