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Q: COMPRESSION Force On Anchor Bolts/Rods

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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

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?


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.


William L. Polhemus, Jr. P.E.
Polhemus Engineering Company
Katy, TX, USA
Phone (281) 492-2251
FAX (281) 492-8203
email bill(--nospam--at)

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