Well yes there is the "user defined section" possibility in the Structural Modeler. However, the expansion joint presents a problem, and I'm sure there will be others. So while you could use the Structural Modeler to create an element or two (to be included in a piping model), I think this would be overly cumbersome to use for the entire model.
Aren't there software packages out there that specialize in "ducting"?
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Regards,
Richard Ay - Consultant