HISTORIC COVERED BRIDGE PRESERVATION PLAN
Vermont ’s covered bridges are widely recognized as important cultural, economic, educational, aesthetic, and historic resources. Most are owned by towns and continue to serve the state’s network of roads. Although public support for preserving them is high, many are vulnerable to well-meaning attempts at repair that result in inappropriate or inconsistent application of preservation standards.
The factors threatening Vermont ’s historic covered bridges are complex. For one thing, structural systems must be adequate for the increasingly heavy volume of traffic and loads that use these bridges. Unfortunately, the strength of their timber frames is often difficult to calculate. Replacement of original or existing materials, alterations that force changes to overall bridge dimensions and reinforcements that discourage maintenance of original structural systems are the harmful and unnecessary results. In short, the historic integrity of these bridges is at risk.
The goal of this preservation plan is for Vermonters to be able to say, fifty or a hundred years from today, that the state’s covered bridges are truly historic bridges, not a collection of covered bridges that were largely rebuilt as new bridges at the turn of the 21stcentury.
To pursue this goal, the Historic Covered Bridge Preservation Committee has been established. This committee will review the status of all historic covered bridges and make specific recommendations for each, relying on the priority of uses explained in Part 1 and the priority of treatments explained in Part 2. Organization of the committee and its responsibilities are described in Part 3. Finally, integration of the preservation plan and the Vermont Historic Bridge Program is described in Part 4. The committee’s recommendations for each bridge will be developed over time, as required, and will appear in Appendix A. As new recommendations are developed or existing recommendations modified, Appendix A will be amended accordingly.
The committee has established five principal objectives in developing the plan and its priorities for uses and treatments.
1. The historic physical and structural integrity of covered bridges should be preserved to the maximum extent possible.
2. Covered bridges should remain in use on the state’s network of roads whenever possible.
3. Towns provide the best opportunities for continued stewardship of covered bridges. Partnerships between towns and the State of Vermont should be established to assure consistent application of appropriate preservation practices.
4. The Historic Covered Bridge Committee will implement this plan through participation in the development and review of all projects involving historic covered bridges when state or federal funding is used.
5. An effective management system must be implemented and sufficient funding obtained. This strategy should be balanced, on the one hand, identifying bridges in very good condition and maintaining them adequately and, on the other, identifying bridges in very poor condition and preventing deterioration from becoming irreversible.
PRIORITY OF USES
The following uses for historic covered bridges are listed in order of priority. Preferences have been established to achieve two objectives. The first is a desire to maintain the historic use of these bridges as part of Vermont ’s network of roads. The second is a desire to preserve the structural integrity of historic members of these bridges to the maximum extent possible. The load capacity for each category varies.
(A) Special Use on Roads. Bridges will remain in use on roads but will be limited to very light traffic, primarily cars. This category of use assumes that alternative routes are available or are capable of being built at locations near enough to historic bridges to minimize inconvenience and to eliminate, or reduce to an acceptable level, any risk of damage by overweight vehicles. At the same time, alternative routes must not compromise the settings for these historic bridges. Creative designs for bridge approaches, intended to prevent use by overweight vehicles, are encouraged. However, these designs must also avoid damage to settings.
Structural Integrity. This category of use assumes that the structural integrity of historic members of these bridges will be preserved to the maximum extent possible. If bridges suitable for this category of use have been compromised by extensive alterations, they should be restored to their original design.
Capacity: The capacity sought is the maximum amount obtainable under the preservation treatments permitted for bridges in this category. Most bridges should be confined to one-lane traffic.
Preservation Treatments: Superstructure preservation treatments 1, 2, 3, and 4, in that order of priority, are preferable for bridges in this category.
(B) Limited Use on Roads. Bridges will remain in use on roads and will be limited to vehicles that do not exceed 40,000 lbs. This category of use also assumes that alternative routes are available or are capable of being built at locations that accommodate vehicles weighing in excess of 40,000 lbs. The proximity of alternative routes, the degree of risk that bridges will be damaged by overweight vehicles, and the historic structural integrity of bridges are the decisive factors in choices between Category A and Category B – Limited Use on Roads.
Structural Integrity. This category of use assumes that the structural integrity of historic members of these bridges will be preserved. If bridges suitable for this category of use have been compromised by extensive alterations, restoration should be considered.
Capacity: The maximum capacity for bridges in this category is 40,000 lbs.
Preservation Treatments: Superstructure preservation treatments 1, 2, and 3, 4, 5, and 6, in that order of priority, are preferable for bridges in this category.
(C) Alternative Transportation Use. Bridges will be adapted to alternative uses at their existing sites and restored. The setting of the bridge, including its approaches, should be preserved to the maximum extent possible. This category of use will require alternative routes. However, the design of any new bridge in close proximity to the historic bridge should avoid changing the setting of the historic bridge and should avoid interfering with views of the historic bridge. This category of use assumes that the structural integrity of historic members of these bridges will be preserved to the maximum extent possible.
Structural Integrity: This category of use assumes that the structural integrity of historic members of these bridges will be preserved. If bridges suitable for this category of use have been compromised by extensive alterations, restoration should be considered.
Capacity: The minimum capacity for bridges in this category is that required to carry dead load, snow load, and anticipated pedestrian or snow machine loading.
Preservation Treatments: Superstructure preservation treatments 1, 2, 3, and 4, in that order of priority, are preferable for bridges in this category.
(D) Relocation. This alternative presumes that none of the three preceding categories of use is feasible, and it is an alternative of last resort. Relocation may be considered for any of the first three categories of use, and all requirements for the category selected will apply.
(E) Bridges Subject to Exceptional Constraints. The circumstances of some bridges pose exceptional constraints. A few have been so drastically altered that repairs required for an acceptable carrying capacity would necessitate reconstruction of the entire bridge. In other cases, bridges have been reinforced by systems that make the historic structure redundant. Bridges subject to such constraints shall be assigned to this category with the hope that acceptable alternatives for preservation will develop in the future. During the interim, a greater variety of preservation treatments are available for preserving these bridges.
Preservation Treatments: All preservation treatments will be considered for bridges in this category.
PRIORITY OF TREATMENTS
The following superstructure treatments for historic covered bridges are listed in order of priority. Preferences reveal a desire to preserve the structural integrity of historic members of covered bridges to the maximum extent possible. Many of Vermont ’s covered bridges display ingenuity in timber craftsmanship and incorporate unique treatments designed to address specific problems on specific bridges; this tradition of ingenuity should be carried forward as well. Prioritization allows identification of treatments that are appropriate for the specific categories of use. Treatments should be considered in their order of priority to the maximum extent possible. Treatments should also be applied in order of priority to individual elements of bridges.
1. Retain all existing historic materials that have not deteriorated beyond the point of repair. Where existing rot or other damage is not severe enough to require replacement, the materials should be repaired rather than replaced. This treatment should be applied to each member individually, and deterioration of a large number of bridge elements should never justify the replacement of any single member capable of being repaired.
2. Replacement of existing materials in kind, meaning identical in species, quality, and dimension to the maximum extent feasible, or restoration of original materials and design. Preferably, material origins should be from the Northeast region of the country. If a different species or quality is considered and/ or materials from the Northeast are not available, substitutions may be considered with justification.
3. Application of historic methods of strengthening such as the application of sister lattices in Town lattice truss bridges.
4. Introduction of glu-laminated beams as a co-functional, reversible structural system. The beams must be designed to work in conjunction with the historic structural system to achieve required load capacity, and the historic structural system must be restored according to Preservation Treatments 1, 2, and 3.
5. Replacement of limited pieces of existing load-bearing members with materials identical in species, quality, and origin, preferably from the Northeast region of the country, to the maximum extent feasible. Dimensions may be larger but must not cause alterations to the dimensions of any other important bridge components. For example, increasing the depth of bottom chords of Town lattice trusses may increase capacity without requiring alteration to either overall bridge dimension or the design of the floor system.
6. Replacement of existing load-bearing members with glu-laminated members (beams or chords) of identical dimension.
7. Reinforcement of load-bearing members with non-obtrusive modern materials such as steel rods or plates, glass fiber, carbon plates, or other materials.
8. Protection of load-bearing members by the introduction of steel beams that provide a safety-net for the bridge. The redundant structure must allow the existing timber frame to continue functioning, and a minimum clearance between steel beams and floor beams should be designed. The purpose of this treatment is to protect the historic bridge in case of structural failure, not to increase carrying capacity.
9. Replacement of load-bearing members with, in order of priority: (a) timber of larger dimension but otherwise identical in terms of species and quality; or (b) timber of larger dimension and different species.
10. Replacement of existing load-bearing members with modern materials.
The following substructure treatments for historic covered bridges are listed in order of priority. Preferences reveal a desire to preserve the historic structural integrity of these abutments to the maximum extent possible and to use masonry materials that are consistent with existing materials whenever possible.
1. Masonry abutments, whether rubblestone or ashlar, shall be retained whenever possible and repaired rather than replaced. Repairs should be undertaken with like-kind materials, and all repointing should apply appropriate mortar. Bearing seats should be repaired in kind whenever possible. Drainage tubes or weep holes should be installed to channel runoff in all cases to avoid hydrostatic pressure behind the abutments.
2. If masonry abutments, or portion of abutments, have deteriorated to the point where repair with like-kind materials is not feasible, alternative materials may be considered. Where abutments have been undermined by stream flow, concrete underpinning may be installed. Where bearing seats are inadequate, concrete caps may be added.
3. If masonry abutments have deteriorated beyond the point of repair, they may be reconstructed with modern materials such as concrete. Ornamental treatments to produce texture such as veneers, form liners, acid washing, pneumatic blasting, bush-hammering, mechanical stamping or special form-work may be considered.
4. Existing concrete abutments should be repaired whenever possible or replaced in kind if deteriorated beyond the point of repair.
HISTORIC COVERED BRIDGE COMMITTEE
Mission . The mission of the Historic Covered Bridge Committee is to insure that the historic integrity of Vermont ’s covered bridges is preserved to the greatest extent possible. Toward that end, the committee must balance the needs of each project, including historic integrity, traffic volumes, vehicle weights, overall traffic needs, and local concerns.
Composition. The permanent members of the committee shall include the Vermont Agency of Transportation (VTrans) Structures Engineer, the VTrans Bridge Management Engineer, the VTrans Historic Preservation Officer, a VTrans special consultant, the two Co-Managers of the VTrans Vermont Historic Bridge Program, and two designees from the Vermont Division for Historic Preservation.
Meetings. The Co-Managers of the Vermont Historic Bridge Program shall call meetings and prepare a record of committee discussions and recommendations. Copies of meeting records shall be distributed to each participant after the conclusion of each meeting. VTrans project managers and staff will be invited to attend meetings when specific projects are being discussed.
Town Participation. One or more representatives from a town owning a covered bridge may be invited to attend any committee meeting convened for the purpose of making recommendations for appropriate uses or preservation treatments concerning that bridge.
Participation by Other Organizations and Individuals. The committee may invite other individuals or organizations to participate in meetings concerning specific projects. Such participants may include, but are not limited to, local historical societies, timber-frame restoration specialists, and the Vermont Covered Bridge Society.
Recommendations and Coordination with Section 106 of the National Historic Preservation Act of 1966 and its amendments. Committee recommendations reached by consensus of all participants are desirable. In the event that consensus is not achieved, a recommendation will be reached by vote. Permanent members each will be entitled to one vote. Towns owning the bridge in question will be entitled to one vote. The committee recommendation will be recorded and notification given to the VTrans Project Manager. Objections raised at meetings by non-voting participants will be noted in the record. Any voting participant may request further review of the recommendation and may take advantage of the provisions for resolving disputed projects, as outlined by the Vermont Historic Bridge Program Programmatic Agreement dated July 7, 1998 .
Committee recommendations meet the Secretary’s Standards for Rehabilitation and therefore are consistent with the requirements of Section 106 of the National Historic Preservation Act (Section 106). The VTrans Historic Preservation Officer is responsible for reviewing federally funded covered bridge projects under Section 106 through a "Programmatic Agreement ... Regarding Implementation of the Federal-Aid Highway Program in Vermont ." (PA). Committee recommendations will be recorded in matrix format and will be attached to the letter prepared by the VTrans Historic Preservation Officer as required in the PA. The letter will state that if circumstances necessitate departures from Committee recommendations or if there are significant project changes, the VTrans Historic Preservation Officer will justify and document them in writing. The VTrans Historic Preservation Officer may consult with individual members of the Committee or the entire Committee regarding project changes that might arise after the Committee’s review of the project.
Responsibilities. The committee will oversee implementation of the Historic Covered Bridge Preservation Plan and its incorporation into the Vermont Historic Bridge Program. Specific tasks will include:
1. Preparing and advocating for an agenda that fully funds and implements this plan as part of the Vermont Historic Bridge Program.
2. Identifying suitable funding programs for special needs such as stabilization and repair of bridges threatened with collapse.
3. Submitting recommendations for assigning each historic covered bridge to a specific category of use together with the appropriate preservation treatments applicable to that category.
4. Advocating that a plan is prepared for the maintenance and repair of each historic covered bridge.
5. Resolving any conflict among various parties concerning specific bridge projects.
6. Participating as needed during both the design and construction phases of each bridge project affecting an historic covered bridge.
7. Participating in presentations to the VTrans Transportation Research Committee when appropriate.
8. Evaluating the merits of innovative products and ideas available for the preservation of historic covered bridges.
9. Such other tasks as the committee, from time to time, determines are appropriate.
Communication. Participation by the committee is vital, and the committee must be given adequate information to determine whether design proposals are appropriate. Effective methods of communication with engineers, contractors, town officials, and members of the public who work with historic covered bridges are essential. Toward that end:
1. Project managers assigned to work on specific bridge projects should provide all documentation necessary for committee consideration. This may include, but is not limited to:
(a) identification of all major structural problems and major failures;
(b) estimates of structural capacity assuming all components are in good condition; and
(c) engineering standards for determining all features and materials to be replaced in order to achieve the required capacity.
2. In the case of projects for which detailed plans have not been developed by an engineer, the bridge owner should require the contractor to provide a detailed statement identifying all features and materials to be replaced. That report should be supplemented by one or more field inspections by the committee during any disassembly phase.
3. Contractors or engineers should notify the VTrans Historic Preservation Officer immediately in the event that new information is uncovered during the construction phase.
4. Towns should notify the committee about any proposed work prior to the commencement of that work.
INTEGRATION OF HISTORIC COVERED BRIDGE PRESERVATION PLAN
AND THE VERMONT HISTORIC BRIDGE PROGRAM
Ownership of Bridges. Partnerships among the respective towns, the Vermont Agency of Transportation (VTrans) and the Vermont Division for Historic Preservation (VDHP) are the preferred methods of stewardship. Towns will remain owners of these bridges but may enroll them in the Vermont Historic Bridge Program when projects involving substantial repairs are developed, provided bridges remain in highway use. Enrollment will take place when an easement agreement has been conveyed from the town to VTrans, formalizing the town’s commitment to preserving the bridge in perpetuity for highway use. After bridges have been enrolled in this program, VTrans will pay for all future costs of rehabilitation and major maintenance to the extent that federal and/or state funds are available for such work. VDHP will cooperate with VTrans in monitoring compliance with the preservation easements. Towns will be required to conduct the following routine maintenance tasks as part of the easement agreement:
1. Cleaning bridge components with compressed air and removing with hand tools any deposits of debris or dirt that may hold moisture.
2. Keeping drainage areas free of debris and channeling deck drains and approach run-off away from bridge elements below.
3. Removing all small trees and shrubs growing in, on, or near substructure units or under bridges.
4. Removing any debris that accumulates in the channels beneath bridges.
5. Maintaining proper load posting and advance warning signs and keeping all signs visible.
6. Maintaining a water-tight roof system and repairing any damaged siding.
7. Removing any accumulated snow when such snow is of a depth to cause concern for the stability of the structure.
8. Maintaining smooth transition between approach roadway and bridge decks, maintaining straight and continuous rails, and repairing minor damage caused by accidents.
9. Reporting significant problems concerning bridges to the co-managers of the Vermont Historic Bridge Program and to the appropriate District Transportation Administrator.
10. Consulting with the co-managers of the Vermont Historic Bridge Program prior to initiating any emergency repairs.
All historic covered bridges owned by the State of Vermont will become part of the Vermont Historic Bridge Program upon execution of this preservation plan and are the responsibility of state government. Partnerships between state agencies owning covered bridges may be developed in the future to address the question of financial responsibility for maintenance and rehabilitation.
Public Education. Success of the preservation plan will depend on public awareness about the value of Vermont ’s covered bridges and about threats to the historic integrity of these bridges. Toward that end, the program and other departments in VTrans should collaborate with VDHP to accomplish the following:
1. Educate towns about the importance of consistent preservation treatments for the state’s collection of covered bridges and encourage town officials to enroll eligible bridges in the Vermont Historic Bridge Program.
2. Establish and enforce a consistent policy regarding state and federal funding for town-owned covered bridges that are not repaired according to treatments recommended by this preservation plan.
3. Increase public awareness about the need to enforce load restrictions on historic bridges, undertake any legislative initiatives required to assure enforcement, and develop attractive, familiar signage that will notify truck drivers at appropriate locations to select alternative routes.
4. Conduct periodic training programs for state employees, town officials, and other interested individuals or organizations covering a broad range of topics, including but not limited to project administration and maintenance.
Objectives. Success of the preservation plan will also depend on continued growth of the Vermont Historic Bridge Program and its ability to address the specific concerns associated with preservation of covered bridges. Toward that end, the following should be accomplished:
1. Develop VTrans engineering expertise in the preservation, restoration, and rehabilitation of historic covered bridges.
2. Develop the ability to apply new technology and treatments to the preservation of historic covered bridges, as those technologies and treatments become available.
3. Conduct research and develop design specifications for the preservation, restoration, and rehabilitation of historic covered bridges. Research should focus on analysis of structural materials and the interaction of components and should lead to evaluation of structural components in place through non-destructive testing.
4. Develop design specifications for the construction of new timber framed covered bridges and the rehabilitation of existing historic covered bridges.
5. Prepare design criteria for roadway approaches to historic bridges that remain in use on the state’s network of roads and for bridges that are placed in alternative transportation uses. Criteria should address a number of issues including, but not limited to, discouraging large vehicles from using these bridges. For bridges in alternative transportation use, criteria should seek to preserve the bridge’s original setting to the greatest extent possible.
6. Develop an appropriate policy for privately owned historic covered bridges. Preservation of these bridges is desirable, but a public interest in these structures must be assured through partnership agreements, preservation easements, or outright conveyance. Legislative initiatives may be necessary to achieve these objectives