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Knee Braced Frame-Effective Column Length

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Dear James Fulton:

If the depth of the knee connection is short compared
to the length of column, I believe you can compute the 
flexibility of the connection and add it to the flexibility
of the girder; then you can use the AISC chart.

At some point the axial load in the column segment 
within the knee will significantly reduce its flexural
stiffness. For example, if there is no axial load, the 
flexural (that is, rotational) stiffness of that segment is 
3EI/L (three EI over L), assuming a pinned top.  If the 
axial load is the Euler buckling load of the segment, 
its stiffness is zero.  If it is half the buckling load, the 
stiffness is about 1.83EI/L (one point eight three EI/L).
A computer program which uses the geometric 
stiffness matrix will be able to approximate this effect.
Classical stability methods can also be used.  

Also, story-stability effects (the B-sub-two moment
magnification factor, LRFD equation H1-5 or H1-6 in the 
first edition, C1-4 or C1-5 in the 1993 Specification) are 
bound to be significant for a knee-braced structure 
subjected to high seismic loads.  The LRFD commentary
has a thorough explanation of how this can be used to 
modify the K-factor, although moment magnification is 
probably of more use.

I hope this helps.

Rafael Sabelli