Structural Details of the Bridge Wrights Covered Bridge, Claremont New Hampshire
Ithiel Town, in his 1835 patent, already foresaw doubled lattice trusses. Occasionally they were built as highway bridges, but the principal use was for railroads. Snow pointed out that single lattice trusses could safely carry railroad loadings in spans up to 80', but they tended to warp under heavy loads. The double lattice was more rigid because it functioned something like a box girder.
Many authorities have called the doubled truss a Town-Pratt railroad lattice, on the theory that T. Willis Pratt was involved in its development. Pratt worked for a time as an engineer with the Eastern Railroad, one of the many lines that later formed the Boston & Maine system. In the early 1870s, he patented three little-known bridge designs, apparently inspired by his railroad work. One of these, patent no. 137,482 of April 1, 1873, was for a wooden plank truss assembled with treenails that is sometimes loosely described as a lattice. Pratt is said to have used it for at least one railroad bridge, and from these facts has arisen the designation "Town-Pratt lattice."
Pratt's 1873 patent was really a Warren girder in form, with no resemblance to a lattice except in its construction details (use of plank assembled with large treenails). Snow himself discussed the origins of the doubled Town lattice in his 1895 paper, and he made no mention of T. Willis Pratt. Instead, he says he followed the precedent of David Hazelton (1832-?), who had been Master of Bridges and Buildings for the old Boston & Lowell Railroad. Hazelton was a bridge wright, not a college-trained engineer, but Snow had a warm admiration for his abilities. Hazelton no doubt drew upon the craftsman tradition of New England bridge building tracing directly back to the influence of Ithiel Town himself. The designation "Town-Pratt" for the doubled lattice truss seems to be in error.
J.P. Snow understood the capabilities of the doubled Town lattice truss, and he varied it to suit different applications. In some structures, such as the former Hillsborough, New Hampshire railroad bridge, he varied plank size through the length of the span, using wider plank towards the ends. His major refinement in truss design was an ingenious solution to the problem of critical least section in lower chords. These members are in tension, and unless the chords are single sticks through the entire length of the span, the joints are a problem. In Town trusses the chord planks butt directly together end for end, with no scarfjoint. There are several thicknesses of plank in the chords and the joints are carefully staggered, but where there are joints, those planks that continue solid are the only ones carrying the load. Snow devised a method of connecting the plank ends by means of jib bars bridging the joint, fastened with rods bored through the width of the timber.
Because of the long span of Wright's Bridge, Snow used a different modification. He included a mammoth laminated arch in between the two sets of lattices. The lattice itself is lighter than usual and does not have secondary chords on top. This suggests that the arch was intended to carry most of the load. Construction plans for Wright's Bridge have not been located, but the former railroad bridge at Goffstown, New Hampshire was similar. Its plan included a note plainly stating that the arch was to carry most of the load.
The arches are built up of twenty-two leaves of mostly 2-3/4" x 9" plank, but the bottom three leaves are thinner. The bottom two measure 7/8" thick and the third is 1-3/4". This feature probably facilitated bending the plank to the required curve. The total assembled depth is 4'-9 3/4", not 6' as sometimes stated. The iron hanger rods are of 1-3/4" diameter stock on 4'-6" centers, which corresponds to the spacing of the lattice joints. They do not support the floor beams directly, but instead pass through holes bored in the central section of the three-part lower chord and are fastened below this with large washers. The rods are all equipped with turnbuckles to permit the correct load to be transmitted from the chord to the arch.
Because of the arch, the lower chords differ from other railroad lattices such as the nearby Pier Bridge, one mile to the east. At Pier, the lower primary chord uses tripled 3-3/4" x 14" plank inside and outside, and doubled 2-3/4" x 14" for the central part between the two lattice webs; iron jib bars connect the plankjoints at their ends. At Wright's, the inside and outside parts of the lower primary chords use two 2-3/4" x 11-3/4" planks. The center uses the same size plank but it is tripled to make room for the arch. The lattice webs are thus spaced further apart than usual. There are no jib bars at the chord joints in Wright's Bridge.
The lower secondary chords are of the same dimensions as the lower primaries for the center section, but use only 2-1/4" x 11-3/4" plank for the inside and outside parts. The top chords were not measured because of difficult access, but there are no secondary chords, and the primaries appear to be of the same dimensions as the lower secondary chords. This light construction indicates that the arch was intended to do most of the work.
The lattice planks vary from 2-1/4" x 11-1/2" to 2-1/4" x 11- 3/4", with a slight increase to 2-1/4" x 12" at the very ends of the bridge. The nearby Pier Bridge, which does not have an arch, uses thicker plank, 2-3/4" x 12" in the centers of the spans, increasing to 2-3/4" x 12-1/2" and finally 2-3/4" x 13" at the ends. Joints at Wright's Bridge are spaced apart 4'-6" on center for width, and about 7'-4 1/2" for height. The joints for the two webs are staggered, which is the usual practice. The truss is three diamonds high.
The upper lateral bracing is of Howe style, but with the addition of four heavy wooden cross timbers to which ship's knees are fitted as sway bracing, a very elegant feature. The central two ship's knees foot upon the arch, while the end two foot upon vertical posts added to the truss. The lower lateral bracing is also of Howe style.
The floor beams are 7-1/2" x 17-1/2" with some variation, on 2'-3" centers, i.e., half the lattice spacing. Since the lattice webs are staggered, each can support a floor beam in its place. The beams are suspended below the lower chords by short hanger rods from wooden blocks, which bridge alternately between inside chord and center or center chord and outside, as the lattice web allows. Stringers measure 6"xl0"; there are four, grouped in two pairs under the former rails. They rest atop the floor beams, and are cut out around the lower lateral bracing. The railroad ties are still in place, but the rails are gone. There is now a longitudinal plank deck for the recreational trail.
The abutments are of large granite blocks in regular courses, and mortared. They were probably built new for the present bridge, as the previous one was 16' shorter. The arches end on large wooden skewbacks footing upon stones that jut out from the abutment wall specifically for this purpose.
