Slab on Deck - Camber, Ponding, Deflection Etc
Slab on Deck - Camber, Ponding, Deflection Etc
(OP)
As an example take a typical steel floor beam with a 38 ft. span supporting slab on composite metal deck (using 3.25 ltwt over 3 deck):
- L/240 for total load over 38ft is close to 2", at what point do you limit total to some absolute number? If you add in the girder deflection you're over 2". Assume a dropped ceiling so it's not visually an issue. At spandrels I can see an issue for facade attachments, but is there any reason to have a limit at interior beams?
- The pre-comp deflection is about 1.25". Assuming the contractor is working to a constant elevation rather than constant thickness this is an additional 12psf at midspan (probably more if accounting for the accumulated deflection of girders). How do people handle this - assume an average of say 3/4" extra concrete for ponding and carry that in the pc dead load?
Is camber worth the savings in loading from ponding concrete?
- L/240 for total load over 38ft is close to 2", at what point do you limit total to some absolute number? If you add in the girder deflection you're over 2". Assume a dropped ceiling so it's not visually an issue. At spandrels I can see an issue for facade attachments, but is there any reason to have a limit at interior beams?
- The pre-comp deflection is about 1.25". Assuming the contractor is working to a constant elevation rather than constant thickness this is an additional 12psf at midspan (probably more if accounting for the accumulated deflection of girders). How do people handle this - assume an average of say 3/4" extra concrete for ponding and carry that in the pc dead load?
Is camber worth the savings in loading from ponding concrete?






RE: Slab on Deck - Camber, Ponding, Deflection Etc
In my opinion:
1) In general, no.
2) I only usually worry about LL + LT for interior beams.
3) If your concrete ponding increases your concrete weight by more than about 15%, I'd start to worry about stability.
4) If possible, I try to keep LL + LT less than the deflection that can be accommodated by a deep leg deflection track.
Pretty much. I go 1". Or 15%.
Meh. I've been leaning towards no lately:
1) By the time that I adhere to all of the "when not to camber" rules, there often doesn't seem as though there are very many beams left for most buildings.
2) One must be cautious in camber lest the camber not come out enough to provide adequate stud cover. This cautiousness tends to limit the value of camber somewhat.
3) Natural camber probably gets you part way there anyhow. Although I guess the owner reaps no benefit from that.
Some excellent background information, most of which you're surely already familiar with.
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I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Slab on Deck - Camber, Ponding, Deflection Etc
Adding 1" of ltwt adds about 10psf, my deck/slab is 46 psf - about a 20% increase. I'm surprised that shored construction isn't the answer but I don't see many in favor of that option.
Where are you at with rebar. Same deal - in the past we've shown top bars but now I'm thinking it's a waste in most cases.
RE: Slab on Deck - Camber, Ponding, Deflection Etc
1) Crack control if it's exposed.
2) Performance of the studs on the girder if those exist (I don't usually have studs on the girders).
If it's an exposed concrete floor scenario where folks may care about the appearance, I'll go with a light mat of top steel all over. Or WWF. Or fibermesh. Kinda depends on who I'm dealing with and what their priorities are.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Slab on Deck - Camber, Ponding, Deflection Etc
You always do non-composite girders? Savings on studs outweighs steel uptick?
RE: Slab on Deck - Camber, Ponding, Deflection Etc
1) I worry about aligning flutes with the girders or doing special deck things to make that happen.
2) Even with the rebar, I worry about the concrete being all split up around the studs due to negative flexure.
3) My sense is that, the larger the member tributary load, the less ROI you seem to get out of the composite action.
Sometimes I'll throw some nominal studs on the girders (drag strut stuff etc) but I rarely design them as composite.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Slab on Deck - Camber, Ponding, Deflection Etc
AISC Design Guide 3 also recommends limiting floor live load deflection to L/360 with a limiting value of 1 inch using 50% of the design live load. You may recall that using 0.5L for serviceability checks is discussed in the Commentary to Appendix C of ASCE 7-10.
I have found composite construction to be very economical and don't hesitate to use it when it makes sense.
RE: Slab on Deck - Camber, Ponding, Deflection Etc
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Slab on Deck - Camber, Ponding, Deflection Etc