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# A CBEAM question

## A CBEAM question

(OP)
Been trying to learn more abou this. Can CBEAM element compute shear stresses due to torsion?

1. I think CBEAM element is not capable of computing shear stresses due to torsion alone. For example, I modelled a simple circular C/S using CBEAM element. Apply a moment along axis of beam on one end and fix the other end. The analysis will run but the stress outputs are shown 0. Perhaps, by specifying scalar points (Entries SA & SB in PBEAM), Nastran can compute shear stresses due to torsion?

2. Does anyone know how to create Scalar Points (SPOINT) in Patran GUI? In Femap, scalar points are created automatically but I am not able to find out how to create/specify them in Patran.

### RE: A CBEAM question

Well, the answer is no, and this is a well documented fact in the nastran element reference guide that a CBEAM cannot compute shear stresses due to torsion as NASTRAN does not know your cross-section when 'J' is defined on the PBAR/PBEAM card.

The usual CBAR/CBEAM stress outputs are (P/A)plus/minus(Mc/I) where 'c' gets calculated from the user specified stress recovery points, but torsional stresses are highly cross-section dependent and if you happen to have a non-circular cross-section you'll have the effects of warpage causing the stress distribution to be non-linear.

You can do it by hand using the CBAR/CBEAM force output but if its just a simple solid circular section that you're modeling then you can use a CROD element which computes torsional stress as c*T/J where 'c' is a user specified scale factor in the PROD card. So in this case your 'c' would be the radius of the circular section to compute the correct torsional stress from nastran.

### RE: A CBEAM question

(OP)
Thanks for the reply. I wanted a confirmation about CBEAM's inability to compute shear stresses due to torsion.

>> You can do it by hand using the CBAR/CBEAM force output.

Can you please elaborate more on the above?

### RE: A CBEAM question

Do it by hand meaning do a hand-calc using the applied torque or the internal force from the CBAR/CBEAM element to evaluate the shear stress.

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