TRUSS ARCH BRIDGE

A 'truss arch bridge' combines elements of the truss bridge and the arch bridge. The actual resolution of forces will depend upon the design. If no horizontal thrusting forces are generated this becomes an arch-shaped truss, essentially a bent beam — see moon bridge for an example. If horizontal thrust is generated but the apex of the arch is a pin joint, this is termed a 'three-hinged arch' - see The Iron Bridge for an example. If no hinge exists at the apex, it will normally be a 'two-hinged arch'.
In the ''Iron Bridge'' shown below, the structure of each frame emulates the kind of structure that previously had been made of wood. Such a wood structure uses closely fitted beams pinned together, so the members within the frames are not free to move relative to one another, as they are in a pin-jointed truss structure that allows rotation at the pin joint. Such rigid structures (which impose bending stresses upon the elements) were further developed in the 20^th century as the Vierendeel truss.

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Some bridges of this type

Some bridges of this type

★ The Iron Bridge (1791), the first metal bridge of any kind, still standing.

★ Maria Pia Bridge (1877), Gustave Eiffel's pioneering two-hinge arch.

★ Navajo Bridge An older (1929) and newer (1995) bridge of the same general construction, each built as unsupported cantilevers joined with a central pin.

★ I-35W Mississippi River bridge (1967) bridge that collapsed in 2007 in Minneapolis, Minnesota, USA.