By Edith Tucker
December 23, 2015
RANDOLPH — The 1918 Snyder Brook pony truss railroad bridge, located on the Presidential Recreational Rail Trail east of the Appalachia trailhead on the south side of Route 2, was lifted by a crane onto rebuilt abutments before noon on Monday, Dec. 14. It had been lifted off its abutments so they could be repaired a year ago on Dec. 5, 2014.
The project is sponsored by the state Bureau of Trails and the state Bureau of Historic Sites, both in the Department of Resources and Economic Development (DRED).
Extensive stabilization and repair was done this fall by large-project contractor Dennis Thompson and his crew from Northern New England Field Services (NNEFS) LLC, of Stewartstown because the fast-flowing Snyder Brook that tumbles down from the col between Mt. Madison and Mt. Adams had undermined its east abutment, so that some of the cut stones had fallen onto the streambed.
Last December, Thompson, whose crews often work on seawall and other large-rock projects, numbered the cut stones that made up the dilapidated east side abutment.
An excavator retrieved some that had fallen into the brook; others were recovered later when water levels were low. The state Division of Historic Resources had rejected the idea of pouring new concrete abutments as being historically incorrect, although both cement and rebar were placed behind the longer stone wall to avoid future freeze-and-thaw damage.
As Thompson discussed the project it became clear that this kind of high-stakes repair work depends on science and engineering plus art and experience.
Guided by those with knowledgeable eyes, the bridge settled perfectly onto new wood beams when the crane slowly swung it back into place. This winter snowmobilers, cross-country skiers and trail runners will be able to cross the bridge. The sheathing that covered the wood beams for nearly a century was removed for the move, temporarily replaced by sheets of plastic for this winter.
The bridge will be re-sheathed in the warm weather months. The protective siding that boxed the trusses of the bridge were removed a year ago before the bridge was lifted from the rail trail, allowing the slings to be threaded through the diagonal wood trusses and vertical bolts and engineers to watch out for signs of undue tension during the move.
The rail trail was closed to public use at the bridge site, and last winter snowmobilers and other users had to use a short NH Corridor 12 reroute in the White Mountain National Forest (WMNF).
James Garvin, retired state architectural historian, inventoried the bridge for the National Society for the Preservation of Covered Bridges.
“The Snyder Brook bridge is believed to be the last surviving railroad boxed (covered by shiplapped vertical wooden sheathing boards) pony truss bridge in substantially original condition in the U.S.,” Garvin writes. “The Boston & Maine Railroad (B&MRR) was pre-eminent among major American rail lines in maintaining the use of wooden bridges into the 20th Century.”
“This type of railroad bridge is a rare survivor as a historic asset,” said Historic Sites director Ben Wilson. “The bridge is an important interpretive resource in explaining the development of railroads in New Hampshire.”
Garvin writes that the clear span of the Snyder Brook bridge is 28 feet, nine inches. Its width, from centerline of truss to centerline of truss, is 17 feet, 10 inches. The truss has two lengthened floor beams that serve as outriggers; these are connected to the upper chords by diagonal wooden braces and steel tension rods and maintain the trusses in plumb (exactly vertical) condition.
These outriggers increase the total width of the bridge to about 31 feet.” “The floor beams are suspended below the bottom chords in a fashion that was employed on most B&MRR wooden bridges and are placed slightly under two feet on centers. The stringers, ties, and rails that rested on top of the floor beams have been removed to convert the bridge to trail use, and the beams [are] covered by a floor of wooden planks, laid diagonally.”
Garvin points out that the concept of using wood for compression members and iron rods for tension members in trusses was adopted in the U.S. for both for roof trusses of buildings and in a popular and successful bridge truss, the Howe truss, patented in 1840 by William Howe of Mass., more than 50 years later than in England. Steel gradually was used in long spans to replace wood, but continued to be cost-effective for use in short spans into the 20th century.
Benjamin Wilder Guppy (Class of 1889, M.I.T.) worked for the B&MRR from 1890 to 1950, Garvin reports, and had become bridge superintendent when the Snyder Brook and Moose Brook bridges were both built in 1918.
“The Snyder Brook bridge is larger and heavier in its details than a typical Howe pony truss bridge used on the B&M in 1895 and was capable of bearing the loads imposed by larger classes of locomotives that were introduced in the early 20th Century,” Garvin writes. “These included Pacific locomotives, weighing approximately 380,200 pounds with their tenders, which became the workhorse of the B&M system in the early years of the century. Snyder Brook bridge continued to sustain the loads imposed by all B&M rolling stock until service on the Berlin Branch was discontinued in 1996.”
The Snyder Brook bridge “retains integrity of location, setting, materials, workmanship, feeling and association... and that the recreational trail [with its] undeviating straightness... and its bed of ballast and cinders preserve the integrity of feeling and association despite the change in transportation function,” Garwin says. “The timbers of the Howe trusses, the vertical steel tension members, and the cast iron fittings that create the panel points of the trusses all remain intact and in good condition."