I would say it depends on the analysis itself, the loading conditions and the results you want to to obtain. The "go to" approach in most frame applications is to just represent the tubes as beam elements with the proper section area set up. But, if you need a more detailed (local) analysis or you need to evaluate and obtain shear results (directly from ASM) due to torsion, beam elements are not the way to go (I'm still curious about this limitation in ASM!). If this is the case, I would say that it is appropriate to use shell elements to represent the tubes. I would not recommend solid (brick nor tet's) for this kind of geometry. Hope It helps.
