NCEES Practice Exam, 2011, Structural, Problem 802

[tenguy23]

Part B - I'm calculating that the Brace force is a tad over the 70% limit of the AISC Seismic Provision limit of 30-70% of the total story force. Has anyone done this problem & is anyone getting the same result?

Also, the "solutions" do not even check this limit. I'm under the impression that for Braced frames, this 30%-70% limit must always be checked.

Advice or help is welcome! Perhaps i'm making an error...

 

[captaincaution]

That provision states that 30%-70% of the total horizontal force along a line of braces is to be resisted by braces in tension. In the frame in question, you have one brace resisting the force in tension, and one in compression. The symmetry of the braces means each brace is resisting 50% of the horizontal force, though the actual forces in the braces will be higher.

 

[tenguy23]

OK, and thank you for the reply...for some reason, I was reading that the converted magnitude of the inclined brace in tension couldn't be < 30% or > 70% of the lateral load along the top of the braces.

So, I agree with the solutions that 56.6k is the brace force, but then 56.6 / 80 k total = 71%.

Am I reading this wrong? I understand that the load is distributed 50% / 50% to a symmetric brace, but it is the angle of the brace that dictates the magnitude of the force. If the brace is angled even more, the force in the brace can get higher.

The way I read the spec (which definitely could be wrong) is that this brace does not satisfy 13.2c....

 

[captaincaution]

You're reading way to much into the provision. Although the brace force is 56.6 kips, the brace is resisting 40 kips of the horizontal force. It's not the magnitude of the force in the brace that you look at when checking that provision, but the magnitude of the horizontal force being resisted. Yes, if the brace is angled more, the total force in the brace would be higher, but that wouldn't change the amount of the horizontal force that brace would resist.