How to determine if an existing metal deck is Composite or Non-Composite

[jochav52802]

I'm working on a project where we need to add a new concentrated load to an existing 6" overall slab on 3" metal deck.

I contacted the Steel Deck Institute (SDI) with a sketch and dimensions of the underside of the deck, and was told definitively that since there are no visible embossments, the deck is a non-composite deck.

I then computed the maximum positive moment based on the dead and live loads specified in the original general notes drawings as well as based on the (2) live load patterns specified in ACI 318-14.

Next, I computed the positive moment capacity based on ACI 318-14 and determined that the slab alone had no where near the amount of capacity required to support the existing loads without some sort of composite action.

Any thoughts on how I might go about verifying the true capacity of the floor? Does it make any sense to take the original dead and live loads, pattern them per ACI 318-14, and use the resulting maximum positive and negative moments as a basis for what the floor was designed for?

The design live load was 100 psf, and I'm thinking of de-rating that value for the client in order to accommodate the new concentrated loads. This is a an office space, which per ASCE 7-16, only requires a live load of 50 psf, so I'm thinking that I can potentially de-rate the floor by as much as 50 psf to make the new concentrated loads work.

Any thoughts or ideas are much appreciated, thank you!

 

[JedClampett]

I'm not clear. Do you know for certain that there's no reinforcing in he slab?
Back in the 70's, when I started, to now, I was trained to ignore the composite properties of the deck, even when it was composite and to reinforce it.
I've worked with my share of incompetent idiots, but incorrectly designing a slab for 100 psf seems pretty unlikely.

 

[jochav52802]

Hello JedClampett,

The top 3" of the slab is reinforced with #5 bars @ 16"; other than that there is no positive moment reinforcement in the flutes. With that said, my main concern is regarding SDI's adamant claim that the deck is not a composite deck (which is based on the absence of embossments), as the slab's positive moment capacity is far to little to carry the total load of 166 psf specified on the design drawings. This seems to imply to me that there is some other composite action being employed by the metal deck that is not visible, (e.g. studs may have been welded inside the deck.)

Your second sentence is aligned with what SDI told me, which was that composite metal deck was a new product/concept in the 70's and it took a lot of convincing to get Engineers in the era to rely on composite action, so many didn't use it.

My main question is whether or not I can just use the total load of 166 psf, break it down into it's 66 psf dead load and 100 psf live load components per the existing drawings, derive the maximum positive moment using the (2) ACI 318-14 load patterns, and use that resulting value for the moment capacity of the slab, which would represent the composite moment capacity of the slab and deck. From there, I'd add the new concentrated load and reduce the 100 psf live load such that the resulting moment matches the moment capacity of the slab.

 

[JLNJ]

I have never heard of welding studs to deck to make the deck composite. I wouldn't hang my hat on that one.

If it's an office space and can't/won't be reconfigured into assembly or a corridor, then de-rating it is reasonable.

If you have doubts about the deck capacity, then I don't think you can "assume" it's good for 100 psf and just "match" that. Think about trying to explain that to the jury if something went wrong. "Yea, I knew I couldn't get the capacity to calculate out but I went with the 100 psf listed on the drawings anyway because it was convenient."

3" deck has a fair amount of capacity on its own. Do you know if you have a 2 or 3 span condition? What's your span? Best to exhaust all your options to figure out how it works. If it still doesn't, you need to have a conversation with the Owner.

 

[jochav52802]

Thanks JLNJ,

Per my communications with Verco deck, welded studs, welded mesh and other forms have been used in the past. Definitely not trying to hang my hat on that, but merely point out that the discrepancy between the slab's moment capacity and the applied moments suggests there's something in the deck that's acting compositely with the slab.

We were considering identifying the de-rated area with permanent paint or taping and signage that noted the reduced capacity in the area.

Your point is well take; I don't want to assume anything hear, merely just trying to hash out all possibilities. The drawings clearly indicate the service dead and live loads for the floor, and it seems unlikely that the drawings are wrong or that non-composite decking was accidentally installed given that the floor has successfully carried office loads for 45 years now. I guess in the end, I can't tell for sure without some kind of local demo or GPR to see what the deck composition looks like, so I'm best served in pursuing plan-b, which is to add new framing to support our new concentrated loads.

Per my communications with SDI, we're not supposed to use the combined capacity of the deck and slab together. This makes sense since the two elements have different stiffnesses, which will not allow the deck to fully develop it's strength since the slab is much stiffer. Additionally, I found it interesting that SDI Standard NC-2017 requires that the slab support it's own weight when it's supported by decks that are uncoated or painted. Galvanized decks are allowed to support the weight of the slabs. I suppose this is to account for the possibility of the deck corroding and loosing it's capacity over time. A bit of a bummer for me as there's no sign of deck corrosion after 45 years.

 

[BAretired]

Plan-b sounds like the safest and simplest solution.

BA

 

[bones206]

If you strongly suspect the structure was not properly designed to resist the 100 psf live load, isn’t there an ethical obligation to present those findings to the owner and possibly de-rate the structure entirely?

 

[BAretired]

bones 206,

Before doing that, it might be a good idea to do a little radiographic or other non-destructive investigation. If that doesn't produce conclusive results, perhaps a little destructive investigation in an appropriate location where the full live load is unlikely to be present.

BA

 

[bones206]

Agreed. Whatever course of action is taken, the owner should be aware of the load rating issue and it should be seen through to resolution.