Note: Subsequent discussion revealed that the bridge may have been built as early as the 1880s as an open bridge, and trusses mayor may not have had protective weatherboard cladding. Roof system probably added c. 1904. The bridge may be one of the last surviving examples of Vermont's once-ubiquitous open bridges, some of which were subsequently protected by a roof. So few of these bridges have been available for study that little is known about their design variations, typical span lengths, joinery, or methods for protection against the elements. The bridge is unique.
Structure is half-through multiple king-post truss with floor system of wood floor beams supporting plank deck. However, the deck system has been extensively altered and the wood floor beams are supported by steel stringers and floor beams, in turn supported by two large steel I-beams running through the bridge longitudinally. Secondary frame on top of trusses is a four-panel, multiple king-post truss and supports the roof. Clearance at knee braces is 7 feet 9 Inches.
Trusses do not carry their own dead load and the flex of the steel beam under live load stresses the trusses, which suddenly must support their own dead load. Thus, steel beams are not compatible with the timber truss. The existing truss bottom chords do not have sufficient capacity to carry timber dead load and cannot be rehabilitated to carry dead, live, and snow loads without replacement of truss members with new, larger, and modern timber materials.
Note: Reinforcing historic timber truss systems with a secondary structural system of steel is not considered an acceptable treatment under the preservation plan. Steel may be used only as a safety net.
Site elevation does not meet the 10 year flood requirement, and overflow elevation is roughly equal to the elevation of the bottom chord. It might be possible to raise the bridge, but the broad overflow area provides a measure of protection.
Proposal for rehabilitation is to introduce a secondary co-functional, reversible structural system using glu-laminated beams extending through the bridge longitudinally. Two options are available:
Rehabilitation could also increase vertical clearance approximately 16 inches by placing the 6 inch nail-laminated deck, protected by 2-inch thick runners, directly on new 10-inch square floor beams, which will be spaced closely at 32 inches. Increasing vertical clearance would not alter the reveal of the historic trusses. As required under Preservation Treatment 4, the existing trusses and deck system will be restored in accordance with Preservation Treatments 1,2, or 3. Replacement percentages are as follows: truss verticals: 17%; truss diagonals: 26%; truss chords: 0%; floor beams: 100%.
The bridge's substructure is in fair condition but scour exists at the base of the easterly stone abutment. Steel sheeting backed by a concrete poured to the top of the piling will be installed as a shield for the stone, extending slightly above the water line. In addition, a stone cap will provide a new bearing pad on that same abutment.
Discussion continued on the question of whether a nail-laminated deck required the protective surface of runners shown on the proposed plans by John Weaver, which would increase retention of moisture and add maintenance problems for the town. Discussion also extended to the relative merits of using runner planks as an alternative. Concerns about traditional appearance, driver safety, and damage to the nail-laminated deck by snowmobile cleats were voiced and no amendment to the motion was offered on this point.
Meeting Adjourned 10:45 A.M.
[This article was originally posted December 7, 2007]