The vertical-lift bridge over Arthur Kill between Staten Island in New York City and New Jersey has a span of 170 m (558 ft) and can be raised 41 m (135 ft) above the water. Swing bridges have the advantage of not limiting the height of passing vessels, but they do restrict the horizontal clearance, or width, of passing ships. The longest swing-bridge span is that of a railroad and highway bridge crossing the Mississippi River at Fort Madison, Iowa. This bridge has a span of 166 m (545 ft).
H Floating Bridges
Floating bridges are formed by fastening together sealed, floating containers called pontoons and placing a roadbed on top of them. A pontoon typically contains many compartments so that if a leak occurs in one compartment, the pontoon will not sink. Some floating bridges are constructed using boats or other floating devices rather than pontoons.
Floating bridges were originally developed and are most widely used as temporary structures for military operations. For everyday use, floating bridges are popular when deep water, bad riverbed conditions, or other conditions make it difficult to construct traditional bridge piers and foundations. A concrete-pontoon bridge carries a highway across Lake Washington, near Seattle, Washington. It consists of 25 floating sections bolted together and anchored in place and a span that can be opened to permit the passage of large ships. The floating section of the bridge is 2.3 km (1.4 mi) long.
I Combination Bridges
Combination bridges include crossings consisting of several types of bridges or both bridges and tunnels. For example, the Chesapeake Bay Bridge-Tunnel in Virginia includes two tunnels that are each 1.6 km (1.0 mi) long along its 28 km (17 mi) length from shore to shore. The Triborough Bridge in New York City is actually a network of bridges connecting the boroughs of Queens, Manhattan, and the Bronx. These bridges meet over Randall’s Island. Seven truss spans stretch over Bronx Kills, and three truss spans and a vertical lift extend over the Harlem River. A viaduct and a suspension bridge also make up part of the Triborough Bridge.

III BRIDGE PLANNING AND CONSTRUCTION
New bridges are built either to replace old structures that no longer meet the demands of modern traffic or to cross obstacles on a new transportation route. Old bridges are replaced when repairs cannot be made economically or when traffic becomes too heavy for the old bridge. New transportation routes are built when traffic levels have outgrown the capacity of existing routes or simply to make it faster to get from one busy place to another.