Design of Beams in Special Concentric Braced Frames

[CivilSigma]

Hello,

I'm studying steel seismic design from the textbook: Seismic and Wind Forces: Structural Design Examples, 5th Edition.

I am on Example 3-3 which illustrates the seismic design forces on a beam in a chevron braced frame.

Per AISC 341 Section F2.3, we need to check the beam for a) Expected tensile brace force + Expected compressive brace force and b) Expected tensile brace force + 0.3*Expected compressive brace force. Section F2.3 tells you how to calculate these forces.

However, in the example on page 288, the author is using the true nominal brace capacity in compression to design the beam. Please see the below screenshot. I disagree with this. Do you agree with me or am I missing something?

Thank you,

 

[Ron247]

I do not have the publication you cite, and the picture is a little fuzzy but I noticed the formula for Pr looks like it has Pn, not Pu in it. Pn above is 149. But I also noticed the formula for Pn looks (again fuzzy) like Pn = phic*PN/phic. If so, that looks odd. The numerator and denominator have the same symbol.

 

[CivilSigma]

Hi Ron, please see the attached PDF snippet.

What the author was doing is using the design tables in AISC 360 to determine the compression capacity of the HSS brace, and then dividing by the saftey factor to get the unfactored capacity. I don't think that's how we determine the seismic capacity limited force on the beam though.