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RE: Flat Slabs

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Mike,

I would start off by suggesting that you just build the slab on grade using
any good slab-on-grade design reference.  The only reason that I can think
that you are putting it on a slab supported by piles/columns is that you may
have either really bad soil (like in Thailand) or very expansive soil (like
in Colorado).  I will predicate my comments supposing that you have one of
these 2 conditions.  I don't have any good references, but I have designed a
few of these.  My last similar application was for some heavy industrial
equipment that could not tolerate movement, and the soil was very expansive.

Start by determining the load on the pile/column.  Your slab thickness will
be driven by the concentrated punching shear loads of the column or the fork
lift wheel load.  Drops are expensive to form, and I would avoid them.  Once
you have the column sized, check the punching shear and size the slab.

On the last one of these brutes I designed, I had a heavily loaded area
about 20' x 80'.  I used a one way slab on beams.  I used the same thickness
of slab as I had for my beams.  I was working for a design build firm at the
time, and started out using a deeper beam than the slab, but the forming
costs caused me to go to the same depth slab as the beam.  You build the
form deck (hard board on void forms), and then just tie 2 layers of rebar
with the base on one horizontal plane.

The advantage of the beams is that you can add stirrups to take the shear.
I tried to use the stud rails and the flat slab, but I had all kinds of
point loads which did not let the rails work very well.  Your fork lift will
do the same thing as the point loads I struggled with.

If the area is really big like a stamping plant, then you would be better
off with no beams (the rebar tying costs will outrun the extra concrete
costs) and just design it as a flat 2 way slab, but I would still drop the
drops.

Regards,
Harold O. Sprague

> -----Original Message-----
> From:	Michael Zaitz [SMTP:mzaitz(--nospam--at)hgbd.com]
> Sent:	Thursday, September 20, 2001 2:00 PM
> To:	Structural Engineers Association
> Subject:	Flat Slabs
> 
> Hello all;
> 
> We are looking at a preliminary design for a flat slab floor (at grade)
> supported by precast piles at 12'-6" o.c. each way.  The floor must
> support a 800 psf live load.  We also have a forklift carrying a 7000
> lbs roll of paper (trying to get more information on this).  They have
> not informed us of any rack storage in the building.  At present we are
> looking at a 12" slab as a starting point.  Having only done one flat
> slab project (actually checking an existing building to insure it was
> designed for vertical expansion) I am trying to find out about any
> pitfalls I may need to look out for.  Based on limited investigation
> there may be a need for drop panels (don't think so for sure but is a
> possibility).  Also would love to get some ideas for references on the
> design of flat slabs.
> 
> Mike
> 

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