I am not a structural engineer, but am interested and involved in it's concepts and some calculation processes.
When it comes to structural constructions, you might take look at this: It's a bridge created completely in Rhino by using a Grasshopper.
Rhino is a 3d modeller. The power that it gets from Grasshopper is that you can model something in a way that each part of your model could be changed by a simple slider. So either an engineer could be a designer by altering those sliders, or he can even let the Grasshopper decide the design outlook, by applying some criteria - for example you could ask for the shape that has the smallest deflection or mass, elastic energy. It could define the places where it would be most convenient to put supports, so that some of the upper mentioned goal could be achieved, and so on. Check some of these videos: http://www.youtube.com/watch?v=33EIvtbONsk