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Re: Concrete in uplift

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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)>
To: <seaint(--nospam--at)>
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)> 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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