Check out this website from Tata Steel Construction, a British firm:
http://www.tatasteelconstruction.com/en/
The educational section is under the "reference" heading. There's a specific "Student Guide to Steel Bridge Design" somewhere on the site, but I don't remember where exactly. The site is a good resource on all things related to steel construction.
You're on your own for finding resources on reinforced concrete beams, but I will give you a hint. Think about where some bridges are located and in what climates, and think about why it wouldn't be a good idea to use steel beams in those locations.