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Re: Anchor Bolts in Flexure

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Nels,
 
A good discussion on the issue is presented in AISC "Column Base Plates" (AISC D801). 
 
Appendix B "Some Practical Aspects of Column Base Selection" by David Ricker says:  "Anchor bolts should not be used to resist shear forces at a column base."  A detailed discussion, including the issue of grouting and the recommended solutions, follows. 
 
At the same time, on page 30 ("Design for Shear"), the main text of the publication is less restrictive, and allows (although not recommends) some limited use of anchor bolts in shear.  Several research and testing sources are noted.  Bolt hole oversizing (only portion, say, 50%, of the bolts being loaded), limited (say, to 2 kips) shear/bending capacity, and other factors are discussed.
 
AISC generally recommends the shear issue to be resolved through shear lugs, with consideration to friction, shear friction, and column embedment in the footing. 
 
In my practice, I am fortunate to have the shear on the anchors practically always in combination with a huge clamping force (say, generated by the seismic moment only).  In this case, shear is resisted by the non-gravity friction, and I do not have to worry about the bending on the anchors (holes are oversized).  When grouting is required, I design the shear to be resisted by 50% of the anchors (2 per baseplate) in shear and bending. 
 
I calculate the section modulus based upon the net (inside the threads) section of the bolt.  Usually, such bolts are quite large (dia.>1"), and provide substantial resistance.   
 
V. Steve Gordin, PhD
Registered Structural Engineer
Irvine CA
 
----- Original Message -----
Sent: Friday, August 27, 2004 5:44 AM
Subject: RE: Anchor Bolts in Flexure

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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