American Truss Bridge Connections in the 19th Century. I: 1829-1850

In 1829 Stephen Harriman Long built a wood truss with details well suited to the mechanical behavior of wood. The truss was statically indeterminate, prestressed by wedges and required no tensile connections for the diagonals. In 1840 William Howe and Amasa Stone improved Long's design by using threaded vertical iron rods for prestressing. In 1839 Long patented a truss with diagonals that were pretensioned by using wedges on the vertical members. Long's method of prestressing the diagonals in tension was improved by Thomas and Caleb Pratt, who prestressed by tightening threaded iron diagonals. However, their design was not immediately successful for bridges with wood chords because prestressing often caused local crushing of the wood. Therefore, the dominant American truss at mid-century was the Howe truss, wonderfully adapted to the properties of wood, statically indeterminate, prestressed, easily adjusted and maintained, and with precompressed diagonals bearing on joint castings without positive connections to the chords and verticals. All these features were destined to be abandoned by American engineers over the next 20 years.