huangyhg

two bracing questions on the 13th edition of aisc.
first off, i'm in the process of going from asd 9th to asd 13th, so my knowledge of the 13th edition has gaps (ergo my question).
1. what (by the 13th edition) constitutes a braced frame (in terms of stiffness)? until now, i have used various theorems/rules picked up [over the years] from the likes of yura and my salmon and johnson text book on steel structures. (from the latter the equation k=sigpn/h comes. where sigpn =all loads to be carried in columns resisting sway; h= story height; and k=required story stiffness. yura had a similar equation.) appendix 6 has a similar approach, but it is more for individual members.
2. on p.16.1-193 (sect. 6.3) the required bracing stiffness is given for lateral [beam] bracing. (i am considering my system as nodal bracing.) the thing about this is: in an evaluation of the stiffness of these points, the weak-axis strength of the beam itself is contributing significantly [the beams are pretty heavy: in the w36 range]. i kind of doubt this is what was intended; it's sort of like saying it is bracing itself. thoughts?
thanks in advance!

for question 2, you can't count on the weak axis bending of the section to brace the beam because that strength is already part of the ltb equation.
for question 1, i don't think there is a stiffness requirement. any braced frame is inherently stiffer than a moment frame by many factors.
that's what i figured for question #2.
because many frames are mixed (braced at some stories, not braced at other stories) it's not clear (always) what you can/would call a [story in a structure] sway or nonsway frame. that is the idea (for example) behind aci 318's stability index "q" (see sect. 10.11.4.2). i'm just not sure what is official for the 13th edition of aisc.

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