Elevated concrete floor: shore it up or demo?

[BillyAsh]

Hello my name is Bill. I’m an engineer working on my own house requesting advice. I’m not a structural engineer. The house has a 400 sqft room over an added-on attached garage that was built in the 1940’s. The room has a concrete floor supported by wood joists.

I’m stuck in a “can’t make a decision” mode about how to deal with the floor. The floor has through-cracks in a few areas and feels a little bouncy when I jump on it. There is no significant displacement at the cracks but they seem more like a system of cracks then one clean crack. The floor is out of level/out of flat by up to 1 inch, probably from new.

My decision is do I shore it up from underneath with steel beams or just demo it and replace with a modern wood floor system. My main concern is the ~50PSF concrete dead load supported by 2x8, 12 OC wood joists spaning up to 12.7 ft. All the wood joists have a 45 bevel on top reducing the cross sectional area. See the graphic attached. I assume the concrete has some amount of spanning capability since it has a little reinforcement but this much dead weight doesn’t seem wise on 2x8 wood joists.

Any thoughts would be greatly appreciated.

 

[jayrod12]

I haven't seen this type of construction before, and I've been doing residential work for 16 years.

That being said, I'd be inclined to agree that the floor system is a bit taxes if the concrete and the joists aren't acting compositely, which I don't think they would be without dedicated connection between the concrete and the wood. And even then, composite action between concrete and timber is a pretty rare situation.

There could be an option to just remove the concrete topping and then consider the condition of the existing joists before going full removal and replacement. 2x8 @ 16 spanning 13 feet is far, but not incredibly so, so they may be sufficient without the 50 PSF of additional dead load caused by the concrete.

 

[BillyAsh]

Thanks jayrod12, I agree the floor construction is very unusual. The wood and concrete are mainly only connected by gravity but the slab does overlay the masonry walls below on 3 sides sort of like a platform framed structure. It will take some precise surgical cuts in the concrete around the edges. Good thought on reusing the existing joists with a wood deck but if I demo the concrete I'd probably reframe the floor to gain headroom below.

 

[StrucPatholgst]

Relying on the concrete for added strength over the span is out the window because of the cracking and the wood chairs running the lengths of the joists. Plus that concept only works when the deck is rigidly affixed or pinned to the supporting beams (joists in this case). With that said, on paper this all fails for moment.

 

[csd72]

Are you sure that is concrete on top? Could be self levelling screed, it is difficult to tell the difference with an untrained eye.

I expect those joist would have a visible sage with that amount of weight on it.