Page:America's Highways 1776–1976.djvu/430

 wrought-iron and steel plates increased. The first American railroad girder bridge with a 100-foot span was built in 1887. Just 7 years later, a 182-foot span was constructed, a major span even by today’s standards. The use of wrought-iron and steel I-beams for short spans was initiated by the rolling of deeper beams.

Details of a 200-foot Finley chain link suspension bridge.

Still, early plate girder highway bridges were limited to short spans because of the inconvenience and cost of transporting and erecting long members except when the bridge site was near railroad transportation. Thus, long spans still belonged to the truss, the suspension span, and the arch.

The first American plate girder arch bridge was the Washington Street Bridge over the Harlem River in New York City, built in 1886–88. It had two 510-foot arch spans with plate girders 13 feet deep.

By the early 1870’s, the size of I-beams rolled was large enough to use for stringers in the floor systems of short paneled truss bridges. This, together with the development of plate girders for floor beams, led to the general discontinuance of the use of wooden stringers by 1875. However, wooden stringers were used on short- and medium-length spans until 1890 and later. During the period 1874 to 1890, the maximum depth of American rolled I-beams increased from 10½ inches to 20 inches.

This most fascinating of all bridges first appeared in America in 1801 when Judge James Finley built a 70-foot chain link suspension bridge over Jacobs Creek near Greensburg, Pennsylvania. The chains were made from 1 inch square wrought-iron bars. The links varied from 5 to 10 feet in length so as to match the distance between floor beams. The timber floor system was stiff enough to distribute live loads to several hangers and to resist deformation, undulations and vibrations from the live load and wind loads. The bridge failed under a six-horse team in 1825 but was repaired. A number of bridges were built under the direction of Judge Finley or his licensees. The maximum span lengths were probably about 150 feet. The cable supports were usually timber towers on stone masonry piers. A suspension bridge at Lehigh Gap, Pennsylvania, apparently the last Finley type in use, was replaced by a modern structure in 1933.

Suspension bridge built by Charles Ellet in 1849 over the Ohio River at Wheeling, W. Va.

One of the largest of the chain link structures was the Point Bridge over the Monongahela River at Pittsburgh, Pennsylvania. Built in 1875–77, it was a stiffened chain suspension span 800 feet long. The main and backstay chains were of wrought-iron link bars 20 feet 6 inches long.

The first highway bridge in America with wire suspension cables was designed by Charles Ellet and completed in 1842. It replaced Wernwag’s wooden trussed-arch Upper Ferry Bridge over the Schuylkill River at Philadelphia. As would be the case for all American suspension bridges until the Brooklyn 424