Historic Structures

Bridge Description Contoocook Covered Railroad Bridge, Hopkinton New Hampshire

The Contoocook Railroad Bridge is a continuous two-span, double-web Town lattice truss covered wood bridge on a mortared stone pier and abutments. The total length of the bridge is 157'-3", with spans of 63'4½ " and 68'-9 ¾". The truss is 20'-0" high from the top of the upper chord to the bottom of the lower chord and 21'-0" wide overall, with a width of 15'-8" between the trusses. Vertical clearance is 19'-4".

The spruce trusses are framed in the manner patented by Ithiel Town in 1820 and modified in 1835. The upper and lower chords are triple lines of 3x10" planks. There are secondary chords (three pairs of 3x10" planks) above the lower chord and below the upper chord, as patented in 1835. There is a third chord (three pairs of 3x10" planks) above the lower secondary chord. The chords sandwich offset double lattice webs, composed of 3x12" planks, three diamonds high. The webs are fastened at each lattice intersection with two 2" diameter treenails, at the upper chords with three 2" diameter treenails, and at the lower chords with four 2" diameter treenails. There are paired 6x12" timber posts at the ends of the trusses and two pairs of timber posts (four per truss) flanking the trusses at mid-span to carry the vertical forces to the pier and abutments.

The upper lateral system consists of 8x8" transverse tie beams seated on the upper chord at the lattice intersections, spaced approximately 5' apart. There are 6x6" lateral cross braces between every three struts and sway braces between the tie beams and upper secondary chords.

The floor system is composed of 10x15" floor beams suspended from the lower chord with 1½" diameter bolts which pass through a plate under each beam and up through a wooden block on top of the chord and are fastened with a nut and washer assembly. The floor beams support two lines of stringers, 6x12" timbers laid flat. Originally, the railroad ties and tracks would have been fastened on top of the stringers, but these have been removed. In their place is a timber walkway composed of 5x7" timbers placed transversely every 12" along the middle of deck.

Variable width vertical board siding covers the exterior of the bridge to 2' below the upper chord. The sheathing is fastened to nailers on the outer faces of the lattice webs. Rafters, measuring approximately 2x6", frame between a 5x5" timber on the outer edges of the tie beams and a 4x6" ridge pole resting on blocks on top of the tie beams. The rafters support longitudinal sheathing, to which is fastened a very low-pitched metal roof. The roof has overhanging eaves and exposed rafter tails. The portals are straight with hipped openings, pilaster moldings and shelter panels. The pediments have an outward curve at the eaves where the rafters would otherwise be exposed.

Design

Ithiel Town was born in Thompson, Connecticut in 1784 and died in New Haven in 1844. As a young man he learned carpentry and later studied architecture at Asher Benjamin's school in Boston. For most of his life, he practiced architecture, primarily as a partner in the New York City firm of Town & Davis. Town designed a number of noteworthy buildings, including Christ Church in Hartford (1825), the New York City Custom House (1837), the Yale College Library (1842), and the Virginia State Capitol at Richmond (1842). Although he is primarily recognized as an architect, Town also made a significant contribution to the field of engineering when, in 1820, he was granted a patent for a truss bridge. As he explained in his 1821 pamphlet, "A Description of Ithiel Town's Improvement in the Construction of Wood and Iron Bridges," this new method of bridge construction was designed to be "the most simple, permanent, and economical, both in erecting and repairing."

Town's design consisted of two layers of overlapping planks running perpendicular to each other, with each layer arranged at an angle to the chords, forming a lattice fastened together with wooden pins or treenails at each intersection. The most significant feature of this design was that it could be quickly erected and utilized sawn planks instead of heavy hewn timbers. In 1821, Town published a description of this design in The American Journal of Science & Arts.

In this he specifically proposed spans of 120 to 160 ft. made of planks 10 to 11 in. wide by 3 to 3½ in. thick set at 45° forming relatively open lattices (only double or triple intersection), with a span:depth ratio of 10:1. The lattices were to have three or four 1½ in. diameter treenails through each intersection. To stabilise [sic] the lattices Town proposed single stringers on either side at top and bottom for spans up to 130 ft, but for longer spans he proposed adding others.

The lattice design actually functioned as a series of overlapping triangles so that the load in any one triangle affected distribution of stress in all other triangles. Because the webs were fastened at every intersection, no triangle could function independently, and, as bridge historian Richard Sanders Allen points out, "Therein lay the great strength of the Town truss. It was a real invention, not resembling any design advanced for wooden spans in the thousands of years before its time that bridges had been built." Because it did not rely on European precedents, the Town lattice is considered "the first truly American design" for a bridge truss.

Town took out a second patent in 1835, adding a second lattice web. While single lattice trusses could safely carry railroad loadings to 80', they tended to warp under heavy loads; by doubling the lattice web, the truss became more rigid and resistant to warping. The secondary chords were used in both railroad and highway bridges to help stiffen the truss.

Builder

On most railroads, metal trusses gradually superseded timber trusses in the late nineteenth century. The Boston & Maine Railroad, however, continued to maintain and build timber bridges into the early twentieth century. This was largely due to the efforts of Jonathan Parker Snow (1848-1933), an advocate of timber bridges, who served as an engineer for the Boston & Maine from 1888 to 1911.

Early in his railroad work, Snow became convinced that wooden truss bridges should be maintained in service as long as possible, instead of being replaced with iron trusses. In 1895, nearly 70 percent of the bridges on the Boston & Maine Railroad were wood. It was accepted that they might have a shorter service life, but they could be easily reinforced if necessary, and they gave ample evidence of distress long before failure. An iron single-track bridge of 120' span cost about $5,300, but a spruce lattice truss was only about $3,500. Snow advocated the double-web Town lattice truss, pointing out that while single lattice trusses tended to warp under heavy loads, the double lattice stayed rigid because it functioned like a box girder. In 1900, there were an estimated one hundred Town lattice truss covered bridges on Boston & Maine lines. Although no longer in service, four of these covered bridges survive: Contoocook (1889), Wright's (1906), Pier (1907) and Fisher (1908).