Historic Structures

Burr Arch Truss Barrackville Covered Bridge West Virginia

Theodore Burr was born in Torringford, Connecticut in 1771 and died at Middletown, Pennsylvania in 1822, but spent most of his life in Harrisburg, Pennsylvania. His father was a millwright, so Theodore learned construction at an early age. As a young man, Burr moved to Chenango County, New York, established a saw and grist mill, and shortly thereafter built his first bridge, a timber stringer span, in Oxford, New York in 1800. Within a short time, he began getting requests to build bridges throughout the northeast. Among the early spans he erected were a 440' drawbridge at Catskill in 1802, a 330' arch bridge across the Mohawk at Canajoharie in 1803, and a five-span bridge over the Delaware River at Trenton, New Jersey in 1806.

Until the mid nineteenth century, mathematical engineering analysis was virtually unknown. Bridges were designed by empirical method, that is, by a combination of intuition, experimentation and practical experience. The combining of arches and trusses in bridge design is seen as early as the sixteenth century in the writings of Italian architect Andrea Palladio (1518-1580), who is reportedly the first to illustrate and build a truss bridge. In 1764 a wooden bridge was built in Switzerland with "trusses consisting of rectangular frames supported by massive arched ribs." America's pioneer bridge builders Timothy Palmer and Louis Wernwag, who preceded the work of Burr, built bridges that were highly indeterminate combinations of arches and trusses. Nevertheless, Theodore Burr is credited with being the first to separate the truss and arch to make a bridge with a level deck.

In 1806 and 1817 Burr took out patents for a multiple kingpost truss with arched 1 7 reinforcing ribs. Burr's predecessor, Timothy Palmer (1751-1821), who is believed to be the first American bridge builder to advocate weatherboarding and roofing bridges to protect them from exposure to the elements, also designed combination arch-truss bridges, but Burr took the concept further by carrying the below-deck arch ribs of Palmer's last bridges up into the truss, thereby leaving both the deck and truss horizontal. By combining the arch and truss into a single structure, it appears that the strengths of each form are called into play, and the two structures work simultaneously as an integral system, each carrying part of the load, thereby increasing rigidity and reducing deflection under load.

In an effort to reduce the need for complex joinery, Burr in his 1817 patent recommended eliminating mortise connections at the posts and diagonals, advocating instead, "merely butting suitably mitered ends to save much of the carpentry effort and expense." Thus, ordinary carpenters were able to erect Burr arch-trusses, which only increased the popularity of the type.

The Burr arch-truss was popular in the mid-nineteenth century for railroad bridges and highway spans of 100' or more. According to one source, thousands of such bridges were constructed, particularly for longer spans. The longest single-span wooden covered bridge on record, the 360' McCall's Ferry Bridge in Pennsylvania, was a Burr arch-truss. Today, there are as many as 215 wooden covered bridges classified as Burr arch-trusses remaining in the United States. The vast majority of these are located in Pennsylvania and Indiana.