Page:America's Highways 1776–1976.djvu/434

 The Morgan Bulheley Bridge over the Connecticut River at Hartford, Conn.

Although concrete bridges are part of the modern era, the early use of concrete was limited. Natural cement entered into bridge construction as mortar for stone masonry and for unreinforced concrete footings and substructures as early as 1850. Although many cement users switched to the more uniform and superior portland cement after its first manufacture in this country at Allentown, Pennsylvania, in 1871, natural cement has continued to be used to a lesser extent even to this day.

The Clefridge pedestrian underpass was built of concrete in Prospect Park, Brooklyn, New York, in 1871. This structure, said to be the earliest concrete arch constructed in America, is an arch that follows the design of stone arches and has a radius, of 10 feet.

The first reinforced concrete building in this country was built by W. E. Ward in New York State in 1875, and the first reinforced concrete bridge in this country was a 20-foot span built in Golden Gate Park, San Francisco, California, in 1889.

Other early reinforced concrete bridges were the Eden Park Bridge, Cincinnati, Ohio, a 70-foot arch span built in 1894–95; the five-span arches over the Kansas River at Topeka, Kansas, built in 1896 with a maximum span of 125 feet and a total length of 539 feet; and the 36-foot span stone-faced reinforced concrete arch built according to the Melan method near Rock Rapids, Iowa, in 1894. This latter bridge was moved to a roadside park when the bridge was replaced in 1964.

The first 10 years of the 20th century saw a phenomenal growth in the use of concrete structures on both the highway and railroad systems. Many long-span concrete arch bridges with either plain or reinforced arches were built in populated places, probably because of the architectural improvement over the truss and trestle bridges of the time. The use of reinforced concrete for deck girders, culverts, slabs for steel truss and girder spans, and bridge abutments and piers soon became commonplace. Of special importance was the use of reinforced concrete slabs, still in use today, for bridge decks instead of timber, steel, or cast iron.

Through the years, the methods, materials and equipment for bridge construction had developed and improved with experience and invention. Steam-powered equipment had supplanted men and horses for heavy lifting and excavation. Deep foundation excavation in open cofferdams, cellular cofferdams, open caissons and pneumatic caissons had been developed and successfully used. Steam piston and pulsometer pumps had supplanted the chain and ship pumps of colonial days for draining foundation excavations. Improved timber sheet piling, such as the Wakefield type, and steel sheet piling were developed which resulted in relatively watertight cofferdams.

Many steel fabricating shops had become established with power equipment for handling and fabricating the steel bridge members. Power shears, punches and drills were employed. Heavy hydraulic and pneumatic riveters were developed for driving tight rivets. Relatively efficient and structually satisfactory standards for riveted and pinned connections and other structural details had been developed and were in use, by 1916.

Efficient and safe methods had also been developed for erection of steel superstructures. Steam-powered cranes, derricks and gin poles were available for

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