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.
Return to Top Return to T.O.C.
5.0 TRAFFIC EVALUATION
5.1 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.
5.2 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.
5.3 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
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.
Total Daily Volumes (Two Way).
6816/7≈ 974 average daily traffic (ADT)
1001 ...average weekday traffic
906 average weekend day traffic
Return to Top Return to T.O.C.
6.0 STRUCTURAL EVALUATION
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
The bridge is posted for a legal load limit of 16,000 lb.
Return to Top Return to T.O.C.
7.0 CONSIDERATION OF PRESERVATION OPTIONS
Referring to the preservation options outlined in subsection 1.1 of this
report, considerations are summarized as follows:
- 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
- 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
- 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.
- 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
- 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.
Return to Top Return to T.O.C.
8.0 CONCLUSION AND RECOMMENDATIONS
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
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
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
- 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.
Return to Top Return to T.O.C.
COVERED BRIDGE STUDY TEAM
- B&B Engineered Timber
Dr. Robert (Ben) Brungraber, Owner
Timber Materials Specialist
- Restoration and Traditional Building
Jan Lewandoski, Owner
Covered Bridge Reconstruction Specialist
- Bridge Software Development International, Ltd.
Dann Hall, Principal
Refined Computer Analysis of Brown Bridge
Return to Top Return to T.O.C.
GLOSSARY OF TECHNICAL TERMS
(Reference, in part: Covered Bridges of the Northeast by Richard Sanders Allen,
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
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.
Return to Top Return to T.O.C.
GENERAL RECOMMENDATIONS FOR MAINTENANCE AND REPAIRS
OF COVERED BRIDGES
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.
- 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
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
- 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
- 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
- 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
- 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.
Return to Top Return to T.O.C.
Joe Nelson, P.O Box 267, Jericho, VT 05465-0267, firstname.lastname@example.org
No part of this web site may be reproduced without the written permission of Joseph C.
This file posted November 29, 2001, revised March 18, 2002