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RE: Anchor Bolts in Flexure

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Look at ACI 318 Appendix D (Anchorage to concrete) or the source document
from ACI (PCA?).  It talks about shear and distance between top of concrete
and bottom of base plate.  If you are off the top of the concrete, you get a
reduced shear capacity.  There is no direct moment check, but there is a
reduction.  As I read it the grout doesn't provide confinement.  All you get
is bearing area for the base plate.  Personally, if the column is outside I
don't even use the grout for bearing.  I have seen too many exterior columns
without any grout after a few years.

Jake Watson, P.E.
Salt Lake City, UT

-----Original Message-----
From: Nels Roselund, SE [mailto:njineer(--nospam--at)att.net]
Sent: Thursday, August 26, 2004 11:09 PM
To: seaint(--nospam--at)seaint.org
Subject: Anchor Bolts in Flexure


Anchor bolts secure a steel base plate to the top of concrete.  The base
plate is installed some distance above the top of concrete and grout is
placed to fill the space between the top of concrete and bottom of steel
base plate.  Lateral load in the baseplate is transferred to the concrete
through the grout by the anchor bolts.
1.  Is it reasonable to base the design of the anchor bolts on shear only,
as if the grout provides fixity?
2.  Or should the anchor bolts be designed to resist flexure through the
grout?
3.  Is there a minimum grout edge distance needed to be able to assume
fixity?
4.  Does the answer have to do with preparation of the top-of-concrete
surface to assure effective bond between the grout and the concrete?
5.  If the anchor bolts should be designed for flexure, how is their flexure
capacity figured?  Section modulus based on the tensile-stress area listed
in the AISC Manual [as on page 4-147 of the 9th edition]?  Is maximum stress
based on Fy of the anchor bolt material?

In the past, I've assumed fixity at the top of grout, but I don't know how
to justify that assumption.

Nels Roselund
Structural Engineer
South San Gabriel, CA
njineer(--nospam--at)att.net



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