Shear Transfer into Anchor Rods with Minimally Over-sized Holes. 1

[waytsh]

I am designing the anchor rods for a metal building which is going to have some very large horizontal forces. This manufacturer typically only oversizes the holes in the base plates by 1/8" (3/16" for 2" diameter and larger). With oversizing this minimal is it permitted to consider all the anchors acting in shear at the same time? This seems reasonable to me, although if I would do this I would conservatively ignore the shear friction contribution between the base plate and the concrete. Thoughts or code references addressing this?

 

[Lomarandil]

To be clear, oversized by 1/8" compared to standard or oversized by 1/8" compared to the bolt size?

You can always consider the effect of that much movement at the base of your column on the rest of your structure. Anchor rods and base plates can be pretty ductile -- so it's possible you can handle the kind of movement required to mobilize all anchors.



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The name is a long story -- just call me Lo.

 

[dik]

If shear is an issue to be transferred by anchor rod, then have a fitted plate washer welded to the column base plate.

Dik

 

[waytsh]

I should have clarified that the 1/8" over-sizing is compared to the bolt sizing. In my case I have 2" anchor rods so the holes are 3/16" larger (2 3/16" holes).

Dik, normally I do that when the holes are larger and am not opposed to it. I will need to check if the bolts are ok for bending. I am also entertaining the option of a shear lug or building the pier up and pocketing it to embed the column. It would be a lot of extra work if it didn't need to be done. I didn't want to dismiss what might be a common and acceptable practice just because I had not seen it.

Thank you for the responses.

 

[dik]

If your shear load is sufficient... I try to avoid shear lugs.

Dik

 

[bones206]

I also prefer the welded, fitted plate washer approach, but I've gotten fairly consistent push back from contractors (and my boss) for using that detail because of the added cost of field welding.

I've wondered how fitted plate washers would perform without welding, but instead with the prep and installation of a slip-critical connection.

 

[waytsh]

That's an interesting thought bones. Not sure how it would pan out in the end since they would need to be considered torqued anchors.

I have also heard of using welds or grout to fill the gaps between the plates and the anchor rods. Not sure that is any easier than the plates.

Back to my original question, I think I am ok assuming that all the anchors will engage at the same time. As Lomorandil said the base plate and the anchors are pretty ductile and the worst case movement of 1/8" is not drastic.

https://www.eng-tips.com/viewthread.cfm?qid=438545

 

[hokie66]

I don't think I have ever seen a contractor set plain anchor bolts accurately enough to accommodate the hole sizes your manufacturer uses. Sleeved anchor bolts might be the answer.

 

[dik]

bones206... still cheaper than shear lugs... just not as disguised...

Dik

 

[bones206]

The shear breakout section of ACI 318 Chapter 17 (previously Appendix D) discusses different cases of load distribution to the anchors, which depend on the anchor spacing and edge distance, and whether or not the anchors are welded to the plate. For non-welded anchors, the standard assumption is 50% shear on the front anchors and 50% on the rear. If the anchor spacing is greater than the edge distance, than you need to also check the case of the rear anchor taking all of the shear. This accounts for when the front anchors start to form a breakout cone and lose stiffness, while the stiffer rear anchor row attracts more load.

For non-welded anchor spacing LESS than the edge distance, it is recommended to assume the front row takes all of the shear.

I believe the code allows the engineer to select the critical anchor row for checking breakout, but the commentary suggests these assumptions based on available testing and literature.