Bridge building companies prefabricated metal truss bridges. They formed, drilled, assembled, and shop-riveted the component parts of a metal truss bridge in their foundries and erecting shops. The parts were then shipped to the bridge site, usually by railroad, where the bridge company’s own crew, its agent, or a local subcontractor assembled the bridge on the substructure. The bridge abutments and piers which formed the substructure were almost always erected by a third party. They were generally formed from cut stone or, after ca. 1910, from poured reinforced concrete.

       Most metal truss bridges constructed prior to 1900 used pinned connections. In a pin-connected bridge, a cylindrical metal bar joins the truss members together. This connection method required holes in the ends of each member that were then aligned together, so that the pin could be driven through all the holes to form a structural connection. Pinned connections were popular because they allowed the rapid erection of the trusses and made it easier to analyze the stresses in the truss members. However, they were susceptible to loosening, especially under the shaking caused by fast-moving vehicular loads.

Diagram of a pinned connection.

       Fueled by improvements in pneumatic field riveting equipment, builders of metal truss bridges made the transition from pinned connections to riveted connections around 1890. Panel points were connected at gusset plates. Hot rivets were placed in pre-drilled holes and hammered to flatten the outside rims. As the metal cooled it contracted, pulling the joint tight.

Diagram of a riveted connection.

        Riveted connections also benefited from stronger steel, which was developed in the closing decade of the nineteenth century. Where pinned connections permitted stress points to move, the new, stronger riveted steel could accept the stress without fracture.

Diagrams taken from the T.A. Comp and D. Jackson 1977 leaflet, Bridge Truss Types: A Guide to Dating and Identifying.