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Anchor Bolts
2

Anchor Bolts

Anchor Bolts

(OP)
Does anyone recall what code the minimum length for anchor bolts/rods is in? I cant find it, and I know I have seen it somewhere

RE: Anchor Bolts

ACI-318 App. D deals with anchor bolts.  I don't recall a specific minimum length, though.

RE: Anchor Bolts

(OP)
I checked App. D, I couldnt find anything. I remember something about 12*the Dia, but maybe that was something else.

RE: Anchor Bolts

As JStephen has mentioned, in ACI-318 app. D, I do not see any minimum embedment requirements.  On the other hand the app. mentions a maximum diameter of the anchors and max embedment depth for which the equations in this chapter  can be applied.

RE: Anchor Bolts

I don't believe there is any Code prescribed minimum.  I have an article that says embedment of 12*diameter fully develops an anchor bolt in tension.

DaveAtkins

RE: Anchor Bolts

These are published in the first edition of the AISC Base Plate Design Guide (12d for A307 and A36; 17d for A325 and A449).  These are also absent from the second edition of this same design guide, so there is probably something to that.

RE: Anchor Bolts

I believe AISC's 2nd LRFD edition (the 2-volume silver books) had a table with edge distances and embedment lengths (I don't have that edition with me right now).

The 3rd LRFD Edition refers to IBC and ASCE 7 (page 14-9)

Sections 1911 and 1912 of the 2006 IBC addresses anchorage to concrete.

RE: Anchor Bolts

DaveAtkins:  12*diameter to fully develop an anchor rod? Surely not.  It takes 44 bar diameters to develop a #4 bar, and that's a deformed bar, not a smooth rod.  Can you shed some light on this article you have?

RE: Anchor Bolts

nutte-
DaveAtkins may be talking about a headed anchor bolt, not necessarily a straight bar with no anchorage.

RE: Anchor Bolts

Even so, 12 rod diameters seems awfully light to fully develop the rod in tension.  I'd still like some clarification from DaveAtkins on the article he mentioned.

RE: Anchor Bolts

The second edition recommends an embedment length of 17dia, but gives an example of the concrete cone resistance for an embedment distance of 6" for a single rod, uncracked conditionof 19.5K for hef <= 11" and f'c=4ksi.  It's based on the calculated capacity of the concrete cone.  I think the 17dia reflects ACI guidelines, but am not certain.

Another publication by the AISC related to the design of headed anchor rods has formulae for the calculation of the embedded length that is approximately 12dia for A36 and 17dia for higher strength materials.  In any event, the cone development is what determines the capacity.

If a threaded rod is used, then the end should be peened to prevent the nut from loosening during installation. In addition, the concrete should have sufficient strength to restrain the nut end.

Dik

RE: Anchor Bolts

I did mean headed anchor rods, and the article is "Design of Headed Anchor Bolts," by Shipp and Haninger, from the Second Quarter 1983 AISC Engineering Journal.

And StructuralEIT is correct--12 times the diameter is for A36 and A307 rods.

DaveAtkins

RE: Anchor Bolts

Thanks for the article information.  You're right, it says 12d to fully develop an anchor bolt in tension.

I contend, however, that this does not comply with today's code, namely the hardest to follow mess ever pressed onto engineers, the ACI 318 Appendix D.  Run through that procedure with 12d embedment, and you'll surely have less capacity than the rod's tensile capacity.

RE: Anchor Bolts

I just follow whatever the manuf. said with the design you have.  They give you the axial, shear with various embedment, min spacing and min edge distance (or reduce it with shorter spacing and edge distance).  If the # met your design load then you should be good.

Never, but never question engineer's judgement

RE: Anchor Bolts

This convoluted Appendix D surely must be work in progress, clearly rushed into production and printing so as to charge more for the ACI Code.

The unintended consequences of this unworkable mess has yet to be fully realized.

RE: Anchor Bolts

COEngineer-
You can only do that with epoxy anchors.  I don't know of a standard anchor bolt manufacturer that publishes values with tables for reductions based on spacings and edge distances.  This is done with ACI Appendix D.  If you only have a few anchor bolts maybe it wouldn't be a problem, but when you have a job with 100 columns and 400 anchor bolts, I imagine it can become cost prohibitive.
I have only ever used it for field fixes when the contractor mis-placed the anchor bolts.

RE: Anchor Bolts

The ASCE "Guide to Wind Loads and Anchor Bolt Design in Petrochemical Facilities" calls for a MINIMUM embedment of 12Db, but the method behind the design is to transfer the tension from the bolt into the reinforcing.  I agree with everyone else...12Db is NOT enough to develop the bolt tension in plain concrete according to today's code.  However, using reinforcing to help develop the bolt tension will greatly reduce your embedment depth.

RE: Anchor Bolts

The example clearly shows that 12dia is sufficient to develop the capacity of the bolt.

Dik

RE: Anchor Bolts

Most popular on the market inserts (pre installed or post) are Dayton superior, Hilti, Meadow Burke, etc. For most of their inserts or lifters they provide the capacities and reduction factors for edge distance, spacing, embedments etc. Even if few of them are missing, you can always call them and get the information you need.

RE: Anchor Bolts

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.

DaveAtkins

RE: Anchor Bolts

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...

Dik

RE: Anchor Bolts

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?  

RE: Anchor Bolts

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.

RE: Anchor Bolts

The ACI 'cone formulae' have two ranges... one less than 11", the other greater...

Dik

RE: Anchor Bolts

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.  

 

RE: Anchor Bolts

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?

Dik

RE: Anchor Bolts

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.

http://www.aisc.org/Template.cfm?Section=ePubs_Engineering_Journal1&template=MembersOnly.cfm&ContentID=24108

http://www.aisc.org/Template.cfm?Section=ePubs_Engineering_Journal1&template=MembersOnly.cfm&ContentID=31880

RE: Anchor Bolts

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.

RE: Anchor Bolts

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.  

RE: Anchor Bolts

er F1554s

RE: Anchor Bolts

FM also requires (4) 1" diameter anchor bolts.

RE: Anchor Bolts

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.

RE: Anchor Bolts

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.

Daniel Toon

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