Deep epoxy embedment in foundation wall

[bnickeson]

I have a podium-style apartment building where some large cast-in hold down anchor rods were omitted from the top of the foundation wall (despite my huge note on the drawings saying not to forget these). The location that was omitted was, of course, the shear wall with the highest uplift load on the entire building. Approximately 35 kips ultimate, though that was using Exposure C and realistically we're probably B. So you can keep in mind there was some conservatism there.

In any event, the foundation is a 12" thick wall and about 12'-0" tall. The uplift anchor rods are 1 1/8" diameter ideally placed 3 1/4" from the outside face of wall to the center of the bar. I can increase this to around 4" or so if needed. I am trying to post-install a single #9 bar in epoxy with a deep embedment to use development theory to resist this uplift load. I have enough vertical #5 bars in the foundation wall to prevent any tension breakout as these bars are developed where they theoretically cross the breakout cone. Looking at ESR 3814 for Hilti RE-500 V3, Table 31 gives a development length of 32" for a #9 bar and I am thinking of using 36". Section 4.2.3 states for embedment greater than 20db, that required minimum edge clear distance is to be 1 9/16" which isn't much. My biggest concern is just the proximity of the bar to the outside face of the wall and I'm wondering if this "development" of the bar is still adequate or if some other failure mechanism - like side face splitting or bond - would lower the capacity. I talked to Hilti's engineering help desk and they weren't any help.

Does anyone have any further experience with this? Will a 36" embedment in the top of the foundation wall be adequate to develop the strength of the bar? I don't believe we really have any other potential solutions for this location other than cutting a section of the wall down and re-pouring, which isn't an option since the walls are already partially backfilled.

Thanks.

 

[jhnblgr]

Can you add a sketch of the reinforcement present in the wall? Did you use the hilti program to check edge break out?

 

[Adamian]

You can add a plate at the end of the rod to increase the size of the the breakout cone.

 

[Aesur]

I have always had an issue with this approach because it seems to me that the old Chapter D checks still apply for side face blowout etc. However I could maybe buy it if you have equal area adjacent reinforcing to lap too that drags down to the foundation, in which case you are really a lap splice and not development length.

 

[bnickeson]

Can you add a sketch of the reinforcement present in the wall? Did you use the hilti program to check edge break out?

My apologies, extremely busy today so I can't put a sketch together, but the exterior layer of reinforcing is at 18" o.c. and the interior layer is at 12" o.c. There are also two pairs of vertical bars both faces of the wall about six inches away from where the rod will be installed, so we should have plenty of vertical rebar developed through the failure cone plane.

Since this is deep embedment development theory, the Hilti program doesn't apply using the typical anchorage to concrete module. You have to use the concrete-to-concrete module which is not geared well towards a single anchor. I tried that and the input just didn't make much sense. But it didn't give any warnings for installing an anchor this close to the edge of the wall, so at least there's that.

 

[EngDM]

...the Hilti program doesn't apply using the typical anchorage to concrete module. You have to use the concrete-to-concrete module which is not geared well towards a single anchor.

I loath that calculator of Hilti's. The design report is so hard to follow and not organized well at all. Doesn't show loads in the bars, just says that it exceeds the capacity. I've stuck with modeling it as baseplates.

 

[bnickeson]

You can add a plate at the end of the rod to increase the size of the the breakout cone.

That's actually a really good idea as long as cutting/coring the opening is feasible, but the problem in my instance is that the exterior face of the wall has already been backfilled. The new anchor would be about 3 1/2" from the exterior face, so we'd only have access to the interior face of the wall which would require an opening almost the entire way through the wall.

If all else fails though, I may propose this. Thanks!

 

[bnickeson]

I loath that calculator of Hilti's. The design report is so hard to follow and not organized well at all. Doesn't show loads in the bars, just says that it exceeds the capacity. I've stuck with modeling it as baseplates.

Yes, it's truly terrible. I modified the input as best I could to match our conditions. It should have been more than enough to resist the loads but it was telling me it failed prematurely. The output gave zero indication of what was controlling the design or why it was failing. It was completely useless.

 

[ANE91]

Be careful — ACI is clear that Chapter 17 doesn’t apply when embedments exceed 20db, as you’ve already figured out from the ESR. Whatever you come up with needs to comply with Chapters 12 and 21. Doing this by hand isn't too difficult.

Cast-In Starter Bars vs Post Fixed Epoxy - Edge Breakout - Structural engineering general discussion

Hi All, Does anyone know why the effect of edge breakout is more severe in post-fixed rebar with epoxy compared to cast-in rebar? Say you're drilling a post-fixed rebar the same length as what the development length of the cast-in rebar, if you check it in Hilti or similar software, the...

www.eng-tips.com

Only other point I'd add is that the contractor should be using a drilling template for a post-installed element this deep. This is one of those rare times where you should absolutely specify the means and methods, else you'll be back out to the site to try and retrofit a wonky bar that's too far out of tolerance. Don't forget that making them redo it is always an option, regardless of backfill, regardless of cost. It's their mistake.

 

[EngDM]

Be careful — ACI is clear that Chapter 17 doesn’t apply when embedments exceed 20db, as you’ve already figured out from the ESR. Whatever you come up with needs to comply with Chapters 12 and 21. Doing this by hand isn't too difficult.

Cast-In Starter Bars vs Post Fixed Epoxy - Edge Breakout - Structural engineering general discussion

Hi All, Does anyone know why the effect of edge breakout is more severe in post-fixed rebar with epoxy compared to cast-in rebar? Say you're drilling a post-fixed rebar the same length as what the development length of the cast-in rebar, if you check it in Hilti or similar software, the...

www.eng-tips.com

Only other point I'd add is that the contractor should be using a drilling template for a post-installed element this deep. This is one of those rare times where you should absolutely specify the means and methods, else you'll be back out to the site to try and retrofit a wonky bar that's too far out of tolerance. Don't forget that making them redo it is always an option, regardless of backfill, regardless of cost. It's their mistake.

Anyone familiar what the CSA A23.3 equivalent to this requirement past 20db is for epoxy?

 

[ANE91]

Anyone familiar what the CSA A23.3 equivalent to this requirement past 20db is for epoxy?

The requirement lives in ACI 355.2, which A23.3-14 refers to on page 21. If there's wording in A23.3 equivalent to ACI 318-19(22) 17.1.2(e), then I imagine that the requirements are the same. Note that I am wholly unfamiliar with Canadian codes.

I'll lay out the code path here, once more, since it's not abundantly clear at first blush:

1. ACI 318 states clearly that the provisions of Chapter 17 only apply when an anchor has met the assessment criteria of one of two other documents. For adhesive anchors, it's 355.4. For mechanical anchors, it's 355.2. Simply, if you think something is an "anchor," but it's not qualified by ACI 355.2/4, then it's not actually an anchor in the eyes of ACI 318. A simple example of this is mechanical anchors smaller than 1/4-inch in diameter. Tiny "anchors" like that are not subject to the provisions of Chapter 17.

2. Looking to ACI 355.4-19 reveals early on in the scoping section that it does not apply to embedments greater than 20*db. See 1.2 and 1.2.1. Thus, post-installed reinforcing bar embedded more than 20*db cannot be qualified for design in accordance with ACI 318 Chapter 17.

 

[ANE91]

@EngDM look at D.1.3(d)...looks like this is in part of the code that I would call non-statutory were it an American code. Reading the note makes me think that it can indeed be adopted as statutory on a jurisdiction-by-jurisdiction basis.