Exceeding maximum unsupported span of metal deck

[jdbpe]

I have a project where a 2 inch, 20 gage metal deck was specified to support a 5 1/2 total concrete slab depth. During the design phase, column center lines increased and the spacing of beams increased. It turns out that the 2 inch metal deck that was specified has an unsupported span of 8'-10". The deck tables indicate for a three span condition that the deck can only span unsupported during construction 8.45 feet. Our clear span is 5 inches more than the table allows. The concrete has been poured. How big of a problem is this ? Analysis indicates that our total load will be 170 psf. The tables indicate that it can support 285 psf as a composite structure.

 

[271828]

You should try to figure out where their 8.45' came from. That number probably is just to make sure the deck is strong and stiff enough to support the wet concrete. If that's the case, then the number seems less important now, assuming that there is no sign of distress (deck local buckling for example) or excessive deflections. I suspect that you're fine.

FWIW, Vulcraft's equivalent deck is listed at 8'-11" clear in their catalog.

IMO, it's good practice to only tentatively select the deck based on the catalog and make the deck mfr actually design it and take responsibility for it. I generally list the depth and call it out something like "20 gage, min." I prefer to leave it off totally and make the deck mfr pick it using loads and deflection criteria I give, but the contractor often needs it at DD for pricing.

I like the performance spec approach because there are inevitably weird cases in which there might be a single span or whatever. The deck guy will know where these are and I won't.

 

[lkjh345]

The 8.45 feet was the maximum construction loading under wet concrete and (I believe) a 20 psf live load for finishers and equipment. Once the concrete cures and becomes composite with the deck, this distance has no relevance.

The method described by 271828 above is a good way of preventing a similiar problem in the future.

 

[msquared48]

The only long term problem will be extra dead load deflection since it was unshored.

Mike McCann
McCann Engineering

 

[JAE]

If it sagged during concrete placement, it could be that you have a bit more dead weight of concrete than you originally figured, but that might not be a big enough amount to affect safety.

 

[Lion06]

I expect there might be some (probably a small amount) of unaccounted for DL due to increased deflection of the deck and an increased amount of concrete placed to get it flat.

 

[whyun]

Its not a collapse issue. Extra deflection due to 6" longer span is negligible. Now you know to leave some room in your design next time around (for requested changes in the middle of design phase).

 

[JKStruct]

I agree with the comments of 271828. If I'm not mistaken, those tables in deck catalogs are usually based on the clear span distance between beams -- i.e. if you're beam centerline spacing is 8'10", you could subtract bf from that number to arrive at the clear span of the deck. I usually design the decks based on the 2-span condition for a personal comfort safety factor. I know of a project that called for 22ga (non seismic area -- I think CBC requires 20ga min.) and was designed for the 3-span case. There was one bay where the deck only spanned one bay, and the deck collapsed when they poured the slab.

Jae and StrlEIT make good points about the ponding load. I always include around 8 psf of ponding load for deck and beam deflection. Don't know if the deck manufacturers include this allowance, but they should.

Our office uses 20ga at a minimum, and often places notes similar to 271828. We have a general metal deck note that reads something like "Metal deck should be installed continuously over 3 spans (4 supports) wherever possible. Where not possible, the contractor shall increase the deck gage as required to support the same load as the 3 span condition".