The Eads Bridge is a combined road and railway bridge over the Mississippi River at St. Louis, connecting St. Louis and East St. Louis, Illinois. The bridge is named for its designer and builder, James B. Eads. When completed in 1874, the Eads Bridge was the longest arch bridge in the world, with an overall length of 6,442 feet (1,964 m). The ribbed steel arch spans were considered daring, as was the use of steel as a primary structural material: it was the first such use of true steel in a major bridge project. The Eads Bridge was also the first bridge to be built using cantilever support methods exclusively, and one of the first to make use of pneumatic caissons. The Eads Bridge caissons, still among the deepest ever sunk, were responsible for one of the first major outbreaks of " caisson disease" (also known as "the bends" or decompression sickness). Fifteen workers died, two other workers were permanently disabled, and 77 were severely afflicted. On 14 June 1874, John Robinson led a "test elephant" on a stroll across the new Eads Bridge to prove it was safe. A big crowd cheered as the elephant from a traveling circus lumbered towards Illinois. It was believed that elephants had instincts that would keep them from setting foot on unsafe structures. Two weeks later, Eads sent 14 locomotives back and forth across the bridge at one time. The Eads Bridge, which became an iconic image of the city of St. Louis, from the time of its erection until 1965 when the Gateway Arch was constructed, is still in use. The bridge crosses the St. Louis riverfront between Laclede's Landing, to the north, and the grounds of the Gateway Arch, to the south. Today the road deck has been restored, allowing vehicular and pedestrian traffic to cross the river. The St. Louis MetroLink light rail line has used the rail deck since 1993.

The Eads Bridge was built by the Illinois and St. Louis Bridge Company, with the Keystone Bridge Company serving as subcontractor for superstructure erection. The domination of the river trade was no longer as important as before the American Civil War, and Chicago was fast gaining as the center of commerce in the West. The Bridge was conceived as a solution to reverse this new found eminence. Meanwhile in an attempt to secure their future, steamboat interests successfully lobbied to place restrictions on bridge construction, requiring spans and heights previously unheard of. Ostensibly this would maintain sufficient operating room for steamboats for the then foreseeable future beneath the bridge’s base. Their unproclaimed purpose was to require a bridge so grand and lofty that it would be impossible to erect according to conventional building techniques. The desired consequence would be the prevention of any structure, and continued sole dependence on river traffic to sustain commerce in the region. A bridge of this magnitude would require a radical design solution. Though the ribbed arch had been a known construction technique for centuries. The triple span, tubular metallic arch construction was supported by two shore abutments and two mid-river piers. Four pairs of arches per span (upper and lower) were set eight feet apart, supporting an upper deck for vehicular traffic and a lower deck for rail traffic. Construction involved varied and confusing design elements and pressures. State and federal charters precluded suspension or draw bridges, or wood construction. There were also constraints on span size and regarding the height above the water line. The location dictated a change from the low Illinois floodplain of the east bank to the high Missouri cliff on the west bank of the river. The bedrock was extremely deep. These pressures resulted in a bridge noted as innovative for precision and accuracy of construction and quality control. Utilization of cast chromium steel components is arguably the first use of structural alloy steel in a major building construction. (Though the bridge as actually completed contained large—and unknown—amounts of wrought iron.) Eads argued that the great compressive strength of steel was ideal for use in the upright arch design. This decision resulted from a curious combination of chance and necessity, due to the insufficient strength of alternative material choices. The particular physical difficulties of the site stimulated interesting solutions to construction problems. The deep caissons used for pier and abutment construction signaled a new chapter in civil engineering. Unable to construct falsework to erect the arches because they would obstruct river traffic, Eads's engineers devised a cantilevered rigging system to close the arches. Although recognized as an innovative and exciting achievement, the Eads Bridge was undercapitalized during construction and burdened with debt. With its focus on the river, St. Louis had a lack of adequate rail terminal facilities, and the bridge was poorly planned to coordinate rail access. An engineering and aesthetic success, the bridge was bankrupt within a year of opening. Granite for the bridge came from the Iron County, Missouri quarry of Missouri Governor and U.S. Senator B. Gratz Brown who had helped secure federal financing for the bridge. The Merchants Exchange eventually lost ownership to the Terminal Railroad Association of St. Louis. The Exchange, fearing a Terminal Railroad rail monopoly on the bridges, would then build the Merchants Bridge (which in turn would eventually be taken over by the Terminal Railroad. The Terminal Railroad transferred the bridge to the City of St. Louis in 1989 in exchange for the MacArthur Bridge. Eads Bridge had long hosted only passenger trains on its rail deck. By the 1970s the TRRA had abandoned its Eads trackage, as the bridge had lost all remaining passenger rail traffic to the MacArthur during the early years of Amtrak; the dimensions of modern passenger diesels were incompatible with both the bridge and the adjoining tunnel linking the Union Station trackage with Eads. In 1998, the Naval Facilities Engineering Service Center investigated the effects of the April 4, 1998, ramming of the bridge by the barge Anne Holly. The ramming resulted in the near breakaway of the SS Admiral riverboat casino; several recommended changes reduced the odds of this happening in the future.

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