V.A.O.T. Historic Bridge Committee Meeting

Historic Covered Bridge Committee Notes of October 24, 2008 for:

Taftsville Covered Bridge, Woodstock

3rd Floor Conference Room 1, 9:00 - 11:30 A.M.

• Attendance:
  • J.B. McCarthy
  • Warren Tripp
  • John Weaver (project manger)
  • Wayne Symonds
  • Eric Gilbertson
  • Nancy Boone
  • Bob McCullough (recorder)
  • Joe Nelson (Vermont Covered Bridge Society)
  • Ron Joy (McFarland-Johnson, Consulting Engineers)
  • Neil Daniels (restoration contractor)
  • Note: Phil Swanson (Woodstock Town Manager), invited but unable to attend
• Introduction by Warren Tripp: Project identified via application to FHWA Covered Bridge Program (Senator Jeffords legislation) in the amount of 1.5 million, which may not be enough. In addition to consulting services provided by McFarland-Johnson, stone artisan Michael Weisberg of Thistle Stoneworks issued a report regarding the center pier and abutments. The project involves a number of difficult problems.
• Project Presentation by Ron Joy: The Taftsville Bridge is a two-span, modified multiple king-post truss built in 1836 by Solomon Emmons, and reinforced by laminated arches, introduced during the early 20th century. Arches and trusses work independently for much of the load. The east span is 88 feet (arch contains 10 laminates) and the west span is 100 feet (arch contains 12 laminates). Longitudinal distribution beams were added c. 1970, and the deck is a nail laminate surface protected by runner planks.
  

  The bridge is on a Class 2 town highway, carries approximately 1,000 vehicles per day, and is posted for 8 tons. Other crossings are located nearby, but the desirable load rating is 10 tons. Vertical clearance is 10 feet, limited by metal knee braces that the town would like to remove. Elevation clears the 100 year flood mark, reaching to just below the lower truss chord.

  Note: Subsequent discussion revealed that the bridge is distinguished by its many curious modifications to both the superstructure and substructure. These modifications are character defining features of the bridge, relating directly to its historic significance - a chronicle of the varied techniques used to keep the bridge in service. The town of Woodstock takes very good care of the bridge and many of the modifications may have originated from projects initiated by the town.

  • Superstructure: Arches lean, and laminate defects include impact damage, weathering, splitting, rot, and distortion. The springing points of the arches are contained by uncertain methods in the stone center pier and abutments. Tie rods restraining lateral movement of arches are not effective. Trusses sag and portions of the hand-hewn lower truss chords reveal rot. Some diagonals reveal splits, checks, and rot. Some posts reveal splits, holes, and rot. Portions of the upper chords reveal splits, rot, and holes.
    • Proposal Presented by McFarland Johnson: Introduce new laminated arches capable of carrying 10 ton load, and install concrete pedestal to contain springing points of the arches. Repair or replace deteriorated truss elements. Remove distribution beams (redundant structurally and add to dead load). Replace steel knee-braces with timber struts. Install lateral bracing system.
  • Substructure: Stone abutments and pier are dry-laid stone of various types and courses. Both abutments are set on ledge and have been reinforced by a concrete skirt to protect against scour erosion; concrete bearing pads support truss chords and bolster beams. Some stone courses in the center pier appear to be damaged, and the pier also has been reinforced by a concrete skirt; timber cribbing provides bearing points for the truss chords. Strength, condition, and design of housing for arch springing points is unknown.
    • Proposal Presented by McFarland Johnson: Consider rebuilding stone abutments and center pier. Alternatively, for center pier consider installing a concrete cap to better distribute bearing loads from truss chords. Design a concrete pedestal to house springing points of arches. Least costly option employs minimal concrete work to abutments, including new backwalls and bearing seats. Alternatives available require further study regarding costs and methods.
• Discussion: Discussion ensued, focusing on several topics, including: 1. the merits of replacing the arches; 2. the integrity of the pier housing for the arches; 3. the integrity of the lower chords; 4. integrity of the floor beams; 5. integrity of the abutments and center pier, particularly the latter; and 6. construction methods, schedule and availability of funding.
  1. Arches: Before replacement may be considered, preservation plan requires a showing that historic materials have deteriorated beyond the point of repair. Various options for repair should be considered including partial replacement of deteriorated laminates; injecting epoxy consolidation; reinforcing bolt system; strengthening the x-bracing system. Consultant's report indicates arches are capable of carrying an 18 ton load, but the bridge only needs to carry 10 tons, which creates greater flexibility for options. The Pulp Mill Bridge in Middlebury has similar arches, which will be repaired. Difficulties of repair include partially disassembling the laminate. Concern about disassembling the bridge - and the difficulty of reassembling it without engaging in extensive replacement of materials - was also voiced. More information is needed.
  2. Arch Housing/Pedestals: Various options were considered for securing the arch springing points in a predictable manner by creating some form of housing, either concrete or steel, or with a pin-connection. Concerns about concrete wicking moisture into the wood were voiced, and various means for creating a moisture barrier were discussed. Merits of the existing system were also considered - it may be more functionally efficient than it appears. More information is needed.
  3. Lower Chords: Hand-hewn lower chords are important, and if arches carry most of the load, preservation of the lower truss chords becomes easier. Where severe deterioration exists, splicing or other appropriate treatments should be considered as a way to strengthen those chords.
  4. Floor Beams: The consultant - MJ - has yet to determine/present the extent of floor beam replacement or re-spacing of existing floor beams.
  5. Center Pier and Abutments: Proposal to install concrete cap, or lid, to provide more consistent distribution of bearing load, to establish a flat bearing surface, and to stabilize the center pier seems to be credible. However, concern about the long-term effects of such a cap on the pier's overall stability were voiced. Recommendation to install longer bolster beams was also discussed. Consultant recommended rebuilding the pier, but didn't adequately address the option of partial repair. What are the advantages to keeping the surviving stone? Is it possible to construct a shield or tall fender in front of the center pier, upstream, to protect it against the force of high water. Would that require additional maintenance by the town?
     

     More information is needed, and inquiries should identify experts who specialize in repairing stone piers and abutments. Abutments present fewer problems here, but repairs are needed in a few specific locations.

  6. Construction Methods, Schedule, and Funding: Temporary support systems will be required during construction, and those support systems should not be vulnerable to high water. New Hampshire has used a Bailey bridge as an effective means of internal, rather than external, support. Construction will not occur next year, and the funding available will not pay for the costs of repair to the superstructure.
• Summary: Given the bridge's many unusual structural components and systems introduced at various periods during the bridge's long history, and acknowledging that these unusual features are character defining and thus historically significant, the challenge is one of finding a balance between preserving those features and reaching an acceptable risk management. Is it better to leave well enough alone and watch the bridge cautiously, or to introduce improvements that may reduce risk of catastrophic loss but at the same time may destabilize systems that have been functioning reasonably well for a very long time.

More information is needed and a site inspection was scheduled for Friday, November 14th at 1:00 P.M. Town representatives will be invited and some means of traffic control will be necessary. J.B. McCarthy will communicate with the town.