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Hollow rectangular Beam  rigidity and max torque 
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#1
Apr2111, 10:27 AM

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Revision that is turning into a learning from scratch session. Some help would be welcome!
 I have a hollow rectangular beam. 0.150m by 0.110m with uniform thickness walls at 0.003m. It is 3m long.  It has a modulus of rigidity (G) of 27 GigaPascals.  It has a max permitted shear stress of 200 MegaPascals. What is the torsional rigidity? What is max permitted torque (with a safety margin of 2.0)? What would be the twist angle for the 3m length? Torsional rigidity = G x second area of inertia. I have G so need 1xx. 1xx = Each side at ((0.003x0.110^3)/12) = 3.33 E07 Top & base each at (using parallel axis theory) ((0.003 x 0.144^3)/12) + (0.003 x 0.144 x 0.062^2) = 2.41 E06 Add 4 sides = 5.48 E06 Torsional rigidity = 27 E09 x 5.48 E06 = 148,000 (units???) Is this right and what are the units? As for the other 2 parts – I am lost. Any pointers? 


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