12d Minimum Anchor Depth Requirement?

[Spanky7]

I recently had an engineer that I work with ask me if there were any code requires for minimum anchor depths to be maintained. He was specifically asked about 12d (i.e. 12 times the anchor diameter). As far I know the 12d is an old “Rule of Thumb” and is not a current code requirement. Does anyone have any insight into where the minimum anchor depth requirement of 12d originally came from?

 

[WillisV]

You are correct that is not a current requirement, but for historical perspective that was the limit set forth by Shipp and Haninger (EJ 1983) to develop an A307 anchor rod based on ACI 349 cone failure provisions in the early 1980s. Their paper is attached. This method was then adopted by AISC as indicated in the 2nd Ed. LRFD manual, Volumne 2, p. 8-88 thru 8-90. Table 8-26 indicates the 12d rule for headed A307s. All of this is now superseded by ACI Appendix D, though I tend to still use these values as minimums for gravity columns.

 

[WillisV]

Attachment.

 

[ishvaaag]

With the design methods used lately here for mechanical and adhesive anchors you use to have 9 to 11 diameters inside the concrete. At this point the propietary systems (normally with hight stregth steels) have got what they think right to ask of the concrete. Hence an older, generic value of 12 diameters seems perfectly reasonable for ordinary concretes somewhat weaker.

 

[Spanky7]

I emailed Dr. Burdette of UT Knoxville the same basic questions I posted here earlier today. Dr. Burdette was my advisor when I got my Masters Degree. The following is my email to Dr. Burdette and his response.


From: "Burdette, Edwin G"
To: TRAVIS C SMITH
Date: 7/27/2010 10:41 AM
Subject: RE: Is there a Minimum Anchor Depth Requirement by Code?

Travis,

I agree with you that ACI 318 App. D can be used to calculate required depth, and I am unaware of a minimum depth requirement. But it seems to me that there would logically be a minimum depth. As to 12d, I think that came from the old way we used to calculate pull out capacity using a 45 degree cone. And a good rule to assure that the pull out strength would be at least as great as the ultimate strength of the anchor that is, the bolt would break before it pulled out was to embed it 12d. That still is not a bad rule of thumb to assure ductile behavior.


EGB


Edwin G. Burdette
Professor of Civil Engineering
115 Perkins Hall
The University of Tennessee
Knoxville, TN 37996 2010



________________________________

From: TRAVIS C SMITH
Sent: Tuesday, July 27, 2010 10:02 AM
To: Burdette, Edwin G
Subject: Is there a Minimum Anchor Depth Requirement by Code?

Dr. Burdette,

I was wondering if you could help me answer a question? I was recently had an engineer that I work with ask me if there were any code requires for minimum anchor depths to be maintained. He specifically asked about 12d (i.e. 12 times the anchor diameter). As far I know the 12d is an old "Rule of Thumb" and is not a current code requirement. The only code requirement that I know of is that the strength requirements of ACI 318 Appendix D should be met. I would assume that one could use Appendix D to calculate a minimum embedment depth for a give anchor.

Do you have any insight into where the minimum anchor depth requirement
of 12d originally came from?

Thanks for your help.


Regards,
Travis C. Smith, P.E.
Senior Structural Engineer
Mesa Associates, Inc.
10604 Murdock Drive
Knoxville, TN 37932