Bearing strength of thru-bolt in concrete

[ajk1]

Any suggestions of how to calculate the ultimate bearing capacity of a threaded rod (say 1/2" diameter) inserted into say a 5/8" diameter hole drilled thru a 5"± wide rib of a joist, as shown in the attached?

One possibility is perhaps to take the concrete code (CSA A23.3) capacity for baring strength, including the two times multiplier when the supporting surface is wider on all sides than the loaded area. But that still leaves the question of how much of the bolt rod length should be considered as bearing.

If say consider that half the 5" rib width is the bearing length for the bolt (loaded from one side only of the rib), then

Bearing resistance is:

0.85 Φc f'c A = 0.85 x 0.65 x 4 ksi x (0.5 x 5/2) x 2

= 5.5 kips.

That would give more than enough capacity, but is that the right analysis.

Also, would 5/8" diameter be the right hole diameter to drill for the thru bolt (rod) in the concrete?

 

[jplay2519]

I don't see an attachment.

 

[ajk1]

I forgot. Sorry. Here it is now.

 

[KootK]

That's the difficulty with trough bolts. Nobody seems to be in possession of any kind of "official" evaluation procedure. If it were me, I'd do one of the following;

1) Ditch the bracket and just go with a Hilti adhesive anchor for which you can get reliable capacities. My guess is that, properly evaluated, the through bolts won't calc out much better anyhow.

2) Deal with tension on the far side of the bolt via some rational procedure. Then adapt some appendix d anchorage checks to evaluate shear. Here you would be hoping the the appendix d adaptation and assumed shear/tension decoupling would be valid.

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.

 

[ajk1]

To Kootk:

The thru bolts are much superior to Hilti anchors for resisting tension in this case of small edge distances, and I suspect that they may be not much worse at resisting shear although perhaps not. But tension is by far the major part of the load. I am not sure that I understand part 2 of your answer. I don't see any way to make any of the Hilti fasteners work in this situation, with such small edge distances, although I should use PROFIS to verify my hand calcs. I hate to abandon what I and at least one other person on this forum believe is a good system that addresses all the points that I have been struggling with, just because it is not readily analyzable. I woud think that I could come up with a free body diagram of the thru bolt and analyze all the reactions and stresses, and then take a conservtive apprach. It seems strange that there is no analysis for thru bolts. But if you have not come across such, then I am not likely to find such an analysis either.

 

[BadgerPE]

I know there has been a long discussion on the bracing portion of this problem and I have not kept up with it. It appears that you are struggling with resolving the tension force from the brace at your anchor. Could you simply brace the steel beam on both sides? Therefore, the brace force would always be in compression and you wouldn't have to worry about the connection to the rib (which I would avoid if possible). It would make your connection much simpler.

 

[KootK]

It's weird. There was a time, not so long ago, when through bolting was very common. Somehow, however, nobody seems to have written anything down. Whenever this comes up here, I cross my fingers that someone will show up with the silver bullet. In chapter 11 of the Moroccan building code, there's this handy little formula...

In more than one place, I've seen the assumption that the bolt could be assumed to distribute bearing stress over six bolt diameters of bolt length. Whether or not that's valid, I can't say. It seems a bit sketchy in light of modern anchorage design provisions.

I'd be surprised if there isn't something out there in the precast universe.

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.

 

[BUGGAR]

Maybe this is why App. D explicitly excludes through bolts. ACI can't figure them out either.

 

[KootK]

About a decade ago, I was designing a connection for a brilliant, super senior engineer. He was recovering from cataract surgery at the time. I was having trouble making the anchorage work so he told me to knock it off and just use through bolts. Great, I figured. Finally, somebody will show me how to do through bolts. His understanding was that they worked through friction! Very much like we use shear friction today. I get that there's a mechanism there but:

1) How do you get reliable pretension in the bolts and;
2) There's still a breakout cone to contend with.

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.