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sidesway web buckling - AISC 9th edition

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Ken Peoples wrote:

. > I am checking for stiffener requirements of a beam
. > supporting a column.  I would think that this is a
. > simple one but maybe there is more to it:  I have a
. > 48' long beam with a column sitting on it at 24'.  The
. > beam is 24" deep and is also supporting 18" and 16"
. > beams at 6' spacing (top of all beams are equal).  In
. > order to check the sidsway buckling per K1.5, I need
. > to know what "l" is.  "l" is defined as the longest
. > laterally unbraced length of either flange.  I think
. > this should be 48' for my case but I just wanted to
. > make sure that it would not be 24'.

. > Can someone direct me to a good example of the
. > calculations required to determine if stiffeners are
. > required for such a condition (I am not talking about
. > a moment connection - just a concentrated load like a
. > column sitting on top of a beam).  I'm pretty sure
. > that I am doing it correctly but the allowable loads
. > that I come up with (without using a stiffener)are a
. > lot higher than I would have expected.  It makes me
. > wonder why I see so many stiffeners under concentrated
. > loads.

Ken,

First I will comment on stiffeners under the beam supported column.  I would 
use at least two on each side of the beam web.  Take a look at the beam cross 
section and imagine the bending resistance the web would have to even a 
slightly eccentric load.

With regard to the unbraced length of either flange, what is stopping you 
from bracing the bottom flange where each floor beam is connected?  Combine a 
shear connector and a stiffener at each location where beams connect.

Hope this helps.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

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