Opportunity
West of the town of Marathon in the Thunder Bay district of Ontario, the Little Pic River empties into Lake Superior. At the mouth of the river is Neys Provincial Park, where visitors can see caribou, moose, the ghost town of Coldwell, and the remains of Neys Camp 100, a World War II era military facility.
Just before it reaches the lake, the river is crossed by the TransCanada Highway, which provides a vital road link with Marathon. The largest shore town between Sault Ste. Marie and Thunder Bay itself, Marathon began as a railway community called Peninsula, was later renamed for the company that ran the local pulp mill, and now provides homes for workers at the Hemlo gold mines to the east.
According to the book Pic, Pulp and People: A History of the Marathon District, Marathon had no road connection to the outside world until 1953, when a Bailey bridge was swung over the river. The crossing of the Little Pic River was considered especially dangerous.
In 1958 a permanent bridge was completed, 740 feet (225 meters) long and 180 feet (55 meters) above the water, making it the highest bridge on the Trans-Canada Highway and the largest truss bridge in the province’s Northwest Region.
The bridge underwent a major rehabilitation in 1989, but after twenty more years Ontario’s Ministry of Transportation believed it necessary to assess this important transportation link.
Solution
In 2009, HMM, a joint venture of Mott MacDonald and Hatch, was retained to conduct a site inspection and load evaluation, and to prepare a detailed design for rehabilitating the deck and strengthening the four-span steel-truss and concrete-deck structure. The main span is a suspended configuration supported by four pins. The truss varies in depth from 13 feet (4 meters) in the midspan and approach spans to more than 39 feet (12 meters) at the piers.
The joint venture undertook a detailed structural analysis to assess the load and capacity of the existing truss members and gusset plate connections. To do this, we created 3D structural models incorporating truss sections made by combined bolted plates and angles.
Outcome
With the help of our joint venture, the Little Pic River Bridge was brought up to modern code requirements and its service life was extended by more than 50 years.
Due to the remoteness of the site and the need to accelerate construction, the bridge deck replacement used prefabricated elements. A lightweight precast deck design incorporated fiber-reinforced polymer bars and ultra-high-performance fiber-reinforced concrete in the joints between the panels.
To ensure against the failure of the pins supporting the central span, we developed a hanger design using large-diameter high-impact bars. To eliminate the need for major structural changes, the design retained the existing flow of truss forces. We developed a monitoring system to activate warning lights in the event of pin failure.
Disruption to traffic on the Trans-Canada Highway was minimized through the staging and sequencing of construction.