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Re: Concrete in uplift

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Drew,

Thanks for the observations. I did not address,
explicitly, the capacity of the components in the
entire load path down to soil bearing layers. Morris
indicated that the load is applied on a 4" slab and
not on a pier. If the slab does not pulled out in
"punching shear" the stability will be governed by the
weight of concrete available in the slab. For example,
if the column is attached to the slab through a 4-bolt
connection on a 6" square base plate, the "punching
shear" capacity will be of the order of 10 k.
Therefore, the concrete will not break off.

To resist the uplift of 1500 Pounds with a F.O.S. of
1.2, 36 sft of slab is all that is required. For a
F.O.S of 1.5, 46 sft. is required.

Rajendran

--- "Drew A. Norman, SE" <DNorman(--nospam--at)dnormanse.com>
wrote:
> Mr. Morris,
> 
> Mr. Rajendran seems to have either misunderstood
> your question or
> mis-spoken, and his reply could get you into
> trouble.  It is NOT appropriate
> to use allowable loads for a drill-in anchor without
> consideration of the
> ability of the concrete into which you are anchoring
> to resist the load your
> structure will apply.
> 
> The reply Mr. Rajendran posted implies that a 5/8"
> Kwik Bolt anchor with
> 2.75" embedment in the center of the top of a 3000
> psi precast concrete
> foundation pier sitting on the ground beneath a home
> undergoing a seismic
> retrofit could be relied upon to safely resist an
> uplift force of 1,870 lbs.
> Envision that pier as a cube 12" on a side for
> simplicity.  That's one cubic
> foot and it weighs about 145 lbs.  While it is true
> that the ANCHOR should
> be capable of transferring 1,870 lbs to the pier, it
> should be obvious that
> when the earthquake hits (and even though Jim-Bob
> the residential
> improvement contractor may jab his finger repeatedly
> at the 1,870 lb
> allowable load written right on the side of the box
> the anchor was in when
> he bough it at Wal-Mart) that little old pier is
> going to pick up and move
> away from New Madrid.
> 
> In re your original inquiry, knowing that the
> concrete is at least 4" thick
> is not enough to allow you to make a reliable
> estimate of uplift capacity.
> If (and only if) I knew that it had at least minimal
> two-way reinforcing,
> AND if I knew that it was at least 2500 psi
> material, AND if I had seen it
> and it appeared to be in good condition (free of
> significant cracking), then
> I might assume a circular anchoring mass with a
> radius equal to eight times
> the thickness or 32" (this via an admittedly
> generous application of the
> Westgard theory used to determine effective flange
> widths for "T" beams).
> That would give a theoretical upper limit of about
> 1,100 lbs. You would have
> to be careful however of edges and overlapping
> effective areas.  If the
> information available was insufficient (it sounds
> like it is), or if the
> load exceeded a few hundred pounds (you indicate
> you've got 1,500), I would
> go with the helical pier, "blob" or some similar
> approach.
> 
> Or you could led Jim-Bob sign the drawings himself.
> 
> Had to speak up because we have the same first name.
> 
> Drew Norman, S.E.
> Drew A. Norman and Associates
> Pasadena, California
> 
> ----- Original Message -----
> From: "Padmanabhan Rajendran" <rakamaka(--nospam--at)yahoo.com>
> To: <seaint(--nospam--at)seaint.org>
> Sent: Thursday, October 18, 2001 5:30 AM
> Subject: Re: Concrete in uplift
> 
> 
> > Hilti Kwik Bolt II Expansion anchor will do the
> job
> > for you. As you have indicated, assuming that the
> slab
> > is only 4" thick, 5/8" dia. Kwik Bolt II embedded
> > 2.75" in concrete can provide an allowable
> capacity of
> > 1870 Pounds in a 3000 psi concrete. The capacity
> is
> > independent of reinforcing. The only other
> > requirements are:
> > Minimum edge distance to the anchor = 4.5"
> > Minimum anchor spacing, for multiple anchors =
> 5.5"
> >
> > The above recommendation assumes that the anchor
> does
> > not experience any shear.
> >
> > Rajendran
> >
> > --- Drew Morris <dmorris(--nospam--at)alaska.com> wrote:
> > > We were having a discussion at work on anchorage
> to
> > > a concrete slab.
> > > The uplift force is 1500 # and the slab is at
> least
> > > 4" thick.  We have
> > > no information on the slab thickness or
> reinforcing
> > > (if present).  What
> > > tributary area can I use for resisting the
> uplift
> > > (if any at all)?  I'm
> > > planning on using a helical screw anchor or saw
> cut
> > > the slab and use the
> > > "blob of concrete as dead load" concept.
> 
> 
> 
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