Tied-arch bridge

A tied-arch bridge is an arch bridge in which the outward-directed horizontal forces of the arch(es) are borne as tension by a chord tying the arch ends rather than by the ground or the bridge foundations. This strengthened chord may be the deck structure itself or consist of separate, independent tie-rods.

Thrusts downwards on a tied-arch bridge deck are translated as tension by vertical ties between the deck and the arch, tending to flatten it and thereby to push its tips outward into the abutments, like for other arch bridges. However, in a tied-arch or bowstring bridge, these movements are restrained not by the abutments but by the strengthened chord, which ties these tips together, taking the thrusts as tension, rather like the string of a bow that is being flattened. Therefore, the design is also called a bowstring-arch or bowstring-girder bridge.

The elimination of horizontal forces at the abutments allows tied-arch bridges to be constructed with less robust foundations; thus they can be situated atop elevated piers or in areas of unstable soil. In addition, since they do not depend on horizontal compression forces for their integrity, tied-arch bridges can be prefabricated offsite, and subsequently floated, hauled or lifted into place. Notable bridges of this type include the Fremont Bridge in Portland, Oregon,^{[citation needed]} and the first "computer-designed" bridge of this type, the Fort Pitt Bridge in Pittsburgh, Pennsylvania.

Both the tied-arch bridge and the self-anchored suspension bridge place only vertical loads on the anchorage, and so are suitable where large horizontal forces are difficult to anchor.

Some tied-arch bridges only tie a segment of the main arch directly and prolong the strengthened chord to tie to the top ends of auxiliary (half-)arches. The latter usually support the deck from below and join their bottom feet to those of the main arch(es). The supporting piers at this point may be slender, because the outward-directed horizontal forces of main and auxiliary arch ends counterbalance. The whole structure is self-anchored. Like the simple case it exclusively places vertical loads on all ground-bound supports.

An example is the Fremont Bridge in Portland, Oregon which is the second-longest tied-arch bridge in the world and also classifies as a through arch bridge. The Chaotianmen Bridge in Chongqing is a tied-arch, through arch and a truss arch bridge.

Contrarily, the Hart Bridge uses a cantilevered trussed arch, it is self-anchored, but its arch is non-tied. In particular the bridge deck is suspended, but does not tie the arch ends.

Tied arch bridges may consist of successively lined up tied arches in places where a single span is not sufficient. An example for this is the Godavari Arch Bridge in Rajahmundry, India. It has four separate supports on each pier and carries the South Central Railway Line of India. It was designed for 250 km/h rail services.