Post Installed Anchors

[ingenii]

I have read the structural forums on this site for many years and found many an answer to my own questions. However I now have one which has not been addressed before.

I am wrestling with trying to anchor a shear wall into the top of a narrow foundation wall. Because of the limited width, I can only develop half of the required load using typical adhesive or mechanical fasteners inserted into the top of the wall. Using steel straps down both faces of the wall has its own drawbacks, so I am considering a different approach whereby I drill a hole 1/4" larger than the rod down into the wall, and at a depth to be determined, core through the wall so I can add a thick plate and washers to the bottom of the rod. The entire pocket would be filled with high strength non shrink epoxy grout. With such a detail, I am expecting to check three failure conditions: bearing of the plate to grout, bearing of the grout to concrete, "punching" shear type failure (ie resistance of the homogenous concrete to transfer the tenson in the rod). The way I see it, this is not your typical post installed anchor of the type governed by ACI 318 CH 17, and I would not be checking for side face blowout as the load transfer mechanism is comparable to a base plate on a pedestal. I'm looking for anyone's opinions on the proposed detail and analysis.

 

[bones206]

This is an interesting idea - kind of reminds me of barrel bolts for timber frame connections. I think the issue is that it is maybe too novel, and might fall out of the standard of care without testing to back it up. You may also face significant pushback from contractors as coring could make it too cumbersome and expensive.

I usually specify cast in place anchors for hold downs and such on stem walls, but contractors typically ignore my detail and post-install something of their choosing and force me to buy off after the fact. Your detail would make a nice "punishment", but I'm not that vindictive.

Simpson has some engineered anchor bolts specifically for stem walls, but again that assume they actually get installed per your detail (

https://www.strongtie.com/bolts_anchorbolts/category).

Ideally there would be a high capacity post-installed solution out there that the contractor would actually go for. This has actually been on my mental to-do list as well.

 

[ingenii]

Thanks bones - the problem is the contractor already poured the walls without the anchors in place (didn't install the hold-downs for the shear walls either) - so now I'm stuck trying to fix it. I would happily be that vindictive if that contractor had to do the repair at his expense, but he is off the job (still on his nickel though).

 

[driftLimiter]

I don't really see it as vindictive when no post-installed solutions will work. The alternative is to remove and replace and I'm sure they wouldn't like that any more than what you've come up with here.

Is it really a punching shear check? Doesn't really fall into a middle, edge, or corner condition.

Seems like a pretty typical concrete blowout with edge distance per Ch 17. Perhaps 1-way shear on concrete would also be appropriate.

From a strut and tie perspective you would need your nodal region to develop two compression struts going up and away form the anchor. Maybe looking at the required dimensions of those struts(and their bearsings at the node) would help rationalize this problem per code.

 

[Eng16080]

I would check all the requirements of Ch. 17 as if this were a cast in place anchor, except I might use the strength reduction factors for a post-installed anchor. I think the 3 failure modes that you mention will essentially be covered if you meet Ch. 17, but you might as well check those as well if you think there's a unique situation which might not be covered by Ch. 17.

In terms of your 3 failure modes:
bearing of the plate to grout - This should be covered by checking the pullout strength per Section 17.4.3
bearing of the grout to concrete - I would start by using the lower concrete strength for the calculations and would not worry about the small area with the higher strength grout. Perhaps it has some benefit but I would be hesitant to use it.
"punching" shear type failure - This should be covered by checking the concrete breakout strength per Section 17.4.2

And yes, I would check the various other failure modes too including side-face blowout. I can't imagine that will be limiting if I'm understanding correctly that you have a massive 4"x4"x1" thick plate washer!

 

[dvd]

Why not just grout the anchors in?

 

[bones206]

I think ACI-318 explicitly exclude grouted post-installed anchors in the first or second paragraph of Chapter 17. That’s why I think you’d need an ESR report or something to back this up.

 

[ingenii]

Thank you for all of your insights. I like the strut and tie approach since the steel in the top of the wall will act as a suitable tie, giving me greater capacity than the shear strength of the concrete alone supplies. However, that doesn't help me if the plan reviewer insists this is a post installed anchor subject to Chapter 17 checks. Thank you also therefore for pointing out the exclusions in Ch 17. as this could help deflect any plan reviewer comments that I must provide a side blow out check.

That said, my 1x4x4 bearing plate was purely hypothetical to ensure uniform bearing, but I wasn't thinking that it would also provide me with an extremely high side bursting capacity (since eq. 17.4.4.1 is based on the SQRT of the bearing area- which essentially solves my problem, so thanks to Eng16080 for pointing that out. With a suitably sized plate, the side bursting issue can be eliminated entirely.

 

[bones206]

You could also look at undercut anchors, as they are most likely to provide sufficient capacity out of all the various post-installed types. Probably less work overall for the contractor as well and it’s well defined in the code.