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Re: CONC Slab Uplift

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Sickler wrote:

> I am evaluating a 4" slab on grade with no edge
> beams or footings.
> At one column location, there is an uplift load
> of 2.5 kips.
> Given that the slab is thin and flexible, it
> would appear that
> the uplift load would engage a portion of the
> slab in bending.
>
> The highest uplift reaction occurs in the middle
> of the slab, away
> from the edges; but I am also interested in
> behavior near the edges.
>
> How would you determine what portion of the slab
> resists the uplift
> load and then how would you evaluate the design
> moments in the slab?

In my own (meager) experience I have encountered a
wide variety of views on using slab for uplift.

Personally, I will somtimes count on the slab
concrete that lies directly over the footing for
uplift resistance if I need it.  Sometimes a
couple of feet beyond (I'm usually working with a
5" min slab with welded wire fabric).  After
reading the posts here, I think I will be more
cautious about using those extra couple of feet in
the future.

I have seen some metal building foundation designs
(note that the slabs (6") were reinforced and the
perimeter column footings tied about 10 to 20 feet
back into the slabs by rebar "hairpins"), that use
a lot of slab.  The argument that I have heard,
(which I generally disagree with), is
approximately as follows:

1.  The mode of failure for overturning is for the
building to overturn.
2.  In order for the building to overturn
completely, you will end up picking up much of the
slab, one way or another.  Even if the slab cracks
and buckles in bending, the slab reinforcement
will still provide a sort of draping action off
the footing which will carry most of the slab
concrete up too.
3.  The 1.5 safety factor ensures that you will
rarely test this extreme case of overturning.  (I
suppose that this implies that the less extreme
uplift case, where you are talking about damaging
the slab and building, but not overturning the
building, is fair game for a 1.0 safety factor.)

Anybody concur in whole or in part?



Stan Johnson
BS, EIT, "No matter which way you turn, you're
backside is always behind you,"  Squire (Jons?) .
:)