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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.


    1. Purpose and Objectives
    2. Bridge Location and History
    1. Introduction
    2. Structural Evaluation Methodology
    3. Traffic Evaluation Methodology
    1. Study Area of Influence
    2. Study Area Land Use
    1. Existing Roadway System
    2. Future Roadway System
    3. Alternative Route Evaluation
    1. Existing Traffic Volumes
    2. Projected Traffic Volumes
    3. Traffic Analysis
    1. Purpose and Objectives

      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:

      1. Close the structure to vehicular traffic, with traffic diversion to the existing transportation network,
      2. Continue use of bridge for light vehicular traffic, with heavier truck traffic diverted to other routes in the local network,
      3. Close the structure to traffic and construct an adjacent bypass structure,
      4. Rehabilitate the structure to safely support moderate traffic, or
      5. Other options, such as moving the existing structure to a nearby preservation site with structure replacement on the existing 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.

    2. Bridge Location and History

      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
      Sidewalk Provided None
      Approach Roadway Surface Asphalt
      Load Posting 16,000 pounds

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    1. Introduction

      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.

    2. Structural Evaluation Methodology

      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.

    3. Traffic Evaluation Methodology

      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:

      1. Undertake a field review at the bridge site, and make a determination whether detailed traffic counts were required (either 24-hour or intersection peak hour movements). This determination was based on volume of traffic observed, classification of the road approaching the bridge site, and observation of the surrounding land use and potential traffic generators.
      2. Review survey responses relative to existing and future land use, traffic generators, and bridge specific construction activity. Determine how anticipated land use, within the study area, will impact the existing covered bridge.
      3. Obtain from the VAOT estimated existing and future traffic volumes, the bridge inspection report, and the bridge inventory list. If the volume of traffic warrants a traffic analysis, calculate the roadway's quality of traffic operational conditions using the "Highway Capacity Manual Special Report 209" guidelines.
      4. Draw conclusions from appropriate data and make recommendations.

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    1. Study Area of Influence

      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.

    2. Study Area Land Use
      1. Existing Land Use

        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.

      2. Existing Zoning

        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:

        • Village Residential (VR) District was created to encourage residential centers that would serve as the nucleus for future residential development within the Town. Thetford currently has the following village residential districts: Thetford Hill, Thetford Center, Post Mills, North Thetford, and Union Village.
        • Rural Residential (RR) District was created to maintain a low density, rural landscape composed of primarily farms, residences, and woodlands.
        • Community Business (CB) District was created to allow for the development of business centers at central locations to meet the commercial needs of the community and serve the motoring public. There currently is one such district located in the Village of East Thetford east of Exit 14 of Interstate 91.
      3. Anticipated Future Development

        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.

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    1. Existing Roadway System

      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.

    2. Future Roadway System

      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.

    3. Alternative Route Evaluation

      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:

      • The shortest detour (bridge-to-bridge circuit) on established roads is 10.1 miles of Class 2 T.H. However, it crosses another covered bridge (Union Village) and is therefore not an appropriate detour for trucks.
      • The next shortest detour is 14.8 miles.
      • No load restrictions were posted at any bridge on the detour route at the time of our transit. Further, VAOT information indicates that the posting capacity for the one bridge on the detour is 30 tons (which provides sufficient capacity for this detour to be acceptable).
      • No vertical clearance restrictions exist at any of the bridges on the detour.
      • A local site bypass may be possible if necessary, on the downstream side of the existing covered bridge; however, this issue was not studied in depth.

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    1. Existing Traffic Volumes

      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.

    2. Projected Traffic Volumes

      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.

    3. Traffic Analysis

      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
      (Two Way)
      Monday 1021
      Tuesday 961
      Wednesday 1006
      Thursday 972
      Friday 1044
      Saturday 987
      Sunday 825
      total weekly traffic 6816
      average daily traffic (ADT) 974
      average weekday traffic 1001
      average weekend day traffic 906

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    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:

    • Heavy wear to timber decking at Abutment No. 1.
    • Impact damage to kneebraces.
    • Steel superstructure members are rusted and corroded.

    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.

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    Referring to the preservation options outlined in subsection 1.1 of this report, considerations are summarized as follows:

    1. Close the structure and divert traffic:
      This structure currently carries light traffic adequately. An unrestricted detour is quite long (bridge-to-bridge circuit) of 14.8 miles. This option is not judged to be acceptable.
    2. Continue use of bridge for light traffic:
      This option is acceptable. However, repairs are noted as being necessary and are estimated to cost $85,000, including deck replacement, guide rails and signs, and steel painting. No judgement is offered herein as to the potential need of stabilization measures. The Town is expected to maintain the covered bridge in good condition.
    3. Close structure and construct an adjacent bypass:
      A permanent bypass structure may be possible at this site, if considered necessary. The bypass would permit unrestricted use by all legal vehicles. The cost of construction with a two-lane structure is estimated to be $515,000. Additional right-of-way costs may range from a few thousand dollars, to much more, depending on the particulars at this site. We have assumed a ROW allowance of $5,000. Therefore, the total cost of this option, without stabilization noted above, is estimated to be $520,000. This option appears to be unnecessary.
    4. Rehabilitate structure for moderate traffic:
      This structure has been rehabilitated to safely support heavy vehicles. The bridge has previously been reinforced with the addition of four steel beams allowing it to safely support heavy vehicles. No additional major rehabilitation of the superstructure floor system is anticipated. This option is not necessary.
    5. Relocate the structure to a preservation site and build a new structure at the existing site:
      Since a bypass structure may be possible, if required, this option is unnecessary.

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    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:

    • Replace timber deck.
    • Timber repairs as necessary.
    • Clean and paint steel superstructure members.
    • Provide additional guide rail as required on each approach for compliance with VAOT Standards.
    • Install new signs to replace missing or damaged signs indicating "One Lane Bridge", vehicle weight limits, vertical clearance and object markers in accordance with VAOT standards and the Manual of Uniform Traffic Control Devices (MUTCD).

    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.

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  • Primary Consultant:
    • McFarland-Johnson, Inc.
  • Subconsultant:
    • B&B Engineered Timber
      Dr. Robert (Ben) Brungraber, Owner
      Timber Materials Specialist
  • Subcontractor:
    • Restoration and Traditional Building
      Jan Lewandoski, Owner
      Covered Bridge Reconstruction Specialist
  • Subcontractor:
    • Bridge Software Development International, Ltd.
      Dann Hall, Principal
      Refined Computer Analysis of Brown Bridge

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(Reference, in part: Covered Bridges of the Northeast by Richard Sanders Allen, 1985)

  • ABUTMENT - The shore foundation upon which a bridge rests, usually built of stone but sometimes of bedrock, or concrete.
  • ARCH - A structural curved timber, or arrangement of timbers, to support a bridge, usually used in covered bridges together with a truss. Most commonly used with a multiple kingpost truss. Thus, a supplemental or auxiliary arch is one ;which assists a truss; a true arch bridge is entirely dependent upon the arch for support.
  • BEARING BLOCKS - Timber components used to shim between two components (e.g. blocking pieces between a bolster beam and truss chord).
  • BEDDING TIMBERS - Timber components typically located between the top of abutment/pier and the underside of the truss bottom chord. Intended to serve as sacrificial components to be easily replaced when deteriorated from rot; thereby protecting truss components from similar deterioration.
  • BOLSTER BEAMS - Longitudinal timber components beneath the truss bottom chord that project past the face of the abutment. Intended to provide additional support of the truss. Most commonly used beneath Town Lattice trusses.
  • BRACE - A diagonal timber in a truss which slants toward the mid-point of the bridge.
  • CAMBER - A slight convexity, upward bowing or "hump" of the chords, built in to allow the bridge to be level after it settles.
  • CHORD - The top (upper chord) or bottom (lower chord) member or members of a bridge truss; may be a single piece or series of long joined pieces. Town Lattice trusses typically contain two levels of top and bottom chords; hence, there may be upper and lower top chords and upper and lower bottom chords.
  • COMPRESSION MEMBER - A timber or other truss member which is subjected to squeeze. Often a diagonal member such as a brace of counterbrace. Also a top chord.
  • COUNTER-BRACE - A diagonal timber in a truss which slants away from the mid-point of the bridge (opposite from brace).
  • DISTRIBUTION BEAMS - Longitudinal timber components aligned below, and supported by, the floor beams of the structure. Intended to force participation of several floorbeams in supporting axle loads of vehicles. Rarely effective.
  • FACE OF ABUTMENT - The side of the abutment toward the center of the stream.
  • FLOOR BEAM (OR FLOOR JOIST) - Transverse beam between bottom chords of trusses on which longitudinal joists (or "stringers") or decking are laid.
  • GOOD CONDITION - Indicator of physical configuration and material properties similar to that at time of original construction. Having no significant defects, such as: cracks, crushing, buckles, rot, insect attack, or impact damage.
  • JOIST (OR STRINGER) - Timbers laid longitudinally on the floor beams of a bridge and over which the floor planking is laid.
  • KNEE BRACES - Transverse timber components connecting the upper portion of the truss with the transverse tie beams, usually positioned at a 45 degree angle.
  • LAMINATED ARCH - A series of planks bolted together to form an arc; constructed in such a manner that the boards are staggered to give extra strength.
  • LATERAL BRACING - An arrangement of timbers between the two top chords or between the two bottom chords of bridge trusses to keep the trusses spaced apart correctly and to insure their strength. The arrangement may be very simple, or complex.
  • LONGITUDINAL - Direction parallel to the bridge.
  • PIER - An intermediate foundation between abutments, built in the stream bed, for additional support for the bridge. May be made of stone, concrete, wood, etc.
  • PORTAL - General term for the entrance or exit of a covered bridge; also used to refer to the boarded section of either end under the roof.
  • POST - Upright or vertical timber in a bridge truss; center post is the vertical timber in the center of a truss; end post is the vertical timber at either end of the truss.
  • RAFTER - One of a series of relatively narrow beams joined with its opposite number to form an inverted V to support the roof boards of a bridge.
  • ROT - Deterioration of timber material evidenced by soft spots/areas as a result of poor ventilation and/or excessive moisture.
  • RUNNING PLANKS - Longitudinal timber planks on the top of the deck intended to provide an easily replaceable wearing surface. Also tends to guide vehicles along the center of the bridge and causes traffic to reduce travel speeds.
  • SAG - Opposite of camber; permanent downward deflection of trusses at middle of span.
  • SISTER - Additional Town Lattice web member inserted adjacent to a damaged or deteriorated existing web member that provides additional strength to the truss without replacing the existing member.
  • SKEWED BRIDGE - A bridge built diagonally across a stream.
  • SPAN - The length of a bridge between abutments or piers. Clear span is the distance across the bridge, measuring from the face of one abutment to the face of the other. The length usually given is for the truss span, i.e., the length between one end post of the truss and the other, regardless of how far the truss may overreach the actual abutment. Bridges of more than one span are called multi-span bridges.
  • SPLICE - A method of joining timbers, especially end-to-end, by means of a scarf or other joint, sometimes with keys or wedges inserted to give additional strength and stability to the joint. A splice-clamp is a metal or wooden clamp designed to hold two spliced timbers together.
  • TENSION MEMBER - Any timber or rod of a truss which is subjected to pull or stretch.
  • TIE BEAM - Transverse timber component connecting tops of top chords. A part of the upper lateral bracing system.
  • TRANSVERSE - Direction at right angle to bridge (i.e. 90° to bridge), opposite of longitudinal.
  • TREENAILS (TRUNNELS) - Wooden pins which are driven into holes of slightly smaller diameter to pin members of lattice trusses together. (Pronounced "trunnels").
  • TRUSS - An arrangement of members, such as timbers, rods, etc., in a rigid form so united that they support each other plus whatever weight is put upon the whole. Covered bridge trusses, including arch trusses, employ a triangle or a series of combined triangles, since this is the form which cannot be forced out of shape by external pressure. Truss is also used to refer to just one side of a bridge.

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Regular maintenance and proper repairs can help preserve these unusual structures for an indefinite period of time. The following discussion highlights good maintenance measures.

  • Maintaining a waterproof roof and side boarding system is an extremely important measure that can prolong the life of these bridges.
  • The buildup of dirt and debris, tracked onto the bridge from vehicles or introduced by poor roadway drainage, should be regularly removed to help prevent opportunities for decay to develop. The material would ideally be removed with air pressure. Use of water jets to remove the dirt is effective; however, it introduces moisture into areas of the bridge that are hard to dry and would otherwise have stayed dry.
  • The trusses should be raised above direct contact with the foundation units, via timber bedding timbers or bearing blocks. The inevitable deterioration of those components can be addressed with much less expensive replacements whenever necessary.
  • The bridge structure should be elevated above the approach roadway so that road drainage does not flow onto the floor system. If elevating the structure is neither possible nor practical, then significant and effective drainage collection systems should be installed on the uphill end of the bridge to minimize the amount of drainage entering the bridge.
  • All timber components should be kept in like new condition and the structure should be "tight". A structure that is loose enough to distort, will undergo an accelerated rate of deterioration. The diagonal compression members of Multiple Kingpost structures are occasionally so loose as to be subject to handshifting by a person. Knee bracing and top lateral bracing in the roof area is often damaged by oversized vehicles, and should be repaired as it is discovered.


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.

  • Town Lattice trusses derive much of their exceptional strength from long chord components. The shorter the components become, the more the structure is dependent on the trunnels for load transfer among the individual components. Although failure of trunnels is uncommon (or at least not readily observable nor often noted by repairers), short chord components lead to excessive deformations of the trunnels and/or holes, so that the "gaps" between chord members enlarge. It is good practice for rehabilitation of such structures to require replacement components to be as long as possible.
  • Town Lattice diagonals often exhibit cracking along the axis of the member, beginning at the end of the member, and passing through trunnel holes. In many instances, bottoms of lattice members may also be damaged from ice and/or flooding impact forces. In such cases, lattice members have been "spliced" in the past by cutting the member off above the upper lower chord. A replacement bottom end has been joined to the existing upper portion by the use of steel bolts (with or without steel "shear plates"). In many instances, the splice is made with a pair of bolts (often only 3/4 inches in diameter) in either end of each timber component. Not only is the bolted connection weaker than a corresponding connection with trunnels, but the end distance of the bolt is often substantially less than required by Code.
  • Several Queenpost trusses have been rehabilitated through installation of steel "heel plates" at the end post to bottom chord joint. In most examples, the bolting patterns do not appear to conform to timber design specifications. There are other, more subtle problems with these added steel plates.
  • Moisture condenses on these large steel plates and can cause decay in the concealed wood surface behind. The larger plates can be inducing large perpendicular to the grain load components in the bolts, through eccentricities in the forces being transferred. Unless the steel plates are drilled in place (a difficult procedure) it is very tough to get the holes in the wood aligned with those in the steel. The "hole oblonging" this causes when installing the bolts can seriously compromise the capacity of the designed joint, as well as allow an unexpected amount of deflection in the "repaired" structure.
  • Use of these plates seems to be inspired in efforts to avoid authentic restoration techniques or extensive timber chord replacement. Skilled timber craftsmen are often able to restore the capacity of these critical joints without resorting to the use of bolts, and usually produce a stronger connection.
  • Distribution beams have been added to the underside of the floorbeams on many covered bridges. The longitudinal members were intended to force participation of several floorbeams in the support of axle weights of vehicles. Specific installations may include one or two lines of members hung beneath the floorbeams by steel U-bolts. In practice, the members are usually ineffective due to several reasons. The relative stiffness of the distribution beam is usually much less than the floorbeam, and hence cannot perform its intended function. The connections are usually sufficiently loose so that the floorbeam beneath the axle deflects without fully engaging the distribution member.
  • The positive benefit of added resistance to ultimate failure of the floor system caused by an overload vehicle does not offset the adverse effect that it represents due to its own weight. Existing distribution beams should be removed when a structure is rehabilitated. No new beams should be installed. New replacement floor systems should be sized and detailed to properly support vehicle loading by conventional design practice. Some bridges contain features that make the installation of a new floor difficult and may require special attention.

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