Iron Wire of the Wheeling Suspension Bridge
Wheeling Suspension Bridge
The modern suspension bridge is an American development. The first modern suspension bridge was the 1801 chain bridge designed by Judge James Finley (1756-1828) across Jacob’s Creek in western Pennsylvania. Though it had a span of only 70 feet, it became the model for
European bridges such as Thomas Telford’s famous Menai Straits Bridge of 1826 in Wales. In 1822 the French began replacing chain with cables made of parallel bundles of iron wire only 3.1mm in diameter. Wire has several advantages. It is both strong and reliable; drawing iron into wire can make it nearly twice as strong, but should the iron be flawed it will break in the attempt to draw it and be discarded.
The first interstate highway in the United States was the National Road. Begun in Baltimore in 1806, it reached the Ohio River at Wheeling, Virginia, in 1816. The Ohio River is a tributary of the Mississippi, so cities along the Ohio, such as Pittsburgh, have direct access to the sea. Not until a bridge high enough to allow the stacks of the steamboats of the day to pass beneath was
Woodcut of Bridge
the main shipping channel crossed, in 1849. The Wheeling Suspension Bridge was designed by Charles Ellet (1810-1862), who used 12 cables, each 1380 feet long, having a total weight of 455,400 pounds of wire. The cables were laid out in the main street of Wheeling. The wire was laid parallel and selvaged (wrapped tightly with more wire) to
The wire is 140mils diameter (ca. 3.5mm) and each cable is 7½ inches (ca. 19cm) in diameter. The Bridge is 1010 feet long (307.85m) and, at the time it was built, it was the world’s longest. Today, it is the oldest ‘long-span’ suspension bridge in the world that still carries vehicular traffic. (A ‘long span’ is over 1000 feet, or 304.8m)
View of the Wheeling Suspension Bridge, modern traffic signs
Modern steel was not available in 1849. The Bessemer process was not invented until 1856 and the open hearth was developed for steelmaking even later. Instead, the wire for the cables in the Wheeling Suspension Bridge was drawn from wrought iron. This is the sort of material that was used by the blacksmith, and is characterized by the presence of mineral inclusions called slag.
Wrought iron has not often been tested with the methods that were developed and became standardized after the introduction of modern steel. During the recent rehabilitation of the Wheeling Suspension Bridge samples of original wrought iron wire from the cables became available, so it was decided to investigate them. The following are initial results.
Metallographic samples were sectioned longitudinally and axially, prepared by standard techniques and etched with nital (initially 3% nitric acid in alcohol, later 1%.) They have a typical wrought iron structure of ferrite with slag stringers, moderate elongation of the grains and no evidence of heat treatment subsequent to drawing.
Photomicrographs of cross sections, 3% nital etch
The most conspicuous feature of these sections is the uneven distribution of the slag particles in and among the ferrite grains. This is more evident in the cross section, where the remnants of piling can be seen. Initial analyses by EDX (energy-dispersive x-rays) in a SEM (scanning electron microscope) at magnifications up to 110,000x of separate inclusions suggest that their size is composition-dependent, the larger inclusions (ca. 25µm) being iron scale (iron oxide) but the finer ones (ca. 5µm) chiefly of alumina, from the furnace lining or possibly the ore. A search of the documents has not yet identified the source of the iron or the ore.
The iron itself shows only a trace of carbon and strontium but no detectable sulphur and manganese, indicating that this iron was charcoal smelted. In the nineteenth century charcoal smelted iron was recommended for wire drawing1.
Hardness indentations were used to estimate the tensile strength. The Rockwell B hardness on this wire was 85, which is roughly equivalent to about 82,000 psi (~565MPa.) This shows that work hardening during wire drawing increased the strength of the iron by more than 30,000psi, since the strength of wrought iron ordinarily does not exceed 50,000 psi.
There is general interest in the materials of this historic structure as a milestone in the history of engineering. The data we are gathering will also prove useful in the maintenance of the Bridge, which as we pointed out is still in service. In that regard it is not unique. Many other structures of wrought iron like the Wheeling Suspension Bridge are still standing and in use. Prominent examples are the wrought iron framework that supports the Statue of Liberty (1886), the world’s first curtain-wall building, and the 300m, wrought iron Eiffel Tower (1888) that, until 1930, was the world’s tallest manmade structure built since the pyramids of Egypt.
Eiffel Tower and detail
These structures. like the Wheeling Suspension Bridge, were conceived and built in wrought iron. Contrary to our usual assumption, high-rise buildings and long-span bridges were not a response to the availability of modern tonnage steel. The concepts were already there, before The Age of Steel, and had already been realized in wrought iron.
1. Frederick Overman, A Treatise on Metallurgy, New York, 1852, p. 551.