Post Tensioned Slab Beams

[ron9876]

I am looking for some input for the design of PT slab beams. When I say slab beams what I am referring to is relatively wide beam and shallow beams.

Are there any limitations to the width used? Should the design also consider T-Beam action?

My specific condition is a 72' span over a large open space with beams at about 30' c/c and I am considering a 8' wide x 4' deep slab beam. I know this approach is used for parking garages in some instances but I haven't had the opportunity to use this approach before. Any input is appreciated.

 

[Retrograde]

Generally I stick to a limit of 2400mm, which is pretty much the same as 8'.

Your initial suggestion of 8' wide x 4' deep does not sound unreasonable for a 72' span.

You should definitely consider T-Beam action.

 

[rapt]

Ron,

Yes it should consider T beam action, but using the limited flange width in ACI 318 for RC beams or even better the Eurocode effective flange width, not the full panel width as suggested by some in USA.

For width, I would normally limit it to about .25 - .3 of the transverse span length. But because your beam is so deep it is really a beam and not a band beam. The width/depth ratio does matter in this definition

 

[KootK]

Are there any limitations to the width used?

1) Shear for beams very much wider than their supporting beams requires some special / two way consideration. That's not unique to PT systems however.

2) Will you also be prestressing the slab between beams? Probably. I think that's a good idea to maintain prepression in the beams.

3) Perhaps give some thought to the effects that axial shortening may have in any supporting structure that will offer restraint.

Other than that, the concept seems solid. Cool span.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.