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RE: Seismic bracing system- Knee-braced Frames
- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: RE: Seismic bracing system- Knee-braced Frames
- From: "Carter, Charlie" <carter(--nospam--at)aisc.org>
- Date: Fri, 6 Dec 2002 18:54:00 -0600
>Well, this is a question I've been long
delaying to pose to
>Charlie Carter, so thank you for raising
it. Just where do
>Knee-braced Frames get defined? As far
as I've been able
determine, there is no specific prohibtion to them, yet
no category to which they seem to be assigned, nor (code)
>literature dealing with them.
R=?? Are the ductility requirements
detailing per eccentric braced frames?
to think about this one a bit. I don't know of any specific prohibition on knee
bracing. But there is also no code provision that explicitly defines an R factor
for such a system. If are in Seismic Design Categories A, B and C, you can use
R=3 and do just about anything you want. The forces are higher, which
theoretically offsets the transition to a system of normal
wanted to use knee bracing with a higher R factor, you'd have to justify it, I
think. Would it behave like an EBF? moment frame? Might have to do testing.
Would it be worth it? One possible alternative might be to switch from an
R-factor-based approach to the FEMA 356 M-factor approach, which is useful for
systems that do not have definable elements that will act as a fuse. I think
you'd have to sell thatone to the authority having
>Moment Resisting Frames are allowed (to
impose a concentrated
>force where the bottom chord meets the
column) would the same
true for a KBF?
STMF has fuse members that protect the column. I'm not sure it would be possible
to protect the column with K-bracing. Would it?
Ephraim G. Hirsch, S1200