Spotting Bridges 101 - A Basic Guide To Bridge Design

Frequently passing under bridges for a living and over them in the family sedan we, like most mariners, have a fascination with bridge design. In fact one of the most popular posts on gCaptain is “The 5 Most Ambitious Bridge Designs Of Today“. Today we bring you a great link from the Bridge and Tunnel Authority of Pittsburgh Pennsylvania titled “A Spotter’s Guide To Bridge Design“. Here’s a preview:

A cantilever is a structural member which projects beyond its support and is supported at only one end. Cantilever bridges are constructed using trusses, beams, or girders. Employing the cantilever principles allows structures to achieve spans longer than simple spans of the same superstructure type. They may also include a suspended span which hangs between the ends of opposing cantilever arms.

Some bridges which appear to be arch type are, in fact, cantilever truss. These may be identified by the diagonal braces which are used in the open spandrel. A true arch bridge relies on vertical members to transfer the load to the arch. Pratt and Warren bracing are among the most commonly used truss types.

The classic cantilever design is the through truss which extends above the deck. Some have trusses which extend both above and below the deck. The truss configuration will vary.