Hi,
I took a look at your model. I am not sure why the general cross-section is not working, so I have asked someone else to take a look at it. In the mean time, you may be able to use one of the shapes from the AISC library. It appears that your shape is very close to an L8x4x1/5 angle [4 inch (101.6 mm) x 8 inch (203.2 mm) x 0.5 inch (12.7 mm)].
The second thing to do, whether using a pre-defined shape or the general cross-section shape, is to use a beam offset. By default, the centroid of the theoretical cross-section is located at the line drawn as the beam element, so this places part of the cross-section above the shell and part of it below the shell. You want to specify the offset so that the theoretical centroid is the proper distance above the midplane of the shell.
Last, you may want to change the thickness of the shell. 15 m thick seems too large
😉
John Holtz
Product Design Engineer, Algor Simulation
Autodesk, Inc.
John Holtz, P.E. Global Product Support
Autodesk, Inc. If not provided already, be sure to indicate the version of Inventor Nastran you are using!"The knowledge you seek is at knowledge.autodesk.com" - Confucius 😉