composite slab on cantilever beam

[johndeng]

Just wondering composite slab on cantilever beam, the slab will crack in tension, so cantilevered portion of beam can not be treated as composite, can only be checked with non-composite.
For the non-cantilevered portion of beam, any problem to be treated as composite?

 

[manstrom]

Correct. If the top of the beam is in tension, the concrete won't do anything for you.

 

[structSU10]

If there is a load case in which the cantilever could have tension across the whole backspan (load only at one end) you will likely be stuck with non-composite unless you add reinforcing in the concrete specifically for the negative moment. If you add rebar it can be considered composite, and you would ignore the concrete contribution for that situation.

 

[johndeng]

Thanks, but my problem is not for the cantilever part.
The part that's between the two supports, both ends are negative moments, should I check with middle non-cantilever part as composite or non-composite?

 

[jayrod12]

For positive bending it can be considered composite. For negative bending causing stresses above Fr then non-composite. You could account for the reinforcing in negative bending however most people don't

 

[JAE]

I would concur with jarod12 - for load combinations with positive bending across the whole span then you could consider the composite action for those load combinations.
If a partial distance of the span is in negative bending - we usually ignore the composite action. I believe that RISA and maybe RAM simply negate any composite if there is any negative moments at all.



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[johndeng]

JAE/Jayrod12,

That's exactly what I want to hear, thanks!

 

[Agent666]

Even if you do try account for the reinforcement to account for some composite action in the negative moment regions, be aware that you need a lot of reinforcement to make any real impact both in terms of strength and increasing the second moment of inertia. If you think about the area of Steel in the top flange vs reinforcement area in the slab locally above the beam, you can see to get a significant increase you need a reasonable amount of dedicated reinforcement. This is why it's not really worth looking at if you are just trying to account for the normal distributed bars in the topping to have some effect.