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Re: Q: COMPRESSION Force On Anchor Bolts/Rods

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Bill et al,

This is what I do, when it comes to designing
anchorages in compression. Calculate the cevelopment
length required to transfer compressive forces per ACI
318, Section 12.3. I am comfortable with this approach
mainly because of what I have seen for anchorages
provided for tall light poles. You may have seen 30'
tall light poles, with 4 luminaires on top, with a
base plate configuration similar to what you have
indicated. Yes, wind forces are critical for that type
of structure, but two of the four bolts will be in
compression. Typically, I have seen hooked bolts, 54"
long for such poles! If you used PCA methodology, you
will never need such deep embedments. But the anchor
design for the light poles have worked for years, all
over the country.

Your question is just only one of several others that
begs an answer from PCA publication(or ACI).Frankly,
anchorage design is as confusing to me today, as it
was, say, 10 years ago, notwithstanding the latest
publication on the subject from PCA. The philosophy of
cone pullout etc. does make sense. However, the
interpretation of all the rules in various practical
anchor bolts layout is still fuzzy. The examples at
the back of the publication raises many questions.

The publication does not address, specifically, cases
where concrete will be limited to the extent that it
cannot provide the required strength to resist the
tensile loads. It seems to me that PCA is based on the
assumption that adequate amount of concrete will be
avilable around the bolt(s) to provide the required
strength. (ACI 349R recommends hairpin reinforcing for
tension and horizontal reinforcing for shear to cover
such cases. However, implementation of ACI 349R can
become a nightmare).

PCA does not address the situation where a few of the
bolts may be in tension and the remaining part of the
system may impose compressive stress on top of
concrete. That part of concrete which is in
compression may provide a much larger resistance to
cone pull out arising from the bolts that are in
tension.

Are there cases of anchorage failures over the years
that have resulted in the development of more and more
sophisticated and complicated analytical methods for
design of anchorages? Has there been any catastrophy
due to anchor failure? I have not heard of any.

Rajendran

--- Bill Polhemus <bill(--nospam--at)polhemus.cc> wrote:
> QUESTION ONE.
> 
> I am obliged to design the anchor rods for a base
> plate which is NOT grouted
> so that ALL forces are transmitted to the anchor
> rods via hold-down AND
> leveling nuts. I gather this isn't an altogether
> uncommon thing since the
> baseplate design program I'm using actually allows
> it as a condition for
> design.
> 
> My problem is in considering the embedment of the
> bolts and the capacity
> thereof to resist compressive forces. If I have a
> steel-plate anchor
> mechanism, it seems to me that the anchorage and the
> encapsulating concrete
> are going to see the same kind of effect with the
> bolts in compression as
> from tension, only in reverse (i.e. there will be an
> effective conical
> "pushout" area BELOW the anchorage).
> 
> As you might expect, because of the gravity loading
> the compressive force is
> actually GREATER than the tension force in a given
> bolt.
> 
> It seems to me as a first approximation, then, that
> you'd need at least as
> MUCH concrete below the anchorage as would be
> required above it, to keep the
> anchor rod from "plunging" or the anchorage zone
> from blowing out.
> 
> Has anyone looked at this in detail? If so, what
> design assumptions did you
> make and why?
> 
> QUESTION TWO.
> 
> Using the publication "Strength Design of Anchorage
> To Concrete" by Ronald
> A. Cook, published 1999 by the Portland Cement
> Association, and which
> purports to be a "late draft" of the proposed
> provisions for design of such
> anchorages to be included in the next edition of ACI
> 318, I have NOT been
> able to determine if the designer may utilize
> surrounding reinforcing bars
> that cross "concrete cone failure" boundary to
> increase the concrete
> breakout strength.
> 
> I have seen this utilized in other instances, and it
> is inferred in ACI 318
> ? 11.7.7 (esp. see the accompanying Commentary), so
> it seems logical you
> could do that in this instance, but I don't see
> where Cook explicitly
> mentions it.
> 
> Any comments are very welcome.
> 
> Thanks.
> 
> 
> 
> William L. Polhemus, Jr. P.E.
> Polhemus Engineering Company
> Katy, TX, USA
> Phone (281) 492-2251
> FAX (281) 492-8203
> email bill(--nospam--at)polhemus.cc
> 
> 
> 
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