Prestressing of 19th Century Wood and Iron Truss Bridges in the U.S.

Many of the well-known American truss bridges built in the first half of the 19th century were prestressed. Specifically, the Long, Howe, Pratt and Rider/Moulton forms, built entirely of wood or iron or using a combination of wood and iron, were prestressed. The prestressing was achieved by driving wood or iron wedges or by tightening nuts on threaded iron rods. The level of prestressing was controlled only qualitatively, probably by observing if any elements became slack when a heavy live load traversed the span. The significant advantages of prestressing were that connections were simplified, some wood tensile connections were eliminated and, if all elements did not loosen, the stiffness of a bridge was increased. This paper describes studies of the Eldean Bridge in Miami County, Ohio. The Eldean Bridge is a wooden Long truss, built in 1860 by James and William Hamilton. The experimental and analytical studies address the actual magnitudes of prestress forces achieved by driving wedges, the effects of prestress on the structural behavior, the loss of prestress from wood shrinkage and creep, and the need for periodic retightening. Also presented are studies of the Pine Bluff Bridge in Putnam County, Indiana. The Pine Bluff Bridge is a classic Howe truss, built in 1886 by Joseph Albert Britton. The studies quantify the actual magnitudes of prestress forces achieved by tightening nuts and examine the same issues as those for the Long truss. After the Long and Howe trusses, two significant prestressed truss forms were patented in the U.S. They are the truss of Thomas and Caleb Pratt and the truss of Nathaniel Rider and Stephen Moulton. These designs are briefly discussed, to provide a more complete view of the early 19th century prestressed truss forms.