Pittsford/Proctor's Gorham Bridge

(WGN 45-11-04)
Inspection Report - May 1995
Highlights

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 service.

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 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 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 are summarized:
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.

Preservation Options:

1. Close the structure and divert traffic: This option is not acceptable due to the moderate traffic usage and lack of a nearby substitute route.
2. 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.
3. Close structure and construct an adjacent 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 time.
4. 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 structur
5. Relocate the structure and build a 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.