Excerpt:
The first iron bridge was constructed in 1779 at Coalbrookdale, a cast-iron arch. Cast iron then began to replace stone, quite successfully in arch bridges, but the attempt to use cast-iron beams ended in the failure of Robert Stephenson's Dee Bridge. Thereafter, cast iron was used for compressive members only, and engineers turned to wrought iron as a tougher, more reliable material, using it first in massive tubular girders riveted together from the small plates which were all that were available at the time. Wrought-iron link chains were used for suspension bridges by Telford and Brunel.
In the United States, iron was expensive and largely imported from Britian because of the primitive state of the domestic iron industry. Wood, however, was abundant and cheap, and it was good hardwood that made an excellent material of construction. Wood, therefore, was the material most often used for even major bridges, with spans greater than 50 feet. We are not talking here of the trestles and king-post trusses used for minor bridges, but bridges that crossed the rivers and gorges that abounded. Two kinds of bridges were in common use, the lattice truss and the trussed arch, of which there were as many varieties as there were builders. Bridge builders were self-taught craftsmen and amateurs, for the most part.