Building a covered bridge is as much of an art as it is a science. Each timber and joint in the frame must fit precisely to evenly distribute the load. Stan Graton II, president of 3 G’s Construction, Holderness, NH, is up to the challenge. Graton comes from a family of covered bridge experts. His grandfather, the late Milton S. Graton, is a nationally recognized covered bridge expert and his father, Arnold Graton Sr. is also a noted expert.
Old World technology combined with state-of-the-art engineering will give the Smith Bridge of Plymouth, NH, the distinction of being the world’s strongest covered bridge, according to the Society for the Preservation of Covered Bridges. The bridge, which spans the Baker River, was closed to traffic in May of 1991, and later destroyed by fire on April 16, 1993.
The $3.1-million dollar reconstruction project is funded 80 percent state and 20 percent by the town of Plymouth. The bridge is slated for completion in the spring of 2001.
The 176 ft. (53 m) cedar clapboard and steel roofed bridge will have its original long truss with added arches design and will be able to hold 425 tons (383 t). This is quite an improvement from the original bridge constructed in 1850 with a clear span of 140 ft. 6 in. (42 m), which held 6 tons (5.4 t) and was, according to the New Hampshire Department of Transportation, built at a cost of $2,720.
The new bridge will also have the distinction of being the only two-lane covered bridge in the state, according to Graton. In addition the bridge will feature Interstate Grading HS 20 legal load capacity for Federal Interstate Highways, illuminated walkways and travel-ways, and have several fire prevention devices.
“The bridge will be so strong that four loaded tractor trailers can travel on it at the same time. The old bridge used 6 ft. by 6 ft. vertical posts and the new one will use 12 ft. by 12 ft.,” said Graton. “It should last at least 150 years.”
The $1.3 million timber portion of the project is being handled by 3 G’s Construction. The 250,000 board ft. (75,757 m) in timber had to be cut and framed by hand.
3 G’s Construction set up a temporary workshop in Sydney, NY, at the Unidyl Company to cut the timber. Next, the timber was sent to Canada for lamination. Laminated timber is used to help offset weathering and aging and is much stronger than untreated timber.
According to Graton, E.D. Swett Inc., general contractor for the project, drove 100 piles to a depth of 120 to 160 ft (36 to 48 m). Other contractors involved in the project were Hiltz Construction, responsible for the earthwork and P&G Scaffolding installed 20 ft. (60 m) of scaffolding for the project.
Abutments for the bridge are New Hampshire granite and were quarried at Swenson Quarry in Concord. The granite was salvaged from a previous bridge in Concord. Each block weighs 1 ton (0.9 t) according to Graton. The stone masons for the project are Hayden Hillsgrove. E.D. Swett provided a 35-ton (31.5 t) Grove crane and a 90-ton (81 t) American crane for rigging.
Herman Marcy of Littleton, NH, builder of the Smith Bridge in 1850, would be surprised to see modern technology and Old World techniques working hand in hand.