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Abaqus General Beam Element - Torsional Rigidity 1

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tlewin

Structural
Jun 30, 2009
3
Hello,

When defining the profile for a generalized beam element in Abaqus, it asks for J. In the documentation, J is defined as torsional rigidity. However, from mechanics torsional rigidity is the product of the polar moment of inertia and the shear modulus, GJ. Does the program need J or GJ entered in that area?

Thanks,
Tom
 
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You should need to enter both, J is a property of the cross-section while G is a property of the material. Do not use GJ for the section input otherwise there will be a double-up of stiffness when it complies the stiffness matrix.

What material are analyzing?
 
I'm using post-tensioned circular concrete columns. Additionally, is there a way to enter a tapered section in Abaqus?
 
Post-tensioned circular concrete columns. Are they resisting large uplift forces. I know of a football stadium in my city that has four prestressed concrete columns in each of the four corners to hold down the roof for uplift wind. Is this similar to your design?

Be careful when modelling torsional stiffness of concrete sections. If you find after you have performed your analysis that the torsion attracted by the column by modelling 100% of the torsional stiffness (J) exceeds the cracking torsion there will be a large fall away in strength.

The MacGregor textbook on reinforced concrete suggests up to 90% reduction in torsional stiffness after the section has cracked.

I am not to savvy when it comes to modelling higher-order beam elements, but for a tapered section that you have described, I would subdivide the section into smaller sections each with different stiffnesses.
 
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