Partially Encased Steel Beams

[kam5001]

Hello Everyone. I am working on an increasing the live load capacity of an existing structure. I would like to remove the existing 4" concrete floor slab and replace it with a 6" concrete floor slab at the original elevation, thereby encasing the beam flanges in 2" of concrete. Does anyone know if it is possible to look at this beam as a partially composite beam in some way? I just want to rely on the friction between the two materials and not add any shear connectors. Thanks for your help.

 

[ishvaaag]

I wouldn't rely just on friction. Weld transversally deformed rebar to development length, dimensioned by shear friction concept. This way you will get a operative composite beam.

 

[ishvaaag]

I normally would also add either concrete or steel angles to forestall the weakness in the concrete hanging under the flanges.

The rebar in the prior post could even take the shape of closed "clip" loops, to further engage the concrete head.

 

[kam5001]

I am going to put continuous angles on each side of the beam web to pick up the concrete under the flanges. I was just hoping not to add more steel to the project. I need the 6" slab for the new live load. I was hoping it would help the moment capacity of the beam. But if that ends up not being the case, I will just reinforce the existing beam.

 

[port125]

If you detail the welds for the shears the continous angle will reinforce the beam, maybe you could size this angle a little heavier and not rely on the concrete at all?

 

[ishvaaag]

An on the same detail of welding transverse rebar, it could take the shape of "serpent" greek pattern.

 

[hokie66]

Adding angles only 2" below the top of the top flange sounds like your biggest problem. Easy to draw, but how do you expect the welder to do it?

 

[JAE]

And you would be restricting any shrinkage between beams as well - possibly creating a crack problem along the beams.

I agree with the above - composite action without shear connectors is not acceptable. AISC doesn't acknowledge composite action without the studs or other shear connector.

 

[WillisV]

"AISC doesn't acknowledge composite action without the studs or other shear connector."

Well...sorta...if you fully encase the beam in concrete you can use elastic superposition of the composite section up to first yield of the steel beam without shear connectors per I3.3a of 13th ed. though I have not seen this done in practice. You can also ignore LTB if fully encased. Note that the detailing provisions associated with a fully encased beam were left out of the 13th ed, but you can go back to the 3rd and see the minimum cover requirements.

 

[JAE]

WillisV - sure - fully encased beams and composite action in AISC. I agree. But I wouldn't personally use something like that in a design myself. I've repaired far too many encased beams where the concrete eventually just fell off the beam.

One chunk - about the size of an American football, fell through the windshield of a brand new Lexus in a parking garage and was held in place halfway through the glass, about an inch from the dashboard.