Concrete bearing capacity in bolt holes

[yakpol]

Hi, I am designing steel bracket attachment to existing concrete using thru bolts. The bolts will be epoxied in the holes prior to installation. My question: are there recommendations for concrete bearing capacity in such situation?
Thank you all!

 

[racookpe1978]

Your pull out (tension) or pull down (shear) forces expected are what?

 

[MotorCity]

If you are thru-bolting, why are you using epoxy? Just drill the hole slightly larger than the bolt, no epoxy needed.

ACI addresses bearing capacity on concrete, but makes no distinction between bearing due to say a baseplate versus bearing to a bolt or anchor rod. You may find something buried somewhere in Appendix D, but Appendix D does not apply to thru bolts.

I would simply use the bearing strength equation in section 10.14 of ACI 318.

 

[KootK]

In the 15 or so years or so that I've trolled this forum, I've seen this question asked at least three or four times per year. In all that time, I've not once seen anyone post reference to an "official" through bolt design method from any jurisdiction. Some of my old mentors said to assume that a bolt length of six bolt diameters should be considered in bearing. Others have claimed that through bolts should be pretensioned and that through bolting was really just shear friction before codes picked up the concept.

As far as I'm concerned, there's functionally no such thing as through bolting. There's appendix D stuff for which I would absolutely grout/adhesive the hole and, if you need better pullout numbers, punching shear can be mobilized via the backside anchor plate if you have one.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TehMightyEngineer]

I do the same as KootK.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[TLHS]

For a significant shear load, I'd probably epoxy the thing, but realistically there is some amount of lateral capacity available in the bolt installed through a slightly oversized hole.

Take a look at something like a mechanical expansion anchor. You drill a hole, insert the anchor and then engage the wedge at the bottom. That gives you an obvious tension load path, but your shear load path is some combination of that tension and shear capacity at the very bottom of the hole along with bearing along the normal anchor material that's been installed in a slightly oversized hole.

Realistically, this is the same situation you'd have in place with a through bolt. You have tension restraint and probably some friction at the bottom, and then bearing along the shaft.

Maybe investigate the technical documentation behind Hilti's Kwik Bolt? Their European technical stuff is more detailed than the North American stuff, maybe they go into their assumptions somewhere in that.

There are situations where I'd likely be comfortable using a through bolt with shear load without epoxying or grouting it. I'd want a large factor of safety on top of conservative assumptions, though, unless I can find good documentation of testing.