The Burr Truss
Theodore Burr built a bridge across the Hudson River in Waterford, New York in 1804. There, he used a combination truss and segmented arch that became the basis of his 1817 patent. The patented truss consisted of parallel chords tied together by vertical posts and stiffened by crossed braces. Each timber-arch segment, lap-jointed on both ends, is fitted into the next on both sides of the posts and braces. The segments are "through-bolted" to the vertical posts, sandwiching the truss. The ends of the arches are received by the abutments below the bed timbers.
The Vermont versions, except for the bridge serving the Shelburne Museum, differ from Burr's patent in that the ends of the arch are supported by the ends of the lower chords rather that at the faces of the abutments. Also, a multiple-Kingpost truss is used rather than the cross-braced truss.
The Burr truss is not a true arch-bridge in that the parallel-chord truss system and the arch act in concert to support the bridge and its load. The multiple-Kingpost truss serves the purpose of aligning the arch segments while carrying the road-bed. The arch serves to make the whole structure more rigid, and the combination is capable of longer spans than is the multiple-Kingpost truss alone. The Burr-type bridge is best classed as an auxiliary arch system. In a true arch bridge, the roadway is supported only by the arch.
How the Burr truss actually works is "a thorny topic of debate," according to Gilbert Newbury, an engineer with the Vermont Agency of Transportation. Newbury, who is trained in timber engineering, planned the repair of the Burr-arch bridge on Gates Farm in Cambridge. He explains that one school of thought has it that the arches do the work, the other believes it is the parallel chord truss.
Newbury's computer analysis and field observation indicate that it is the truss that carries the majority of the bridge and roadbed weight, with the plank arch acting as a stiffening element for the truss.
From Spanning Time Vermont’s Covered Bridges, by Joseph C. Nelson