Historic Covered Bridge Committee Meeting
Cambridge Junction: Poland Covered Bridge
Guilford: Green River Covered Bridge
Attendance: VTrans: J.B. McCarthy, John Weaver, Mark Sargent, Pam Thurber, Judith Ehrlich, and Bob McCullough (Historic Bridge Program); Preservation Trust of Vermont: Eric Gilbertson; Vermont Division for Historic Preservation: Jamie Duggan; and Vermont Covered Bridge Society: Warren Tripp.
Introductory Report: J.B. McCarthy began the meeting with a summary of the three projects and the minor amount of work required for each.
Pulp Mill Covered Bridge: The bridge suffered damage when a Green Mountain Power Company truck, estimated weight 10,000 lbs, struck one of the vertical posts and diagonal web members near the base of the vertical member on the upstream truss. The bridge is posted for four tons. Two small sections of one of the laminated arches were also damaged, possibly a different occurrence. One of the bolts extending through the laminated arches was bent and a portion of its washer extends out from the arch. Following a discussion, committee members agreed to the following treatments.Other Discussion: Eric Gilbertson explained that officials from the Town of Montgomery had notified him that the old Longley Covered Bridge, which has been replaced and is being stored, would be available for reconstruction at a new site.
Approved Treatments: The two-piece diagonal member will be replaced with a single member, and a protective steel plate angle, half-inch, will be installed over the damaged section of the vertical member, with spacers or holes drilled in the plate to allow air circulation.Poland Covered Bridge: Due to overloading of this bridge, posted for three tons, the arches have become distressed at the point of ties to the bottom chords, the area where the arch laminates are weakest. The most severe stress is visible in racking of the northeast arch at its bearing area. Following discussion, the following treatments were approved by committee members.
Approved Treatments: Arch members on the northeast arch will be replaced to the point of the first arch joint. All arch bearing areas will receive reinforcing steel plates mounted outside and inside of the arch planks. To reduce deadload, the existing heavy wheel runners will be removed and replaced with one-inch oak runners for all season use and with additional plywood runners during winter when the bridge is closed but used by VAST.Green River Covered Bridge: This is an ongoing project, and the town of Guilford has requested that the project engineers, Hoyle, Tanner and Associates, prepare a load rating study outlining the repairs needed to increase the load capacity of this bridge to eight tons. The engineers anticipate that this will require strengthening lattice plank ends, removal of the longitudinal distribution beams, and replacement of the deck. Town officials questioned whether, if town funds are used to pay for this additional work, review by the Historic Bridge Committee would be necessary. Committee members noted that the project is currently receiving federal and state funding and the work should not be separated for purposes of avoiding review by the committee. The committee will review final plans when they become available.
Discussion then focused on the feasibility of strengthening lattice plank ends, with observations by John Weaver that the testing of spliced planks has shown that the planks attain only about twenty per-cent of their strength in tension and about eighty percent in compression. In sum, splicing is an ineffective treatment for strengthening lattice planks. John Weaver also noted that the floor beams are not spaced very far apart, and thus a plank deck would probably be adequate, making heavy laminated deck systems unnecessary.
Approved Treatments: Arch members on the northeast arch will be replaced to the point of the first arch joint. All arch bearing areas will receive reinforcing steel plates mounted outside and inside of the arch planks. To reduce deadload, the existing heavy wheel runners will be removed and replaced with one-inch oak runners for all season use and with additional plywood runners during winter when the bridge is closed but used by VAST.