Before the advent of railways, highway bridges in America were made of wood, and called trusses. Few of them existed before railways. The large rivers and estuaries were crossed in horse-boats, a trip more dangerous than an Atlantic voyage now is. A few smaller rivers had wooden truss bridges. Although originally invented by Leonardo da Vinci, in the sixteenth century, they were reinvented by American carpenters. Some of Burr’s bridges are still standing after more than one hundred years’ use. This shows what wood can do when not overstrained and protected from weather and fire.
The coming of railways required a stronger type of bridge to carry concentrated loads, and the Howe truss, with vertical iron rods, was invented, capable of 150-foot spans.
About 1868 iron bridges began to take the place of wooden bridges. Die-forged eyebars and pin connections allowed of longer panels and longer spans. One of the first long-span bridges was a single-track railway bridge of 400-foot span over the Ohio at Cincinnati, which was considered to be a great achievement in 1870.
The Kinzua viaduct, 310 feet high and over half a mile long, belongs to this era. It is the type of the numerous high viaducts now so common.
About 1885 a new material was given to engineers, having greater strength and tenacity than iron, and commercially available from its low cost. This is basic steel. After many experiments, the proper proportions of carbon, phosphorus, sulphur, and manganese were ascertained, and uniformity resulted. The open-hearth process is now generally used. This new chemical metal, for such it is, is fifty per cent. stronger than iron, and can be tied in a knot when cold.
The effect of improved devices and the use of steel is shown by the weights of the 400-foot Ohio River iron bridge, built in 1870, and a bridge at the same place, built in 1886.
The bridge of 1870 was of iron, had panels twelve feet long, and its height was forty-five feet, and span 400 feet.
The bridge of 1886 was of steel, had panels thirty feet long, and its height was eighty feet. Its span was 550 feet. The weights of the two were nearly alike.
The cantilever design, which is a revival of a very ancient type, came into use. The great Forth Bridge, in Scotland, 1600-foot span, is of this style, as are the 500-foot spans at Poughkeepsie, and now a new one is being designed to cross the St. Lawrence near Quebec, of 1800-foot span.
This is probably near the economic limit of cantilever construction, but the suspension bridge can be extended much farther, as it carries no dead weight of compression members.