There is no need to use AutoCAD if you have Inventor. Create part, derive, select your assembly, save.

Do you really have an assembly of surfaces though? Or when you say plate you mean solid models of plates with a physical thickness?

You can use Inventor assemblies in Simulation though, however I prefer to use multi-body part files that work the same in simulation. I can only think that simulation cannot find the part files that are associated with your assembly.

If you are deriving into a part file and you want to run a shell model then I prefer to extract my mid-surfaces manually in Inventor before bringing into Simulation.

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Do you really have an assembly of surfaces though? Or when you say plate you mean solid models of plates with a physical thickness?

Yes, the entire model is a plate model (zero thickness).  Tech support at Simulation said the problem with plate assemblies is a known issue.

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If you are deriving into a part file and you want to run a shell model then I prefer to extract my mid-surfaces manually in Inventor before bringing into Simulation.

Are you suggesting building the models as solids and then extracting mid-surfaces before bringing into Simulations?  Are there any problems or gaps that develop where an edge meets a surface?

ok, fair enough. Next time try add all surface bodies in the one part, it will be easier to work with too.

No I am not, that was only if you had solids. Yes there can be so that is why I did not recommend it.

All you need to do is what I initially said: Create new part, derive, select your assembly, save.

HIH

I'll give it a try...thanks

Becareful when using plates/shells in this software. Although you specify a plate thickness in the element definition, the software only uses this as a mathematical quantity when forming the stiffness matricies. It does not acount for the shell thickness in your model, which maybe important when doing for wxample a modal analysis. Your alternitave is to model as solids but deal with the order of magnitude increase in solve time...

Thanks for the tip.  We most do static stress analysis so I do not think it has become an issue yet.

I just installed Simulation 2013 and the issues with plate assemblies have been fixed.

Yes, it is listed in the 2013 release notes.

http://wikihelp.autodesk.com/Simulation_Mechanical/enu/2013/Help/0031-Autodesk31/0886-Release_886/0887-Introduc887/0888-V2013_Re888