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RE: Anchor Bolts in Flexure

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I don't like putting any significant amount of shear through the anchor bolts. I also do not like using shear lugs at the base of columns. Those are the lessons of an iron worker converted into a structural engineer.

1. Is it reasonable to base the design of the anchor bolts on shear only, as if the grout provides fixity?
No.  There is a significant bending component.

2. Or should the anchor bolts be designed to resist flexure through the grout? Yes. Unless you specify a flowable grout that is pumped under the base plate and you have grout holes in the base plate for consolidation. Dry pack (the most common) is very poorly consolidated under the base plate especially around the anchor bolts.

3. Is there a minimum grout edge distance needed to be able to assume fixity? If you use grout to provide some fixity, the grout edge should be some function of the thickness of the grout. As a rule of thumb you should use the diameter of the anchor bolt plus the thickness of the grout.

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? Maybe. A wetted, raked surface is best for bond, but then that makes setting the shim stack an issue. Most grouts give good bond to almost any concrete surface. Get a Five Star Book.

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? Yes. Yes. You should check the COMBINED shear, flexure, and tension (if applicable) ratios.

I have used anchor bolts to resist shear loads, but rarely. Base plates will have oversized holes. The bolts will not go into bearing simultaneously. That is why you need plate washers which will have a hole that is the standard 1/16" over the the bolt diameter. The plate washer is then welded to the base plate in order to transfer shear. Now you have the moment arm as the base plate thickness plus the grout bed plus the point of counterflexure in the concrete embedment. Combine that with shear and tension, and you are really pushing the anchor bolt.

You are much better off just using the friction developed between the bottom of the base plate and the grout. RE: AISC Design Guide 1 "Column Base Plates", page 47, 4th paragraph, "Anchor bolts should not be used to resist shear forces in a column base." This is not a code, but it is a good idea.

If the shears are too large for friction to handle, there are several options. You can encase the base in concrete. This is easy for architectural interior applications where the base plate is often recessed below the floor. If the shears get very large, you can then use a setting template with shear lugs welded to the setting template. Shear transfer bars are then welded from the base plate to the setting template. I do not use shear lugs. The grout is almost never properly consolidated around the lugs, and they interfere with the practice of using shim packs.

I had the opportunity to see many shear and tension anchor bolt failures on one project alone. A wind storm pushed over a large ware house under construction. The precast walls were up, the steel was erected, but the deck (diaphragm) was not in place, and the precast erectors removed their shores, because they were done with their work. Anyway the hole thing collapsed after a wind storm. The anchor bolts were not designed for this, and they formed a classic S pattern and fractured just below the surface of the concrete at the base of the threaded portion of the bolt.

Regards,
Harold Sprague





From: "Nels Roselund, SE" <njineer(--nospam--at)att.net>
Reply-To: <seaint(--nospam--at)seaint.org>
To: <seaint(--nospam--at)seaint.org>
Subject: Anchor Bolts in Flexure
Date: Thu, 26 Aug 2004 22:08:31 -0700

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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