Introduction

        Metal truss bridges first appeared in the United States in the 1840s, and are still being used today. The metal truss bridge was the most commonly built long span bridge type between the 1870s and 1925.  The six Northwestern Pennsylvania counties in the Pennsylvania Department of Transportation's (PennDOT) Engineering District 1-0 (Mercer, Crawford, Erie, Warren, Forest, and Venango) have more metal truss road bridges than any other region of the state.  Mercer County currently contributes 12 examples, ranging in age from 1884 through 1953.

        Metal truss bridges, however, are relics of earlier times and are often not well-suited for modern transportation systems.  Narrow widths, poor approaches and visibility, decades of wear and tear, weight and height restrictions, high cost of painting, and expensive maintenance and repair costs are just some of the problems.  PennDOT, the Federal Highway Administration (FHWA) (which mandates a biennial safety inspection of all highway bridges), and the Mercer County Engineer have as their first priority the safety of the traveling public.  For this reason, truss bridges must sometimes be replaced.

Height restriction sign on the Carlton Road Bridge.

       A truss bridge uses many small pieces, or members, to make a beam long and strong enough to span great distances.  The individual components are connected in a series of triangles.

       The truss form dates to ancient times. In America, the first truss bridges were wood truss bridges like the Kidds Mill Covered Bridge in Pymatuning Township. Wood was cheap, plentiful, and readily understood by local craftsmen. A transition to composite wood and wrought iron trusses began in the 1840s. All-metal truss designs appeared within just a few years, but they were not widely used until after the Civil War, when they were embraced by the rapidly expanding railroad industry. Truss bridge technology was transferred to the nation’s road and highway system in earnest in the 1870s.

The Kidds Mill Covered Bridge.

       Until the 1920s, metal truss bridges faced little competition from other bridge types for spans of greater than 100 feet. Metal trusses were stronger and more rigid than wood bridges, and they were fire resistant. They were also affordable and easy to construct, due to standardized designs and competition among bridge firms. Other bridge building technologies, such as steel beam and reinforced concrete, did not fully develop until the early twentieth century, leaving metal truss bridges as the best bridge type to span great distances until that time.

Diagram taken from David Weitzman's 1982 book Windmills, Bridges, and Old Machines: Discovering Our Industrial Past.

Note: This site was produced as mitigation for the replacement of two National Register of Historic Places Eligible or Listed truss bridges in Mercer County, the Quaker (Williamsons Crossing) Bridge in Hempfield Township and the Milburn Road Bridge in Springfield Township.  Mitigation is required as a result of Section 106 of the National Historic Preservation Act of 1966, as amended.