Construction Joint in Elevated Composite Slab at Cantilever

[structuralengr89]

Please see attached sketches. For those of you that have experience with cantilevered composite framing, is it common to have a Joint at the cantilever to reduce cracking in the slab? I typically don't use Joints in elevated slabs, but I typically use bar joist framing.

Do you see any issues with having a Joint where indicated in the attached section?
Thanks.

 

[JAE]

You are changing the effective width of concrete on that one particular composite beam (shear flow is interrupted so the full "T" of the concrete slab can't be engaged).

But other than that I see no issue with what you are doing.

We don't use control joints in elevated slabs for shrinkage but in this case you are trying to control the crack due to flexure of the underlying steel framing.
If the slab will eventually be covered with carpet or other floor covering, the straightness vs. randomness of the crack probably doesn't matter.



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[structuralengr89]

Thank you for the information JAE...I appreciate it!

 

[hokie66]

The W24 is an edge beam part way anyway, so it will just be an edge beam full length.

To me, using a control joint there shows a good thought process.

 

[KootK]

For those of you that have experience with cantilevered composite framing, is it common to have a Joint at the cantilever to reduce cracking in the slab?

Not to my knowledge. Frankly, this would be my first time hearing of it.

Do you see any issues with having a Joint where indicated in the attached section?

I don't see any shear flow issues at the location being considered because any composite action would be between the beam and any slab rebar, not between the beam and the slab concrete. The issue that I do see is simply the effectiveness of this as crack control. Control joints for temperature and shrinkage are effective because the development of a planned crack will usually eliminate the development of unplanned cracks. That's much less the case when you're trying to prevent flexural cracks. Wherever you have studs, you're going to have composite curvature. Therefore you're nearly as likely to have cracks develop a foot from your girder as you are to have cracks develop over your girder. And it's important to recognize that your moment connections here make this quite different from the stock infill beam / girder connection where you get a gross curvature discontinuity across the girder.

I think that the better and more conventional solution would be to simply provide top steel across the girder to control the inevitable cracking.

The very real issue that I've encountered most commonly with cantilevered composite floor systems is unwanted, upwards deflection of the cantilever. This can come about as a result of camber, back span deflection, and concrete shrinkage. I'd recommend keeping an eye on that.

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.

 

[JAE]

Good point, hokie66, on the rest of the beam being an edge beam anyway.

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[structuralengr89]

KootK,
Thank you for your thorough reply. I had originally looked at doing as you suggested, placing rebar across the cantilever along Grid line 5. However, it would take quite a bit of reinforcing to handle the tensile stresses in the slab across the joint. In addition, I'm trying to have the loads handled by the WF beams and not the slab at the cantilever.

What if I placed like a 1/2" wide flexible joint where I have indicated?

I will definitely consider the upwards deflection at the end of the cantilever. That is a very good point!

Thank you.

 

[KootK]

Thank you for your thorough reply.

You are most welcome. As you can see, opinions vary.

However, it would take quite a bit of reinforcing to handle the tensile stresses in the slab across the joint. In addition, I'm trying to have the loads handled by the WF beams and not the slab at the cantilever.

No problem. Here's what you do:

1) DO design the wide flange beams to take all of the loads at the cantilever. This a choice.

2) DO NOT design the slab rebar to deal with tensile stresses associated with composite behavior. It's unnecessary.

3) DO design the slab rebar for nominal crack control. In practice, see a lot of straight up 0.002 Ag.

What if I placed like a 1/2" wide flexible joint where I have indicated?

I don't really see how this would alleviate any of the concerns that I raised.

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.

 

[structuralengr89]

Thanks KootK!