Effective embedment length for partially debonded anchor bolt

[BridgeEngineer21]

I am reviewing a proposed connection for a tower foundation that has the detail shown in the sketch below. The contractor wants to use corrosion protection tape at the top portion of the anchor bolt. They did not provide any info on the tape, and from the manufacturer's website I can see it is mainly geared towards wrapping pipes and things like that so they don't give any structural properties or friction coefficient for it.

Therefore, I believe the length of the anchor bolt covered in this tape should be assumed to be completely debonded from grout. This means that for anchor bond strength, only the length ld,effective should be counted on.

Now my question is, what about for other checks such as pullout, breakout, or pryout? Intuitively, it seems those are all based on the depth of concrete (or in this case, grout and rock) above the bottom of your anchor, and there's no need to discount the debonded portion of the anchor. But I've never dealt with a case like this before, and would like to get a little confidence with some second opinions.

A related question is having to do with the check of bending in the exposed portion of anchor bolt. Intuitively, it seems the moment can just be V*e1, since below e1 the compressive strength of the grout should prevent the bolt from deflecting at all. But then I looked at the situation as basically equivalent to the one sketched below, which I again have never really analyzed before, and now I'm a bit in doubt if I'm not missing some other aspect.

Would appreciate anyone's thoughts on this.

 

[BridgeEngineer21]

The fixity question is a tricky one, especially with the wrap. I think there are equations for the depth to lateral fixity for bars/bolts embedded in concrete, but I'm not sure where to find them. However, that depth would increase some because of the wrap, and it would vary depending on how thick/tight the wrap is at the surface of the concrete.

I was able to find this, in EN 1992-4. a3 is defined above as 0.5*d, so its not a very large additional depth.

The contractor right now is calculating bending in the bolt by taking a moment with the lever arm all the way to the end of the bolt, and then subtracting a moment due to triangular loading that represents the rock compressive stress starting from the surface of the rock. Essentially that method has similar logic (but not exactly equivalent results) to just using a lever arm (la) as defined in b) or c) above. I think at a minimum they should be using a). It seems a bit too conservative to measure la all the way to the end of the tape (about 1/4 of total embedment), but I don't know a good way to find a suitable in between value.

 

[BridgeSmith]

I think you could make a conservative estimate of the 'clearance' between the bolt and the concrete due to the wrap, then calculate the length of the bolt that would needed for it to bend that far at your allowable or yield stress in the bolt, and add that to the a3 distance.

 

[Lomarandil]

If I'm understanding that right, the type of anchor I have here is a post-installed screw anchor

No, a grouted threaded rod would be a post-installed adhesive anchor, where the grout is your "adhesive". A screw anchor would be drilled in directly and has coarse threads that cut into the concrete.

Eurocode (EN 1992-4) is a little less explicit, but is the governing code I have to follow here. I am thinking that the combined pullout and concrete cone failure mode, as you posted above, is the main failure to check here. I'm wondering if there's any significance to the fact that in the Eurocode figure that's the only failure mode they showed a non-headed anchor with..

Exactly