Hilti has some info on post-installed development and splice lengths in their technical guide.

I think you can create a "lap" with the bars in the wall below, but be prepared to drill pretty darn deep and wrestle with the "we've never had to do it that way" criticism.

Are we talking about #7 bars as localized zones in the corners? Or #7 @ 16" all of the way around? If it's just zones, I'd consider chipping away enough of the stem walls to get mechanical couplers on the bars in there. If a "stem wall" is like a 4' tall thing and it's #7 all of the way around, then I'd be inclined to just demolish the stem walls and rebuild them. Yeah, the lapping stuff can sometimes be made to calc out. If you're doing it all round the stem wall perimeter, however, there just isn't going to be anything left of that stem wall worth having.

I'm not overly concerned about the contractor squawking if we make them drill deep. Unfortunately we're looking at #7 @ 16" all the way around for these guys which make it a little worse. We're at the bottom floor of an elevator core so we're looking at a 5'-6" concrete wall (10" thick) that forms up the elevator pit that we're sitting on top of.

Well, at least it's not 8' OC. As JNLJ mentioned, Hilti's got some decent info in their design guide on post installed rebar: Link. You're effectively doing a lap splice and an offset lap splice at that. So it's going to be those kinds of depths.

In addition to that option, I'd propose hydro demolishing the tops of the stem walls down 8", so that you can get mechanical couplers on the bars, and the building the walls back up with repair material. I feel that this might be economically competitive with the drill & adhesive solution. More importantly, to me, is that I feel as though so much drilling, so deep, is going to beat the heck out of your wall. If you're working the core this hard in flexural tension, then you're working your zones similarly hard in flexural compression. I worry that your wall will wind up splitting apart like barrel staves when that happens as a result of internal cracking developing from all of the closely spaced drilling. I don't know how realistic such a failure really is but it's the picture that pops into my head. It's not as though this is something that Hilti will be considering.

Sure enough, the Hilti guide has a pretty great graphic of the situation.

Thanks, Koot, much appreciated. I had been looking at the Hilti post-installed rebar guide and it seems at least plausible. Would definitely be a pain for them to drill and epoxy every 16" with a 30"+ embed depth. That said, I do see your concern about potentially weakening the concrete there. I'm a little confused on the graphic they're showing - do you get the idea that they assume the presence of horizontal reinforcing (of which we have due to typical horiz bar) will act to limit that splitting mechanism? Seems like something that is hard to real numbers to, but I agree that it potentially just doesn't feel great.

You're most welcome Nulukkizdin.

Quote (Nulukkizdin)

I'm a little confused on the graphic they're showing - do you get the idea that they assume the presence of horizontal reinforcing (of which we have due to typical horiz bar) will act to limit that splitting mechanism?

No, I believe that they are showing the horizontal bars simply because they complete the strut and tie model implied by the offset lap splice condition. Such bars would surely help with vertical cracks oriented perpendicular to the wall but they wouldn't do anything for the the through thickness cracking that moderately concerns me. Maybe you just agree to have some dowels installed and pull tested over 4' length of wall to see how it goes before you commit to the whole thing.