Hi Old Horse,
If you only need to draw beams, coloumns, plates, etc, then most of 2D CAD programs can assist you well, especially AutoCAD, it is mainly used in civil engineering adn architec. I have seen many cranes and elevators were designed with AutoCAD.
If you have concerning to automate your design later, it means you jaust only give some design change for your previous design and the CAD should give you this result, then AutoCAD is the answer, it has AutoLISP and C++ Application Programming Interface (API), that you can costumize it.
I hope, this would make it clearer for you.
regards