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Re: CONC Slab Uplift

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The solution is easy..
Write a memo to your p.m. stating your objections, keep a copy and/or
have the contractor or your Civil project manager sign the drawings.

How many times have we heard from contractors "I've never had to do
this or that before" or "I've just finished an _identical_ project than
we didn't have to do this or that" ?

What are you being hired to do? Just nod and say "yes" to every non-techie?

I'm starting to remember why I stopped working for other people. Today, I
no problem telling a contractor, when he wants to deviate from the
design and I'm uncomfortable with the changes, "If this is what you want to
_you_ sign it."

Yes, Nina, I wouldn't use the slab to resist 2.5k in uplift.

Bill Allen

> From: Sickler <sickler(--nospam--at)>
> To: seaoc(--nospam--at)
> Subject: Re: CONC Slab Uplift
> Date: Friday, August 01, 1997 11:06 AM
> Joe McCormick wrote:
> > 
> > ... it seems like the 4" slab
> > would not be all that far off from being **cabable** of resisting 2.5k
> > uplift even without reinforcing as long as the column-to-slab
> > doesn't fail!
> ...
> > Even if the numbers work out, is this a good design, though? maybe and
> > not...guess that's ultimately up to the EOR.  I would personally be
> > about using the floor slab to resist uplift and not do it because IMHO
> > many floor slabs get completely beat up and riddled with cracks over
> > 
> 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