truss bridge

truss bridge, bridge with its load-bearing structures composed of a series of wooden or metal triangles, known as trusses. Given that a triangle cannot be distorted by stress, a truss gives a stable form capable of supporting considerable external loads over a large span. Trusses are popular for bridge building because they use a relatively small amount of material for the amount of weight they can support. They commonly are used in covered bridges, railroad bridges, and military bridges.

Form and mechanics

The individual pieces of a truss bridge intersect at truss joints, or panel points. The connected pieces forming the top and bottom of the truss are referred to respectively as the top and bottom chords. The sloping and vertical pieces connecting the chords are collectively referred to as the web of the truss.

The component parts of a truss bridge are stressed primarily in axial tension or compression. A single-span truss bridge is like a simply supported beam because it carries vertical loads by bending. Bending leads to compression, in the top chords (or horizontal members), tension in the bottom chords, and either tension or compression in the vertical and diagonal members, depending on their orientation.

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Early trusses were built without precise knowledge of how the loads are carried by each part of the truss. The first engineer to analyze correctly the stresses in a truss was Squire Whipple, an American who designed hundreds of small truss bridges and published his theories in 1869. Understanding precisely how loads were carried led to a reduction in materials, which by then were shifting from wood and stone to iron and steel.

History and uses

There is no evidence of truss bridges in the ancient world, but the 13th-century sketchbook of the French architect Villard de Honnecourt depicts a species of truss bridge, and the Italian Andrea Palladio’s “Treatise on Architecture” (1570) describes four designs. Several notable covered bridges, which are enclosed truss bridges, were constructed in Switzerland. The Kappel Bridge (1333) of Luzern has been decorated since 1599 with 112 paintings in the triangular spaces between the roof and the crossbeams, depicting the history of the town and the lives of its two patron saints.

In the 18th century, designs with timber trusses reached new span lengths. In 1755 a Swiss builder, Hans Grubenmann, used trusses to support a covered timber bridge with spans of 51 and 58 metres (171 and 193 feet) over the Rhine at Schaffhausen. He and his brother also built a notable arch-truss bridge over the Limmat River in Baden with a clear span of 61 metres (200 feet).

In North America the covered truss bridges underwent further evolution. From simple king-post trusses, in which the roadway was supported by a pair of heavy timber triangles, American carpenters in the 18th and 19th centuries developed bridges combining simplicity of construction with their other economic advantages. The first long covered bridge in America, with a 55-metre (180-foot) centre span, was built by Timothy Palmer, a Massachusetts millwright, over the Schuylkill River at Philadelphia in 1806. A New Haven architect named Ithiel Town patented the Town lattice, in which a number of relatively light pieces, diagonally crisscrossed, took the place of the heavy timbers of Palmer’s design and of the arch. Another highly successful type was designed by Theodore Burr, of Torrington, Connecticut, combining a Palladio truss with an arch. Burr’s McCall’s Ferry Bridge (1815; on the Susquehanna River near Lancaster, Pennsylvania) had a record-breaking span of 108 metres (360 feet). Numerous Town and Burr designs remained standing throughout North America into the early 21st century, some dating back to the early 19th century.

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With the increasing importance of locomotive transportation in the 19th century, iron was adopted for covered bridges to carry the heavy loadings of the railroad. At first metal was used for only part of the truss, in either vertical or diagonal members, and later for the whole truss. Cast iron and wrought iron were soon replaced by steel, and a principal form of the modern railroad bridge rapidly evolved. The metal truss did not require protection from the weather and consequently was not covered. The two systems most commonly used are the Pratt and the Warren; in the former the sloping web members are parallel to each other, while in the latter they alternate in direction of slope.

Steel truss bridges have also been built as part of highway systems around the world. The longest continuous-truss bridge in the world is the Ikitsuki Bridge (1991) in Japan, with a main span of 400 metres (1,300 feet). The Astoria Bridge (1966), spanning the mouth of the Columbia River between the states of Oregon and Washington in the United States, consists of three spans reaching a total length of 6,545 metres (21,474 feet) and including a main span of 376 metres (1,232 fee); it is the second longest continuous-truss bridge.

Truss bridges have been used in military operations, particularly where riverbanks are steep or navigation must be kept open. They are usually made up in panels that can be readily transported and quickly bolted together. Such military truss bridges were pioneered in World War II by the highly successful British-invented Bailey bridge, which played an especially important role in the Allied campaign in Italy.