A tied-arch bridge is an arch bridge

Arch bridge

An arch bridge is a bridge with abutments at each end shaped as a curved arch. Arch bridges work by transferring the weight of the bridge and its loads partially into a horizontal thrust restrained by the abutments at either side...

 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 abutment

Abutment

An abutment is, generally, the point where two structures or objects meet. This word comes from the verb abut, which means adjoin or having common boundary. An abutment is an engineering term that describes a structure located at the ends of a bridge, where the bridge slab adjoins the approaching...

s 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

Soil

Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...

. 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

Fremont Bridge (Portland)

The Fremont Bridge is a steel tied arch bridge over the Willamette River located in Portland, Oregon. It carries Interstate 405 and US 30 traffic between downtown and North Portland where it intersects with I-5...

 in Portland, Oregon

Portland, Oregon

Portland is a city located in the Pacific Northwest, near the confluence of the Willamette and Columbia rivers in the U.S. state of Oregon. As of the 2010 Census, it had a population of 583,776, making it the 29th most populous city in the United States...

.

Issues

In a 1978 advisory issued by the Federal Highway Administration

Federal Highway Administration

The Federal Highway Administration is a division of the United States Department of Transportation that specializes in highway transportation. The agency's major activities are grouped into two "programs," the Federal-aid Highway Program and the Federal Lands Highway Program...

 (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

Electroslag welding

Electroslag welding ' is a highly productive, single pass welding process for thick materials in a vertical or close to vertical position. is similar to electrogas welding, but the main difference is the arc starts in a different location...

, 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.