Back to Grants.
Pittsford/Proctors' Gorham Bridge(WGN
Inspection Report - May 1995
Description: One of four covered wood bridges remaining in the town of Pittsford,
the Gorham Bridge was built in 1841 by Abraham Owen and Nicholas M. Powers. Powers, who
became. "Vermont's most famous covered bridge builder," served his apprenticeship under Owen.
The Gorham Bridge represents later collaboration of the two master builders. Of the many bridges
which Powers built during a career of more than forty years, only the Gorham and two other
bridges survive in Vermont.
The Gorham Covered Bridge is currently listed on the
National Register of Historic Places.
Bridge Characteristics: Truss Construction - Town Lattice; Number of Spans - 1;
Measured Length (End to End) - 114.8 feet; Measured Horizontal Clearance - 16.67 feet;
Measured Vertical Clearance at Truss - 9.5 feet; Measured Vertical Clearance at Center of Bridge
- 12.33 feet; Load Posting - 16,000 pounds.
Traffic Volumes: According to 1994 VAOT data, the estimated average daily traffic
(ADT) volume at the bridge site for the Year 1992 was approximately 600 vehicles per day. An
estimated average daily traffic volume of 840 vehicles per day on the bridge is projected by the
VAOT for the Year 2013.
Traffic Analysis: The Gorham Bridge warranted a traffic analysis for several reasons.
VAOT traffic volumes confirm, given the existing study area's land use and Town Highway 6 and
14 serving as a link between the two Towns, 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 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 two-lane rural road, the related percent time delay criteria for each level of
service is applied. 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. 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
The projected volume of 840 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
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 840 ADT.
This two-lane, Class 3, town road provides access between the two Towns with good Levels
of Service defined by a range of A through B.
For one week in June 1993, 24-hour traffic counts were taken at the Gorham Bridge. The counts
Total Daily Volumes (Two Way): 430 - - Monday; 480 - - Tuesday; 500 - - Thursday; 480 - -
Friday; 500 - - Saturday; 390 - - Sunday.
2780/6 = 460 average daily traffic (ADT); 470 - - average weekday traffic; 450 - - average
weekend day traffic.
Alternative Route: The shortest detour (bridge-to-bridge circuit) on established roads
(minimum of Class 3 T.H.) is 3.9 miles. No load restrictions were posted at any bridge on the
detour route at the time of our transit. However, VAOT information indicates that the maximum
posting capacity for a bridge on the detour is only 10 tons (which does not provide sufficient
capacity for this detour to be acceptable). Further, a vertical clearance restriction exists at a
railroad underpass on the detour.
The next shortest detour (approximately 4.4 miles)
crosses another covered bridge (the Cooley Bridge) which is restricted to 8 tons and a vertical
clearance of 12.0 feet. This detour is not acceptable.
A local site bypass may be possible, if necessary, on
either side of the existing covered bridge; however, this issue was not studied in-depth.
Structural Evaluation: During a visit to the bridge site in March, 1993, an evaluation
of various maintenance was performed to facilitate continued use of the structure as a covered
bridge. At that the following deficiencies were observed:
- Some lean of trusses at Abutment 2; more lean at Abutment 1.
- Numerous lattice members were spliced, many lower ends cracked:
- Rotten bearing blocks at Abutments.
- Some evidence of settlement, stone failure at Abutment
- Size and description of truss and floor system members were also recorded by the Engineer. The
following pertinent information was noted:
- - Nail laminated timber decking 5 1/2" thick
- - Floor beams 6" x 11 1/2 , spaced at 1'-9"
- - Truss upper bottom chord 2 1/2" x, 11 1/4" 4 per chord
- - Truss lower bottom chord 2 1/2" x 11 1/4", 4 per chord
- - Truss upper top chord 3" x 11 1/4", 4 per chord
- - Truss lower top chord 3" x 11 1/4", 4 per chord
The analytical investigation described under Section 2.2 of this report concludes that the floor
system has sufficient capacity to support vehicle weights of only 20,000 pounds. However, the
analysis of the trusses indicates a significant weakness. Theoretically, the trusses are over stressed
under the influence of the self-weight of the structure.
A. Close the structure and divert traffic: This option is not acceptable due to the
moderate traffic usage and lack of a nearby substitute route.
B. Continue use of bridge for light traffic: This structure is subjected to use by relatively
heavy vehicles (snow plows, fuel trucks, emergency vehicles, etc.). Our analytical investigation,
subject to limitations noted herein, indicates that this structure may contain a significant weakness
of the trusses. Hence, further investigation is necessary. This option is not appropriate.
C. Close structure and construct new bypass: A permanent bypass structure may be
possible at the site. An estimate of construction cost for a two-lane structure is approximately
$475,000. However, stabilization of the existing structure may be required to avoid failure from
loads imposed by the self-weight of the structure and snow loading. As mentioned herein, the
need for special stabilization measures are not determined by this investigation; hence, no costs
are estimated for this issue. Additional right-of-way costs are also not estimated herein. Although
this option is feasible, further investigation into Option D appears more appropriate at this
D. Rehabilitate structure for moderate traffic: As noted above, this structure potentially
contains a serious weakness of the trusses. Our limited analytical evaluation was unable to identify
specific recommendations for improvement of structural capacity; however, it would appear that
strengthening of this structure should be possible to obtain sufficient capacity, for a much smaller
investment than the cost of a bypass structure. A more refined engineering investigation will be
required to determine details and a cost estimate for this option.
E. Relocate the structure and build new: Since a bypass structure may be possible, if
required, this option is unnecessary.
Recommendations: Having considered the traffic needs at this site, condition of the
structure, and merits of various preservation options, we urge adoption of Option D as the most
appropriate course of action to provide for preservation of this covered bridge for the future. That
is, conduct a more refined structural engineering evaluation of the bridge to prepare appropriate
strengthening of the bridge to support moderate vehicle weights.
Although our limited evaluation performed for this
study identifies an apparent significant weakness of the trusses, it would appear possible and
financially practical to strengthen this bridge to permit use by vehicle weights up to 40,000
pounds. However, until further investigations are concluded that successfully verify more capacity
of the existing structure, or until strengthening is performed, we recommend that a restricted load
posting be implemented at the bridge (6,000 pounds).
We recommend the following interim actions to
improve the current conditions and to support the commitment for long-term preservation:
- Install new signs to replace missing or damaged signs indicating "One Lane Bridge", advance curve
warning, intersection warning, advisory speed, vehicle weight limit and vertical clearance in
accordance with VAOT standards and the Manual of Uniform Traffic Control Devices (MUTCD).
- Provide guardrail on west approach in compliance with VAOT Standards.
Return to top
Joe Nelson, P.O Box 267, Jericho, VT 05465-0267, firstname.lastname@example.org
This file posted November 18, 2001