Steel Truss Bridges
The truss bridge reflected many of the qualities of America's industrial society by the early twentieth century: steel-made, mass manufactured, practical and unglamorous, sturdy and dependable, cost-competitive, and highly versatile. Such structures were, wrote David Plowden in his splendid Bridges: The Spans of North America, "appropriate icons to our industrialization." For the developing state of Oklahoma, steel trusses held symbolic value too, for they provided tangible proof to a wider world of economic possibilities.
The Principle upon which all trusses rely is that the triangle is the strongest and most rigid geometric Figure. By arranging the framework of triangles in patterns that varied from designer to designer, the structure acquired different appearances and served different purposes depending on the needs of bridge builders. The pony truss, the smallest type and ordinarily confined to lengths under 140 feet, with most under 100 feet, is distinguished by its low profile and absense of bracing above the roadway. The through truss by comparison is greater in length and height and consists of a tunnel-like structure that carries traffic through a system of overhead bracing which ties together the upper chords of the bridge.
Designs created by English inventor - engineers Pratt and Warren, after demonstrating their strength and versatility on the railroads, became the preferred choice for building highway bridges. After 1890 almost exclusively fabricated of steel, connected at their joints with either pins or rivets, and became common sights on roads across the country and the epitome of America's dependance on the steel truss bridge. No other nation modified and perfected these designs or made fuller use of them than the United States. The truss bridge was the American bridge. In the Pratt, vertical posts inserted between parallel chords carried compression while diagonal members sustained the stresses produced by loads moving across the bridge. The Warren, whose popularity peaked in the twentieth century, employed a series of triangles in the web to support both compressive and tensile forces.
The pony truss, prefabricated in the shops of national and regional companies and shipped by railroad to a location near the construction site, proved itself the bread-and-butter structure of the industry. While larger bridge firms owned furnaces and manufactured their own materials, most purchased standard rolled shapes from steel mills and fabricated the span by sizing, cutting, and punching holes for the bridge members. (Someone who wants to "read" a bridge can carefully search for the name of a steel mill stamped on its main parts, i.e., Carnegie, Cambia, Jones & Laughlin, or Colorado). Since producing and erecting smaller pony trusses involved a straightforward and routine method, many companies vied for a share of the market in these short span steel companies.