Bog Covered Bridge - Cilleyville Bridge, Andover New Hampshire

Date added: March 20, 2024 Categories:

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Built in 1887 by local carpenter Prentice C. Atwood, the Bog Bridge served as a highway bridge until 1957, when it was bypassed by a new state highway. Although now used as a pedestrian bridge, little changed since its construction, save for the necessary replacement of its roof in 1982. The bridge's two Town trusses, which seem to have survived intact, are representative of those used in the state's fourteen Town through truss highway bridges. The only major difference is the omission of the upper secondary chord, an omission common to the five shortest Town truss bridges.

Today, the bridge is better known as the Cilleyville Bridge, the name commonly used in covered bridge books and lists. That name is historically confusing, as another nearby covered bridge, built in 1883 and replaced in 1909, was known as the Cilleyville Bridge. Bog Bridge was the name used at the time of the bridge's construction.

The first covered highway bridges in New Hampshire appeared in the 1820's. The covered wooden truss bridge remained in common use throughout the 19^th century and the early 20^th century. In the late 19^th century and the early 20^th century, the wooden truss bridges were increasingly challenged by the new bridges of iron, steel, and concrete. But, in rural New Hampshire, the new bridge materials came into use very slowly. The Town of Andover, for example, did not build its first iron bridge until 1909. As late as 1937, a covered wooden highway bridge was built over the Contoocook River between Hancock and Greenfield. Wooden covered bridges once numbered in the hundreds in the state of New Hampshire. However, the heavy automobiles and trucks of the 20^th century required wider and stronger bridges. And the number of covered highway bridges has declined dramatically. Today, there are only forty-three covered highway bridges surviving in the state. (Eight of these bridges, including the Bog Bridge, have been restricted to pedestrian traffic.)

Fourteen of the forty-three remaining covered highway bridges employ the Town truss, one of the most popular truss types in the 19^th century. Patented by Ithiel Town in 1820, the Town truss was both versatile and relatively easy to erect. The truss, composed of a lattice of diagonal overlapping planks, held together by horizontal chords, could be built in any length and did not require any special skills of its builders in bridge design or construction. Any competent and experienced carpenter could build a Town truss. The versatility of the truss is demonstrated in New Hampshire by its use in both the Prentiss Bridge in Langdon, now the shortest covered highway bridge in the state, and in the Cornish, NH-Windsor, VT Bridge, now the longest covered bridge in the United States. The popularity of the Town truss can be seen in Andover, where it was used for all seven of the town's covered highway bridges, including the two survivors, the Keniston Bridge and the Bog Bridge.

The Bog Bridge was built in 1887 to replace an earlier bridge over Pleasant Brook on Bog Road, a back road connecting the villages of West Andover and Cilleyville. The bridge was erected by local builder Prentice C. Atwood (1834-1903)9, described in his obituary as "a natural mechanic, spending most of his time carpentering", who lived in Andover from the Civil War until his death. In 1883, Atwood had built another covered Town truss bridge over the Blackwater River in the village of Cilleyville, barely eight hundred feet from the Bog Bridge. So, he was already experienced in the field of bridge construction. The only contemporary record of the construction appears to be the financial report in the Town of Andover's annual report. The payments for the bridge were made between September 24^th, 1887 and February 27^th, 1888. However, the principal payments, for the lumber on October 3^rd, and for the labor of Atwood and three of his four major helpers on October 17^th, show that the bridge was completed in the fall of 1887. Of the total cost of $465.00 for the actual construction of the bridge, materials accounted for $331.81, labor for $130.22, and a freight bill for supplies for the remaining $2.97. Atwood himself received $39.50 for "19 3-4 days' labor on bridge." Additional payments totaling $57.63 were made for fixing the abutments, "grading and railing" the bridge approaches between October 3^rd, 1887 and February 27^th, 1888. The total cost for the bridge project to the Town of Andover was $522.63.

The Bog Bridge has seen relatively few changes since its construction. The bridge did develop a slight lean soon after its completion. A local tale attributes the lean to disgruntled carpenters, who became angry with Atwood and cut some timbers short to eventually produce the lean. A far more likely cause was the lack of lateral bracing in the bridge structure, which would allow the trusses to lean. The lean disappeared in 1957 when the bridge was straightened up, and other minor repairs were made. After the 1938 hurricane and the accompanying flood, the damaged west abutment was rebuilt with cement mortar. But the rebuilt west abutment does not now appear very different from the untouched east abutment. The addition of supplemental metal bolts in the trusses was probably after the construction, but cannot now be dated. And presumably, the sheathing and the floor planks have been renewed when necessary. The most serious damage to the bridge occurred on March 9^th, 1982, when the roof collapsed under the weight of an unusually heavy snow load. At the 1982 annual town meeting that soon followed the roof collapse, the voters decided to repair the bridge. Contractor Donald Evans built a new roof and resheathed the gables, completing his work in July. The new roof was modeled on the old roof. So, there is little evident change on the exterior, although, the new roof trusses and ceiling are obvious in the interior.

Bog Bridge's survival and fine state of preservation can be attributed to its relative unimportance. The bridge served Bog Road, a back road connecting the two small villages, and Elkins Hill Road, a dead-end road with few houses. Five of the seven covered bridges in Andover, all built in the late 19^th century, were replaced by new concrete, iron, and steel bridges to accommodate the automobile and truck traffic of the 20^th century. However, the light traffic over the Bog Bridge did not justify either its replacement or the strengthening of the bridge with additional elements, such as steel girders. In 1956 and 1957, Route 11, the main east-west road through the town of Andover, was relocated to bypass the villages of Potter Place and Cilleyville. The new state highway crossed the Blackwater River north of its junction with Pleasant Brook and passed the Bog Bridge, barely a hundred feet to its north. North of the new Route 11, Bog Road was replaced by another relocated state highway, Route 4A. The new roads eliminated the need for Bog Bridge. But, Andover's voters decided to keep the bridge, voting at the annual town meeting in March 1956 to close the bridge to all but foot traffic, when the new highway was completed, and, at the annual town meeting in March 1958 to close the approaches to the bridge and to preserve the bridge as a local landmark. The 1982 decision to replace the collapsed bridge roof demonstrates the continued determination of the townspeople to preserve this now little-used bridge as a reminder of a past era.

The Bog Bridge is the youngest of the fourteen covered highway bridges using the Town truss still surviving in the state, the others ranging in date from the 1827 Bath-Haverhill Bridge to the 1885 Edgell Bridge in Lyme. It does not, however, differ significantly from the other bridges. The principal features of the truss design, such as the dimensions of the timbers and the angle of the lattice planks, are typical of the other Town truss bridges. The one major difference is the omission in the Bog Bridge trusses of the upper secondary chord. Most of the state's Town truss bridges have, besides the primary top and bottom chords, two secondary chords, one just above the bottom chord, and one just below the top chord. However, on the five shortest bridges, one or both secondary chords were omitted. On three bridges, the Waterloo Bridge in Warner, the Cold River Bridge and the Prentiss Bridge in Langdon, there are no secondary chords at all. The two Andover bridges, the Keniston Bridge and the Bog Bridge, have only lower secondary chords on their trusses. The shorter spans of these five bridges, and the consequent reductions in stress, apparently convinced their builders that the secondary chords could omitted without reducing the strength of the structures. The other features of the Bog Bridge's design, the granite abutments, the floor beams resting on the bottom chord, the heavy floor planks, the board sheathing on the sides and in the gables, and the long wooden shingled gable roof, are found on the majority of the state's covered highway bridges. The one other uncommon feature of the Bog Bridge's design is another omission, the lack of lateral bracing between the trusses. The typical covered highway bridge in New Hampshire has transverse and/or diagonal wooden beams, and sometimes metal tie rods, between the two top chords and the two bottom chords of its trusses, to keep the trusses vertical and correctly spaced. Save for the two metal tie rods between the bottom chords and, possibly, the portal beams between the top chords, the Bog Bridge has no lateral bracing. The omission of such bracing between the trusses on the bridge was probably an economic move, justified, no doubt, by the shortness of the bridge. And, despite the slight lean that eventually developed, the bridge does not seem to have suffered from the lack of lateral bracing.

Bridge Description

The Bog Bridge is a covered bridge that crosses Pleasant Brook (also known as Pleasant Stream) near the junction of the brook with the Blackwater River in the village of Cilleyville, in the town of Andover. The bridge was built at its present site in 1887 as a highway bridge, but has been a pedestrian bridge since 1957, when the bridge and a short section of Bog Road, on which it stood, were bypassed by a new state highway, Route 11. The wooden bridge is a Town through truss bridge, with two Town lattice trusses set on granite abutments. The bridge, rectangular in plan, is sheathed with vertical boarding and covered by a wooden shingled gable roof. The bridge is set on a west-north-west/east-south-east line, with its easterly portal facing the village of Cilleyville and its westerly portal facing nearby Route 11 and Elkins Hill Road. The bridge floor is 53 feet long and 13 feet wide (measuring from truss to truss). The outside length of the bridge is approximately 56 feet, measuring from gable to gable, and the outside width is 16 feet, measuring from outer wall to outer wall. The clear span between the bridge abutments is approximately 49 feet.

The two abutments are built of rough-cut granite blocks, laid in rough courses. The east abutment is laid without mortar. However, the west abutment was rebuilt with cement mortar after the 1938 hurricane and flood. The abutment walls are continuous with similar granite block retaining walls that stretch along the bridge's approaches. As the approaches were a little wider than the bridge floor, the stone retaining walls typically continued the lines of the bridge walls. However, the southerly retaining wall of the western, approach does run off at an angle, bearing more to the south. Both approaches are now covered with gravel and grass and occasional bushes. The trusses usually rest on large wooden blocks, set on the abutments. Only the east end of the downstream (north) truss rests directly on the granite abutment. The bottom chords of the trusses overlap the abutments by about two and a half feet at the east end, but only by about one and a half feet at the west end.

The two Town lattice trusses, each about 15 feet high, are built primarily of planks, usually 2½ inches deep and 9½ inches wide. The lattice is composed of two overlapping sets of diagonal planks, sloping in opposite directions, but all at an angle of approximately 60 from the horizontal. Parallel planks are placed about two feet apart. The overlapping lattice planks are joined at each junction by two wooden trunnels. The lattice planks are connected by three horizontal chords, a bottom chord (set about one foot above the bottom of the lattice), a top chord (set about one foot below the top of the lattice), and a secondary chord (set about two feet above the bottom chord and about one foot above the bridge floor). The chords are composed of planks of the same dimensions as the lattice planks, running horizontally on both sides of the lattice. The bottom chord has three inner planks and three outer planks, the top chord has two inner planks and two outer planks, and the secondary chord has just one plank on each side of the lattice. The junctions of the bottom chord and the top chord with the lattice planks are each marked by three trunnels, joining the truss members together. However, the junctions of the secondary chord with the lattice planks only have two trunnels. The wooden trunnels have been supplemented in all three chords by metal bolts holding the chords together. The trusses are trapezoidal in shape, as the top chords are both approximately three feet longer than the bottom chords, the top chords overhanging the bottom chords by approximately one and a half feet at each end. The slanted ends of the trusses are each marked by a slanting endpost, a heavier timber than the lattice planks, tied into all three chords by metal bolts. The lattice planks at the ends of the trusses are simply butted up against the slanting end posts. The bottom chords of the two trusses are tied together by two metal tie rods spanning the entire width of the bridge, one near each abutment. The top chords are joined at each end by a horizontal beam spanning the portal and resting on top of the ends of the two top chords. Each portal beam is supported by two heavy braces, rising from the slanting endposts at each end.

The heavy transverse floor beams rest on the bottom chords of the two trusses. The floor beams usually span the entire width of the bottom chords fitting into the triangles formed by the lattice planks and the bottom chords. (The floor beams are usually two and a half to three feet apart.) At the ends of the bridge, however, this pattern is broken, the last two floor beams at each end resting only on the inner planks of the bottom chords and not penetrating the lattices. The last floor beam at each end is also supported by the abutment, resting on the granite abutment at the east end, on a poured concrete sill atop the granite abutment at the west end. The heavy longitudinal floor planks are laid on the transverse floor beams. A second layer of longitudinal planks, about ten feet wide, was laid in the center on top of the first layer of floor planks, to create a travel lane. Another second layer plank, a transverse plank, spans the full width of the bridge floor at the west end of the floor. A sill of rough granite blocks is found at the east end of the bridge floor.

The long lateral sides of the bridge are sheathed with vertical boards nailed onto the chords and horizontal timbers on both trusses that run the full length of the bridge about three feet below the top chord. Save at the very ends of each lateral side, the vertical boarding does not extend above this horizontal timber. The sheathing is interrupted by thirteen untrimmed, tall, narrow rectangular openings on each side. (Some of the lateral sheathing is now missing.) The portal ends have newer vertical boarding, dating from the roof replacement of 1982. Both gables are sheathed with such boarding, as are the triangles formed by the end-posts, the portal beams, and the portal braces. Each portal opening is about twelve feet high in the center. (At the west portal are found two simple wooden railings on the sides of the bridge approach, each supported by the endpost of the truss and an outer wooden post.)

The long gable roof, replaced in 1982, is supported by light kingpost trusses, each composed of a horizontal beam (which rests on the top chords of the two trusses), two rafters, and a kingpost, all bound together by plywood gussets. The rafters extend beyond the lateral walls to support the roof overhang. The board roof is sheathed with wooden shingles and trimmed by ridge boards. The open eaves, overhanging on the sides and the gables, have exposed rafters and fascia boards.