Anchor rod embedment through pier into footing

[StrucDesignPE]

Subject line says it all.

I am working on designing some fixed base column base plates and piers. In order to keep my pier size down from what App. D says I need for concrete breakout, I am electing to develop anchor reinforcement for tension and shear via the requirements of sections D.5.2.9 and D.6.2.9, respectively.

I am planning on detailing stirrups for the shear, but was thinking that I may just run the anchor rods all the way through the pier and down into the footing for tension development instead of detailing tension reinforcing.

Your thoughts?

 

[SlideRuleEra]

...run the anchor rods all the way through the pier and down into the footing...
Your thoughts?

The anchors are kept in only the pedestal for constructability. Accurately positioning the anchors in a pedestal being poured on top of a completed footing is much easier than accurately positioning them when the footing is poured.

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[SgtYui]

Typically the footing and pedestal are poured separately with the pedestal reinforcement developed into the footing. The anchor rod embedment should only be considered from below the transition not from the top of the pedestal. What height of pedestal are you considering that would make detailing out anchor tension reinforcement difficult?

 

[Archie264]

Not to presume to speak for the OP but I think the issue is the edge distance requirements of Appendix D / Chapter 17. It's caused a lot of long-accepted practices to be unjustifiable on paper. I think his intent is to get the anchors down into the footing where they're not near any edge. It seems like Appendix D / Chapter 17 has a 25" cut off but I'm not sure of its exact wording at the moment.

My favorite way to address this issue is one I read on this board: switch to masonry and use the masonry code's guidance.

But, perhaps the best way out of hot water is using supplemental reinforcement, as the OP referenced but would (understandably) rather avoid. Alexander Newman's book "Foundation and Anchor Design Guide for Metal Building Systems" has an excellent discussion of the topic and is well worth the money.

 

[SgtYui]

Widianto, Patel, and Owen's paper titled "Design of Anchor Reinforcement in Concrete Pedestals" is another good reading on the subject. The PDF is available through a quick google search.

 

[StrucDesignPE]

Not to presume to speak for the OP but I think the issue is the edge distance requirements of Appendix D / Chapter 17. It's caused a lot of long-accepted practices to be unjustifiable on paper. I think his intent is to get the anchors down into the footing where they're not near any edge. It seems like Appendix D / Chapter 17 has a 25" cut off but I'm not sure of its exact wording at the moment.

My favorite way to address this issue is one I read on this board: switch to masonry and use the masonry code's guidance. 2thumbsup

But, perhaps the best way out of hot water is using supplemental reinforcement, as the OP referenced but would (understandably) rather avoid. Alexander Newman's book "Foundation and Anchor Design Guide for Metal Building Systems" has an excellent discussion of the topic and is well worth the money.

Thanks Archie. This is what my goal is. I don't want to detail the tension reinforcement unless I absolutely have too. I understand the constructability issues and don't think it will be a problem.

My office has a copy of the Newman book and I have used the Widianto, Patel and Owen paper to design the shear reinforcement with the strut and tie model.

I'm just trying to understand if embedding the anchor rod into the footing is a generally acceptable alternative to a rather large appendix D pedestal or supplemental tension reinforcing.

 

[SlideRuleEra]

I don't want to detail the tension reinforcement unless I absolutely have too. I understand the constructability issues and don't think it will be a problem.

I am not criticizing your decision, but consider how many times the pedestal design detail is used and/or the required accuracy of anchor placement. The answer may be different on various projects.
If it is "just a few, with liberal tolerances" perhaps saving engineering costs is best.
If it is "many times and/or with high accuracy" maybe some time & effort spent on design will save the Client, the Contractor, and the Inspectors money and "grief" during construction.

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[canwesteng]

I don't think dropping them into the footing adds any substantial breakout capacity, although it would help with side face blowout. You're going to need to have enough vertical bar in the pedestal to resist uplift anyway - I don't think you need to make any special detail here to develop it to prevent concrete breakout, and the anchor embedment can likely stay in the pedestal.

 

[Archie264]

>>>I don't think dropping them into the footing adds any substantial breakout capacity<<<

Ah yes, I forgot about that aspect of it. The deeper you go the deeper the theoretical breakout cone for one anchor so the more likely the breakout cones of the anchors within the group are to interfere with each others' breakout cones.

That's what I refer to as chasing one's tail, running on a treadmill, pushing on a rope...anything like that where the output is not linearly related to the input. In this case it's worse than that; at some point increasing the input reduces the output. It may be correct but it's a bit counter-intuitive, to say the least.

 

[KootK]

I've seen this done. In those situations, however, the pier reinforcing was retained and the anchor rods were extended into the footing simply because, at that time, folks were having a heck of a time figuring out what should be done and dropping the anchors to China seemed pretty foolproof, if a bit ridiculous. You can control the position of the anchors at the footing level with a template but, baring highly unusual measures, you will not be able to exert much control over the plumbness of the anchors. So they will be misplaced more than usual at the pier level and you'll need a plan for dealing with that. That said, you've made it clear that you've considered constructability so I'll not harp on that.

Technically, this bears similarity to a question posed here recently: Link. And I think that there's a salient point to be drawn from that. I agree with you that embeddment into the footing would increase capacity with respect to edge distances etc. However, I would then argue that your embeddment depth is really the depth of embeddment into the footing, not the embeddment into the pier plus the embeddment into the footing. And maybe that works, it would depend on how deep that footing is I suppose. Essentially, your pier becomes just a pass through element with the real moment resistance occurring at the footing.

It's also worth noting that the App D method isn't the only way to anchor a bar. STM etc are also valid, perhaps more so.





I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[XR250]

Seems like you could but a templated bearing plate on the embedded anchors and then cast the pier around the assembly. The entire assembly could still be off a little, but maybe better than having two or three individual anchors misplaced. Most of the jobs I do are small enough that they would be happy to field weld a column to that plate if it ain't galvanized. May not make sense for big boy jobs.

 

[StrucDesignPE]

The deeper you go the deeper the theoretical breakout cone for one anchor so the more likely the breakout cones of the anchors within the group are to interfere with each others' breakout cones.

This is what I wasn't sure about. I realize through different calcs/projects I have done that increasing embedment can sometimes make the edge distances increase and anchor capacity be reduced. My big concern is concrete breakout.

I agree with you that embeddment into the footing would increase capacity with respect to edge distances etc. However, I would then argue that your embeddment depth is really the depth of embeddment into the footing, not the embeddment into the pier plus the embeddment into the footing. And maybe that works, it would depend on how deep that footing is I suppose. Essentially, your pier becomes just a pass through element with the real moment resistance occurring at the footing.

I was thinking the same thing. My piers are likely around 1'-4" tall. This is short and to me seems like keeping the anchor rods plumb would not be so much of an issue. I feel like it is best to run the anchor rod into the footing and design it to be deep enough to get embedment in the footing alone. Since the footing is rather large, edge distance is not an issue. My embedment depth would only be the depth in the footing so my assumed breakout cone would be smaller than if I consider embedment all the way through the pier too. Just wanted to see if this makes sense to anyone else but me. I have also considered just setting the baseplate on top of the footing without a pier.

 

[Once20036]

I`ve taken the approach described in this thread to develop the tensile capacity of anchor rods at braced frames.
I think that it's worth a quick discussion/reminder regarding what happens with the shear forces in these locations.
If you tensile resistance comes from the top of the footing, now you have a shear force applied 1'-4" above this point. This will create additional moment and additional tensile forces that need to be resisted by the anchor bolts.

 

[cliff234]

"I understand the constructability issues and don't think it will be a problem."

Respectfully, you may not think it will be a problem, but you won't be the one building it. I would encourage you to provide headed anchor rods and lap splice them with vertical reinforcing in the pedestal. And provide plenty of confining ties. I assume that you have vertical reinforcing in your pedestal, so you just have to make sure it is sized and configured to lap with the anchor rods. Make sure your pedestal is tall enough to accomplish the lap.