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

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In addition, the strengthening system takes substantially less time than traditional methods like saddling, and its effect on traffic is minimal. According to the manufacturer, the installed reinforcing rods have been independently age-tested in the United States and Europe, with a predicted long-term durability of at least 120 years.

Design and Construction

A detailed survey of the bridge focused on the road and arch barrel surfaces, and control points were established for setting up the drilling rig during operations. Engineers visually inspected the arch intrados (interior curve of the arch), spandrel and wing walls, and the railings to assess the general condition of the bridge, and took core samples.

Then the designers modeled the properties of the materials and the behavior of the material contacts, and applied loading in accordance with the British standard. Next, they loaded the survey data into the program to generate a three-dimensional CAD model including the road and arch barrel surfaces, the position and length of the reinforcing rods, the angles of insertion, and utilities.

After establishing the optimum design, construction began in September 2004. Workers drilled holes 6.5 centimeters (2.56 inches) in diameter parallel to the roadway and at an angle along the arch. The holes were drilled to the precise angles specified in the design with a small core drill, which operated at a slow enough speed to preclude any potential damage to the structure due to vibration. The drill rig was bolted to the road through the paving surface to prevent movement during drilling. After a hole was drilled, the reinforcing rod and fabric sleeve were inserted into the hole, and the drilling rig was then moved to begin the next hole. In all, 26 reinforcing rods (13 on each side of the arch crown) were then inserted into the arch barrel. With two drilling rigs, workers were able to drill two holes a day.

"We had not seen the technology before," says Karyn LeBlanc, communications specialist with DDOT. "This was really an interesting engineering feat. We took other engineering feat. We took other engineers out to view the process as it was going on because it was so innovative."

In the end, workers were able to strengthen the Wisconsin Avenue Bridge in less than 3 weeks--2 days ahead of schedule. The strengthening process cost about $350,000 (construction) plus design. Replacement, which was out of the question because of the historic value of the bridge, would have cost many times that, with greater disruption to the local community and traffic.

Additional improvements planned for 2005 include a new concrete slab over the arch to function as a riding surface, and the restoration of the iron railings, stone work, and sidewalks, further enhancing the safety and beauty of the bridge.

A Smooth Operation

The experiences on the Wisconsin Avenue Bridge project yielded several significant lessons for future projects. First, the innovative strengthening process could be a viable option for dealing with other historic structures and can be considered when addressing the requirements of historic preservation, while also satisfying modern engineering, safety, and environmental requirements. Before the strengthening project, the weight restriction on the bridge was posted at 22.7 metric tons (25 tons), according to an inspection report from February 1997. After the renovation, the rating is HS25, or 40.8 metric tons (45 tons) under AASHTO guidelines.