Thanks for your response.
That makes sense, but then wouldn't you also have to consider the horizontal force from the braces from the floor above the beam? (2nd floor braces - 1/2 goes to 3rd floor beam and 1/2 to 2nd floor beam)
This brings up another point:
What if the first floor braces were a "V" configuation (as opposed to an inverted "V")? Then the second floor beam is not intersected by braces and thus section 13.4 would not apply to that beam. So the beam wouln't be designed for the unbalanced force unless you're splitting braces forces as described above.
Also similarily, if you have a single story "V" braced frame the beam would not have to be designed for the unbalanced horizontal load per AISC 341 (although it seems it should?).
My second question about the effective length of the beam: SEAOCC design example uses 1/2 length of beam for Lx (effective length for axial force in beam). See p. 53. It seems they are using the chevron braces as a lateral support point for the unbalanced horizontal load. This seems incorrect; 1 brace is buckled, 1 is in tension.
The load from the braces that frame into the beam-column connection will go directly into the column, so the beam will not receive the axial load from that brace (see cxn design sketch on pg 3-66). I believe if you were to have a one story V-braced frame then the beam would not need to be designed for an unbalanced load. It's my understanding that you design the beam for the unbalanced load when there is a brace framing into the middle of the beam. Someone correct me if I'm wrong.
I don't have the SEAOCC book, so I can't see the example problem, but I don't think you can consider the brace as bracing the beam for the reasons that you stated.