the first step you need to do is understand the type of bridge structure it is?
Suspended Cable Bridge? arch? Continous Beam?
What are the predominent forces being used in each of the structures?
Is it Compression or Tension or a combination of both.
Once you have those questions answered we can look at each material and anlyse each based on each materials capacity to carry the various forces.
[sorry i'm abit tired.. but i hope those few points help?]