kL/r for tension-only bracing in seismic design
kL/r for tension-only bracing in seismic design
(OP)
We've long had this discussion at work, and have never really come to a conclusion. So here's the thing.
For seismic design, most steel design codes allow kL/r for tension-only members of 300, and for tension-compression members the limit is 200. Given that compression braces can use a k value of 0.5 in the case of x-bracing, should I also use k = 0.5 for x-braces in tension? Obviously the compression member doesn't provide any lateral support to the tension member, which leads me to think k = 1 for the member in tension.
But if I use k = 1 for the tension side (L/300), and then k = 0.5 for the compression side (0.5L/300), the member in compression automatically gets a maximum kL/r of 150, thus becoming able to resist forces in compression, which renders the tension-only concept, at least in my mind, inapplicable. Is this some kinda catch 22? Does anyone have a reference explaining this irregularity?
For seismic design, most steel design codes allow kL/r for tension-only members of 300, and for tension-compression members the limit is 200. Given that compression braces can use a k value of 0.5 in the case of x-bracing, should I also use k = 0.5 for x-braces in tension? Obviously the compression member doesn't provide any lateral support to the tension member, which leads me to think k = 1 for the member in tension.
But if I use k = 1 for the tension side (L/300), and then k = 0.5 for the compression side (0.5L/300), the member in compression automatically gets a maximum kL/r of 150, thus becoming able to resist forces in compression, which renders the tension-only concept, at least in my mind, inapplicable. Is this some kinda catch 22? Does anyone have a reference explaining this irregularity?






RE: kL/r for tension-only bracing in seismic design
RE: kL/r for tension-only bracing in seismic design
It is only L/r - see AISC 360, section D1 (and the User note under D1)
RE: kL/r for tension-only bracing in seismic design
Dik
RE: kL/r for tension-only bracing in seismic design
RE: kL/r for tension-only bracing in seismic design
The slenderness is still a requirement due to compression loading, so any restraint that would force it into the next buckling mode would count as restraint for calculation of the slenderness in the Canadian code.