My understanding from the article I mentioned is that when a headed anchor rod is out in the middle of a spread footing, 12*diameter WILL fully develop an A36 or A307 rod. When you are in a pier, you may need a longer length of rod to develop the bars in the pier (a lap splice), but the rod itself is still developed at 12*diameter.
Dave, further to your comments, the two articles have a method for determining the development length based on the force and proximity to the pier reinforcing as well as the hook length for the pier reinforcing...
Dave and dik, what research is given to back up those equations and guidelines from the Shipp and Haninger article? They should have a list of references. Would you mind posting a few of those? I can't find a copy of that article; it is about 25 years old afterall. I would like to see for myself what is behind these numbers.
There would be no reason for both the ACI and the PCI to waste a bunch of money and come up with some new method if using 12d works. How does the 12d account for cracking and ductility, or the presence of reinforcement, or the variability in the strength of the materials, and other things?
I am working on a anchor bolt design at this moment myself. I am using a book titled "Anchorage to Concrete" ACI SP-103, edited by Hasselwander, copyright 1987.
It states "a 15D embedment length can be considered a satisfactory minimum embedment lenght." Then it goes on to give an equation for load capacity of an anchor bolt under simple tension. The equation is only valid for embedment lengths over 15D.
All of these references for 12*db or 15*db are outdated and no longer valid. Information of this nature was intentionaly removed from the last few editions of the Steel Manual. Anchor rod embedment should be designed per ACI Appendix D which, though I agree is very confusing and could be presented better, is still based on a large volume of legitimate testing and is the current state-of-the-art.
ACI approves and backs this method as does AISC. The 2nd edition of AISC's Design Guide on baseplates, which was just recently produced and is therefore the most current AISC stance on the subject, differs to ACI Appendix D for embedment calculations.
The old 12*db stuff was based on a breakout "cone" which has been replaced by the CCD method of a breakout prism in Appendix D.
I agree that 12*db might be a good minimum for anchor rod embedments on gravity columns, but it certainly is not adequate to meet today's codes to develope the tensile capacity of the rod.
Sorry Willis... by conc cone, I meant the CCD method... just have difficulty wrapping my ears around a square 'cone' and the calculations for a single 3/4", 6" embedment was for example 4.4... 0.7*1.25*24*sqrt(4000)*6^1.5 = 19520#
For 3/4" headed bolts, I commonly use 12" min embedment; what is your suggestion?
UcfSE, the article (and discussion) can be found at the link below for AISC members. The equations it references are from ACI 349, Supplement 1979 (!). I don't have access to ACI 349, so I'm unable to look into these equations further.
Regardless, as noted by myself and others, this approach doesn't work today with the ACI Appendix D procedure.
Oh I'm sure it doesn't work for today. I was interested in comparing the two methods, old and current, to see the differences. When I have time, I try to look into the history of code provisions as well as what to do. It's interesting.
Dik - I use the same general rule as you, 12" minimum embedment on gravity columns. I also tend to use 1" diameter GR36 F1154s as a minimum (instead of 3/4") solely for the reason that contractors seem to like to play target practice with their backhoes and it is more difficult to bend over a 1" bolt.
Just look for the design manual of any type of anchor you wanted to use. From there you will find that for every size/diameter of the anchor bolt theres a specific length/depth required for it.
WillisV hit it on the head. The current code addresses the requirements of modern research on cracked sections and seismic performance criteria.
We also seem to have confusion between cast anchor rods and post installed anchors, which are both addressed though the post installed anchors are not as thoroughly reviewed and sufficent research does not exist for many types of anchors and many current anchor cannot function in cracked concrete.
Appendix D is significanyly changed from the past. It is not perfect, but it exists to fill a void and ensure the safety of the public.
As with anything, engineering judgement must be used with its application, but to call it out as a money grab and ignore it is irresponsible.
There are seminars available from both ASCE and ACI as well as others to review the changes and methodology of App D.
Additionally, Hilti and Simpson have both made efforts to provide reviews and seminars on the topic. The seminar I have attended by Simpson was very useful and remarkably unbiased. I would seacrh for their website and see if there are seminars near you.
