Page:America's Highways 1776–1976.djvu/435

 lifting heavy members. Relatively light pneumatic riveting guns were developed for driving field rivets so that only small and minor structures had hand-riveted or bolted connections.

Most important of all, these construction improvements were documented in the literature, and engineers and technicians well versed in their application were available for bridge construction.

About 1870 engineers began developing shear and moment analyses to determine stresses for structural design. Before that, structures were “proof loaded” before acceptance, that is, highway bridges were subjected to carts loaded with pig iron or stone, while railroad bridges were subjected to two locomotives in tandem. While this primarily tested the quality of workmanship and protected against the most gross errors, it did not provide for overloads or fatigue failure. These proof-load requirements persisted in the specifications until the turn of the century.

However, engineers were aware that the proof-load was no substitute for a rational design and, as their knowledge grew, began to write design specifications. Whipple had already recommended a design load of 100 pounds per square foot, equivalent to the whole roadway area covered with men. In 1875, as a result of a series of bridge failures, an American Society of Civil Engineers committee recommended live loading values for both railway and highway bridges. For highway bridges, loads varying from 40 to 100 pounds per square foot were given, depending on span and type of usage. While railroad and “bridge companies” issued their own specifications for highway bridges, a concerted effort for a specification of national scope had to await the Operating Committee on Bridges and Structures of the American Association of State Highway Officials in the 1920’s.

In 1910, the Office of Public Roads established a Division of Highway Bridges and Culverts. This new division, upon request from a State or local authority, assisted in bridge design and construction and reviewed and advised on bridge plans and specifications prepared by States, local authorities or bridge companies. It also prepared and published bulletins on highway bridge and culvert design and construction, including typical plans for reinforced concrete culverts, abutments and piers and discussions on the use and design of steel truss and plate girder bridges.

In 1913, the Office of Public Roads issued Circular No. 11, Typical Specifications for the Fabrication and Erection of Steel Highway Bridges. This circular specified a live loading of interurban electric cars or a 15-ton road roller plus a uniform live load on the portion of the bridge deck not occupied by the roller.

It was said earlier that the opening of the Eads and Brooklyn Bridges was the beginning of the great bridge building era. As noted, it continued slowly with the Williamsburg and Manhattan Bridges.

The Hell Gate Railroad Bridge in New York City, built in 1916, and designed by Gustav Lindenthal, spanned 977½ feet between support pins and was the longest and heaviest arch in the world. When it was built, it was considered an engineering marvel because of the location conditions and the long span. It was to have a profound effect on steel arch design.

At almost the same time, the long-span continuous truss had finally come to America in the form of two continuous spans of 775 feet over the Ohio River at Sciotoville, Ohio. (Strictly speaking, a continuous truss had been used in the approaches to an earlier structure, but Sciotoville was the first use of it for main river spans.) Coincidentally, the longest simple span truss, 720 feet, was built over the Mississippi at Metropolis, Illinois. Both of these were railroad bridges.

Not long after, a major cantilever truss with two cantilever spans of 1,100 feet, designed by David B. Steinman, crossed the Carquinez Straits near San Francisco, California.

The long-span records continued to fall through the 1920’s and 1930’s. Detroit’s Ambassador Bridge, 1,850-foot suspension span surpassed the Quebec cantilever’s 1,800 feet. In 1931, Othmar Amman’s arch over the Kill van Kull between Staten Island,New York, and Bayonne, New Jersey, set a new arch record of 1,652 feet.

In that same year (1931), the George Washington Bridge was opened. This bridge across the Hudson River in New York City reached 3,500 feet between towers. At one bound, it had virtually doubled the Ambassador’s span. Not only that, but it was one of the strongest bridges ever built, with the greatest capacity—eight highway traffic lanes on the upper deck and provision for future rapid transit below (ultimately six highway lanes were added instead) when the stiffening trusses would be added. While the increase in technical knowledge was one factor, the most important reason behind this great leap in span length was, as Roebling had realized years before, the great improvement in the quality of the steel wire.

Since then, the George Washington Bridge has been surpassed several times in span but not yet in strength and capacity. The three longest spans are the 4,200-foot center span of the Golden Gate in San Francisco (1937), the 4,260-foot span of the Verrazano-Narrows (1965) in New York City and also an Amman design, and the 3,800-foot center span of the Mackinac Straits Bridge which joins Michigan’s upper and lower peninsulas.

The George Washington Bridge. The upper roadway has eight traffic lanes; a lower deck with six lanes was added later. 429