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RE: Seismic bracing system- Knee-braced Frames

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>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
>to determine, there is no specific prohibtion to them, yet there
>is no category to which they seem to be assigned, nor (code)
>literature dealing with them.  R=??  Are the ductility requirements
>and detailing per eccentric braced frames?
I had 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 ductility.
If you 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 jurisdiction.
>If Special Trussed
>Moment Resisting Frames are allowed (to impose a concentrated
>force where the bottom chord meets the column) would the same
>be true for a KBF?
The 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?
Hope this helps.

-----Original Message-----
From: EphHirsch(--nospam--at) [mailto:EphHirsch(--nospam--at)]
Sent: Wednesday, December 04, 2002 8:16 PM
To: seaint(--nospam--at)
Subject: Re: Seismic bracing system- Knee-braced Frames

Ephraim G. Hirsch, S1200 (CA)