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Re: CONC Slab Uplift

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Joe McCormick wrote:
> 
> ... it seems like the 4" slab
> would not be all that far off from being **cabable** of resisting 2.5k of
> uplift even without reinforcing as long as the column-to-slab connection
> doesn't fail!
...
 
> Even if the numbers work out, is this a good design, though? maybe and maybe
> not...guess that's ultimately up to the EOR.  I would personally be nervous
> about using the floor slab to resist uplift and not do it because IMHO too
> many floor slabs get completely beat up and riddled with cracks over time.
> 

This is exactly the issue I am wrestling with.  My question stems from a
situation that came up about two years ago.  We were designing a 170'
long
slab for airplane T-hangars where the center line of columns were spaced
at 21' and had alternating uplift loads of 6.8 kips and 0.8 kips
(unfactored).

Our design included a 6" slab with a 5' wide by 12" deep grade beam
under
the center column line to resist the uplift loads.  The contractor
adamantly
insisted to the owner (our client) that the slab was grossly
overdesigned.
Our project manager (a civil P.E.) weakly defended the design and agreed
to
some reductions - reduced slab to 5", increased rebar spacing slightly.

Apparently, the contractor had installed similar hangar slabs with much
less
in the way of "formal" uplift resistance.  Using "common-sense" and
figuring
that concrete will behave as detailed (concrete does not "know" whether
it
is a slab on grade or a structural slab), one could devise a model of
the
slab's behavior that could explain how the slab could resist the applied
loads.

On the other hand, as many folks have pointed out, there are real-life
factors such as cracking of the slab, unknown material quality, etc.
that
could make it unwise to depend on a slab-on-grade alone for uplift
resistance.

I wanted to avoid the mistake of "over-conservatism" in the case of this
4" slab,
but I am getting the feeling that most of you would insist on a footing
to
resist the entire uplift load.

Thanks for the input.

Nina Sickler