After a winter's worth of wear and tear, bridges can take a beating in Vermont. This time of year cities and towns, as well as the state, take stock of how the bridges are holding up. The bridges that raise the most concern tend to fall into two categories: the oldest and the busiest. When a bridge fits under both those headings, the problems can be far more complicated.
Case in point is Montpelier's Granite Street Bridge. The Montpelier Public Works Department recently issued a condition report on the bridge, which spans the Winooski River by the intersection of Berlin Street and River Street. It is a Baltimore through truss bridge, built in 1902, according to bridgehunter.com - a website dedicated to historic and notable bridges in the United States.
The Granite Street Bridge underwent a rehabilitation project in 1992. The wood decking was replaced during that project, but the bridge has seen a lot of traffic since then. According to the condition report, the city has been hard pressed to find a decking surface that will hold up to the wear and tear.
Currently the deck wearing surface is worn, exposing the wood to tire abrasion. The rehabilitation project included a hot mix asphalt wearing surface which failed and was reapplied a short time later. In 1999, a revised asphalt mix design known as a chip-seal was used with better results but also relatively short lived. In 2008 , an epoxy resin system with aggregates and known as a High Friction Surface was applied which is what we see on the deck today. This system has shown better results but has also proven to be a short lived solution.
While the wood decking maintains the integrity of the bridge as an historic structure, it is not ideal for the load of today's traffic.
It’s important to keep in mind that wood bridge decks are uncommon on bridges with high traffic volumes such as Granite Street but the wood deck was thought to be an important component for historic preservation purposes matching the original design and for other structural reasons. Wood decks typically have planks attached on the deck aligned along the wheel path of vehicles to protect the deck surface . The planks can be readily replaced when they deteriorate. However, wood is a poor surface with little skid resistance and can be hazardous for bikes or motorcycles and is not recommended in this instance. This deck is also not suited for a plank system due to the exposed splice plates.
The good news is, the bridge is overall structurally sound. Aside from the problem of finding a decking surface that will last, the work that needs to be done on the bridge is largely cosmetic. And one issue that had been reported by passersby turns out not to be a major concern:
Some people have noticed what might appear to be a loose and broken steel bridge member on the upstream side of the bridge. This is not a part of the bridge structure. It’s a privately owned plastic (pvc) conduit for tele-communication lines which was damaged as a result of high water during tropical storm “Irene”. The owner of the conduit has been contacted and asked to repair or remove it.
The report also noted the bridge requires routine maintenance such as cleaning, painting, and tightening of its bolts.
