Concrete Anchorage

[ivanga7]

Please see the attached file for reference. When designing for the concrete breakout capacity, is the edge of concrete considered "a" or "b"? This condition has always thrown me off. Thank you in advance.

 

[NorthCivil]

A good question. For shear as you have drawn it, I would go with the short distance, B. only because the failure of the concrete would arguably start at the top of the slab. I could see a failure pattern where the top part blows out, but the lower part with the bigger edge distance holds on, and fails after more load is applied.

 

[haynewp]

I had this same case before and used B. If there is something that justifies A I would like to see it.

 

[JAE]

Seems like it would depend on the magnitude of the vertical step would it not?

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[haynewp]

Maybe you could use the larger of the 2 if bolt bending can engage A with the depth leftover assuming B area at the top has failed. But the bolt now has a moment at the top to be resisted by the concrete it is embedded in.

 

[BridgeSmith]

The equations in the AASHTO spec. are based on assumed failure surfaces extending at 45 degree angles from the direction of the applied force. I would expect other codes to be similar. If you want to 'sharpen your pencil' and get a theoretical capacity using an equation that is similarly derived, you would need to draw and measure the area of the failure surfaces for the anchorage block under consideration and multiply by the shear stress capacity portion of the equation. To do that, you'll have to dissect the equation into the shear stress capacity and area calculations. The result would be somewhere between what you'd get using a or b. As JAE pointed out, it will depend on the height of the step (relative to the depth of the anchors and the ratio of b to a).

...Or just use dimension b and save yourself a big headache.

 

[ivanga7]

Do you think that it would be wise to use A if I can justify the bolts resisting the bending as haynewp suggested? I need my concrete breakout strength to be a little higher

 

[haynewp]

The stress distribution in the concrete will change when there is an applied moment in the anchor. I don’t know how to figure that in to the ACI breakout equations.

 

[bones206]

Hilti has a procedure that they use to check bolt bending in Profis. You can dig into the procedure starting on page 198 of their design manual: Link

Sizing bolts for bending can be a challenge though.