Old Engineer Detailing Question: mild rebar in composite concrete slab on metal deck

[BecomingCurmudgeon]

I am trying to recall how things used to be detailed in the early 1990s.

I am looking at a current project where the design engineer spec'd a 4" concrete slab on 2" composite floor deck (6" overall depth). He used Fibermesh and no mild reinf. The slab cracked in tension down the backs of the girders and there are other cracks over the beams. It is going to be difficult to put a floor finish on now. The owner wants a second opinion.

I remember that in the 90s, my first boss always installed 6' long rebar spaced at 48" down the backs of the girders at the top of the slab. He also installed mild reinf over the beams at the top of the slab. We never had cracks. It was commonly done in upstate New York at that time. It was not so much an engineered solution but a conventional practice. I have since moved and the old firm is gone so there are no old colleagues to ask.

I have been looking for a source that discusses my old boss's detailing preference for this. Anyone remember?

 

[cliff234]

We still do that. We specify #4 top bars about 6' or 8' long spaced at about 18" oc, perpendicular to, and over top of the girders. And we specify those bars regardless of whether or not we have WWR or fiber reinforcing (macro fibers) in the slab. We've never had any significant cracks over the girders.

 

[BecomingCurmudgeon]

That sounds like what my first boss used to do. He also put bars over the floor beams as well. It might have been (2) #4 on either side of the beam center line. I'm going to go thru a few boxes this weekend to see if I can find an old print.

The project, that I saw without any mild reinf, used OWSJ floor beams on either side of a WF girder. The OWSJ deflection caused a crack down the back of the girder and had one side of the crack rise up 1/8" above the other side of the crack. Probably due to the different joist spans on either side.

I think using the mild reinf would have prevented this unfortunate outcome. For the want of a nail...

 

[jhnblgr]

Sounds like the contractor didn’t properly shore the deck and possibly ran a ride on power trowel. The concrete had to get hard enough to place the power trowel onto the slab and the weight and vibration caused cracks in the setting concrete that were due to deflection of the steel beams and trusses. The trowel filled in the cracks but the concrete was already set and the cracks popped back up. Seen this numerous times.

 

[271828]

Composite slabs often have cracks over girders and beams at column lines even if rebar is placed there. That's because the slab is thinnest there and has substantial in-plane restraint. It has to crack somewhere. Rebar might reduce the size of those cracks.

I first saw this on a project a couple of decades ago. We designed a six or seven story building, and the top two floors were shell. Unshored construction, so in the shell spaces, the deck was theoretically resisting all of the load up to that point. We had rebar over the girders and beams along column lines. Before any superimposed load was added, it had cracks along many of those girders and beams. The owner was concerned that it might cause issues with the flooring that might go in later.

 

[lexpatrie]

I take it the project is in New York so there is no statute of repose?

But this cracking isn't a structural defect from your description, not being able to put down crappy vinyl flooring doesn't seem like much on the whole economic harm doctrine either.

Unless it's a health code issue, or somehow this cracking endangers the fire rated assembly, carpet.

 

[dik]

He used Fibermesh and no mild reinf.

This is possibly the cause of the cracking... I never use polypropylene fibres for reinforcing.