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Re: Commercial Sign structure

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 >In a message dated 98-04-03 11:26:24 EST, Bruce Bates writes:
 >> I wouldn't recommend RISA-2D for this model, as it will not include
torsional
 >> effects, even if you break it up into several 2D models. You really need
to do
 >> a 3D analysis; also be aware that you need to consider warping stress as
well
 >> (in addition to pure  torsional shear) if your legs are not pipe shapes.
 
 >The problem described by Alex Nacionales appears to be sensitive to 
 >GLOBAL torsional effects only.  Resistance to St. Venant and warping 
 >torsion effects at the MEMBER level is not required to satisfy 
 >statics.  The member-level contribution to global torsional 
 >resistance would be insignificant in this case and reliance on such 
 >effects for truss type structures with open sections is generally 
 >not wise.  Consideration of member-level torsion IN THIS CASE 
 >appears to be a red herring.
 

I think my post was misunderstood. My point was that warping STRESS (at the
member level) should be considered (for an open section), not warping
STIFFNESS at the global level. That's why I said "you need to consider warping
stress".

I put together a simple model similar to the one Alex described with open
section legs (WF) and found that warping normal stresses equal to about 10% of
the major axis bending stresses were generated in the legs. Including or not
including warping stiffness calculations had no appreciable impact on the
model's global displacements.

Bruce Bates
RISA Technologies