EAST FAIRFIELD COVERED BRIDGE
Bridge No. 50
Prepared by: HTA Consulting Engineers, Burlington, Vt.
Prepared for: Vermont Agency of Transportation
April 2006 Revised May 2006
1 EXECUTIVE SUMMARY
Hoyle, Tanner & Associates, Inc. (HTA) has been assigned, through a retainer contract with the Vermont Agency of Transportation (VTrans), the task of preparing an Engineering Study for the rehabilitation of the East Fairfield Covered Bridge. The project's Priority of Uses as defined by the Historic Covered Bridge Preservation Plan is "Special Use on Roads". This use allows the bridge to remain in use but limits use to very light traffic, primarily cars.
The East Fairfield Covered Bridge was inspected and load rated to determine it's current condition as well as its capacity to support a H6 (6 ton) live load rating. Our rating assumes replacement of deteriorated bridge members, new decking, new bottom chords of each truss and supplemental members added to the truss end diagonals. It was determined that the existing trusses have a live load capacity of approximately H5.7 (5.7 tons) at inventory level and the floor system have a capacity of H5.4 (5.4 tons) at inventory.
Additional items are discussed in this report and recommended for the bridge include a new standing seam metal roof; new siding; substructure repairs and replacement; replacement of deteriorated bridge members; the addition of lighting, fire protection, substructure repairs and replacements, bridge approach railing and lateral bracing; paving of the bridge approaches; and realignment of, the trusses. The total estimated cost of all recommended work items, without contingencies, is approximately $477,750 including contingencies.
HTA has been assigned, through a retainer contract with the VTrans, the task of preparing an Engineering Study for the rehabilitation of the East Fairfield Covered Bridge. On November 28,2005 and November 29,2005 an Inspection Team from HTA visited the bridge to perform field observations and gather field data for this Engineering Study.
The East Fairfield Covered Bridge is located in the Town of Fairfield, Vermont and was built in about 1865. The bridge's sixty-eight (68') foot single span timber superstructure utilizes Queenpost spaced at 14'-10" on-center (see Appendix C for existing and portal sections and plan view). The bridge runs east to west and crosses over the Black Creek on Town Highway No. 49. It is supported by abutments originally constructed out of large stones. The east abutment has been partially reconstructed and is encased with concrete. The west abutment has been pointed and is constructed out of large stones. The bridge has been closed to traffic since 1991 and is on the National Register of Historic Places.
4 FIELD OBSERVATIONS On November 28, 2005 and November 29, 2005 an inspection team from HTA visited the bridge to perform in-depth field measurements and gather field data for this Engineering Study. Wright Construction Company was in on-site on November 29, 2005 and provided inspection access to the lower portions of the bridge with a raft. Field observations were used as a basis for this report and expanded as appropriate.
5 WOOD SPECIES IDENTIFICATION Five (5) small wood samples were removed from the bridge for the purpose of species identification. The samples were taken from deteriorated members that will most likely be replaced during the course of the bridge rehabilitation. The samples were forwarded Doug Gardner, Ph.D., a professor of Wood Science and Technology, at the University of Maine at Orono for identification. From analysis, it was determined that the deck and stringer samples were Eastern Hemlock and the chord sample is Spruce. The floor sample is either Eastern Hemlock or Fir, while the rafter is either Spruce or Fir. It is interesting to note that the rafter sample has 40 growth rings per half inch, which indicates very slow growth. This species evaluation is consistent with historical records that indicate native wood was used during construction. A copy of Dr. Gardner's report is included as Appendix A.
The bridge crosses over the Black Creek which runs roughly south to north at the bridge site. At the time of our inspection there was approximately 6-feet of vertical clearance above the water level to the lower chord bottom face elevation of 383feet.
The VTrans Hydraulics Unit completed a preliminary hydraulic study dated November 7, 2005. The conclusion of the study was that both the Q25 (El 381.4') and Q100 (El 382.4') water elevations were below the low chord elevation of 383-feet. A USGS level II Scour Analysis was also performed with a result of 3.1' of contraction scour at the Q500.
7 STRUCTURAL ANALYSIS/REPLACEMENT RECOMMENDATIONS
A structural analysis was performed of all key members of the bridge superstructure. The Service load (Allowable Stress) design method was used for all members as required by the VTRANS Structure Manual (Chapter 22, Section 22.1). Allowable stress values for wood members were obtained from the 2005 National Design Specification for Wood Construction and Supplement. The wood species used in the superstructure was identified through testing (see Section IV). The grade assigned to each member was based on a visual examination of knots, checks and the growth rate characteristics of the wood. All superstructure members are wood unless noted otherwise.
The condition of each member is detailed in the following sections with our initial recommendations for repair of replacement which must be reviewed by the Historic Covered Bridge Committee (HCBC) and the structural and historical issues weighed. We have also identified in the recommendations for sections 7.1 through 7.5 the priority treatment number from the Historic Covered Bridge Preservation Plan to aid in review.
It should be noted that not all members to be replaced can be identified based on our inspection due to inaccessible areas (top face of rafters, etc.). The contract documents and estimate of cost in this study include an additional amount of conditional replacement where appropriate to determine an appropriate budget for the project.
7.1 Roof Framing
The roof framing consists of metal roofing attached to 7/8" thick roof boards. The roof boards are supported by 3" x 5" rafters spaced at 2'-0" on-center. The rafters are notched into a 7" x 7" rafter support beam which spans wood columns supported on the trusses. The rafter and roof board wood species is Spruce/Fir for analysis and they have been assigned a grading of Select Structural.
The roof rafters were analyzed for dead load and a ground snow load of 50 PSF (27.4 PSF roof applied) per the 2005 Vermont Fire and Building Safety Code snow load and the 2000 International Building Code. The rafters and roof boards were found to be adequate for the applied dead and snow loading.
The existing metal roof is in poor condition and we recommend that it be replaced with a new standing seam metal roof. The rafter and roof boards are generally in good condition. We have assumed replacement of six (6) rafters and 30% of the existing roof boards for this study based upon our field inspection. (Priority Treatment Number 2 for the roof boards and 9 for the rafters since local species is select structural will be very difficult to obtain.)
7.2 Lateral Bracing
The top lateral bracing consists of 4" x 4" knee braces and nine (9) 7%" x 8W' crossbeams bearing on top of the rafter support beam. The 4"x4" knee braces connect to the cross beams and either a truss vertical or siding support. There are not any diagonal braces between the crossbeams.
The lateral bracing in the bridge is in poor condition and is not adequate to resist code required wind loads. The bridge trusses are racked between 2" and 5" depending upon the location. Damage (tenon pullout) was noted in at least two (2) crossbeams. Some of the crossbeams are supported on rafter support beam columns that bear on the truss. The connection between the column and the truss member is toe nailed which has a very low lateral capacity.
The upper lateral bracing is very limited and was, not analyzed for this study since additional lateral bracing is recommended for the bridge.
Due to the poor capacity of the bridge lateral bracing, we recommend additional horizontal 'X' bracing be added between the existing crossbeams and strengthening of the connections of the rafter support beam columns.
The knee braces in the bridge are in fair to good condition and no replacement is proposed. There are ten (10) locations where crossbeams were intended in the bridge. Of these:
Since the crossbeams transfer load to the rafter support beam and the roof rafters transfer a horizontal load to the connections, steel may be required to provide adequate lateral capacity. Specific recommendations will be made at a later date after discussions with the HCBC.
Depending on the lateral bracing decisions made, priority of treatment will be treatment number 2 or 5 for new or replaced crossbeams depending on the member size required and treatment number 7 for rafter support beam connections.
The bridge utilizes Queen Post trusses with the truss members in poor to good condition. They were identified as a Spruce species with a grade of select structural assigned for analysis. Steel beams have been added inside of each truss to help support the bridge due to the poor condition of the truss lower chords.
The upper truss members are in fair to good condition and appear to be original construction. The lower chords are in poor condition with large sag deflection, additional members added at mid-span and added through bolting. Large gaps at chord butt joints and rot were also noted in many locations of the lower chord. Steel straps have also been added at the truss verticals and end diagonals where rot is present or the original mortise and tenon joint is no longer functioning.
The trusses were analyzed to determine their current live load capacity. Full dead and live loads were applied and compared to inventory stress levels and full dead, live and snow loads were applied and compared to the higher operating stress level. The following assumptions were made with the truss analysis: 1) the bottom chord is replaced in kind with new members and 2) the end diagonals are supplemented with additional members to increase the live load capacity (a rating of approximately H1 is obtained without this assumption). The live load capacity of the trusses was found to be H5.6 (5.6 tons) at inventory and H5.7 (5.7 tons) at operating both ratings are controlled by vertical member capacity.
Due to the location of the bridge on a low volume Town road with a very short unposted detour and the low vertical clearance of the bridge we have assumed that the bridge will be posted for a live load of H6 (6 tons), which is approximately the truss capacity. Should a higher live load capacity be required, many members of trusses would have to be replaced or strengthened.
Recommendations The portions of the truss above the bottom chord are in good condition except for the end diagonals where rot was found at the lower end and several large splits in some members. Additional splices/supplemental members are recommended for the end diagonals as discussed above. The existing supplemental members are not considered to be adequate due to their poor connection to the diagonals and insufficient length. Epoxy injection into the larger splits is also recommended to lessen further splitting and insect damage to these members. (Priority Treatment Number 1)
The lower chords of both trusses are in poor condition and full replacement is recommended. The members would be replaced with similar sized members which retain similar joints and detailing. (Priority Treatment Number 9)
7.4 Floor Beams, Stringers, and Decking[Existing Deck]
The existing floor framing consists of a 3½" thick plank deck supported by stringers spaced at approximately 2'-8" on center and vary in size from 5" x 7" to 8" x 8". The stringers are supported on multiple piece floorbeams with a total size of 19" wide by 9½" deep. The floorbeam spacing varies but is on average 9' with 3¾" x 5¾" lateral 'x' bracing spanning between floorbeams.
The existing floor beams, stringers and decking were analyzed to determine their live load capacity (see Table 1). All three (3) existing components are Eastern Hemlock and were assigned a grade of No.1 due to large knots and wane in the members.
Table 1 - Floor Framing Capacities
Based upon the low live load capacity and wear in the deck we recommend complete replacement with a new 3W' thick No.1 grade Douglas Fir deck (Priority Treatment Number 9). The live load operating rating of the stringers is H7.3, therefore only replacement of approximately one third of the stringers is recommended due to their condition. The stringer selected for load rating is at the maximum spacing in the bridge while most stringers are at a lesser spacing, which would have a higher capacity. The floor beams are adequate for a six (6) ton live load capacity. We recommend approximately one quarter of the floorbeam board footage be replaced due to condition. We recommend both stringers and floorbeams be replaced with Douglas Fir or Southern Pine since local species in select structural will be very difficult to obtain. (Priority Treatment Number 9)
In addition, we recommend that a new wood curb be added to the bridge to help keep vehicles from impacting the trusses. This curb has previously been used by VTrans on the Union Covered Bridge in Thetford, the Comstock Covered Bridge in Montgomery and the Greenbanks Hollow Covered Bridge in Danville.
The east abutment was originally constructed out of large stones and the exact depth of the abutment is not known. The east abutment has been partially encased in concrete and a concrete backwall added. Each of the east wingwalls have failed; the southeast wingwall stone section has failed and the northeast concrete wingwall has broken with a large vertical crack. Temporary repairs were made to this wall by bracing it off the southeast wingwall with steel wire rope.
Several large stones are no longer in place in the east abutment breastwall near the base. The stones above the void are bridging the opening and the abutment is relatively stable. This section of the abutment appears to be founded on ledge.
The west abutment and southwest wingwall are constructed out of large stones and are fully pointed with an unknown depth. A concrete backwall has been added to the abutment. The northwest wingwall is a concrete wall which was once part of a mill building at the site (see Appendix E for more information). The west substructure is generally in fair to good condition.
The existing abutments were analyzed for overturning and sliding per the VTrans structures manual with an assumed breastwall depth of 5'-0" deep. Two (2) conditions were checked for each abutment. The first condition analyzed the abutments alone with soil loading and no dead load. This condition may occur while the bridge has been removed during construction and the dead load of the bridge is not applied to the abutments. The second condition includes both soil loading and a live load surcharge acting on the abutment without dead load.
We determined that for the first condition, the factor of safety for overturning and sliding met VTrans requirements. For the second condition, both factors of safety were greater than 1.0 but less than VTrans requirements of 1.5 for sliding and 2.0 for overturning. This is considered a temporary condition during construction where a lower factor of safety may be appropriate. When the dead load of the bridge is added to the abutments, they are adequate for dead load earth, pressure and live load surcharge.
The following repair/replacement recommendations are made for the bridge substructure:
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Both bridges approaches are gravel and are in fair to good condition with access to the bridge blocked by piles of gravel at the entrances. There is no guardrail installed at either end of the bridge. There are no bridge closed, vertical clearance signs, or weight limit signs posted within the immediate vicinity of the bridge.
The east approach is tangent until it reaches the bridge where it turns sharply. The roadway is in a sag vertical curve with the grade increasing at the bridge. The west approach is on a horizontal curve to the bridge and is relatively flat near the bridge.
We recommend that 50' of each approach to the bridge be paved. This is recommended to help direct water away from the bridge and to reduce the amount of gravel and sand brought into the bridge.
The existing right-of-way has not yet been conformed within the proposed project limits. All proposed work could be completed within a 3-rod right-of-way and the exact limits will be determined during the conceptual design phase.
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The only known fire protection system at the bridge is a dry hydrant located at the southeast corner. Three (3) fire detection/protection systems are generally used for covered bridges; fire retardant coating (NOCHAR/Poloseal), fire detection system (Protectowire) and a dry deluge sprinkler system.
A fire-retardant coating, such as "NOCHAR", can be used to protect the bridge from fire. This coating works by Dry Hydrant raising the f1ashpoint of the wood and therefore making it difficult to start a fire. The coating is available in colored and clear versions that are applied to the wood by brush or spray. The coating does not affect the strength of the wood.
If a fire is started, it is advantageous to notify the local fire department as soon as possible. The "Protectowire" system works by running a small wire through key locations in the bridge. If a rapid rise in temperature is detected or if a wire is cut, the system alerts the local mutual aid or fire department. This advanced warning can greatly reduce fire damage to a bridge and hopefully prevent the fire from making the bridge a total loss. A deluge sprinkler system is not recommended for this bridge since it is relatively short and water can be directed to the center of the bridge from either end.
For the purposes of this study, we have assumed that the NOCHAR and protectowire systems will be used on the bridge. We recommend that the Town, specifically the fire chief, make the final decision on which systems should be used on this bridge.
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There is no lighting present on the bridge or at the immediate approaches. Lighting can be an effective means to deter vandalism and improve visibility at night. The decision to add lighting to the bridge should be made by the Town. For this study, the cost of adding interior lighting to the bridge has not been included in the cost estimate in Appendix B. Since there is a residence very close to the bridge, if desired, interior lighting could be added and would be high-pressure sodium lights controlled by photocells. This type of lighting provides a light brown color and is the type preferred by state historic agencies.
The only known utilities within the project limits are overhead utility lines that cross Black Creek downstream (north) of the bridge. The lines are located a sufficient distance from the bridge that we do not anticipate any impacts to them fro this project.
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11 ARCHEOLOGICAL RESOURCE ASSESSMENT
HTA retained Hartgen Archeological Associates, Inc. (HAA) to perform an Archeological Resource Assessment (ARA) at the project site. A complete copy of their report is included in Appendix E. Please note that the location (directions) used by HAA vary from those used by HTA and shown in Appendix C, Figure 1. We have referred to the abutments as east and west with the downstream wingwalls called the north wingwalls and the upstream wingwalls called the south wingwalls. The directions refer to the HT A nomenclature
In summary, HAA concluded the following:
The only known and identified historical resource within the project limits is at the northwest corner of the bridge where a concrete portion of a former mill complex foundation serves as a wingwall. The only proposed work to the wingwall is partial depth concrete repairs where scour has occurred. The environmental resources in the area will be identified during the conceptual phase of the project.
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12 ESTIMATE OF COST
An estimate of cost was prepared for this project using the "Estimator" software and past HT A and VTrans covered bridge bid data. The estimate of cost is included in Appendix B and includes the following major items:
The total estimated cost of all recommended work items is approximately $477,750.00 including 5% for contingences. A complete breakdown of this estimate is included in Appendix B.
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The East Fairfield Covered Bridge was built around 1865 and remained in service until 1991 when it closed due to the poor condition of the lower chords. The bridge components above the deck level are generally in fair to good condition, the exception being the rafter support beam which is rotted and broken in several locations. The truss bottom chords are in poor condition with vertical sag and rot in multiple locations. The west substructure is in good condition while the east is in poor condition. The east substructure has two failed wingwalls and a portion of the breastwall stones are missing.
The East Fairfield Covered Bridge was load rated to determine its capacity to support an H6 (6 ton) live load. A live load capacity goal of H6 (6 tons) was used for this study due to the location of the bridge and the short unposted detour. Our rating assumes replacement of deteriorated bridge members, new decking and new bottom chords of each truss and supplemental members added to the truss end diagonals. It was determined that the existing truss has a live load capacity of approximately H5.7 (5.7 tons) at inventory limited by the truss vertical capacity and the floor system has a capacity of H5.4 (5.4 tons) at inventory.
Additional items are discussed in this report and recommended for the bridge include a new standing seam metal roof; new siding; replacement of deteriorated bridge members; fire protection, substructure repairs and replacements, bridge approach railing and lateral bracing; paving of the bridge approaches; and realignment of the trusses. The total estimated cost of all recommended work items is approximately $477,750.00 including 5% for contingences.
Return to Top Return to T.O.C.Joe Nelson, P.O Box 267, Jericho, VT 05465-0267
This file posted 01/02/2008