I have analyzed trusses for class work, but never in professional practice. In homework I am given the geometry and the applied loads, and asked to find the loads or stresses in members, not too difficult. But coming from a blank paper where do you start to design a truss or bridge, with so many types to pick from? For a simple beam I know what to do. For a complex truss, vs suspension bridge how is this decided before all the detail analysis starts? In the attached photo the bridge over the RR tracks (right side) is a truss. Over the road way a different type. I assume the RR  needs more clearance and puttinging the "depth of the beam" above the road way, provides the clearance. But why not a truss across the entire span, or the other type raised higher and extended. I assume there must be a significant cost difference but I have no practical experience to support this. Also I have seen bridges with a truss as show then other with the truss under the road bed or tracks for RR bridge. What drives the design?

Well, I got one good comment from JAG. I presume that means that he and I are the only folks left who don't know everything. I suppose that stands to reason; we are both old men! I had no idea the rest of you were so brilliant and over-informed!

To answer JAG's question, "What drives the design?" let me respond by saying "many factors." We might consider --

* cost and availability of materials (steel, concrete, stone, etc);

* cost and availability of a work force familiar with construction in various materials;

* cost and availability of machines to fabricate and erect the various bridge types;

* local and national laws and ordinances regarding height of structures, visibility blockage ("must not interfere with airport approach," "don't block the view of the mountain or the lake");

* past experience of each designer/erector (some have never worked in steel but have lots of experience in concrete and vice versa);

* opinions on aesthetics of various bridge types (suspension bridges look graceful, etc),

There are probably some more factors, but this is a start.

DrD