Historic Minto Bridges to be restored
Reinforced wood system to replace steel grating
The 110-year-old Minto Bridges that cross the Rideau River are undergoing rehabilitation to repair damage and to make them strong enough for fire trucks.
The $2.5-million project “will help protect this historic structure from further deterioration and extend its service life for many years to come,” says Patrick Mergel, a structural engineer with Delcan Corporation.
The Minto Bridges are made up of three spans. They connect New Edinburgh at Union Street to Green Island, site of the old Ottawa City Hall, and Lowertown at King Edward Avenue. Built between 1900 and 1902, they were part of Ottawa’s first ceremonial route leading from Rideau Hall to Parliament Hill.
Their graceful silhouette is visible from Sussex Drive at the Rideau Falls and is muchloved by photographers. The slender structural members painted white make a surprisingly delicate tracery. A light ornamental webbing suggests an arch over the roadway.
The first section to be restored by the City of Ottawa is Minto Bridge West, a 53-metre long span between Green Island and King Edward Avenue. There are plans to rehabilitate the other two spans but the work has not yet been scheduled.
The heritage-designated bridges are in daily use by cars, cyclists and pedestrians.
However, the steel grate deck has allowed winter road salt to splash onto the structure below causing corrosion.
An innovative reinforced wood system is to replace the steel grating. The fibreglass-and-wood deck, made by Guardian Bridge Rapid Construction, will be the first such bridge deck in Ottawa.
“It’s a very robust and durable material,” says Crawford Dewar, president of Guardian Bridge in Stratford, Ont. “It is 10 times stronger than steel, 30 per cent lighter, and immune to corrosion.
“Most people don’t realize that pound-for-pound wood is stronger than steel,” he says. “We reinforce wood externally with epoxy fibreglass for durability and capacity.”
The deck tapers slightly from the centre allowing water to drain into gutters, protecting the structure below-deck from salt. The run-off will flow into the river. “As the structure type is a truss, which is fairly open without barrier walls, it would not be possible to contain salt splash from the deck from entering the river without significantly altering the heritage appearance of the bridge,” says Mergel.
At 16 pounds per square foot, the fibreglass and wood composite deck is lighter than steel grating, which weighs 28 pounds per square foot.
Reducing the weight of the bridge deck enables heavier traffic to cross. A fire truck, for example, will be able to access Green Island, where federal Department of Foreign Affairs employees work in the John G. Diefenbaker building.
Dewar developed the technology, which has been in use since 1996. His firm has replaced more than 200 bridge decks in the United States and Canada.
Installation in Ottawa is expected to take four days at the end of January. The deck, designed to last 75 years, costs about $550,000.
The rest of the bridge will be renewed also. Greyleith Engineering and Construction, of Carleton Place, is working on steel repairs. Steel members are being stripped back to bare metal, strengthened and repaired. In some cases, they will be replaced. They will also receive a protective coating.
Masons are repairing and repointing the stone abutment walls using matching stone. The existing wooden sidewalk along the east side of the bridge will be replaced with a new wooden sidewalk and curb. There will also be new lighting.
Work is expected to be completed in early February.
“The structural steel below the deck was generally in fair condition with several areas in poor condition,” says Mergel. “The existing deck grating is still generally in good condition, but replacing it with a closed deck system is the most appropriate solution, as it will protect the structural steel members and coating system from costly repairs in the future, reduce noise pollution from vehicles passing over the grating, and will offer a much better riding surface for cyclists and vehicles.”
The bridge deck was originally built with timber planks on a steel floor. In 1978, it was replaced with asphalt on steel. In 1996, the current steel grate was put in.
Built by the Dominion Bridge Company, the bridges were one of the first projects of the Ottawa Improvement Commission which prime minister Sir Wilfrid Laurier set up in 1899 to transform Ottawa into the “Washington of the North.”
The Minto Bridges are named after the Earl of Minto, who was governor general at the time.
Delcan Corporation is the engineering consultant. The project was undertaken in collaboration with heritage consultant Barry Padolsky Architects.