Retrofit Concrete Anchors

[Mathias65]

I've been struggling w/ the design of retrofit concrete anchors using the Simpson and Hilti software packages, and am wondering what the rationale is for why you can't increase the anchor embedment length of the anchor to the point at which you can satisfy the ductility requirements of Appendix D?? If your embedment depth is limited, it forces you to increase the anchor diameter in order to achieve the required adhesive bond strength, which then drives up your required anchorage force due to ductile failure of the anchor itself. Seems like a catch-22, and one that I don't agree with. I could see that if you were specifying a 1/2" dia. anchor that was 8 ft. long you wouldn't necessarily be able to activate the rod at it's very end, but the embedment values that you can specify in the anchor design programs seems to be VERY conservative. Does anyone know what that stems from? Thanks!

 

[CELinOttawa]

I rarely, if ever, use the software. The design method in the Hilti International manual is fantastic, based on the most advanced research available, and results in great details.

I don't know the US ACI Appendix D, but from everything I've read it is nothing but trouble for Yank engineers... I would hope that your local building authority will permit an alternate method. Ask them.

 

[CELinOttawa]

Oh, and for all of my annoyance with US codes, the commentaries (as well as committee discussions) from ACI are beyond reproach. Have a look into which ACI committee was responsible for developping Ap.D and read their last few reports.

 

[Mathias65]

No, I haven't gotten too deep into the background philosophies of the limited anchor embedment length. I was actually just being lazy in the hopes that someone else had some information I could access easily. But thanks for the response and the suggestions. Hopefully someone else out there will be able to provide me with a path of least resistance.

 

[rharting]

Mathias,
I've used the Hilti program quite a bit and have certainly found some instances of them being a bit overly conservative. But, I think for a free program that deals with a some very complex calculations with a very user friendly interface, it's actually a really good program. I've often called technical support and they have often either given me where in the code it states why they are doing something or explained the research that went into it. If I believe their response is overly conservative, I have done hand calcs to prove why I'm doing meets code (there are a few things that are recommendations that Hilti will not permit).

Appendix D is an extremely tough code, however after the Big Dig collapse, ACI (along with the big players in anchorage) needed to specify the required calculations in a more complex way. I've wondered whether Hilti/Simpson had some ulterior motives when they 'helped' aci develop these codes... but I digress

If I were you, I'd call Hilti and ask them about the limit.

 

[Mathias65]

Thanks for the response and background. Yes, I've considered talking w/ Hilti or Simpson directly, but it can sometimes take a bit of work to find the person who can actually answer the question, so I thought I'd look to the forums first.

 

[wannabeSE]

I think Simpson and Hilti software only allows what is allowed in their evaluation reports. And, the evaluations reports are limited by what they have tested and what is allowed in the ICC Acceptance Criteria (AC308, AC193) and/or ACI Codes (318, 355.2, 355.4). Really deep embedments may be similar to overhead applications that require more skill and care during installation. Hilti or Simpson will have the answers (whether you like the answers is a different story). It may have changed, but AC308 limited maximum embedments for adhesive anchors. I haven't delved into ACI 318-11 yet. But it now has requirements for adhesive anchors.

Maybe I am lucky, but Simpson (anchor products) and Hilti have experienced local field engineers who are very responsive end easy to contact. If they don't know the answer, they know who to ask.