Photos courtesy of the Vermont Agency of Transportation.

A Modernized Vermont Drawbridge Maintains the Character of Historic Bridge

• Spans 160 feet, with two 50-foot approach spans and two 40-foot bascule leaves.
• Bridge carries 3,000 vehicles daily during the off-season and twice that number during the summer and fall.
• With normal upkeep, the bridge is expected to last up to 100 years.
• The new drawbridge maintains the unique design of the original bridge.

For nearly 150 years, a movable bridge has connected North Hero to Grand Isle in northern Vermont. Carrying U.S. Route 2, the drawbridge is the only connection between the islands, the only automobile drawbridge in the state and provides a critical link south to the Vermont mainland. It carries 3,000 vehicles daily during the off-season and twice that during the summer and fall. 

A 2014 inspection of the bridge revealed numerous concerns and deficiencies — holes, corrosion, section loss, debris, rust and a failing paint system. Additionally, the bridge’s mechanical and electrical systems failed to comply with modern codes and the bridge could only be opened once an hour to reduce wear and tear instead of the two per hour required by the U.S. Coast Guard.

The Vermont Agency of Transportation (VTrans) brought HDR on board to advance the project through preliminary and final design phases. The new $74 million North Hero-Grand Isle Bridge, which opened in June 2023, spans 160 feet, with two 50-foot approach spans and two 40-foot bascule leaves. 

Maintaining the Character of the Historic Bridge

The new bridge modernizes the equipment and operations, brings the bridge into code compliance, improves safety and reduces maintenance.

During numerous meetings, members of the public and stakeholders overwhelmingly requested that the new bridge maintain the appearance and function of the previous bridge. Working with the Vermont State Historic Preservation Office, we preserved characteristics of the appearance and function of the existing bridge and kept the location of the historic tender house. The new bridge maintains the same 80-foot-wide navigable channel and 15-foot-tall clearance while in the closed position during high water and unlimited vertical clearance when open. 

With normal upkeep, the bridge is expected to last up to 100 years. Stainless steel reinforcement in the bridge deck helps extend the bridge’s lifespan. Concrete in the partially filled grid deck will reduce the rate of structural steel deterioration. And the girders were metalized rather than painted adding additional protection and longevity.

Temporary Bridge

To figure out the best way to keep traffic moving while the new bridge was being built, we helped VTrans assess its options. A complete closure of the bridge would have created a 75-mile detour, but a temporary drawbridge would add $10 million to the cost of the project.

Our economics team completed an economic impact study comparing the costs to the traveling public of the detour and the temporary bridge. We discovered the detour would cost the public $10 million in delays, extra fuel costs and harm the local economy.

With this information, VTrans opted for the temporary drawbridge for the project’s duration.

Improved Safety for Bridge Operators

To extend the useful life of the bridge and machinery, we improved access for the workers performing the maintenance. New walkways allow access to the underside of the deck across the entire bridge with maintenance platforms to access span locks, navigation lights, trunnions, hydraulic cylinders, electrical equipment, live load shoes, counterweight pockets and other items. The team designed a deeper superstructure and raised the bridge profile, which provides additional headroom for maintenance.

Each spot below the deck now includes at least two egress paths in case of fire or emergency. An enclosed stairway beneath the control house extends from the bottom of the pit to the roadway. We also added enclosed stairs at the three other corners of the bridge extending from the pit to just below the roadway, where ladders and hatches then provide roadway access. Finally, there is an under-deck walkway from pier to pier and stairs that access the fenders.