Tied arch bridge

A tied arch bridge is a truss bridge in which the outward-directed horizontal forces of the arch, or top chord, are borne as tension by the bottom chord (either tie-rods or the deck itself), rather than by the ground or the bridge foundations. Thrusts downward on such a bridge's deck are translated, as tension, by vertical ties of the deck to the curved top chord, tending to flatten it and thereby to push its tips outward into the abutments, like other arch bridges. However in a tied-arch or bowstring bridge, these movements are restrained not by the abutments but by the bottom 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 often 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. [For a nontechnical exposition, see cite book |title= Structures; or Why Things Don't Fall Down|last= Gordon|first= JE|authorlink= |year= 1978|publisher= Penguin Books|location= London|isbn= 9780306400254|oclc= 4004565|pages= 208f ] 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. A notable case of these procedures was the installation of the Fremont Bridge.

Issues

In a 1978 advisory issued by the Federal Highway Administration (FHWA), the FHWA noted that tied arch bridges are susceptible to problems called by poor welds at the connection between the arch rib and the tie girders, and at the connection between the arch and vertical ties. In addition, problems with electroslag welds, while not isolated to tied arch bridges, resulted in costly, time-consuming and inconveniencing repairs. The structure as a whole was described as nonredundant; failure of either of the two tie girders would result in failure of the entire structure.cite web |url=http://www.fhwa.dot.gov/legsregs/directives/techadvs/t514004.htm |title=TIED ARCH BRIDGES: T 5140.4 |author=Federal Highway Administration |date=1978-09-28 |accessdate=2008-07-22]

References

Gallery of Tied Arch Bridges