Slab on Deck - Camber, Ponding, Deflection Etc 2

[bookowski]

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?

 

[KootK]

At spandrels I can see an issue for facade attachments, but is there any reason to have a limit at interior beams?

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.

How do people handle this - assume an average of say 3/4" extra concrete for ponding and carry that in the pc dead load?

Pretty much. I go 1". Or 15%.

Is camber worth the savings in loading from ponding concrete?

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.

 

[bookowski]

Thanks KootK. In the past I've used camber but on this new project I'm questioning it. As you said by the time you eliminate all the 'don't camber' beams (w14, <20ft, moment frames, braced frames, spandrels, webs <= 1/4", etc...) you can end up with a weird mess of flat and cambered.

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.

 

[KootK]

Lately I've been providing 1200 long T&S bars at girders only (not infill beams). You get that painful discontinuity of curvature across girders at the pin-ish ended infill beams that I feel warrants some consideration both for:

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.

 

[bookowski]

In the first link you posted they cite a cost comparison study which found cambering to result in a savings of 4%. That's lunch money - seems to confirm that it's probably not worth it in most cases.

You always do non-composite girders? Savings on studs outweighs steel uptick?

 

[KootK]

I do typically go non-composite girders. Not really sure how the economics pan out. My thinking:

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.

 

[Hokie93]

AISC Design Guide 3 (Serviceability Design Considerations for Steel Buildings) recommends limiting floor dead load deflection to L/360 with a maximum of 1 inch. I personally have used a pre-composite limit closer to 1.25 inch for longer spans and I would consider 38'-0 to be on the long side. The Guide mentions that for composite floors, "the deflection limits should be applied to the instantaneous deflection plus one half of the expected creep deflection". The AISC Specification (360-10) does not require long-term deflection be considered for composite beams and girders. The Commentary to Section I3.2 states that creep deformations "are small unless the spans are long and the permanent live loads large". The Guide also recommends increasing the concrete slab weight by 10% to account for deflections during concrete placement operations. This recommendation is based on a paper ("Ponding of Concrete Deck Floors") by John Ruddy. It was published in AISC Engineering Journal, third quarter, 1986. It is an excellent paper and a fairly easy read if you don't already have it in your library.

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.

 

[KootK]

Nice contribution Hokie. Just added this one to my thread archive.

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.

 

[bookowski]

Thanks Hokie - good stuff. I'll review DG 3 and the Ruddy paper.