Pull-out bearing capacity of a headed stud (ACI-318) 1

[Chaking]

I am now calculating for pull-out bearing capacity of a headed stud.

From ACI-318 code appendix D, it is known that the ultimate bearing capacity = 8*Area of bearing contract*concrete strength

I am wondering how the 8 is coming, if we simply think of the failure mechanism - local crushing of concrete near the head of the stud toward the pull-out direction, isn't the capacity is just equal to area of bearing (Area of head - area of bolt) times the concrete strength? Why will the final result of pull-out bearing capacity become so large?



Thank you.

 

[charliealphabravo]

That's a good question. I suspect it is related to f'c being determined by crushing tall (6"Dx12"H) unconfined cylinders with various possible modes whereas the mechanism for bearing failure under a stud is highly confined, uniform, and basically pure compression.

Edit: Also f'c is specified at 28 days whereas strength for other failure modes may not be required until much later (120 days or more) when the concrete is stronger.

 

[TehMightyEngineer]

I presume that it's because the concrete under compression is confined by the surrounding concrete.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[WARose]

I think it's expected that (under the right circumstances) the bearing load will "spread". Similar in concept to the A₁/A₂ bearing area concept you see in masonry design.