In pursuit of its commitment to preserve the State's historic wooden bridges the Vermont Agency of Transportation initiated a Long-Range Planning Study of Town-Owned Covered Bridges. This Study was conducted by a consulting engineering team led by McFarland-Johnson, Inc., of Binghamton, New York. Mr. Phil Pierce served as the Project Manager for the Study. The focus of the Study was to develop a long-range plan for preservation of all of the bridges for the indefinite future.
The Study included an exhaustive evaluation of all aspects of the bridge, including current and future traffic needs, as well as an evaluation of the structural condition and needs of the bridge. Five preservation plans were investigated at each bridge -- from keeping the bridge open to moderate traffic to rehabilitation of the bridge or constructing a bypass structure if the covered bridge was found to be unable to economically and/or safely support the traffic needs. No covered bridges were to be destroyed -- all to be preserved whether able to remain open to traffic or not.
The Study was completed in 1995 and included seventy-five covered bridges and culminated in a report for each bridge which was provided to its Town owner to help them decide which preservation action to pursue for the bridge.
During the course of the survey, some of the bridges were found to be theoretically unsafe, in which case traffic weight restrictions were recommended until such time as more extensive engineering evaluation was possible.
An example of one such study, that of the Thetford Bridge, is presented here.
The Vermont Agency of Transportation (VAOT) in a continuing effort to promote public safety and accommodate current and future traffic demands, is developing a long-range plan for the historic covered bridges located throughout the state.
The plan provides bridge specific traffic and structural data to local communities. The communities are then able to make better informed decisions involving repair, rehabilitation, or replacement of their covered bridges relative to both local transportation planning and the overall state transportation network system.
This plan has been prepared by a team effort, led by McFarland-Johnson, Inc. with support from several specialty support people/firms. Appendix E presents a listing of participants and involvement.
It is the objective of the VAOT and the Vermont Agency of Development and Community Affairs Division for Historic Preservation to preserve all covered bridges within Vermont. Many preservation actions are possible. It must be recognized, however, that most of the structures included in this study are currently carrying traffic and remain an important part of a community's transportation system. Therefore, practical options must be identified for consideration.
As a result of this Study, a course of action involving one of the following options will be recommended at each site:
It must be recognized that this statewide study of a large number of covered bridges has been ongoing for an extended period of time. Accordingly, this report may not address the latest developments at this particular bridge site, such as accidents, new structural failures, or findings of significance as a result of biennial VAOT bridge inspections.
Since this report deals with a covered bridge, which is a rather unique type of structure, a glossary of technical terms is presented in Appendix F to facilitate the review of this document. The appendix also contains a diagram of various types of truss configurations to further assist the reviewer.
This study addresses the Thetford Center Covered Bridge, located in Orange County in the east-central portion of the State. In the Town of Thetford, the Thetford Center Bridge extends across the East Ompompanoosuc River on Town Highway 29 (Tucker Hill Road), west of State Highway 113, in Thetford Center.
An unusual characteristic of the Thetford Center Bridge is it's structural configuration consisting of a Haupt Truss in combination with a through arch. The Thetford Center Bridge, is believed to be the only remaining covered bridge in the state and in the Northeastern United States to use Haupt patent trusses. The truss, which resembles a combination of the Town Lattice and Multiple Kingpost, was invented by Herman Haupt in 1839.
The bridge was strengthened in 1963. The existing timber deck and floor beams were removed and replaced with a nail laminated timber deck supported by four longitudinal steel beams. The original stone abutments were capped with concrete (the west abutment was also faced with concrete) and a reinforced concrete pier was constructed at the midspan of the bridge.
The Thetford Center Covered Bridge, also known as the Sayres Covered Bridge, is currently listed on the National Register of Historic Places. The National Register is a federal program, administered by the National Park Service, which identifies historic resources of national significance. A detailed account of the structure is contained in the "National Register of Historic Places Inventory - Nomination Form" presented in Appendix A.
A summary of the bridge's physical characteristics is provided below.
|Timber Truss Configuration||Haupt with Arch|
|Number of Spans||2|
|Measured Length (End to End)||127.0 feet|
|Gable Overhang (Each End)||1.0 foot|
|Measured Horizontal Clearance||18.08 feet|
|Measured Vertical Clearance at Truss||9.67 feet|
|Measured Vertical Clearance at Center of Bridge||11.40 feet|
|Approach Roadway Surface||Asphalt|
|Load Posting||16,000 pounds|
The two primary topics involved with this Study are structural needs/capacity and traffic needs/capacity. To obtain the necessary data several techniques were employed. The techniques included site visits, questionnaire surveys, and review of state and federal documents.
For the collection of general data, bridge sites were visited by representatives from the VAOT, McFarland-Johnson, and the Town.
As a service to local communities, the VAOT regularly inspects all publicly owned covered bridges located throughout the State and documents pertinent traffic and structural information. A copy of the June, 1992 Bridge Inspection Report, Bridge Inventory, and Estimated Traffic Volumes are presented in Appendix B.
Bridge and traffic survey questionnaires were sent by McFarland-Johnson to community representatives. The bridge survey addressed the physical characteristics of the bridge as well as local financial resources committed to bridge maintenance and repair. The traffic survey addressed existing and proposed land use relative to traffic volume and circulation patterns. Both survey questionnaires are presented in Appendices C and D.
The Thetford Center Covered Bridge has a timber deck supported by four steel beams which is independent of the bridge's timber trusses. In accordance with the directives of the state-wide study, no structural analysis was required for this structure.
The traffic evaluation considered a variety of issues. These issues included site specific characteristics such as existing and projected traffic volumes, type of vehicle, land use, environmental constraints, and local policies toward development. The evaluation process entailed the following:
The area of influence for this study was defined as the approximate area encompassed by a one-half mile radius around each bridge.
Figures 2 and 3 depict the location of the Thetford Center Bridge in the midsection of the Town of Thetford. Figures 5 and 6 presents general photographs of the structure and approaches.
According to the "Thetford Town Plan" (1993) the dominant land use is woodlands which account for approximately 75% of all Town land use. Approximately 20% of Town lands are used for crop and pastureland.
The Town of Thetford adopted its present zoning ordinance in October 1972 and amended them in March 1992. Currently, the Town has three zoning districts as described below:
Iformation received from the Town indicates there is currently no short or long-range planning issues and no major land subdivision or building applications pending. Therefore, there are no impacts to the Thetford Bridge due to increased traffic associated with new development.
Over the past 13 years an average of 23 new residences have been built per year in the town.
The Town of Thetford "Town Plan" considers itself as not being a major economic center due to limitations caused by lack of municipal services, lack of suitable industrial and commercial locations and the Town's rural character. Thetford functions more as a bedroom community to serve economic centers in Hanover, Lebanon and Hartford.
As shown on Figure 2, the current Town Highway network consists of approximately 63 miles. There are no Class 1 roads, 12 miles of Class 2 roads, and 51 miles of Class 3 roads. The bridge serves a Class 3 road, Town Highway 29 (Tucker Hill Road).
Interstate 91 is the main north-south route in the Town of Thetford. Access to the Interstate is at Exit 14 via State Highway 113. Additional north-south routes include State Highways 113 and 5 and state-numbered Town road (Route 132). The main east-west oriented route is a combination of State Highway 113 and the portion of Route 132 that lies to the west of the hamlet of Rices Mills.
Since Town Highway 29 serves as a connector between the two State Highways 113 and 132, this Town road and the bridge could be considered part of a primary east-west link through the Town.
The VAOT analyzes accidents on State highways. From the most recent data (1983-1987) there are no road segments or intersections within the Town listed as high accident locations. According to the Town Plan, there are approximately 30 accidents per year within the Town of Thetford.
It was observed that traffic crossing the bridge did not reduce speed even with the one lane, limited sight distance of the covered bridge.
A goal of the Town is to maintain and plan for a network of roadways within the Town that will provide safe and adequate transportation balanced with the desire to retain the scenic beauty and natural areas of the Town.
Part of the evaluation of preservation options identified in subsection 1.1 is the consideration of available alternative routes. A transit of the local transportation network led to the following observations:
According to 1994 VAOT data, estimated average daily traffic (ADT) volume on the bridge for the year 1988 was approximately 800 vehicles per day.
As stated in the Town Plan, traffic on Town roads had an approximate growth rate of 4.4% annually from 1972 through 1980 but decreased to a rate of approximately 2% per year during the 1980's.
An estimated average daily traffic volume of 1,120 vehicles per day on the bridge is projected by the VAOT for the year 2009. This represents a growth in traffic of 40% or 2% per year.
The East Thetford Village is the most likely area to become congested with traffic. Two State Highways (113 and 5), Exit 14 from Interstate 91 and the bridge to New Hampshire are all within the Village's immediate vicinity. This Village is however over four miles from the Thetford Covered Bridge and therefore its development will not directly impact the traffic volumes at the bridge.
Following the initial site visit, it was determined that a detailed assessment of traffic issues was necessary for the Thetford Center Bridge. Therefore, traffic counts were taken.
The Thetford Center Bridge warranted a traffic analysis for several reasons. VAOT traffic volumes confirm, given the existing study area's land use with Town Highway 29 serving as a primary link between State Highway 113 and Route 132, that there are traffic generators.
To quantify traffic volume impacts to a road segment's capacity, traffic engineers utilize accepted standards from the 1985 Highway Capacity Manual (HCM). On two-lane rural roads high speed, while beneficial, is not a principal concern. The use of delay, as indicated by the formation of platoons, and the utilization of capacity become more relevant measures of service quality. Percent time delay reflects both mobility and access functions, and is defined as the average percent of time that all vehicles are delayed while traveling in platoons due to the inability to pass. The utilization of capacity reflects the access function, and is defined as the ratio of the demand flow rate to the capacity of the facility.
Since this report is considered a very general planning and policy study of a twolane rural road, the related percent time delay criteria for each level of service is applied. This is considered the primary measure of service quality. The manual has lettered categories A through F with each successive letter describing a progressively deteriorating Level of Service (LOS) for a particular road segment. Specifically, LOS A would provide drivers with delays of no more than 30 percent of the time by slow moving vehicles, LOS B 45 percent, LOS C up to 60 percent, LOS D approaching 75 percent and LOS E greater than 75 percent with passing virtually impossible. LOS F represents heavily congested flow with traffic demand exceeding capacity. The highest volume attainable under LOS E defines the capacity of the highway. Under ideal conditions the maximum service flow rate is 2800 passenger cars per hour, total in both directions. Governmental agencies generally accept levels of service A through D as a measure of quality of service.
The projected volume of 1,120 ADT on a two-lane normal rural highway would indicate a planning LOS B for a rolling terrain and LOS B for a mountainous terrain. Table 8-10 of the HCM was entered with the forecast ADT to determine the level of service. However, since the covered bridge's approach roadway is a two-lane, two-way highway and the bridge is a one-lane structure, one vehicle must stop and yield to on-coming traffic. This situation is not normally addressed by a HCM LOS analysis. Modification of the analysis is the most appropriate method of determining the LOS or operational condition at the bridge site.
For this modified analysis the bridge was considered an unsignalized intersection with the stopped or yielding vehicle considered a vehicle attempting a left turn from a major street. In computing the LOS for this situation, assumptions are dependent upon the distance between vehicles that comprise the on-coming traffic and the behavior of the driver waiting for a gap in the oncoming traffic. This provides a methodology related to general delay ranges from LOS A with little or no delays through LOS E and F with very long traffic delays. Under this scenario, the waiting vehicle would operate under a LOS A, with little or no delay given a projected traffic volume of 1,120 ADT.
This two-lane, town road provides access to an area of the Town with good Levels of Service defined by a range of A through B.
For one week in May of 1993, twenty-four hour traffic counts were taken at the Thetford Center Bridge. The counts are summarized below.
|Day of the Week
||Total Daily Volumes
|total weekly traffic||6816|
|average daily traffic (ADT)||974|
|average weekday traffic||1001|
|average weekend day traffic||906|
Since the traffic loading is supported by the independent, two-span, continuous structural beam system and timber deck, the timber trusses of the covered bridge function only to support the shell of the bridge. As such, a structural analysis of the trusses is not required for this study. However, please refer to Section 8 for further discussion of maintenance of the covered bridge trusses and roof structure.
Review of VAOT load posting information indicates that the current floor system superstructure has a load posting capacity in excess of 60,000 pounds, based on the standard AASHTO two-axle vehicle configuration. This capacity is sufficient to support the types of vehicles typically traveling across this bridge.
An evaluation of various maintenance repairs was performed to facilitate continued use of the structure as a covered bridge. A field survey of the bridge was conducted in September, 1993.
At that time, the following deficiencies were observed:
A discussion about the condition of the structure is contained in the VAOT Bridge Inspection Report, presented in Appendix B. Pertinent bridge dimensions are shown on Figure 7. Photographs of relevant portions of the structure are presented in Figures 8 and 9.
The bridge is posted for a legal load limit of 16,000 lb.
Referring to the preservation options outlined in subsection 1.1 of this report, considerations are summarized as follows:
Having considered the traffic needs at this site, condition of the structure, and merits of various preservation options, we have identified Option B as the most appropriate shorter-term course of action to provide for preservation of this covered bridge for the future. That is, continue to use the structure for vehicle weights of up to 60,000 pounds. A longer-term course of action should include consideration of Option.
This structure has previously been modified, allowing it to be used by heavy vehicles. Additional strengthening is not required; however, large vehicles must exercise extreme caution when crossing the structure due to its limited vertical and horizontal clearances.
The timber trusses of this covered bridge must be maintained in sound condition to safely support the forces imposed by their self weight and that of snow loading. That loading, in and of itself, can be significant, rivaling that imposed by vehicular traffic on authentic, complete covered bridges. Hence, the trusses must be routinely monitored for deterioration and repaired as necessary, to remain in "good condition."
The preceding paragraph makes reference to a structure in "good condition". That terminology indicates physical configuration and material properties similar to that at the time of original construction, i.e. "like new". Good condition components have no significant defects, such as: cracks, crushing, buckles, areas of rot, insect attack, or impact damage. Good condition also implies proper connections including tight and solid joinery and no missing components.
Accordingly, repairs are to utilize "in-kind" replacement. When in good condition, the trusses can be considered sufficiently strong, based on their extended service without collapse. Conversely, covered bridge structures are known to have collapsed from the effects of snow loading, if not in good condition.
We recommend the following repair measures to improve current conditions and to support the commitment for long-term preservation:
The cost for repairs identified in Option B, excluding miscellaneous timber maintenance, is estimated to be approximately $85,000.
To assist the Town in implementing these recommendations, we offer the following general discussion. The State statute limitations for timber deck structures on Town Highways relate to the posted weight limitation of the structure. Operators of vehicles with weights in excess of the posted limitation are required -to obtain a permit from the Town to cross the structure. Section 6.0 of this report provides information on the theoretical capacity of the structure, which may exceed the statute limitations and/or posting capacity, and indicates the maximum weight for permit vehicles. It is important that the Town strictly adhere to, and enforce, the posting and permitting requirements, including all Town-owned vehicles. Use of the structure by heavier vehicles risks damage to, and potential collapse of, the bridge.
Because it is the Town's responsibility to maintain these structures, and because wooden covered bridges require different attention than concrete and steel bridges, general guidance on maintenance and repairs is offered in Appendix G.
(Reference, in part: Covered Bridges of the Northeast by Richard Sanders Allen, 1985)
RECOMMENDATIONS FOR FUTURE MAINTENANCE
Regular maintenance and proper repairs can help preserve these unusual structures for an indefinite period of time. The following discussion highlights good maintenance measures.
RECOMMENDATIONS FOR FUTURE REPAIR PRACTICES
A number of examples of poor quality past repairs are evident in existing covered bridges. The following discussion highlights some of the common problems and the more appropriate repair measures.