Keniston Bridge, Andover New Hampshire

Date added: March 29, 2024 Categories:
Southeast side and northeast portal (1988)

Erected in 1882 by local builder Albert R. Hamilton, the bridge has been in almost continuous use for over a century. The bridge has been repaired and strengthened over the years, most notably in 1949 and 1981. The bridge's two Town trusses, although repaired and supplemented by additional elements, remain largely intact and are representative of those found on the state's fourteen remaining Town through truss highway bridges. The only significant difference is the omission of the upper secondary chord, an omission seen on the five shortest Town through truss bridges. In most other aspects of its construction, such as the floor beams and planks, the lateral bracing, the gables, and the roof, the Keniston Bridge is typical of the covered highway bridges of New Hampshire. It deviates from the majority of the surviving covered bridges only in the partial sheathing of its sides.

The 1820s saw the appearance of the first covered wooden highway bridges in the state. The covered wooden highway bridge remained a common bridge type throughout the 19th century and into the first decades of the 20th century. In the late 19th century and the early 20th century, the wooden truss bridges faced increasing competition from the new bridges of iron, steel, and concrete, which could carry heavier loads. In rural New Hampshire towns, like Andover, however, the newer bridge materials were adopted very slowly. Andover did not build its first iron bridge until 1909. A covered wooden highway bridge, containing some hidden structural steel, was built over the Contoocook River, between Hancock and Greenfield, as late as 1937. Wooden covered highway bridges formerly numbered in the hundreds in New Hampshire. But, the heavier and faster vehicles of the 20th century required wider and stronger bridges. So the old wooden bridges have been largely replaced by bridges of more modern materials and design. Today, there are only forty-three covered highway bridges still standing in New Hampshire. Of these forty-three bridges, only thirty-five are still in actual daily use as highway bridges.

One of the most popular truss types found among the forty-three remaining covered bridges is the Town truss, which was used in fourteen of the bridges. The Town truss is named for its inventor, Ithiel Town, who patented the design in 1820 and promoted its use through both publications and sales agents. The Town truss became one of the most popular bridge types in 19th-century America. Composed of a lattice of diagonal overlapping planks, held together by horizontal planks, the truss could be built to any length by any competent carpenter. A Town truss did not require any special skills in bridge design and construction, and was therefore ideally suited for rural communities, where bridge experts and civil engineers were not to be found. The adaptability of the truss can be seen in New Hampshire, where both the state's shortest covered highway bridge, the Prentiss Bridge in Langdon, and the longest covered bridge in the nation, the Cornish, NH-Windsor, VT Bridge, employ the Town truss. In the town of Andover, the Town truss was used exclusively, for all seven of the town's covered highway bridges, including the two remaining bridges, the Bog Bridge and the Keniston Bridge.

The Keniston Bridge was built in 1882 to replace an earlier bridge over the Blackwater River on what is now called Bridge Road. The bridge was erected by local builder Albert R. Hamilton (1833? - 1912). Hamilton was an experienced and respected builder, who had already built the Town Hall (1879) and two covered Town truss bridges (both in 1880) for the Town of Andover. The only available contemporary record of the bridge's construction is the financial report in the Town's published annual report. The payments for the bridge were made between April 25th and August 6th, 1882. However, most of the payments are dated in May, including those for the lumber (May 27th) and the wages of Hamilton and six other workers (May 16th and 27th), indicating that the bridge was probably completed in that month. The total cost of the bridge was $745.57, of which $521.68 was for materials, $217.64 for labor, and $6.25 for miscellaneous items (freight and the use of a team). A.R. Hamilton received $62.50 for "25 days work".

The Keniston Bridge owes its survival to its location on a little used back road, serving a rural neighborhood with only a few houses. Five of Andover's seven covered bridges were replaced between 1909 and 1935 by new bridges of iron, steel, and concrete to accommodate the automobiles and trucks that were replacing the carriages and wagons. But, the light traffic over the Keniston Bridge could not justify its replacement.

The bridge has seen some changes, both repairs and additions to the structure to enable the bridge to carry heavier loads. Some of these changes are difficult to date, notably the addition of metal bolts in the trusses and the periodic renewal and repair of the floor planking, the sheathing, and the roof. In the spring of 1949, the bridge "started to crack apart and sag toward the river", a failure which the Andover selectmen attributed in their annual report to an overload, as well as the poor state of the roof. Repairs costing $2984.19 were made under the supervision of the state highway department. In their report, the selectmen noted that the repairs included "jacking up the structure, splicing or replacing the defective lattice work, and shingling the roof". Unfortunately, no other town or state records can be located that describe these repairs in further detail. Undoubtedly, the splices now seen in the lattice planks date from the 1949 repairs. And it seems not unlikely that the replacement of the top chord and the removal of the bottom chord on the downstream truss, and the addition of the steel L-beams and supplemental wooden chords to both trusses, also date from this reconstruction. By the late 1970s, the bridge was again in need of further work, with the abutments, some structural timbers, and floor planks all in need of repair or replacement. The 1976 annual town meeting voted that the selectmen appoint a committee to study the bridge and report at the next annual meeting. At the 1977 annual town meeting, the voters, given the choice of paying for the repairs immediately or establishing a capital reserve fund for the future repair of the bridge, chose the latter course. The capital reserve funds and additional funds were finally voted for the bridge repairs at the 1981 annual town meeting. In that year, contractor E.D. Swett overhauled the bridge, according to plans prepared by Scheyd Construction, completing the work in the early fall. The 1981 rehabilitation included the replacement of defective timbers, the renewal of the floor and lateral sheathing, the reconstruction of the upper portions of the abutments and the facing of the northeast abutment with concrete, and the addition of two steel girders under the floor to enable the bridge to carry heavier loads. Despite these changes, the Keniston Bridge still retains its original trusses, albeit repaired and strengthened. The bridge's exterior appearance is little changed since the earliest known views of the structure, as the elements added in the 20th century are rather effectively hidden behind the lateral sheathing and underneath the bridge. Save for the concrete abutments, the casual traveler sees little of the work that has enabled the bridge to continue in use.

Bridge Description

The Keniston Bridge is a covered wooden bridge that crosses the Blackwater River on Bridge Road in the town of Andover. Built in 1882, the bridge is a Town through truss bridge, with two Town lattice trusses set on granite abutments, now partly rebuilt with concrete. The original trusses have been strengthened by added wooden chords and steel beams. (The downstream truss has also seen the removal of its original bottom chord, and the replacement of its top chord.) The trusses have been supplemented by two steel girders that run the full length of the bridge and give additional support to the wooden bridge floor. The bridge, rectangular in plan, is sheathed with vertical boarding in the gables and on the lower portions of the lateral sides. The bridge is covered by a long, wooden shingled, gable roof. The bridge is placed directly across the river on a northeast-southwest line, with the portals at the northeast and southwest ends. The bridge floor is 16 feet in width, as measured between the two trusses, and 65 feet in length. The outside dimensions are 19 feet in width, measured from outer wall to outer wall, and 73 feet in length, measured from gable to gable. The clear span between the bridge abutments is approximately 51 feet.

The abutments were built of roughly coursed granite blocks laid without mortar. The upper portions of the abutments have been replaced by poured concrete abutments set on the remaining lower portions of the granite abutments. The concrete upper abutments are L-shaped in profile, each having a taller inner wall. At the outer ends of the concrete abutments are higher piers which support the trusses. Approximately four feet of each end of the two trusses rest on these concrete piers. The riverside face of the northeast abutment has also been faced by a poured concrete wall. The stone walls of the granite abutments are continued as retaining walls, particularly as upper retaining walls for short distances along the sides of the bridge approaches and continuing the lines of the bridge sides. The northeast abutment's walls are also continued as angled riverbank retaining walls on both sides of the bridge. On the downstream (southeast) side of the southwest abutment is a poured concrete retaining wall that covers the riverbank between the covered bridge and a nearby railroad bridge.

The Town lattice trusses, both originally about sixteen feet high, are each built of two overlapping sets of diagonal heavy planks (each two and a half inches deep and eleven inches wide) slanting in opposite directions, but both set at an angle of 60 from the horizontal. Parallel lattice planks are usually placed about two and a half feet apart. Each junction of two lattice planks is marked by two wooden trunnels, now supplemented by two metal bolts. A few lattice planks on the upstream (northwest) truss and many lattice planks on the downstream (southeast) truss have been spliced, the splices being made with two wooden blocks, one on each side of the lattice plank, tied together by eight metal bolts, four on each side of the joint. Each truss originally had three horizontal chords, each composed of heavy planks of approximately the same dimensions as the lattice planks. The original bottom chord, at the base of the lattice, on the downstream truss has been removed. The surviving bottom chord on the upstream truss has three planks on the outside of the lattice planks and three planks on the inside of the lattice planks. A secondary chord, located about six inches above the bridge floor in each truss, is composed of one plank on each side of the lattice planks. Finally, the top chord, at the top of the lattice on each truss, is composed of two inner planks and two outer planks. Each junction of the top and bottom chords with the lattice planks is marked by three wooden trunnels. But, the junctions of the secondary chord with the lattice planks each only have two trunnels. The original wooden trunnels have been supplemented by newer metal bolts in all of these chords. (The top chord on the downstream truss is a replacement, with four metal bolts, instead of the usual trunnels, marking its junctions with the lattice planks.) The original chords in each truss have been supplemented by another wooden chord (directly above the bottom chord on the upstream truss and in the corresponding position on the downstream truss) and by two steel L-beams, one on each side of the lattice planks between the floor beams and the original secondary chord. The supplemental wooden chord in each truss is quite similar to the original chords, being composed of three inner planks and three outer planks of approximately the same dimensions as the other truss planks, joined to the lattice planks and tied together by metal bolts. Resting on the floor beams are the two steel L-beams of both trusses, each with its flat section on the floor beams, and its vertical section against the lattice planks. The L-beams are joined together and tied to the lattice planks by horizontal metal bolts and are tied to the supplemental chords and the bottom chord of the upstream truss by vertical metal bolts which run through the wooden chords to square metal plates on the undersides of the lowermost chord. The trusses have slanted ends, as the top chords are about eight feet longer than the bottom chords and overhang the bottom chords by four feet at each end. At each end of both trusses is found a slanted endpost, a heavy timber tied into the truss by both wooden trunnels and metal bolts at its junctions with the wooden chords. (The bridge is anchored to the riverbank by two metal lines, formed of long metal rods, with hook and eye connections, one at each end of the bridge, bolted through the top chord of the upstream truss and running to a buried anchor on the nearest riverbank.)

Resting on the supplemental wooden chords of both trusses are the heavy wooden floor beams. The wooden floor beams are now also supported by the two deep steel I-beam girders that are set on the new concrete portions of the abutments. The two longitudinal steel girders are tied together by five small transverse steel girders bolted to their inner sides. The longitudinal girders are also tied to the lowermost chords of the wooden trusses by metal tie rods, five from each girder to the nearest chord. Four of the transverse wooden floor beams have been replaced by steel girders, which, like the wooden floor beams, rest on the longitudinal girders and the supplemental chords of the trusses.

The wooden floor beams have also been strengthened by the addition of heavy planks, one on each side of each beam and joined to it by metal bolts. These strengthening planks do not run the full length of the floor beams, appearing only in the center of the floor beams above the longitudinal girders. At each end of the bridge, on the outside of the truss endposts, are found transverse wooden beams, resting on the inner walls of the concrete abutments, which serve as sills for the bridge floor. Resting on these wooden sills, the transverse floor beams, and the four replacement girders, is the first layer of the bridge floor, heavy longitudinal planks that cover the width of the bridge interior, between the steel L-beams of the trusses. A second layer of transverse floor planks is laid on the first layer to create a ten-foot wide travelway in the center of the bridge. Finally, a third layer of longitudinal planks is found on top of the second layer for the two-wheel lanes, each over a yard wide. (Bridge Road is a gravel road, but short sections of the road at both ends of the bridge have been paved up to the bridge floor.)

The lateral bracing between the two top chords of the two trusses is formed of both wooden beams and metal tie rods. The five transverse metal tie rods, linking the top chords together, are evenly spaced, creating four "bays" in which are found the crossed diagonal wooden beams of the lateral bracing, two beams in each bay, which run from one corner of the bay (the junction of the tie rod and the top chord) to the opposite corner. The lateral bracing is completed by four transverse wooden beams, which rest on top of the top chords and the diagonal braces and are tied to the diagonal beams and the top chords by metal bolts. The board roof is supported by lightweight rafters resting on the top chords. The roof overhangs on the lateral sides and the gables. The open eaves have exposed rafters and fascia boards. The gable roof is sheathed with wooden shingles and trimmed by ridge boards.

The sheathing on the lateral bridge sides is five feet deep, covering the bottom chord, supplemental chord, L-beams, and secondary chord of each truss. The slightly battered outer sheathing of vertical tongue and groove boarding with beveled edges is topped on the outside of the lattice planks by a sloping board sill. On the front of the truss endposts is found horizontal tongue and groove boarding, which rises to the same height as the sheathing on the lateral sides. The two gables are filled with vertical boarding nailed to the roof rafters and horizontal beams set on the top chords of the trusses. The lower edge of the gable boarding is simply shaped, being higher in the center and lower at the ends, the transitions being marked by diagonal sections. The portal opening is approximately thirteen feet high in the center.

Keniston Bridge, Andover New Hampshire Southeast side and northeast portal (1988)
Southeast side and northeast portal (1988)

Keniston Bridge, Andover New Hampshire Southwest portal and southeast side (1988)
Southwest portal and southeast side (1988)

Keniston Bridge, Andover New Hampshire Northwest side and southwest portal (1988)
Northwest side and southwest portal (1988)