2-Way Slab Creep 1

[BadgerPE]

I am working on a preliminary CIP 2-way slab that has large spans of ~ 40' in areas. My superimposed loading is 50 psf dead and 100 psf live (public area) and preliminary slab thickness of 18" is being used (225psf). If I proportion the slab so that the limits of ACI 9.5(c) (318-11) are met, do I need to analyze the slab for creep as well? Per R9.5.1 "For nonprestressed two-way construction, minimum thickness as required by 9.5.3.1, 9.5.3.2, and 9.5.3.3 will satisfy the requirements of the Code". I am unsure if this includes long term deflections or not.

 

[dik]

For those spans, have you considered steel and composite deck? or beam and slab, or post-tensioned slab with drop panels... With 40' spans, you could have a 6" slab with beams at 13'-4" centres of something of that ilk. The economy of structural framing is choosing the system to use in the first place.

Dik

 

[BadgerPE]

Dik,

Thanks for the feedback. Yes all of those options have been considered, but for one reason or another, ruled out. Steel is out due to the environment and as of now PT is out because there is a LOT of CIP on the project and no PT. It would be a small PT job ~7000 sf and therefore the contractor hasn't entertained that possibility yet. However, when pricing comes in for the CIP slab, then PT may show its value.

Any thoughts as to my original question?

 

[dik]

I've not used a slab that thick in all my years... I would think that a beam and slab approach would be far cheaper. If foundations are unusual, the savings increases. I would put together three or four preliminary framing schemes to get a costing on the different approaches.

If corrosion is an issue, then you might consider the use of HDG rebar, too.

Dik

 

[Ingenuity]

BadgerPE:

With your preliminary slab thickness of 18" for a 40' span that is L/D of 27 - which is somewhat less than Table 9.5(c) min. thickness requirements of 33 through 36 (assuming you are using drop panels and f[sub]y[/sub] of 60 ksi), so you may be able to 'skinny' down your slab and still satisfy.

The tabulated values are really just based upon past projects with certain design/load/span parameters, of which you are not privy as the commentary does not give any data on permanent/live load ratios, material props, time to first loading etc . It is a minimum and NOT a 'deemed to comply' type table whereby if you comply you automatically satisfy a given deflection criteria. So in that sense, by satisfying the minimum does not mean you satisfying long-term deflection for your given structure.

So bottom line, I always check long-term deflections.

In my experience of some older 'as-constructed' RC flat slabs, ACI 318 is not especially 'accurate' in its estimation of long-term deflections, using the multipliers of §9.5.2.5, often underestimating real LT deflections.

 

[slickdeals]

I second Ingenuity. For those spans, I would recommend running a true long-term load history analysis that accounts for cracking, shrinkage, creep etc.

 

[BadgerPE]

Thanks for all the feedback. The selection of CIP is one that we have been pushing against, but the owner/arch wants it so here is where we are. It looks though that we are going to add a row of center columns to get spans down in the 20-30' range, so that will help significantly.

 

[JoshPlumSE]

slickdeals -

I'm curious what you mean when you say "true long-term" load history analysis. I have an idea what this would entail for beams. But, for slabs, it doesn't seem as well defined.

 

[rapt]

JoshPlum,

Same logic as for beams, allow for cracking, tension stiffening (not using Branson's formula), creep and shrinkage allowing for the various load stages.

Ingenuity
"ACI 318 is not especially 'accurate' in its estimation of long-term deflections, using the multipliers of §9.5.2.5"
That was very polite!