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# Composite steel beam with a different slab thickness on each side5

## Composite steel beam with a different slab thickness on each side

(OP)
Hi everyone,

When designing a steel beam that supports a steel deck on one side that has different properties than the one on the other side (as shown in the attached figure); what will be the effective thickness of the slab? Should we consider the minimum thickness? I appreciate any advice in this regard.

Thank you.

### RE: Composite steel beam with a different slab thickness on each side

Just determine the depth of compression based on the thicker slab... and as the depth increases, then the shallower slab will come in to play... and you have two compressive forces... one diminished...

Dik

### RE: Composite steel beam with a different slab thickness on each side

Considering the minimum thickness is slightly conservative but it makes little difference to the design of the composite beam, so that is a reasonable approach.

BA

### RE: Composite steel beam with a different slab thickness on each side

(OP)
Thank you very much dik and BAretired for your prompt response!

### RE: Composite steel beam with a different slab thickness on each side

Color me uptight but my preference (in order) would be:

1) Don't bother with composite action.

2) Consider one sided composite action using only the lower side.

Based on the deck direction, I'm guessing this is a girder condition. That means that you're likely to have a steep curvature discontinuity over top of the beam and, because of the slap step, any rebar there won't be particularly effective. I think you'll be dealing with a location where you'll see a lot of slab cracking. To participate, that upper slab need to be shear connected to rest. I'm not loving the look of it in that respect.

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.

### RE: Composite steel beam with a different slab thickness on each side

KootK:
When I did the Cornwall Centre in Regina (a long time back) the difference between composite and non-composite construction was nearly 30c per square foot... first project I used single angle connections (and there were a kazillion of them)... In addition, deflection was greatly diminished... Used a TI58? programmable calculator for composite construction; it had little mag strips.

Dik

### RE: Composite steel beam with a different slab thickness on each side

Right, but I'm not saying to abandon all the composite beams, just this one girder. \$0.000007/SF.

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.

### RE: Composite steel beam with a different slab thickness on each side

(OP)
Thank you very much KootK, Dik and BAretired for your detailed response.

The way I solved this problem is by considering the (Effective Slab Thickness) = (Overall Depth of the Smaller Slab) - (Rib Height of the Larger Deck)

This satisfies the CSA S16-14 code requirement which defines the effective slab thickness as the "MINIMUM thickness of concrete measured from the top of the slab to the top of the steel deck".

### RE: Composite steel beam with a different slab thickness on each side

If you can get ahold of the ETABS composite beam design manual, there is a section that parses out how they treat various situations like yours. It's a good reference for how to analytically approach some atypical composite beam situations. You can take a look but this is how it would treat your beam:

"If two or more deck sections span in the same direction and have the same value of tc * f′c , the program uses the deck section with the smaller tc value."

### RE: Composite steel beam with a different slab thickness on each side

RAM SBeam allows different deck/concrete configurations on either side of the beam.

### RE: Composite steel beam with a different slab thickness on each side

(OP)

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