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Re: Seismic bracing system- Knee-braced Frames

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>From Army Corps of Engineers TI 809-04, Seismic Design for Buildings, 

Chapter 7:

...
(7) Knee-braced frames (KBF).
(a) Definition. A KBF is an assembly of a
beam, a column, and a brace whose ends are
significantly offset from the beam-column joints.
The braces in CBFs are either truly concentric, or
have small eccentricities with the beam-column
joints; accordingly, they induce forces that are
primarily axial, while the braces in KBFs have
substantial eccentricities, and induce significant
shearing, and flexural, as well as axial, stresses in the
columns and beams.
(b) Function. Knee braces were often used
in the past to stiffen beams and to provide a measure
of lateral stability. Their popularity in recent years
has decreased markedly, particularly in zones of high
seismicity, because their seismic behavior has
become recognized as potentially dangerous.
(c) Design considerations. There are two
concerns with KBFs. The first concern involves
gravity load: any change in the load on the beam after
the brace is connected induces forces in all the
components of the frame; moreover, the brace has a
prying effect that can produce surprisingly large
forces in the beam-column joint. The sequence of
erection and the further application of superimposed
loads must be carefully controlled. The second
concern involves seismic loads: another set of loads
is applied, and while the brace does stiffen the frame,
its as-built ultimate capacity may cause bending in
the column of sufficient magnitude to cause collapse.
(d) Design criteria. KBFs shall be designed
in accordance with Section 9.4 of the AISC Seismic
Provisions, and the use of KBFs shall be restricted to
roof structures or to unoccupied storage or other
utilitarian buildings with Performance Objective 1A,
not over two stories in height.
...

I have already asked Charlie for clarification in the next set of seismic
provisions.

-Keith Fix, PE
-Little Rock, Arkansas

--- EphHirsch(--nospam--at)aol.com wrote:
> 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?  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?   etc., etc. Opinions and
> 
> guidance welcomed.  Thank you.
> 
> Ephraim G. Hirsch, S1200 (CA)
> 
> 


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