Anchor bolts - ACI Appendix D (318-05)

[campbelltc]

When it comes to designing large (1 1/2" - 1 3/4") headed anchor bolts subject to both tension and shear on top of a reinforced drilled concrete pier (caisson), I have some questions:

1. Does ACI 318-05 Appendix D cover anchor bolts that are within a reinforced drilled concrete pier if the failure cone intersects the veritical pier reinforcing?

2. Does Appendix D limit the (actual) embedment length of an anchor bolt? I was told it does but I cannot find it.

3. D.5.3.4 seems to allow a bearing strength of 6 times the strength of the concrete for the bolt head for a factored pullout force (8 x f'c x 0.75 phi factor). This seems like a very high value to me and I feel very uncomfortable using such a high value. In 4 ksi concrete, I would have an unfactored pressure of about 9 ksi from the bolt head to the concrete. That seems too high to me. Why does the code allow that high a value? I would feel much more comfortable with a bearing pressure of .85 x 4 = 3.4 ksi by using a washer plate at the anchor head.

4. If I assume a failure cone of 45 degrees (conservative) and it intersects the vertical reinforcing at a certain elevation, I need to have full development of the vertical reinforcing above that location, correct? I cannot use the principal used in section 12.2.5 to lessen the development length of the bars above that height, correct?

Thanks.

 

[campbelltc]

I also would like to know if headed anchor bolts would be considered torqued bolts or untorqued bolts for purposes of interpreting D.8.1.

 

[JStephen]

On Question 1, yes, it should apply, but if you have large bolts in a small concrete area, you may always need reinforcing in the failure cone.

On Question 2, I have made the assumption that if a bolt is adequate when embedded at 25", it's also adequate when embedded deeper. That is, I'll just do the calculations based on 25" even when it's actually deeper. (And you can anchor a pretty sizable load at 25".) I would have to check the wording, but I don't think they prohibit greater depths, they just say you need to develop breakout capacity with rebar instead of the concrete breakout equations in that case. That may be in the commentary rather than the code itself.

On Question 3, this is supposed to all be based on testing, so I have assumed it is realistic. But there's no law that says you can't use a lower value. Using the maximum bearing will give a pretty small bearing area, so there's minimal extra cost to increase it.

 

[charliealphabravo]

Appendix D is based on the simple geometry of shear/breakout cones in plain infinite concrete. The influence of nearby edges is pretty straightforward but the geometry of intersecting shear cones becomes very complex when additional reinforcement is introduced. Appendix D does not rigorously reflect the interaction between primary and secondary reinforcement and the anchor rod. The best you get are a few coefficient points for supplemental reinforcement. You will need another analytic tool if Appendix D becomes overly conservative due to load and space restrictions.

Here is my take on some of your questions and some good points by other's as well from an old post.

http://www.eng-tips.com/viewthread.cfm?qid=310997