Pratt truss

Pratt truss

A Pratt truss includes vertical members and diagonals that slope down towards the center, the opposite of the Howe truss. The interior diagonals are under tension under balanced loading and vertical elements under compression. If pure tension elements (such as eyebars) are used in the diagonals, then crossing elements may be needed near the center to accept concentrated live loads as they traverse the span. It can be subdivided, creating Y- and K-shaped patterns. The Pratt truss was invented in 1844 by Thomas and Caleb Pratt. This truss is practical for use with spans up to 250 feet (76 m) and was a common configuration for railroad bridges as truss bridges moved from wood to metal. They are statically determinate bridges, which lend themselves well to long spans. They were common in the United States between 1844 and the early 20th century.