Prying in AISC 1

[Wanna_be_SE]

In the prying equations the steel manual uses Fu, instead of Fy. On p. 9-10 there is a brief discussion that the design basis was changed to Fu "which provides better correlation with available test data".

I've found some other guidance online for some explanation in what is actually happening in prying, and one source said we're basically ensuring a "plastic hinge" doesn't develop in the plate, otherwise prying forces need be considered in the bolt. While I'm still relatively new in my career, I thought the plastic hinge was Zx*Fy, not Zx*Fu? It seems the prying equations are suggesting no additional tension forces due to prying is present in the bolt, until the plate has reached it's absolute max strength, but I would think prying forces would begin to develop in the bolt after Zx*Fy, because isn't that the plastic hinge? So beyond that no elastic deformation will happen. So as the plate begins to permanently deform in bending, wouldn't that initiate additional prying forces in the bolt?

Maybe the testing and theory doesn't exactly align, and so that's why AISC decided to change to Fu, but did the older versions of steel manual use Fy for prying equations?

 

[JoshPlumSE]

Yes, I think you're essentially correct. For a long time, the theory was based on Fy of the plate. It was a good theory. Keep in mind, of course, that there are LOTS of approximations / assumptions that went along with that:

Fy vs actual yield
degree of fixity of plate at bolt locations
ignoring width of the nuts that clamp the plate.

To me, these approximations are fine as long as they lead to a bit of conservatism in design. That being said, these professors who test this stuff want to be published.... Honestly, they NEED to be published to keep their jobs, get tenure, continue to get research funding, etc. So, when they see something that is significantly conservative compared to test results, they'll look for ways to correlate the results to the "theory" better. To me, this is a case, where they're bastardizing the theory a bit. Likely the lack of correlation on test data is really related to some inaccuracy in one of those other assumptions I mentioned.

That's just my 2 cents though. I'd have to really read those research papers to understand WHY they proposed replacing Fy with Fu.

 

[Wanna_be_SE]

Yea I discovered today that apparently the 13th Edition of steel manual made the change from Fy to Fu.

Thanks for response.

 

[PersonalProfile]

The following states a theory for prying, though I am not aware if is the "latest":

Guide to Design Criteria for Bolted and Riveted Joints

https://www.boltcouncil.org/files/2ndEditionGuide.pdf

Regards.