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Composite action with hollow core slab

Composite action with hollow core slab

Composite action with hollow core slab

(OP)
Hello engineers,
I have a job where I need to remove a portion of a block load bearing wall in level 1 of a 2-storey building.
The attached sketch explains. To meet the required moment and the deflection limit, I wonder if I can assume a composite action between the steel beam and the slab. To make that work, and given that the compression force will be perpendicular to the hollow cylinders, I am assuming the max. moment resistance will be limited to the compression in the top flange of the slab. i.e. the Neutral axis at the top of the holes. Design the fasteners to meet the required horizontal shear.
All comments will be appreciated.

RE: Composite action with hollow core slab

I don't like it. The shear through the web of the plank would get pretty big. I've done something like that before and developing the shear connectors from the steel to the concrete after the fact is tricky. There is too play in the connector and the hole in the steel.

Your better bet would be to add channels to the CMU wall above the plank if you need to add more steel.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

RE: Composite action with hollow core slab

I don't like it much either. Certainly, I wouldn't do it just to shave off steel weight. There would need to be some desperate times to go with that desperate measure. You could probably make a solid argument for composite action with just the bottom flange of the hollow core. Is there access from the top such that you could break open the cores and grout them solid?

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 action with hollow core slab

Quote:

Is there access from the top such that you could break open the cores and grout them solid?
...and do so while the floor is shored from below? And even then it might still be dicey. Add me to the list of those who think it would be better to fully support the system with new steel than to perform heroics to try to achieve composite action.

RE: Composite action with hollow core slab

You could break and grout the cores without shoring in my opinion. If the webs remain in tact, so does the shear capacity. Not that it makes much difference to the overall desirability of the scheme. OP gets points for creative thinking however.

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 action with hollow core slab

(OP)
Thanks all. Let me admit that I am not too keen about it either and certainly not trying to impress or satisfy anyone. I am old enough for that. The issue is constructability. The depth and weight of the beam needed without the composite action is driving this. I have already warned the client of the consequences and provided a couple of options to make it easier to construct, of course not including the ( composite action). I knew the shear connector would be the weakest link. But, don't we reinforce slabs with straps connected to the bottom? I was thinking to use expansion anchors placed in epoxy grouted holes to minimize the slip. If it was a solid slab, I would do it without hesitation.

RE: Composite action with hollow core slab

Quote (OP)

But, don't we reinforce slabs with straps connected to the bottom? I was thinking to use expansion anchors placed in epoxy grouted holes to minimize the slip. If it was a solid slab, I would do it without hesitation.

Yeah, we do. The big difference here is the ability of the hollow core webs to transmit the required horizontal shear up to the top flange, as you've rightly identified. They'll tend to act like small, brittle, unreinforced concrete vierendeel truss webs. Hence my mention of solid grouting the cores above.

Even when the slab is solid, I've always had concerns about the slack between the bolts and the plate. If there's much slip at all, it will take a ton of curvature before the straps will fully engage. A great, although costly, detail that I really like is cored holes in the slab and grouted in place Nelson studs attached to the strap.

If it's an issue of handling weight, could you supply your beam in two halves that get a bolted moment connection in the field? Or use two, stacked beams bolted together at the adjoining flanges?

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 action with hollow core slab

(OP)
Thanks KootK. One of the options I provided is to use two beams, one on each side of the wall. But, Stacked beams is another good option.

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