Abandoned anchor holes near expansion anchor

[MNach]

I have a hanging bridge crane that is connected to an existing concrete structure using expansion anchors. Holes were drilled for these anchors and many hit existing reinforcing before making the required depth. I have contacted a Hilti rep for guidance on this issue and they said that they have not performed any tests on situations like these but provided me a test from the 80's (attached) that came to some conclusions, but also didn't say that Hilti accepts those conclusions (to cover his butt) and that the best way would be to treat the hole as an edge or an anchor and use the appropriate spacing. In another thread I also found the following link that Simpson did on this situation: Link. This study just outlined its conclusions while providing very limited data.

Does anyone have any experience or witnessed any issues with placing an anchor near (min. 2 diameters away) an abandoned hole? I am currently using Hilti's HDA-T M12 anchor to its full capacity (tension and shear) and have very little wiggle room to work with before these failed holes. Both studies appear to say that if the hole is filled and a minimum spacing of 2d is met, there should be negligible effects on the capacity. It's concerning though that few tests have been performed on this and that Hilti has no guidelines, mainly don't want responsibility. Does anyone know of any other tests that have been done on these conditions?

 

[SlideRuleEra]

Both of the referenced studies address abandoned holes that are design full depth.
On your project "holes were drilled for these anchors and many hit existing reinforcing before making the required depth."
How does this project's design full depth compare with abandoned hole depth?

http://www.SlideRuleEra.net

 

[MNach]

The required hole depth is about 5.5" with an embed depth of about 5". Some holes hit steel 1-2" into the concrete and others hit steel at 4.5" in. The 1-2" deep holes do not concern me, but the 4.5" deep ones that are basically full depth are the ones that I am concerned with.

 

[SlideRuleEra]

Concur that 4.5" deep holes are essentially full depth.

Both studies appear to say that if the hole is filled and a minimum spacing of 2d is met, there should be negligible effects on the capacity.

Agree.

...using ...anchor to its full capacity (tension and shear)...

I would not do that for two reasons:
1) Dynamic loading (crane).
2) Assuming multiple anchors are used, load distribution among anchors likely varies... some anchors over loaded, others not.

http://www.SlideRuleEra.net

 

[birneys]

Does Hilti allow the HDA-T for use with dynamic loading? I would be hesitant to use expansion-type anchors in this situation because of the adjacent holes as well as the issues of dynamic loading, vibration, etc. The anchor only works by exerting outward pressure on your hole. In my mind, the combination of dynamic loading, tension loads, and adjacent holes could lead to premature failure of your anchors. If it's sustained tension, then it's even worse. I would suggest looking into other anchor options or consider an alternative detail that would allow you to anchor to sound concrete farther away from the mis-drilled holes.

 

[racookpe1978]

Are the crane loads (regular operation) in tension, or is this a crane rail supported from below? The crane live loads (starting, stopping, traveling, vibration, etc) are tension-compression fatigue-enhancing loads.)

Regardless of orientation, and regardless of appropriate behavior for crane rails with these embeds, why are you not planning on refilling every empty hole back with grout?

 

[MNach]

It is an underhung bridge crane and the supporting framing hangs from a concrete roof structure. The framing is attached to the above structure (which is at a slope)with the HDA-T anchors, so they will always have some tension and shear load (dead load of the structure). Hilti does state that these anchors are suitable for "dynamic loads including seismic fatigue and shock". I do plan on refilling all the miss-drilled holes, I am just wondering if those holes affect the capacity or if it is negligible past 2d like each of the attached studies have come to the conclusion of. Due to the existing structure limits, these anchors have very little flexibility to move locations. In a perfect world I would just provide anchoring away from the miss-drilled holes and call it good, but that is not an option in this case.

 

[bones206]

Undercut anchors seem like the most appropriate solution. I've used them around abandoned expansion anchor holes, because you can sometimes get the abandanoned hole to be completely inside the theoretical failure cone. In that case I would fill the hole and say it's just going along for the ride in the wedge, with no effect on the undercut anchor capacity. I wouldn't be comfortable with expansion anchors for a dynamic load with constant tension, so it's good that you are replacing with undercut anchors.

I actually worked at a plant that had a written policy regarding new anchors next to abandoned anchor holes. I'm pretty sure it was based on the study you referenced and Bechtel may have even commissioned the study for use in that plant. I'll see if I can dig up something for you.

 

[MNach]

Sorry I misspoke, the HDA-T anchor is an undercut anchor that I am using.

 

[bones206]

I managed to dig this up from that plant spec for expansion anchors. UCD refers to "undercut ductile" anchors. I can;t remember the reason for the increased spacing from a hole found already filled with grout, but I think it was because an anchor might still be in the hole under the grout instead of a fully grouted hole.