Page:America's Highways 1776–1976.djvu/437

 During the 1930’s, many other noted long suspension bridges were built, among them two beautiful spans, the Bronx-Whitestone in New York City and the Oakland Bay Bridge, and that fateful structure, the Tacoma Narrows Bridge over Puget Sound in Washington.

The Tacoma Narrows Bridge, like the Whitestone, was built with a plate girder instead of a stiffening truss. In addition, although 2,800 feet between towers, the cables were only 39 feet apart. Just over 4 months after being opened, it failed, literally twisting itself to pieces. The shallow stiffening girder, combined with the light dead load of the relatively narrow bridge, was unable to resist the aerodynamic forces that developed because of the steady winds of Puget Sound. This failure led to considerable research and improvement of future structures, such as the Mackinac and Verrazano-Narrows Bridges, as well as stiffening of existing bridges. Since the Whitestone Bridge showed signs of serious oscillation, a half truss was added to the top of the original girder. It is noteworthy that the George Washington, despite the absence of a stiffening truss, showed few symptoms of instability, possibly due to its great weight and width, as well as to the less constant wind patterns of New York Harbor.

Current technology seems to make it less necessary to build record spans. However, even today, a new steel arch with a record span of 1,700 feet is under construction at the New River Gorge in West Virginia.

These “great bridges” were all toll facilities built by bond issues rather than Federal-aid financing. After enactment of the Federal Aid Road Act of 1916, the Division of Highway Bridges and Culverts, established in 1910, became the Bridge Division of Public Roads. The immediate task of this Division was to set standards for design and construction of bridges to be constructed under the 1916 Act. Under the Act, roadway and bridge planning became a co-operative undertaking with the States initiating, planning, designing and constructing the projects and the Bureau of Public Roads (BPR) advising, approving, committing Federal-aid matching funds for satisfactory plans and specifications and paying such funds upon successful final inspection of the completed projects.

This cooperation was facilitated by the formation, in 1921, of the Operating Committee on Bridges and Structures of AASHO, known popularly as the AASHO Bridge Committee, which was composed of the bridge engineer of each State highway department and a designated bridge engineer from Public Roads.

The Tacoma Narrows Bridge, shouting oscillation 1 hour before its collapse.

Architect’s drawing of the New River Gorge Bridge in West Virginia. The hinged arch has a span of 1,700 feet, the longest of its kind in the world. Construction began in 1973. 431