huangyhg

effective length of column
i would consider the total length for two reasons:
1. it is conservative to do so for slenderness/buckling computations
2. any load (from transverse beam, etc.) below the primary load at the top of the column is just an auxiliary load; i.e., it doesn't matter if it is a beam load or if you're hanging widgets off the side of the column...it is just another load on the same axial member.
as an me i would have a question for ron then:
if the traverse beam is lower than the truss and also provides a bracing force to prevent buckling, wouldn't it serve to stiffen the column and have an effect on the calculation?
or is it standard practice to iqnore the lateral bracing effect and just add the additional vertical load to the column support and buckling calculations?
don leffingwell
if the transverse beam is, in fact, lower than the truss but is framed normal to the plane of the truss and if the beam is itself connected to a flooring which is continuous over several beams then it would appear that one could count on the lateral restraint in that direction (at the intermediate point at the beam).
i would neglect the beam if it weren't fastened such as to prevent lateral movement of the column.
the only other reservation i have is during construction. is it possible to complete the roofing prior to placing the floor framing...in that case i would definitely use the entire column length.
don,
you are correct that it could provide some bracing, but we usually don't consider the affect unless in more than one direction (as qshake alluded). unfortunately, we are giving simplistic answers to partial scenarios. with more info we could probably consider more affect, though not always.
the column, in general, may not have the same 'i' in both directions. it is
therefore necessary to consider the effective length in the two directions
and compute the k*l/r factor for the two directions and take the worse
of the two. note that 'k' depends on the restraint provided at top and bottom.
so, just consideration of a length alone may not suffice. you may have lateral
diagonel bracing (wind bracing) as well in industrial structures. that will
reduce the k to 1.0 whereas the truss at the top may not provide a lateral
restraint at the top in that direction and you may need to use a k of 2.0 or 2.1.

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