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The Howe Truss Covered Bridge

Howe Truss Bridge Truss Diagram

William Howe (1803-52) of Massachusetts was granted his first truss patent in 1840 and a second one later in the same year. His second patent used metal rods as the vertical members of what was otherwise a simple timber parallel-chord, cross-braced truss. This was the first truss patent granted with some major structural components made with metal. The configuration used easy-to-erect and readily prefabricated components that could be assembled on-site and adjusted via threaded connections at the rod ends. Little skilled labor was involved in assembling and erecting this truss type, and it became an immediate success.

Another factor in the success of Howe's truss type was his inclusion of a detailed structural analysis with the patent application. Up to this time, the selection of member sizes, materials, and overall geometry, was generally left to the judgment of the individual bridge builder. The fledgling structural engineering profession was developing rules and relationships to govern such matters, but no consensus had been attained at the time of Howe's patent.

The initial Howe truss bridges had wooden blocks cut to fit at the connections at the ends of the diagonal members against the chords. Later versions converted to the use of cast iron angle blocks. These blocks were simple to construct and install, and they were a major factor in the popularity of this configuration.

The Howe truss is second only to the Burr arch in popularity of extant covered bridges in the United States. There are about 143 bridges supported by the Howe truss, or about 15 percent of all covered bridges. The Howe truss has individual spans that range from an unusually short 6.1 m (20 ft) up to an impressive 61.0 m (200 ft), the longest being only 10 percent shorter than the longest Burr arch. The oldest extant Howe truss was built in 1854, and the configuration remains popular with new authentic examples built today.