Strengthening from within: seamlessly integrating new and old materials helped strengthen a historic bridge in Washington, DC

Public Roads, March-April, 2005 by Christy Darden, Thomas J. Scott

The twin demands of accommodating heavy traffic and ensuring the preservation of the oldest bridge in the Nation's capital proved to be manageable challenges on a recent restoration project in the historic Georgetown area of Washington, DC. When structural analyses showed that the Wisconsin Avenue Bridge over the Chesapeake and Ohio Canal National Historical Park could not adequately support current vehicle loads, transportation officials from the District Department of Transportation (DDOT) and the Eastern Federal Lands Highway Division (EFLHD) of the Federal Highway Administration (FHWA), began exploring alternatives to strengthen the structure.

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Adding to the complexity of the project, the National Park Service (NPS) owns the bridge and DDOT is responsible for the maintenance and control of the road through an interagency agreement, so collaboration and cooperation became essential early on--and remained so throughout the project--to ensure that both transportation and historic preservation goals were met.

"The strengthening project was necessary because DDOT needed to have a reliable and increased load rating for the structure," says Darcel Collins, the FHWA project engineer, "and much of the masonry and iron railing was in very poor condition and needed restoration."

An innovative reinforcing system embedded entirely within the structure offered a solution that helped the team strengthen the stone arch bridge without visibly altering the appearance of the historic structure. Careful planning kept the heavy city traffic flowing on this key thoroughfare throughout construction. An equally significant accomplishment: multiple agencies with widely diverse missions cooperated to make the project a success. Finally, public coordination and communication were critical to the project and helped immensely, even though construction caused some disruptions to the community.

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A Historic Bridge Needs Help

Built in 1831, the High Street Bridge--now called the Wisconsin Avenue Bridge--lies in the heart of Georgetown, a National Historic Landmark District, just minutes from the White House. When the district was established in 1967, the bridge was identified as a contributing element. Resting in the midst of the restored historic district, office buildings, and modern condominiums, the bridge supports a busy city street used by residents, office workers, tourists, and trucks delivering sea-food, vegetables, and meats to stock fashionable Georgetown restaurants.

The single-span arch bridge extends across approximately 16.5 meters (54 feet) over the canal and adjacent towpath. It is constructed of local gneiss (a foliated metamorphic rock, compositionally related to granite), most likely a byproduct of material extracted when the canal was dug, and Aquia sandstone, a building material quarried along the Potomac River during the colonial period. Aquia sandstone was used in numerous historic structures in the DC area such as the U.S. Capitol Building and White House. The original wrought iron picket railing is anchored in large capstones that adorn the top of the spandrel walls and stone wing walls. Large stone end posts topped with ornamental cast-iron spheres stand at each corner of the bridge and at the ends of the railings above the wing walls.

The canal and adjacent towpath historically served as the transportation corridor that carried coal-laden boats, with the aid of mules, into Washington, DC. Today, the towpath under the bridge is a popular resource for pedestrians traversing through the busy office and commercial district and lures recreational hiking, jogging, and bicycling enthusiasts as well. In addition, NPS uses the path to educate new generations through tour boat rides that offer a glimpse into the past, when it was a common sight to see mules towing boats along the canal. There-fore, keeping the path safely open for foot and diverse recreational traffic was an important consideration during the project planning.

The bridge was originally designed to carry horse-and-cart trade across the canal. Today the structure carries the heavy loads of a modern urban bridge--9,400 vehicles per day on average. Although the bridge was not "overtly" structurally deficient, its load rating could not be determined accurately. Therefore, in the mid-1990s, the help of a consulting engineering firm was sought to assess the situation.

After an inspection and load rating analysis, the engineering firm determined that the bridge could not support current vehicle loads at the minimum live load of HS20 recommended by the American Association of State Highway and Transportation Officials (AASHTO), which is 32.7 metric tons (36 tons). DDOT asked the engineering firm to investigate alternatives for strengthening the bridge. In 1999, the department sought assistance from FHWA to complete the project, along with several others in the area, and EFLHD entered into a memorandum of agreement with DDOT.