Trusses are used to stiffen and support a bridge by distributing the loads and forces acting upon the bridge based on the positions of the vertical, horizontal and diagonal chords.  They are based on triangular configurations.  How those chords are arranged identifies the type of truss.  Trusses are also used on cantilever bridges and to support the decks in suspension bridges.  Trusses are "through trusses" when the truss is above the deck, and "deck trusses" when they are underneath, supporting the truss.  Three early American bridgebuilders--Timothy Palmer (1751-1821), Lewis Wernwag (1770-1843) and Theodore Burr (1771-1822) built truss bridges and are known as the "Inspired Carpenters." Palmer is credited among the first to cover the the truss, leading to construction of covered bridges in the United States.  The truss bridge was described as long ago as the sixteenth century by Andrea Palladio in his Four Books on Architecture. 
    
     There are numerous variations and types of truss bridges.  Among the more common is the Warren Truss, patented in 1848 by James Warren and Theobald Manzani and marked by equilateral triangles.  Variations include the subdivided Warren Truss, with vertical members bisecting the triangles, (as in the Uhlerstown-Frenchtown Bridge below), the double warren (shown in the Washington Crossing Bridge  evidencing a diamond-like appearance, and the quadrilateral Warren Truss.  Another common style is the Pratt Truss, invented in 1844 by Thomas and Caleb Pratt.  In this configuration, the diagonals are in tension and the verticals in compression.  (See Calhoun Street Bridge in the images)  The diagonals slant downward and towards the center.  This truss can be used with spans up to 250 feet and was a common configuration for railroad bridges as truss bridges moved from wood to metal.  For more information, see http://pghbridges.com/basics.htm.  
    
     The West Main Street Bridge (1870)(right) in Clinton, New Jersey is an historic pony truss bridge (no top) made of wrought and cast iron   The Historic American Engineering Record (calling this the Lawthrop Bridge, for its engineer), states that this "represents an early type of iron truss that dominated bridge construction from the 1850s to the 1870s." In his 1994 article in American Heritage Magazine, "The Golden Age of the Iron Bridge," Eric DeLony wrote that cast iron and wrought iron bridges are "the rarest and least appreciated . . . Yet in some ways [they are] the most technologically significant." Their heyday was between 1840 and 1880.  Cast iron is an iron alloy with high carbon content that, while in common use in Roman times, first became economical at the beginning of the Industrial Revolution in the mid-18th century.  Wrought iron has lower carbon content and is more flexible. 

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