Pittsford's Cooley Bridge

(WGN 45-11-07)
Inspection Report - May 1995
Highlights

Description:

One of four covered wood bridges remaining in the town of Pittsford, the Cooley Bridge was built in 1849 by Nicholas M. Powers, who was born on a farm in the vicinity of the bridge. Powers became "Vermont's most famous covered bridge builder." The Cooley Bridge is one of three bridges surviving in Vermont whose construction Powers is known to have assisted or directed. The extreme overhang of its portals, which makes the roof 16 feet longer than the 50-foot floor, gives the bridge a distinctive form.

The Cooley Covered Bridge is currently listed on the National Register of Historic Places.

Bridge Characteristics:

Timber truss Construction - Town Lattice; Number of spans - 1; Measured Length (End to End) - 50.5 feet; Gable Overhang (each end) - 8 feet; Measured Horizontal Clearance - 15.25 feet; Measured Vertical Clearance at Truss - 9.83 feet; Measured Vertical Clearance at Center of Bridge - 12 feet; Load Posting - missing.

Traffic Volumes:

According to 1994 VAOT data, the estimated average daily traffic volume at the site for the Year 1992 was approximately 400 vehicles per day. An estimated average daily traffic volume of 560 vehicles per day on the bridge is projected by the VAOT for the Year 2013.

Alternative Route:

The shortest detour length (bridge-to-bridge circuit) is 5.4 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 28 tons which provides sufficient capacity for this detour to be acceptable. No vehicle 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.

Structural Evaluation:

During a visit to the bridge site in April, 1993, an evaluation of various maintenance repairs was performed to facilitate continued use of the structure as a covered bridge. At that time, the following deficiencies were observed:

• Localized areas of rot at ends of bottom chords near abutments
• Bottom ends of lattice members rotten near Abutment #1
• Deteriorated treenails near bottom chords
• Powder post beetle damage and rot at ends of floor beams
• The bridge is currently not posted for a legal load limit
• 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 6" thick
  • Floor beams 8" x 14", spaced at 4'-0"
  • Distribution beams 12" x 14"
  • Truss upper bottom chord 2 3/4" x 11 1/2", 4 per chord
  • Truss lower bottom chord 2 3/4" x 11 1/2", 4 per chord
  • Truss upper top chord 3" x 11 1/2", 4 per chord
  • Truss lower top chord 3" x 11 1/2", 4 per chord

The analytical investigation described under Section 2.2 of this report concludes that the structural capacity of the bridge, when in good condition, is adequate to support a load posting of 16,000 pounds (limited by the capacity of the floor system). The trusses of this structure can support vehicle weights of up to 40,000 pounds (considered to be the maximum prudent limitation for a structure of this type).

Preservation Options:

1. Close the structure and divert traffic: This structure currently carries light traffic adequately. The detour length is approximately 5.4 miles. This option is not judged to be appropriate.
2. Continue use of bridge for light traffic: This structure can safely support vehicle weights up to 16,000 pounds. A floor replacement would be required to safely support heavier vehicles which is considered within the response to Option D, not B. Since this structure may be subjected to occasional use by heavier vehicles, adoption of this option as a permanent solution to the preservation needs of this bridge may be inappropriate. However, selection of this option may be acceptable for the short-term, assuming preparations are undertaken to provide for heavier vehicles in the longer term, through any of the following options. Further, a weight restriction should be installed and measures should be implemented to enforce the load posting. Improvements and repairs are estimated to cost about $40,000 for this option.
3. Close structure and construct an adjacent bypass: If a structure capacity in excess of 16,000 pounds is required, and if unrestricted clearance is important, then a permanent bypass structure may be possible at the site. An estimate of total construction cost for a two-lane structure is approximately $200,000. However, stabilization of the existing structure will also be required to avoid failure from loads imposed by the self-weight of the structure and snow loading. An estimate of appropriate stabilization is $25,000. 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 is estimated to be $230,000. Although this option is feasible, it appears to be less desirable than Option D.
4. Rehabilitate structure for moderate traffic: Rehabilitation of this structure to support loads in excess of 16,000 pounds would require a complete floor replacement. Our limited analytical investigation indicates that the trusses may be capable of supporting vehicle weights of up to 40,000 pounds. An estimate of cost for floor replacement is $40,000. Therefore, the total cost for this option is $80,000 (Option B cost + deck replacement). This option appears to be appropriate.
5. Relocate the structure to a preservation site and build a new structure at the existing site: Since a bypass bridge 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 the various preservation options, we have identified Option D as the most apparent appropriate long-term course of action to provide for preservation of this covered bridge for the future. That is, rehabilitate the structure (including floor replacement) for use by vehicles subject to weight restrictions of 40,000 pounds. Oversize and heavier vehicles must be directed to alternate routes.

In addition to floor replacement, we recommend the following repair measures to improve current conditions and to support the commitment for long-term preservation:

• Bottom chord repairs
• Lattice repairs
• Treenail replacement
• Floor beam repairs
• Provide guide rail on each approach for compliance with VAOT standards
• Install new signs to replace missing or damaged signs indicating "One Lane Bridge", vehicle weight limits, no trucks, advance warning for curve, and vertical clearance in accordance with VAOT standards and the Manual of Uniform Traffic Control Devices (MUTCD)

If the community is willing to adhere to, and enforce, a weight restriction of 16,000 pounds, then Option B can be adopted as a shorter-term course of action for preservation of this covered bridge. Preparations should be undertaken to safely support heavier vehicles via replacement of the floor or construction of a bypass structure as a longer-term preservation plan.

The estimate of construction costs for repairs noted herein is $40,000. The estimate of cost for floor replacement is $40,000.