A suspension bridge is a type of bridge that has been created since ancient times as early as 100 AD. Simple suspension bridges, for use by pedestrians and livestock, are still constructed, based upon the ancient Inca rope bridge. One of the oldest of engineering forms, suspension bridges were constructed by primitive peoples using vines for cables and mounting the roadway directly on the cables. A much stronger type was introduced in India about the 4th century AD that used cables of plaited bamboo and later of iron chain. Suspended well from two high locations over a river or canyon, simple suspension bridges follow a shallow downward arc and are not suited for modern roads and railroads. Advances in materials and design led to the development of the suspended-deck suspension bridge, a modern bridge capable of carrying vehicles and light rail. Instead of the deck following the downward arc of the main load-bearing cables (or chains), these cables are suspended between towers, and vertical suspender cables carry the weight of the deck below, upon which traffic crosses. This arrangement allows the deck to be level or to arc slightly upward for additional clearance.
The design of the modern suspended-deck suspension bridge was developed in the early 19th century. Early examples include James Finley's bridge at Jacob's Creek, in Pennsylvania, in 1801, the Menai and Conwy Suspension Bridges (both opened in 1826) in north Wales; and the first Hammersmith Bridge (1827) in west London. This type of bridge is the only practical type suitable for very long spans or when it would be hazardous to maritime traffic to add central supports. One example frequently cited for its aesthetic appeal is the Golden Gate Bridge at the entrance to San Francisco Bay.
The suspension cables must be anchored at each end of the bridge, since any load applied to the bridge is transformed into a tension in these main cables. The main cables continue beyond the pillars to deck-level supports, and further continue to connections with anchors in the ground (An exception is the Royal Albert Bridge (1859) where the anchors are replaced by an arch between the columns.) The roadway is supported by vertical suspender cables or rods. In some circumstances the towers may sit on a bluff or canyon edge where the road may proceed directly to the main span, otherwise the bridge will usually have two smaller spans, running between either pair of pillars and the highway, which may be supported by suspender cables or may use a truss bridge to make this connection. In the latter case there will be very little arc in the outboard main cables. Without special design this type is generally not suited for heavy rail applications as the bridge will flex under the concentrated load of a locomotive.