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kL/r for tension-only bracing in seismic design

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pmastro

pmastro

Structural
Jan 8, 2013
3
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?
 
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There are papers which discuss checks to see if you indeed can treat the intersection point as a braced point. It is not an automatic thing, as you need the usual minimum strength and stiffness to count it as a brace, and a member with kl/r = 300 may not provide that, at least not completely (K may be .6 or so).
 
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Tension design doesn't use k.

It is only L/r - see AISC 360, section D1 (and the User note under D1)
 
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CSA S16, the Canadian Steel Code, permits kl/r to be 300 except that a greater value can be used if suitable provisions can be made. The seismic component of S16 permits a maximum kl/r to be 200 in 27.5.3.1.

Dik
 
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Let's look at it this way, instead of steel section substitute with cables, kl/r does not apply. However I recall certain building codes not permitting the use of tension-only systems for seismic design.
 
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As dik said, the Canadian code gives you a most stringent slenderness limit for braces if they're part of the seismic force system. You can design them as tension braces, even against seismic forces, but you want enough stiffness so that they'll take a reasonable amount of energy when they buckle. You still have to design the connections to resist the compression capacity of the member.

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.
 
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