ACI Appendix D Larger Pier Smaller Capacity? 2

[rharting]

I recently got a call from a contractor who mistakenly installed 3/4" anchor rods where there should have been 1 1/2". Due to reduced loads during VE, the 3/4" anchor rods actually work on a 20"x22" pier however they will not work on a 20"x28" pier with the same bolt spacing, embedment depth, size etc. However if I decrease my designed embedment depth from 14" to 4", it also works.

I believe this has to do with the Anc/Anco portion of D-5 which stems from the limitation of the Hef to 1.5Camin. I called Hilti to discuss the Profis program (after checking the calculations to make sure of it's accuracy) and they indicated that they get this phone call everyday however there is little they can do about it since it is a code issue (they can't make up their own codes).

This, from just a common sense standpoint, makes no sense. If you increase your embedment or your pier size the capacity decreases? Has anyone noticed this? I think in this instance I'm going to use engineering judgement and say that it's acceptable not to tear out the entire pier and start over.

Thanks in advance.

 

[CELinOttawa]

It is not always the case that this "makes no sense". While I disagree with a great deal of the Appendix D provisions, a deeper anchor bolt can in fact cause a lower strength failure. I have seen it myself when I was the lead research engineer for a seismic anchoring product company in New Zealand.

Interestingly when the standard steel strength went from 300MPa to 350MPa here in Canada, several standard sections went down in strength. This was because, at least theoretically, they were now failing earlier by another failure mode. You're facing something similar, with even less laser-accuracy so it makes it even more frustrating. Like you heard from our fine Lichensteinian friends, you will have to work about it.

I have a few suggestions for you:

- Load test the anchors. This is a perfectly acceptable solution and frequently the bolts can take it. If they cannot, you have them out of the way for a repair.
- Drill the anchors out and install the ones you called for. This may be a huge pain in the 'a', and I never like dealing with smooth shafts, but it may suit your situation.
- Approach the authority, hat in hand, and explain the situation. They are not always as difficult as the "dirty laundry" incidents we all whinge about makes them appear.
- Use the Hilti International provisions and advise all parties that it works, it is just a code provision that most engineers don't buy anyway. (Good luck with this one).

Good luck, and PLEASE come back and keep us up to date. These are the really interesting cases that just seem to disappear after a lot of free advice. Don't be that guy/gal!

 

[CELinOttawa]

Were these cast in place anchors? Appendix D does not apply to grouted or adhesive anchors...

 

[CELinOttawa]

Oops... Was just reading the latest App D, and it seems they've got you on adhesive anchors now too. Sorry.

 

[BUGGAR]

Appendix D reminds me of Microsoft

 

[msquared48]

If Appendix D was top notch, it would have been called Appendix A. I rest my case.

Mike McCann, PE, SE (WA)

 

[LD81]

Check if the controlling mode of failure is applicable to your case. If your anchor is deep enough to transfer the tension to some of the longitudinal bars maybe you can justify that your controlling mode of failure is not applicable. Ref: D5.2.9

 

[JAE]

Perhaps wear this T-shirt (See attached). Might help.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[rharting]

JAE,

I love your posts.

Is that available on the ACI website?

thanks for the laugh.