Hilti Anchors to CMU

[bookowski]

I need to attach plates to an existing cmu wall, my loads are in the 400# range per anchor in tension and about 150# shear. These plates will support cantilever tubes so I want to have a good factor of safety in there. I have some questions about how people typically handle the following:

- Most likely the anchors will hit a mix of solid and hollow cells. If I go with Hilti they recommend a different anchor for each condition - do you spec both and make the contractor switch back and forth?

- Hilti anchors are shown as not applicable at or near the mortar joint, I can't really control the location of these anchors. Do you typically deal with this by adding additional anchors assuming that some will be ineffective?

thanks

 

[frv]

1. Yes- I did precisely that.

2. Yes. Whenever I can, I actually design a Post-installed anchor for double the anticipated load precisely because of the situation you are talking about.

 

[bookowski]

frv - thanks. I see that Powers has fasteners that they specify for both hollow and solid cells (AC100), for the hollow cells you use a screen tube but at least it's the same epoxy for both. Any experience using this? It seems a lot less prone to getting mixed up. They provide loading tables for anchors installed in the joint.

 

[Ron]

Simpson has good info on this as well. Hilti is a little sparse in their info.

In the US, the typical safety factor required by code is 4 for this type of anchorage....further, you have to check unity with shear and tension.

 

[bookowski]

Yeah, I've got the anchors figured out - my question is more about the mixing of anchors for grouted and non-grouted cells in cmu. I have a feeling the contractors won't keep that straight, it would be nice to have an anchor that does both so that I don't have to rely on the contractor keeping it straight. It would also be nice to have more info on anchors into the mortar joint, it's inevitable that this will happen.

 

[hokie66]

The fact that Hilti has little information for anchoring in tension into hollow masonry is a good indicator that it is not a good idea. I would limit tension anchoring to the grouted part of the wall.

 

[anchorengineer]

I used to be an engineer for one of those anchor companies and I can tell you you're getting good advice. Always add a few extra anchors when you can. Base material and installers vary so beware.
What you should do is assume all block is hollow. If you are using an adhesive anchor with a screen tube it really will not matter if you hit a solid block (your capacity will be higher). Also, if you are just penetrating one face of the brick make sure your anchors are short enough. When anchors are too long they tilt up and don't mushroom out behind the block face correctly. If you use Hilti, go with the HIT-A rods. They have squared ends and make a big difference in performance. Finally make sure the guys installing these anchors know what they are doing.

For such small loads you may want to consider a mechanical sleeve anchor. I would only use this if the block is new. Also, once this anchor is set you can not remove the nut because the threads get damaged when you hammer it in and you tend to spin the anchor in the hole which loosens the whole thing.
Good luck!