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# Re: CONC Slab Uplift

• To: seaoc(--nospam--at)seaoc.org
• Subject: Re: CONC Slab Uplift
• From: Joe McCormick <jmccormick(--nospam--at)proaxis.com>
• Date: Thu, 31 Jul 1997 17:49:06 -0700

The slab doesn't meet ACI requirements for a plain footing (ACI 318-95
chapter 22, minimum 8" depth), but that aside, 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!  Consider an 8.5'x8.5' chunk of concrete, 4" thick, assuming
the distance from face of column to edge is about 4 feet.  Design like a
footing.  For flexure as a one-way beam:  S=32 in^3/foot and self weight is
50 PLF/foot.  Mu = (0.75*1.4*50)*4*4/2*(12"/ft) = 5040 lb-in/foot.  (phi)Mn
= (0.65)*(5*(f'c)^1/28*S) = 5(2500)^1/2*32 = 5200 lb-in/foot so bending
seems to be OK.  For beam-action shear:  Vu = 210 lbs/ft, (phi)Vn =
(0.65)*4/3(f'c)^1/2*b*h = 4/3*(2500)*12*4 = 2080 lbs/foot so beam shear
seems to be OK.  For two-way action: Vu = 2.5k (assume this is already
factored?), (phi)Vn = (0.65)*2.66*(f'c)^1/2*BoH (assume Bo is 6"x6" square =
24")=(0.65)*(2.66*50*24*4") = 8299 lbs so punching shear seems to be OK.  Of
course, extra capacity is there too since the floor isn't really an 8.5' x
8.5' chunk, but is probably much bigger than this and will bend like a plate
with constrained edges.  As a reality check, asking myself if I could
install an embedded anchor in a 4" floor slab and pull on it with 2500 lbs
without pulling out the anchor, the answer seems to be "probably".  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.

Joe McCormick

At 03:45 PM 7/31/97 -0700, you wrote:
>Shouldn't be a problem with #4s @ 3-1/2" E.W. x 17'-6" long :o).
>You wouldn't see that on my plans!!
>
>Regards,
>Bill Allen
>
>----------
>> From: Jim Dane <jimdane(--nospam--at)ix.netcom.com>
>> To: seaoc(--nospam--at)seaoc.org
>> Subject: Re: CONC Slab Uplift
>> Date: Thursday, July 31, 1997 1:38 PM
>>
>> Bill Allen, S.E. wrote:
>> >
>> > Don't you need Fres=1.5*Fuplift?
>> >
>> > Regards,
>> > Bill Allen
>> >
>> I assumed that the 2.5 kip uplift included any safety factors.  You know
>> what assume stands for so you are probably correct.  That would mean
>> that 75 square feet of slab would be needed.  I doubt that a 4" slab
>> will have enough reinforcing to handle the force.
>>
>> Jim Dane, P.E.