Page:America's Highways 1776–1976.djvu/429

 the superstructure to his own plans. While many safe and satisfactory bridges were obtained by this procedure, there were also many unsatisfactory bridges. In some cases, the companies furnished weak or inadequate bridges to compensate for underbidding or to make excessive profits.

The Eads Bridge over the Mississippi River was opened to traffic in 1874. Alloy steel was used for the first time in this country for the members of the three arches of this double deck bridge.

Due perhaps to a combination of poor engineering, poor material, and fraud, the numerous failures of iron and combination bridges brought about the complete abandonment of cast iron and led to the acceptance of wrought iron and steel, the flowering of modern truss design, and the rise of the civil engineer.

The first American all steel spans built were the five main spans of the Missouri River Bridge at Glasgow, Missouri. Bessemer steel was used for the 315-foot long trusses. The remaining trusses and trestle work were made of wrought iron.

However, the Glasgow trusses lagged by 10 years the first extensive use of structural steel in a bridge. It was in 1868 that construction commenced on the first of the two “great bridges” of American history, the Eads Bridge across the Mississippi at St. Louis, Missouri.

Until 1855, the “Father of Waters” had never been spanned. Thirteen years later, James Eads, a hero of the Civil War, began building three alloy-steel arches of 502-, 520-, and 502-foot spans, respectively, across the river. This bridge is noteworthy for another reason: The first use of pneumatic caissons in the United States and, sadly, the first death from the “bends” or “caisson disease.” (In all, 13 died.) It is interesting that Captain Eads had never built a bridge before. Despite the magnificence of the Eads Bridge, which is still in use, the future of the steel bridge was with the plate girder and the truss.

After Squire Whipple’s essays on bridge building, the increasing knowledgability of the engineer led to further refinements in the metal truss as iron yielded to steel. Among the changes were the use of bridge rollers and the perfecting of the pinned connection.

The improved metallurgy permitted the evolution of machined steel pins from the old iron trunnions and the use of the now famous eyebar tension member. The eyebar became the trademark of the American truss bridge in the latter half of the 19th century. In this area, the American engineers deviated from European practice, which was turning to the more rigid riveted gusset plate connections. The economy and determinacy of the pinned truss was well suited to the American bridge engineer’s interest in structural analysis. While the riveted joint was finally accepted, the practice of using deeper trusses and longer panels was retained this side of the Atlantic.

Interest in determinate structures led to the use of cantilever bridges. Eventually, cantilevers were so widespread that Europeans referred to them as the “American” bridge.

In 1876, Charles Smith built the first such bridge over the Kentucky River for the Cincinnati Southern Railway with three 375-foot spans. Other long span cantilever bridges were the Monongahela River Bridge at Pittsburgh, Pennsylvania, built in 1904 with a maximum span length of 812 feet, and the Queensborough cantilever designed by Gustav Lindenthal in 1909 in New York City. Its maximum span was 1,182 feet.

The Washington Street Bridge over the Harlem River in New York City.

The use of plate girder bridges for short spans began during the Civil War. While these spans were limited to moderate length by the size and length of the wrought-iron plates and shapes then being rolled, longer spans were built as the available size of 423