Covered timber bridges have been etched deeply into the American landscape over the years. They symbolize the beauty of a unique engineering art which coexists in aesthetic harmony with the natural beauty of a scenic area.
Besides their tradition and heritage, timber bridges also are efficient, graceful and economical. They are probably the only American structures still built with engineering and design techniques dating back two centuries.
The Smith covered bridge over the Baker River in Plymouth, N.H., is a huge, uncommon span that combines the flavor of early America with modern construction technology. It replaces a historic bridge on the same site.
Richard Roy of the National Society for Preservation of Covered Bridges said the new span is the world’s strongest covered timber bridge. It is designed for a HS 20 load rating capacity.
The bridge can handle the maximum legal vehicle-weight limits of New Hampshire, up to 80,000 lb. Describing the two-lane timber bridge, subcontractor Stanley Graton said, "We’re building a bridge that’s similar to one that was here 150 years ago, and hopefully it will still be here 150 years from now." Graton’s family tree includes a long list of timber bridge pioneers. His grandfather started dismantling old timber bridges and became so impressed with their craftsmanship he began a campaign to save and restore covered bridges throughout New England.
Wood work
The Smith bridge is 176 ft long and 39 ft wide, including a 24-ft, two-lane roadway and a 5-ft 4-in. sidewalk. It features a combination buttressed arch and truss design as the main structural members.
The Southern Pine glued laminated timbers in the truss, which is 22 ft high, have multiple 6-in. x 12-in. top chords and 8-in. x 15-in. bottom chords. Vertical columns and diagonal web braces in the truss are double 12-in. x 12-in. laminated timbers.
The glulam arches are 30 in. deep and 10 1/2 in. wide. The massive arches come in three sections which weigh 4,800 lb each.
The rafters supporting the metal roof of the bridge are 4-in. x 12-in. members with 6-in. x 8-in. cross braces and 8-in. x 8-in. diagonal sway braces. The floor joists are 10 1/2 in. x 30 in., supporting laminated deck panels.
The deck is not paved with asphalt or concrete, but is covered with 3-in.-thick oak strip as a wearing course for traffic.
Phil Holowacz, vice president of Unadilla Laminated Products, Unadilla, N.Y., said his firm laminated about 250,000 board ft of timbers with waterproof glue for the bridge. All laminated materials are pressure and fire retardant treated.
The contractor worked closely with Unadilla to lay out precise measurements for the span.
Steve Glines, New Hamphire Department of Transportation contract administrator, said, "The planning group considered a concrete or steel project, but decided that the timber span recaptured the aesthetics and tradition of early New England covered bridges. Also, we found that the timber bridge is cost competitive with other material and should actually last longer than a steel bridge."
The bridge superstructure, including the timber framing, was designed by Ryan-Biggs Associates, Troy, N.Y.
Robert Canham, P.E., of Ryan-Biggs said, "The bridge site is about three miles outside the boundary of the White Mountains National Forest. The White Mountains are famous for harsh weather. We had to consider heavy snow loads and wind loads in our design. The design ground load in the vicinity of the bridge site is about 75 lb per sq ft."
"We used treated structural glued laminated timber for much of the bridge’s fabric, for reasons of strength, durability, appearance and cost efficiency," said Canham.
"Solid sawn timbers of the size and quality required for this bridge are simply not available, and probably never were available on this continent. The only way you might find them is if you were to cut down acres of virgin tropical rainforest to get the giant old-growth Teak trees. Not only would that be untraditional for a covered bridge in New England, it wouldn’t be very environmentally sensitive or economically efficient, either."