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Re: lateral torsional buckling really happens!

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If I understand correctly the bottom flange buckled
due to uplift forces from the wind.  

Lateral torsional buckling occurs when the load is
applied to the compression flange.  Since the uplift
from the wind is applied to the tension face of the
beam these uplift forces will act to prevent lateral
torsion buckling from occurring.  Thus I believe that
lateral torsional buckling cannot occur when the
tension flange is loaded.

Salmon and Jonson has a good treatment of the theory
of LTB.

Likely what we are seeing is a case of the compression
flange buckling due to the lack of lateral support.  
I would imagine the failed structure would look very
similar but the moment at failure would be larger than
in the case of LTB.  In fact the uplift forces acting
on the top (tension)flange might actually act to
inhibit beam buckling thus increasing the failure
moment.

Mark Gilligan

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