The bridge was built in 1865, spans 154 feet over the Otter Creek connecting the towns of Salisbury and Cornwall and is of the town lattice design as patented by Ithiel Town in 1820. It has the widest lattice spacing known of any town lattice covered bridges in Vermont at 5-foot spacing. It was constructed at a royalty of $1/foot to provide Cornwall with a link to railway shipping in Salisbury. The bridge is currently posted for 3 ton with a 15-mile detour. The results of the McFarland-Johnson, Inc. report done for the bridge in 1995 has previously been shared with the towns; the report states that it is likely that the trusses could support an H20 loading if rehabilitated but it is presently limited by the floor system. In 1998 the Agency received concurrence from both towns to move forward with rehabilitation as recommended in the McFarland-Johnson, Inc. study. The Town of Cornwall through their representative at the meeting emphasized that it would like to see the carrying capacity increased to H20 loading. The Otter Creek floods at the bridge and the approach roads are closed for some periods of time each year.
Various repairs have been made to the structure over the years. The Agency oversaw a project in 1969 that involved various timber repairs and the addition of a pier at mid-span designed for H10 loading to level up the bottom chords and remove sag. In addition, the abutments were capped, re-pointed and/ or faced and new treated shear beams were placed. In 1992, the towns hired Jan Lewandoski to rehabilitate the structure. Mr. Lewandoski's work included significant timber repairs such as replacing or sistering all broken/ rotted chord and lattice members with spruce. Abutment bedding timbers were replaced with white oak and other miscellaneous repairs were done. Finally, a new standing seam roof was added to the bridge during the summer of 2002 through an Agency project with some roof sheathing replaced.
It is the intent to salvage the new roof if possible. The bracing system will be analyzed for wind load and the existing rafters for snow load. If structurally adequate, members will be assessed individually for wear and replaced as needed with in-kind members.
Further condition assessment is needed. Some tails of the lattices do not appear to extend adequately below the bottom chord and some of the treenails are in poor shape. It appears that a significant amount of the bottom chords were replaced in 1992 as well as approximately one-third of the top chords. It is proposed that all members needing replacement be replaced in-kind. However, the designers are still struggling with the modeling and structural analysis for this structure and it is not yet known if this will be feasible.
The existing floor system consists of 3" x 12" runner planks over 2" x 6" diagonal flooring over 3" x 12" longitudinal deck planking. The 4"x12" floor beams are at an unusual and varied spacing and there is 2'x 6" lateral bracing. It was proposed that glulam deck panels be installed over 10 1/2"x15 1/8" floorbeams at regular 5'-3 5/16" spacing. Eric Gilbertson stated that he would like to see if the present, irregular floorbeam spacing will work if glulam members are used, even if member sizes need to be increased. It was proposed that running boards not be replaced but curbing was suggested inside the structure to both protect the structure and offer some refuge to pedestrians. New lateral bracing is proposed as well.
The abutments are in pretty good shape and no major work is anticipated. Split or checked shear beams and blocking would be replaced. The pier will need to be analyzed for proposed loading considering existing scour condition. It was proposed that debris be removed at the upstream side of the pier.
It was proposed that either weathering-steel w-beam or steel-backed timber guardrail be used on the approaches. New pavement on both approaches was proposed as well as appropriate signing and miscellaneous drive and grading improvements. As well, stone fill for bank stabilization was proposed.
The siding is routinely damaged at the bottom by floating debris and ice and has been replaced many times. It is proposed that damaged members be replaced in-kind. The siding is not presently continuous in its height; this was discussed along with the probability that members the height of the bridge might not have been available at the time the bridge was built.
The Town of Cornwall is concerned about fire. Various detection and retardant measures were discussed at the meeting and will need more input from the towns. Warren Tripp relayed that we are pursuing a research project on the bridge to measure deflections which will hopefully help develop new distribution factors. Nancy Boone asked if we could wait until the research has been completed before moving forward with design details but, as this is not expected to be completed until sometime this summer, the timing will not work.
It appears that quite a bit of further analysis is required and another meeting is anticipated. The project engineers were looking for input on direction of the project early on and their efforts to date were acknowledged and very much appreciated.
Chair, Historic Covered Bridge Committee
[This article was originally posted February 29, 2004]