Thru bolt loaded in single shear in concrete

[Polecat]

If you have a thru bolt loaded in single shear bearing on a hole in a concrete member, is there any test data or evidence that can show that the stress distribution in the hole is parabolic instead of triangular?

http://www.spiraleng.com

 

[ATSE]

The stresses in the concrete and the steel bolt are very complex. Probably more parabolic than a nice neat triangle.
Just like most other things in structural engineering, we greatly simplify the load distribution, provided there is enough ductility for the connectors and base material to reach "design strength" without fracturing.
Just curious - what is the reason to sharpen the pencil for this condition? Usually, this type of connection is over-designed, and for good reason.

 

[Polecat]

ATSE
The pencil sharpening is because we are pushing the envelope here. Usually, we are able to cast in a steel pipe thru which the bolt would be inserted. This, of course, increases the bearing area to the concrete considerably. This time, however, it is direct bolt-to-concrete bearing, so we have essentially line bearing right at the outside edge of the hole. That, coupled with a required phi factor of 0.65 in bearing, is really making it difficult to meet the required load. That's why I was looking for some justification for a parabolic distribution so I could use an average stress of 2/3 f'c instead of the 1/2 f'c for a triangle.
But, thanks for your input.

 

[dik]

I was involved in a court case about 45 years back, when shear friction, was entering the scene and a local engineer had used it for securing a spandrel beam to a series of shearwalls. The city decided that shear friction wasn't permitted. The prof that was testifying for the city had run a 2D finite element program showing that the bearing at the outside 'end' of the rebar was 13 Ksi.

The other engineer's lawyer asked him what would happen to the stresses if the outside edge were to crush and were to redistribute the stresses in the concrete. He replied that he wasn't able to produce that type of model... but that the concrete had crushed.

He was presented with data from a 3D FEM model that I had written showing the redistribution. Maximum stress could be established. Client lost, but, had fun in the process.

Until redistribution from yielding/crushing the stresses can be quite high and there was little difference with a tiny air gap around the rebar. I suspect a bolt would be the same.

Dik

 

[Polecat]

Thanks for the input, Dik. I would be interested in seeing the model that you presented.
I am convinced that a stress redistribution takes place in cases like this, but have never been able to formally prove it.
Logic tells me this: unless there is a punching shear failure due to the lack of enough concrete wall thickness, then the outer edge of the hole will most certainly start to crush when the f'c stress is exceeded. But, since the load is still present, it then has to go somewhere else, and that would be to crawl inward along the bolt until equilibrium is finally met. If you could plot that path, my guess is that it would look very much like a parabola.
But, just being an ordinary engineer and not a PhD college professor, what do I know?

 

[dik]

We did a full scale load test on the project, after the city had issued a stop work order... just the next step... It was the Court Injunction that should have stopped us, but didn't... Ran the load test anyway... Talked to my lawyer and he suggested that by having a few drinks, there might be mitigating circumstances in the event we were arrested...

It was a different time, and, I was a bit different.

Dik

 

[BridgeSmith]

Manufacturers of concrete anchors have tables giving the shear capacities of their various sized anchors in concrete of various strengths. I would take a serious look at those as a good lower-bound guideline.