ACI-05 APP D Anchor Bolt Supplemental Reinforcement 5

[crunkcrete]

ACI-05 App D uses the CCD method for anchor bolt embedment design. Anchors will small edge distances are penalized in strength making it nearly impossible to develop adequate uplift strength. ACI allows for the use of supplemental reinforcement to provide for additional strength but gives little detail as to how to design and detail this supplemental reinforcement. Does anyone have a good reference as to how to design supplemental reinforcement for tension and shear in column pedestals? My thoughts are to embed anchor bolts to a depth that fully develops the vertical dowels that way when uplift forces crack the concrete the tension forces are transfered into the dowel steel, however depending on dowel size this can lead to some very long anchor bolts. My concept for shear would be to place stirrups across the shear crack and provide for enough area of steel to resist the full shear force. Am I on the right track? Does anyone have a good example reference that details? Thanks in advance!

 

[prsconsultant]

There is an article in Struture Magazine which addresses the exact issue you have described above. Below is the link for that article.

http://www.structuremag.org/archives/2004/july/From Experience.pdf

 

[aggman]

The article in Structure Magazine is quite good. Basically what you have to do is develop the anchor rods. What I typically do for tension is subtract the concrete capacity from the required load and make up the rest using the rebar. You only have to develop the anchor rods the net difference. It is usually quite easy to do and does not require much additional embedment. Deal with shear the same way.

 

[vincentpa]

aggman,
you are not supposed to subtract the concrete capacity to get the Net capacity. If designing the steel to take the load, it must take all the load. Either the concrete takes all the load or the steel. The anchor bolts must be developed and lapped with the vertical pier steel. See AISC Design Guides 1 and 7 for how to calculate the lap lengths properly. for high uplift loads, I don't even bother calculating the concrete capacity anymore. It is minimul in comparison to the steel in a pier. I just lap the anchor bolts to the vertical reinforcement. Remember, that the maximum distance the bars and anchor bolts can be for lap is 6". You might need a special details. I use hairpins. It stinks but that's the code now.

 

[Lion06]

vincentpa-
What exactly do you mean by "the maximum distance the bars and anchor bolts can be for lap is 6""? Does that mean you can't lap them more than 6"? Maximum spacing of 6"? Neither of these make sense, so I am apparently missing something.

 

[Taro]

I believe vincentpa is referring to ACI 318 12.14.3 which says that bars in noncontact splices must be within 6" of each other.

 

[vincentpa]

That's exactly what I mean about 6" apart for a noncontact splice. Thanks for clarifying that Taro. I should have been more specific. The anchor rod embedment will be dictacted by several factors. Embedment = the distance the vertical bars and anchor rods are apart x sin 35 + the cover over the vertical bars + splice length of vertical bars/hairpins. Keep in mind that the embedment is not the total length of the anchor rod. Leave enough room on the rod for a nut and tack weld. The embedment is the clear distance between the top of pier and top of nut.

StructuralEIT,
have your office buy the latest AISC Design Guides 1 & 7 if someone doesn't have them already. If you or someone in your office is a member of AISC, the downloads are free.