a cantilever bridge is a bridge supported by a beam that is anchored at one end and unsupported at the other end. This kind of bridge is not commonly constructed today. An example is the Interstate 80 Carquinez Bridge near Vallejo, California.
cantilever is a bracket or arm that juts out. Cantilever bridges are made up of two such brackets. A diving board is a good example of a cantilever that is anchored at one end and free at the other. The cantilever design provides the strength and rigidity called for in heavy frieght and railway bridges, provides economy of materials and simplifies construction. The Marquam and Ross Island bridges are Portland's two major cantilever bridges.
A cantilever bridge is a bridge built using cantilevers: structures that project horizontally into space, supported on only one end. For small footbridges, the cantilevers may be simple beams; however, large cantilever bridges designed to handle road or rail traffic use trusses built from steel. The steel truss cantilever bridge was a major engineering breakthrough when first put into practice, as it can span distances of over 1500 feet without slumping, and can be more easily constructed at difficult crossings by virtue of using little or no falsework.