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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.

Like other responders, I agree that this is really a 3D problem.  
RISA-2D and other 2D programs cannot capture this type of global 
three-dimensional torsional behavior.  While it is possible to solve 
determinant space trusses by hand using statics, it is usually more 
practical (economical of fee) to use computers.

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Michael Valley                                   E-mail: mtv(--nospam--at)skilling.com
Skilling Ward Magnusson Barkshire Inc.                  Tel:(206)292-1200
1301 Fifth Ave, #3200,  Seattle  WA 98101-2699          Fax:        -1201