column unbraced length 1

[cooksumrice]

Suppose I have a column 30 feet with a beam coming in at 15 feet with a knee brace 5 feet down connected to beam from the column. Would my unbraced length for the column be 10 or 15 feet?

Thanks

 

[dik]

Can you post a sketch?

Dik

 

[cooksumrice]

attached

 

[JoshPlumSE]

Upper unbraced length (above beam) is definitely 15 ft.

Lower unbraced length is more complicated. Does that knee brace a destabilizing or stabilizing effect on the column? In the past, my tendency would have always been to to say 10 ft. But, now that I've been working on a lot more structure stability questions, I'm not so sure anymore.

This is one of the reasons why AISC's Direct Analysis Method can be so useful. If you perform a 2nd order analysis on that column using reduced stiffness and such then you can be confident that the actual buckling will be accounted for. If I were to use the direct analysis method, then I'd use 10 ft as my unbraced length and feel pretty confident in my results.

If I did not use the Direct Analysis Method, then I would probably use 15 ft just to give me a better comfort level.

 

[SethGuthrie]

For behavior in this plane, I think it's best to design the bottom 10' unbraced segment based on the forces in that segment, then the 5' upper segment based on those forces there, but the bottom segment will probably control. In the other plane, the unbraced length might be longer, however, and that should also be checked.

 

[madmantrapper]

I see it braced in one direction (lets call it X), but what about 90 degrees from it (lets call it Y).