A truss bridge is a frame where definite parts are designed to act in tension while other parts act in compression. In addition the top and bottom parts of the frame are normally connected by vertical and diagonal members, which give rigidity to the frame. The truss can be designed as a flat beam structure or curved in arch form.
The concept of the truss in timber was described by Andrea Palladio (1508 – 1580) in his publication “I Quattro Libri dell’ Archittetura”. Between 1595 and 1617 this work was expanded by Faustus Verantus. Isambard Kingdom Brunel (1806 – 1859), who led the expansion of the railways in the UK, applied much innovation to the design of the truss and also constructed a large number of his bridges in timber because he did not trust cast iron. Between 1849 and 1864 he constructed 64 such viaducts in Devon and Cornwall.
The introduction of wrought iron, which was a much better material than cast iron, alongside the development of the railways, led to multiple patents and designs, particularly in the USA. Designers such as S. Whipple, A. Fink and W. Howe gave their names to such patented systems. This type of bridge construction became very popular at the end of the 19th century and the early part of the 20th century.
The truss could be used by itself, such as in a Bailey Bridge, which is used for temporary emergency crossings, or it could be used in conjunction with a cantilever bridge. This latter type of bridge produced some of the longest bridges in the world at that time e.g. The Forth Bridge in Scotland with two spans of 521 metres (1709 ft) each was completed in 1890. Similarly Gustave Eiffel (1832 – 1923), who built the Eiffel Tower in Paris, used the truss principle for the bridge decks on his two famous arch bridges, namely the Garabit Viaduct (1884) in the south of France and the Maria Pia Railway Bridge (1877) over the River Douro near Oporto in Portugal.
The truss was also used in the initial suspension bridges particularly by American designers and it allowed a lower deck to be added for traffic or trains if demand requires it e.g George Washington Bridge in New York. While the truss type of bridge continued to be used for medium construction sizes, the evolution of the cable stayed bridge and cable technology has superseded it.