Tied-arch bridge

Fremont Bridge in Portland, Oregon is the second-longest tied-arch bridge in the world, as well as a through arch bridge. The half-arches on the outer ends meet the bottom ends of the main arch on slender raised piers, where the horizontal forces balance at the ground level foundations.

A tied-arch bridge is an arch 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.[1][2]

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.[3] 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 as well as the first "computer designed" bridge of this type the Fort Pitt Bridge in Pittsburgh, Pennsylvania.[4]

A bowstring truss bridge is similar in appearance to a tied-arch; however, the bowstring truss behaves as truss, not an arch. The visual distinction is a tied-arch bridge will not have substantial diagonal members between the vertical members.

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.

Issues

In a 1978 advisory issued by the Federal Highway Administration (FHWA), the FHWA noted that tied-arch bridges are susceptible to problems caused 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.[5]

References

  1. ↑ See, e.g., U.S. Patent 14,313 (February 26, 1856) issued to P.C. Guiou of Cincinnati, Ohio, for a Truss Bridge, titled on specification as "Girder for Bridges."
  2. ↑ For a description of how bowstring arch and bowstring girder bridges are related, see Margot Gayle & Carol Gayle, Cast-iron Architecture in America: The Significance of James Bogardus, pages 28-29 (W. W. Norton & Company 1998).
  3. ↑ For a nontechnical exposition, see Gordon, JE (1978). Structures; or Why Things Don't Fall Down. London: Penguin Books. p. 208f. ISBN 978-0-306-40025-4. OCLC 4004565.
  4. ↑ "Pittsburgh's bridge". Travel Channel. Retrieved June 14, 2016.
  5. ↑ Federal Highway Administration (1978-09-28). "TIED ARCH BRIDGES: T 5140.4". Retrieved 2008-07-22.
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