Historic Structures

Bridge Description Clark's Covered Railroad Bridge, Lincoln New Hampshire

Clark's Bridge is a single track, ten panel, Howe truss railroad bridge with roof monitor. The 116' bridge currently crosses the Pemigewasset River with a clear span of 107' and a greatest vertical clearance of 20'-6". It rests on granite ashlar abutments moved from a Connecticut River crossing in Coos County, New Hampshire, that still show their yellow paint location numbers. There are 15'-3-1/2" between the trusses. Each of the four sticks that make up the bottom chord are 6-1/2" x 15". Presumably, the top chords are the same. Bolted shear blocks and "C" shaped wood clamps transfer tension around the joints in the bottom chord. Four 7-1/2" timbers serve as end posts. Those on the southwest portal are probably replacements. As is standard with Howe truss bridges, those diagonals leaning towards the center of the bridge are composed of two timbers, while those diagonals leaning away, also known as counters, use single timbers. The former measure approximately 11" x 11-1/2", and the latter are 7-1/2" x7- 1/2." At the middle of the span, however, the diagonals are 6-1/2" x 7-1/2" and the counters are 9-1/2" x 9-1/2". Since they have different finishes, they are probably replacements. Vertical tension rods keep the diagonals in compression, and their size and number vary depending on the panel. There are five verticals at Ul :L1, U2:L2, U8:L8, U9:L9 and three at the other panel points including the end posts. At those panel points where there are five verticals, the outer two verticals are 1-3/4" in diameter and the innermost three are 2-1/8" in diameter. Where there are three verticals the outer two are 2" in diameter while the inner one has a diameter of 1-1/8".

Clark's Bridge has a complete set of upper and lower lateral bracing also employing the Howe pattern. Diagonals of the lower lateral bracing are 3" x 7-1/2", and the tension rods are 1-1/8" in diameter. This lower bracing system is bolted to the stringers. It is not clear whether or not the diagonals are mortised into the stringers. Two pieces of 6" wide lumber resting on top of each other serve as stringers. The bottom timber is notched to accommodate the deck beams. Exclusive of this notch, the effective stringer depth is 1'-4". Deck beams bolted to the underside of the bottom chord, measuring 10" x 15", support the entire deck system. Standard sleepers and rails complete the deck.

The rafters have two collar beams (the uppermost one might be plywood) and a ridge beam. A solid triangular-shaped wood member supports the monitor roofs sheathing.