Page:America's Highways 1776–1976.djvu/428

 First cast-iron bridge in the United States, over Dunlap Creek at Brownsville, Pa.



The second cast-iron arch in this country, completed in 1860, was the Meigs Bridge over Rock Creek in Washington, D.C. Like the Dunlap Creek Bridge, it was also built by the Corps of Engineers. Its two cast-iron pipe arches supported the deck of the highway bridge and carried the water supply of the city over the creek. The 51-inch outside diameter pipes spanned 200 feet. The bridge deck and supports were removed in 1916 and a new bridge was constructed over and independent of the pipes still carrying water.

Ironically, the chief factor in the decline of the cast-iron bridge was its success. The increased use of wrought iron and cast iron for bridges, rails and other related uses caused a boom in the iron industry and created an incentive to develop new processes for producing iron and steel.

The original Meigs Bridge over Rock Creek in Washington, D.C., built in 1860 by the U.S. Corps of Engineers.

Although the Bessemer converter and open-hearth furnace made the steel bridge inevitable, several bridge failures also increased distrust of iron bridges. The most notable was a failure of an iron Howe truss at Ashtabula, Ohio, which took 65 lives in 1876, the worst rail disaster in America so far. Surprisingly, many highway bridges also failed, despite their relatively light live loads. This was due to the pressures for economy put upon county officials who lacked technical expertise and fly-by-night bridge salesmen and promoters, who, sometimes involved in political and business corruption, provided cheap and inadequately designed structures.

From the 1850’s, companies were formed primarily for constructing patented truss bridges, either under their own patents or as licensees. There being very few bridge engineers with a working knowledge of stress analysis and truss design in the early years of iron superstructures, these “bridge companies” became firmly established. Bids let for short- and medium-length span truss bridges allowed the bidder to 422