The Ironbridge at Coalbrookdale, England, completed in 1779, was the first major structure to be constructed entirely of iron. Designed by Abraham Darby III and Thomas Pritchard, the arched structure spans about 30 m (about 100 ft). Scottish engineer Thomas Telford used wrought iron and limestone to design the Menai Suspension Bridge in Wales in 1826. This bridge was the world’s first major suspension bridge. It spanned 176 m (579 ft). During the first half of the 19th century, iron became the premier building material. In addition, truss designs were developed to provide the additional strength needed to bear the massive weight of railroad trains.
The major disadvantage of iron, namely, low tensile strength, was overcome in the mid-1850s, when the Bessemer process of making steel (an alloy of iron and carbon) was developed. The first major structure built entirely of steel was the cantilevered Forth Bridge in Scotland, completed in 1890. Its two record-setting spans of 521 m (1,710 ft) were the longest in existence until 1917. The arched Eads Bridge over the Mississippi River at St. Louis, Missouri, designed by James Eads and completed in 1874, was the first steel bridge in the United States. The Eads Bridge has three main spans. The center span is 160 m (520 ft) long, and the spans on either side are each 153 m (502 ft) in length. At the time the Eads Bridge was built, it was the longest structure in the United States. By 1890 the strength and lightness of steel had made it the material of choice for bridge building.
C Suspension Bridges
The Roebling family pioneered the use of steel in suspension bridges. John Augustus Roebling, a German-born engineer who emigrated to the United States in 1831, is considered the father of modern suspension-bridge design. His major contribution was the development of construction techniques to spin wire cables. He was the first to use cables and stiffening trusses in suspension bridges. Roebling designed the Cincinnati Bridge, over the Ohio River at Cincinnati, Ohio. It was built by his son Washington Roebling in 1866. The Cincinnati Bridge has a span of 322 m (1,057 ft).
John and Washington Roebling also designed and built the Brooklyn Bridge, which was the world’s longest suspension bridge at the time of its completion in 1883, having a main span of 486 m 31 cm (1,595 ft 6 in). The completion of the Brooklyn Bridge marked the beginning of an 80-year period of large-scale suspension-bridge design in the United States. That period ended in 1964 with the completion of the Verrazano-Narrows Bridge in New York City.