WHIPPLE BOWSTRING TRUSS BRIDGE MAP

WHIPPLE BOWSTRING TRUSS BRIDGE MAP

Crossing Normans Kill Ravine, Albany, New York

Designer/Engineer Squire Whipple

Builder Simon DeGraff

Completed 1867

Length 110 feet (34 meters)

Materials Cast and wrought iron Type Metal bowstring truss

With this bridge, a scientific approach to bridge construction emerged.

By the mid-nineteenth century, as rail traffic increased, it became clear that the iron-and-wood truss bridges that supported train lines were not strong enough to bear the sudden, concentrated weight of a locomotive in motion or the vibrations it caused. Many collapsed.

The 1847 publication of A Work on Bridge Building by Squire Whipple (18041888), a self-taught engineer and entrepreneur from New York, profoundly influenced the manufacture of iron bridges. His treatise is now recognized as the first American attempt to supply a theoretical means for calculating stresses in truss bridges instead of the rule-of-thumb methods employed until that time. A scientific approach to bridge building had emerged. Squire Whipple (Squire is his given name and not a title) built his first iron bowstring truss bridge over the Erie Canal near Utica, New York, and patented its design in 1841. His bridgeseconomical, lightweight, and easily shipped and reassembled on-sitewere immediately copied, albeit with slight variations to avoid paying Whipple royalties. Simon De Graff copied Whipple’s design, erecting this cast- and wrought-iron bridge in 1867 in Syracuse, New York. In 1899, the bridge was moved into a farm entrance outside of Albany. A rare extant example of Whipple’s innovative truss, it was entered into the National Register of Historic Places in 1971.

ERIC DELONY, LANDMARK AMERICAN BRIDGE MAPS, 1993

The triangular truss bridge’s strength gave rise to many variations utilizing different combinations of cast and wrought iron, the more prominent of which were named for their inventors, including Wendel Bollman (1814-1884), Albert Fink (1827-1897), and Thomas Pratt (1812-1875). Many more of these filigreed-iron structures were patented and aggressively marketed by anonymous entrepreneurial craftsmen who provided a young America with the bridges she needed to expand west.

By the mid-1870s the myriad truss designs marketed before and after the Civil War evolved into what became known as the American Standard. Descended from Pratt’s truss of 1850, the American Standard was utilized on both sides of the Atlantic and characterized by the use of prefabricated cast-iron sections that could be rapidly assembled on-site by semiskilled laborers.

Of the hundreds of metal trusses built between 1840 and 1880, only a handful survive; those that were not destroyed in the Civil War fell victim to the scrap metal drives of the World Wars or were torn down. According to Eric DeLony, former chief of the Historic American Engineering Recordan organization dedicated to preserving America’s built environment in drawings and photographsanother reason why so few early metal truss bridges remain is that “they are not recognized by the public, preservationists, or the engineering profession as being as significant as the more monumental Brooklyn or Golden Gate suspension bridges, or the quaint and nostalgic stone arches or wooden covered spans.” The pioneering metal bridges of the Civil War era, America’s rarest artifacts, are in danger of extinction. Built by the Wrought Iron Bridge Company in Aurora, Indiana, the Laughery Creek Bridge (1878) is a rare triple intersection Pratt truss, fondly called a “Triple Whipple.” It has a 298-foot (91-meter) span. Joseph G. Henszey designed the Henszey Wrought Iron Arch Bridge (1869) in Cumberland County, Pennsylvania, a rare example of a prefabricated metal truss design.