Post-installed anchor installation needed for missed embedded anchors

[1SEngineer]

This is regarding anchors for a steel column. The contractor installed the threaded anchor rods at the wrong location. We now have to post-install the rods. I cannot get brand-name epoxy anchor systems to work.

4 - 1" x 24" diameter rods with nut at end
6" spacing each way

I am looking for a creative solution to this problem.

I am considering using a grout with larger diameter cores so I can install the nut into the hole also. Have you used this before? I'm not sure if there will be enough bond. And, I'm concerned that large diameter cores may cause concrete cracking. Imagine trying to create 4 - 2 1/2" diameter x 30" deep cores where the anchors are spaced 6" apart.

Another option is to excavate a 24" square x 30" deep chunk of concrete, reset the rods and reinstate with concrete. But, I'm concerned that failure prism will be affected and hence cause a major reduction in breakout capacity. The new-old concrete interface may not necessarily transfer full rupture capacity thorough the interface due to shrinkage.

What are your thoughts? What do you feel is the better option? Do you have a better solution?

Thanks in advance for any help!

 

[jayrod12]

Those are hefty anchors, what kind of loads are we trying to transfer here?

Are we fasteneing into just a giant blob of concrete, or is it a wall?

 

[1SEngineer]

@jayrod12

Loads are:
Uplift = 60 kips
Shear = 20 Kips

Yes, it's a 48" x 48" x 40" deep pile cap

 

[WinelandV]

Are those loads per anchor, or for the group?

EDIT: Also, LRFD or ASD loads?

 

[canwesteng]

What you have for rebar in the pile cap?

 

[bones206]

Take a look at undercut anchors such as Hilti HDA or Drillco Maxi-bolts. High capacity with reliable and predictable load-transfer mechanisms. I don't believe many contractors have experience installing them though.

Are the 1" x 24" rods what was originally specified as cast-in? If so, the post-installed replacement anchor solution will likely be something different, since they have different limit states than cast-in-place bolts. Be prepared to toss out the original anchor design and start from scratch.

My thoughts on excavating and re-casting the anchors in a new chunk of concrete is that it is probably possible, but I don't know of any rational design methods to calculate the capacity with any real confidence. So I'd probably look at getting the anchors load tested after the concrete chunk had cured. Might also be beneficial to "bell out" the excavation as you go down to create a wedge effect. But this method just seems like more trouble than it's worth though, if you have other options. But I wouldn't say it's impossible.

 

[phamENG]

Like bones206 said - forget your original layout. They may have to cut off the original base plate and fab a new one with different size holes and different spacing.

As for replacing the concrete, it can be done. Though with that size pile cap, you may be better off demolishing the cap and starting over since the failure cone envelops the entire surface (24/tan(35)=34.28in diameter around anchor).

The contractor is going to complain, and people will probably accuse you of being over conservative and getting in the way of the project, but it sounds like that's a pretty critical connection. Stand your ground and make sure it's fixed properly. Then again, if your loading is based on over conservative assumptions (completely ignoring the dead load and designing for a 1.0W load "combination" for instance), you may want to think about that.

 

[KootK]

1) I vote for demolishing the pile cap given it's small size.

2) Form a theoretical perspective, if you can't get the name brand, post installed options to work, I would take that as a reason to be suspicious of any other solution that appears to work. Post installed anchorage is capable of mobilizing your full 1.5H:1.0V failure cone. It doesn't get much better than that.

3) Did the original anchor design rely on passing anchor tension from the bolts to the vertical reinforcing in the pile or pile cap?

 

[1SEngineer]

@wine: Ultimate

@can: Typical pile cap reinforcing

@bones: I thought about the bell shape as well. I, too, am reluctant to try to chip all that concrete. It might cause more damage. I have to look into the anchors you mention. Though I do not think they will be sufficient due to the lack of depth. Part of my design methodology is to drop the load farther down into the cap which creates a larger prism and larger breakout capacity.

 

[1SEngineer]

@koot:

1) Right. This may be the best route based on my response #3 below.

2) Well, I think Heff is too small, so a larger failure prism cannot be established. We used our own excel spreadsheet that calls for a projected concrete failure area.

3) We use a plate washer and nut at the bottom of the rod. The plate sits just under the bottom steel of the cap. So the anchor uplift load engages the bottom steel. The whole cap essentially becomes the failure prism.