Exceeding maximum unsupported span of metal deck
Exceeding maximum unsupported span of metal deck
(OP)
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.






RE: Exceeding maximum unsupported span of metal deck
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.
RE: Exceeding maximum unsupported span of metal deck
The method described by 271828 above is a good way of preventing a similiar problem in the future.
RE: Exceeding maximum unsupported span of metal deck
Mike McCann
McCann Engineering
RE: Exceeding maximum unsupported span of metal deck
RE: Exceeding maximum unsupported span of metal deck
RE: Exceeding maximum unsupported span of metal deck
RE: Exceeding maximum unsupported span of metal deck
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".