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Re: WOOD - Load bearing

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Bill Allen:

It appears that the buckling that you are concerned with would be akin to 
"Torsional Buckling" (or, "Torsional-Flexural Buckling").  Timoshenko 
("Theory of Elastic Stability," Second Edition, Timoshenko and Gere, Section 
5.4) states, "There are cases in which a *thin-walled* bar subjected to 
uniform axial compression will buckle torsionally while the longitudinal axis 
remains straight." [emphasis added].  In your case, it would be the edge on 
which the sheathing is attached which would remain straight, not the 
longitudinal axis.

I have run into only one case in which torsional-flexural buckling was a 
factor and that had to do with investigating the collapse of an aluminum 
extension ladder many years ago.  The rails were *thin-walled* sections 
(would you believe that the cross-sectional area of a ladder rail is only 
about 1/16 sq. in.?) and were eccentrically loaded (the shoes are attached to 
the web, not the shear center or centroid) and the bottom rung was about 
1-foot above the shoe.  The buckling occurred in a torsional mode between 
the bottom rung and the shoe.  The capacity in that mode was less than 
both the axial buckling capacity and the pure flexural buckling capacity.  (A 
word to the wise:  Be wary of the posted ratings and capacities of extension 
ladders!)

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