Metal Deck over steel best intermediate basement level

[jdgengineer]

We are working on a multilevel residential basements project. In one area, the basement is roughly two stores deep with 12.5' story heights. We currently have the intermediate level as metal deck with steel beams.

We are running into some problems residing the retaining pressures with the metal deck. Our initial thought was to try and resolve it as a subdiaphragm similar to tiltup construction but we are having issues with the diaphragm shears.

I've discussed using the deck as a compression mber with verco but they weren't entirely helpful and didn't have any testing to back it up. They suggested using only the topping slab (even when parallel to flutes)

How have you been able to make an intermediate basement level work with metal deck? Or is concrete slab really the only feasible option?

 

[calvinandhobbes10]

I would add one more beam, respace, get your unbraced length down so the slab on deck checks. I think your slab axial load is conservative because you have the horizontal wall span anyway. My 2 cents

 

[jdgengineer]

Hi calvinandhobbes10 -- Thanks, we were actually just looking at that. I agree, it's a good thing to look into.

 

[KootK]

I have to ask for my own edification: why not the solution that I posted Friday night? Back to normal deck, no columns on the CIP, and really convincing wall bracing.

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.

 

[jdgengineer]

I have to ask for my own edification: why not the solution that I posted Friday night? Back to normal deck, no columns on the CIP, and really convincing wall bracing.

I believe the shear forces in the horizontal concrete beam were too high to make it work. With 10k/ft x ~ 30' we have roughly 150k in reaction. Also, in looking at the wall in the other direction we are parallel to the beam framing, but if we tried to transfer all the force through the beams at 8'-0" we would have a reaction of roughly 80k into each beam. Obviously the direct connection of the deck helps transfer the compression force, but if we looked at only transferring it through the beam the reaction felt to high for beam pocket.

 

[calvinandhobbes10]

Did you not come off a distance d?

 

[KootK]

So I kinda feel that all roads are really leading us back to a typical concrete slab here. The 6" flat slab on steel is really just a permutation of that. So... perhaps that simply is the right answer then. Regardless, the power of pedantry compels me...

believe the shear forces in the horizontal concrete beam were too high to make it work. With 10k/ft x ~ 30' we have roughly 150k in reaction.

You'd be over using just the concrete shear capacity but, unless I miss my mark, a very light reinforcing ratio in the slab would get the job done.

Obviously the direct connection of the deck helps transfer the compression force, but if we looked at only transferring it through the beam the reaction felt to high for beam pocket.

I've been noodling hard on this aspect of things. Given the reality of concrete shrinkage and creep, I don't think that there's any way around having the axial load wind up primarily in the steel beams. The only question, in my mind, is does the load start off in the steel beams or does it travel through the concrete for a short distance before making its way down?

You may have already come to this conclusion by the sound of it. Regardless, I feel as though this means that there isn't really a need for the deck selection choice to be dictated by the axial load condition in the parallel to beam condition. No matter how robust you make the slab, it's just going to be shedding its load to the beams in short order.

The detail below shows one possibility. It would be perceived as extreme for a residential application in my market. You seem to be afforded a good deal more flexibility in your market however. And who am I kidding? Noboddy's got an indoor curling rink at home where I live. Features of the detail:

1) Keeps the horizontal spanning demand on the wall down. Not sure how 8' was working in previous iterations anyhow.

2) Install a sub-diaphragm six to eight inches below the tops of the beams.

4) Maybe gap the deck edge angle from the wall slightly so the load doesn't start out in the concrete.

5) Change your beam connection detail from wall pocket to embedded plate and double angles. Resistance based on punching shear.

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.