Historic Structures

Bridge Description Sulphite Covered Railroad Bridge, Franklin New Hampshire

The Sulphite Railroad Bridge is a 180'-7", three-span Pratt deck truss covered bridge with timber trestle approaches at each end (the west approach is two spans totaling 23'-0"; the east approach is three spans totaling 30'-0"). The bridge is supported on mortared, cut and dressed masonry piers. The trusses are 10'-0" from the top of the upper chord to the bottom of the lower chord and are spaced 6'-0" apart.

Each truss is composed of six 9'-7" panels, framed in the manner patented by Thomas and Caleb Pratt in 1844. The upper chords are composed of three 6"x12" timbers with shear blocks. The lower chords are composed of three 6"x10" timbers with shear blocks. The upper and lower chords are connected vertically by paired posts, varying in dimension from 6"x6" at midspan to 10"x10" at the ends. Each pair of posts is fastened together with wooden shear blocks and threaded iron bolts. There are diagonal rods in every panel, two 1" diameter rods in panels 3 and 4, two 2" diameter rods in panels 2 and 5; and two 2" diameter rods and two 1 1/2" diameter rods in panels 1 and 6. There are 1" diameter counters in every panel except the end panels. The tension rods come into cast iron assemblies (approximately 2 1/2' x 2' x 10") notched into the upper and lower chords, where they are fastened with plates and nuts. There are two types of cast iron fittings in this bridge, depending on whether they receive four rods or two. The rods bear on washers curved to accommodate different rod angles, suggesting that there may have been similar bridges along the line. The two trusses are connected transversely by cross braces between the upper and lower chords of the two trusses at every other panel point. Connections are bolted with threaded rods.

The lower chords rest on 12"x12" bedding timbers on the pier and abutments. There is 6"x6" lateral bracing between the lower chords and iron rods with turnbuckles placed transversely between the lateral bracing. There are transverse planks on the bracing. These planks support a timber walkway for inspections and maintenance access inside the truss.

The upper lateral system is composed of 8"x8" transverse timbers, spaced 2'-6" apart, bearing on the upper chord. A series of longitudinal timbers on top of the transverse timbers carry the roof and railroad ties. Their dimensions vary from 8"x8" at the ridge to 6"x10" (laid flat) at the center, to 3"x8" (laid flat) at the eaves to form the slight pitch of the roof. Sheet metal roofing is fastened to these timbers. Railroad ties, tapered on the underside, support the tracks just above the roof.

Historic photos indicate that this bridge once had vertical board siding. Vandals burned the siding off in 1981, leaving only the charred trusses and roof intact. Because of its unusual deck truss configuration, this bridge is locally referred to as the "Upside-down Covered Bridge."