Dear all,
I tried to do modal analysis by creating beam elements with beam cross-sectional properties. However, when the modal analysis is done I get only bending modes not the torsional modes. How could I get the torsional modes?
Thanks in advance.

Question

Dear all, 
 
 I tried to do modal analysis by creating beam elements with beam cross-sectional properties. However, when the modal analysis is done I get only bending modes not the torsional modes. How could I get the torsional modes? 
 
 Thanks in advance.

Arthur Hickson · Accepted Answer

Hi,

the reason is that you are using CBAR elements with default settings. This is rather a historical thing but Nastran tends to keep historical default values so that something that was appropriate 20years ago as the default, maybe due to compute power and the need to simplify as much as possible for speed reasons, is still the default.

If you look at the Linear Static user guide on CBAR elements:

https://help.hexagonmi.com/bundle/MSC_Nastran_2022.2/page/Nastran_Combined_Book/linear/msnels/TOC.The.CBAR.Element.xhtml

you will observe the note:

The torsional mass moment of inertia is included in the BAR mass matrix by using NASTRAN BARMASS (398) =1. Note, by default , this term will not be included . For both values of PARAM COUPMASS, this term is added.

Instead of editing the input deck ( or adding in an include card with the entry NASTRAN BARMASS=1 ) in the Patran Analysis setup Solution type/ Solution parameters you can turn on coupmass with the coupled mass entry:

alternatively in the Translation Parameters you can request conversion of all CBARS to CBEAMS. CBEAMS are more generic and do not have the compromises of simpler 1d beam types, computationally more expensive but that is not a real problem these days.

Dear all, I tried to do modal analysis by creating beam elements with beam cross-sectional properties. However, when the modal analysis is done I get only bending modes not the torsional modes. How could I get the torsional modes? Thanks in advance.