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Appendix D / Ch. 17 Question

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Archie264

Structural
Aug 29, 2012
993
On some anchor bolts relatively close to the edge of concrete there's no way to make them work without supplemental reinforcement. And as proposed the embeddment is a little over 12d. Given this is there any advantage to be had in making them longer? Not per Appendix D, i.e. now Chapter 17; I understand that. I just mean is there any advantage to a longer anchor bolt despite Ch. 17 limiting it's advantage?
 
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As you probably know, increasing the embedment won't help the edge controlled cases after a certain point. You might need to resort to some other rational method. Appendix D can be pretty unforgiving since it is based on unreinforced concrete with clumsy modifiers for reinforcement.

Maybe you could neglect some of the edge controlled failure modes as overly conservative if you didn't think they were relevant to your loading cases and if the edge distance improves at some depth (as with a curb or inverted T footing).
 
In shear or tension? In shear the embedment isn't that helpful anyway (capped at 8d in the code iirc), and in concrete once the breakout cone captures the entire section there's nothing more additional embedment can do other than develop bar. Chapter 17 doesn't have a cap on breakout capacity for edge effects iirc, so until the whole section is mobilized into a breakout cone you should get some more capacity going deeper, unless side face blowout is limiting the force on the anchors.
 
Thanks. I got a slight capacity increase going deeper but not enough to make much difference in the design. It just seems so strange to increase the depth for no gain but I understand why; I was just wondering if there might be any intangibles I was overlooking. Other anchors in the group are further from the edge but in the end the ANC / ANCO part of it becomes a tail-chasing exercise. Supplemental reinforcement it is...

Thanks.
 
I don't know how practical it is in your case but you can specify welded base plates instead of bolted base plates. With Appendix D this will get you full credit for the bolts working as a group and sharing the shear load.
 
Take a look at AISC Design Guide 1 if you are still working on this. It includes some more realistic options to Appendix D when dealing with edge distance and pedestal issues.
 
I've heard some people talk about through bolting the slab and then using something like the punching shear equations which tend to give you a lot more capacity than the pull out cones.
 
Thanks, gentlemen. I went through Design Guide 1 but it still didn't bail me out.

Josh, going through the slab sounds intriguing and I wonder if it will change the way some buildings are constructed. For example, piers might become particularly problematic. Since the anchorage seems to depend so much on the edge distance of the breakout cone, well, I can envision one brute force way to address that in some circumstances: instead of spread footings with piers just extend the spread footing up to slab level like a makeshift pile cap without the piles. The contractor might be laughing too hard to ever get it built, though.

In the mean time I've got a ghastly congested nest of rebar...it remains to be seen whether that can be built...[sad]
 
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