Montgomery's Comstock Bridge

(WGN 45-06-04)
Inspection Report - May 1995
Highlights

Description:

The Comstock Covered Bridge was built in 1883 by the brothers Sheldon and Savannah Jewett. The bridge is one of five covered bridges remaining in the Town of Montgomery giving the Town one of the highest concentrations of covered bridges in the State. The Comstock Bridge is currently listed on the National Register of Historic Places.

Bridge Characteristics:

Timber Truss Configuration - Town Lattice; Number of Spans - 1; Measured Length (End to End) - 68.9 feet; Measured Horizontal Clearance - 16.29 feet; Measured Vertical Clearance at Truss - 8.5 feet; Measured Vertical Clearance at Center of Bridge - 11.33 feet; Load Posting - 6000 pounds.

Traffic Volumes:

According to 1994 VAOT data, the estimated average daily traffic volume on the bridge for the year 1992 was approximately 100 vehicles per day. An estimated average daily traffic volume of 140 vehicles per day is projected by the VAOT for the year 2013.

Alternative Route:

A detour of this bridge is very convenient since it is located on a short side street in the Village of Montgomery. The detour length (bridge-to-bridge circuit) is one half mile. VAOT information indicates that the posting capacity for the one bridge on the detour is 31 tons which provides sufficient capacity for this detour to be acceptable.

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 this time the following deficiencies were observed:

• Concrete cap on Abutment #2 has 1/2" diagonal crack under the upstream truss bearing timber.
• Misalignment/shifting of the bottom chord has occurred at the downstream end of Abutment 2, resulting in a poor bearing condition.
• Some areas of rot in the top and bottom chords, a few small punky areas overall and some insect damage.
• A few roof rafters are broken, some boarding missing on doors and a transverse beam is broken.
• Roof has some rust with a few small holes.
• 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 7 3/4" x 11 1/2", spaced at 4'-0"
  • Truss upper bottom chord 3" x 11 ", 4 per chord.
  • Truss lower bottom chord 3" x 11 ", 4 per chord.
  • Truss upper top chord 3" x 11 ", 4 per chord.
  • Truss lower top chord 3" x 10%", 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 12,000 pounds.

Preservation Options:

1. Close the structure and divert traffic: This structure currently carries light traffic adequately. Although the detour is very short, this option is judged to be unnecessary.
2. Continue use of bridge for light traffic, with heavier truck traffic diverted to other local routes: This structure can support vehicles up to 12,000 pounds, following repairs noted herein. Heavier vehicles should be directed to the readily available alternate route. An estimate of cost for necessary repairs/improvements compatible with this option is $55,000. This option appears to be acceptable.
3. Close structure and construct an adjacent bypass: A bypass structure may not be possible immediately adjacent to this covered bridge, nor is a nearby site readily identifiable. The current detour, being only one half mile, is so short as to make a new bypass structure impractical and unnecessary. This option is eliminated.
4. Rehabilitate structure for moderate traffic: Rehabilitation of this structure to support loads in excess of 12,000 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 $60,000. Therefore, the total cost for this option is $115,000 (Option B cost + deck replacement). This option is judged to be unnecessary.
5. Relocate the structure to a preservation site and build a new structure at the existing site: Since a short detour for heavy vehicles is readily available, this option is judged to be unnecessary. However, if this option was to be pursued, a replacement structure on the existing site is estimated to cost $245,000. Stabilization of the existing structure, when moved to a preservation site, would 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 $40,000. The costs associated with obtaining a suitable preservation site and the costs of relocation of the covered bridge would be significant and difficult to estimate at this time. Therefore, the cost of this option, without the site purchase and relocation, is estimated to be $285,000. This option appears to be unnecessary.

Recommendations:

Having considered the traffic needs at this site, condition of the structure, and merits of, we have identified Option B as the most apparent appropriate course action to provide for preservation of this covered bridge for the future. That is, continue to use the structure for light vehicles and direct heavier truck traffic to other routes.

We recommend the following repair measures to improve current conditions and to support the commitment for long-term preservation:

• Jack truss and repair chords
• Realign/repair chord at the bearing area on the downstream side of Abutment #2
• Roof repairs as necessary
• Repair concrete cap on Abutment #2
• 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)

Completion of the repair measures mentioned herein will permit increasing the load posting to 12,000 pounds, subject to concurrence by VAOT and the Town, if additional capacity is desired. The estimate of construction costs for repairs noted herein, compatible with Option B, is $55,000.