Award of Merit: McLean Bridge Replacement

Category: Transportation

AMEC INFRASTRUCTURE /CWA STRUCTURAL ENGINEERS

Highway 864 in Alberta not only provides access to the town of Taber, but is also used by agricultural producers and the oil and gas industry in the region to reach Highway 3. However, the existing McLean Bridge over the Oldman River on Highway 864 was a one-lane truss structure that had a long history of accidents. Plans began to replace it in 1997.

AMEC Infrastructure completed the functional planning and recommended the new bridge’s alignment out of 15 alternatives. However, the project costs threatened to escalate when it came to determining the bridge length and head slope locations. Alberta Environment rejected outright the option of a shorter bridge of 240 metres because it would destroy scarce habitat. They insisted on a longer span of 370 metres to clear the riparian habitat on the south shore. The habitat is a feeding station for deer. A longer bridge, however, was unacceptable to Alberta Transportation because of the higher costs.

AMEC worked with the agencies to find an acceptable compromise and eventually a 301-metre long structure was selected. The bridge has five spans and the end spans are modified to accommodate the concerns over the habitat on the south shore. The configuration is 57.5 m – 62m – 62m – 62m – 57.5m.

With AMEC as prime consultant, CWA Structural Engineers were asked to do the structural design. Maintaining the balance between project costs, environmental concerns, aesthetics became the focus, and the engineers constantly had to provide creative solutions. Aesthetics were important because the bridge passes through Taber’s campground and recreational park.

At the onset it appeared that a steel girder design was the only alternative for the five-span option, but costs again threatened to escalate. The challenge was to find a concrete girder alternative to achieve the span lengths at a reasonable cost.

The engineering team researched a new technology known as the NU girder developed at Nebraska University. The precast girder, 2800 mm deep, has not been used before in North America. The girders are cast in a mechanical form that allows easy access for placing the steel reinforcement. Both concrete and steel girder designs were used in the tenders, and the concrete design was chosen.

In the McLean bridge, custom-formed, welded-wire mesh was used as vertical reinforcement to maximize the use of space in the girder’s 175 mm web, and also to accommodate three post-tensioning ducts, each containing twelve 15 mm steel strands. The girder has a 1000 mm wide bottom flange, which adds stability during transportation.

Staged prestressing of the concrete girders helped reduce the amount of post-tensioning required. The first stage of pretensioning was to counter the weight of the girder. The second and third stages consisted of post-tensioning, alternating with casting of the bridge deck.

Identical splayed piers with twelve-sided columns were designed to support the bridge. The nearly circular bases eliminated the need for the bridge to have a skewed alignment, and the use of the same design for each pier enabled the contractor to re-use formwork — both factors reducing costs.

Extra care was taken with the connection details. Intermediate post-tensioning anchorages were too costly, but careful analysis and the use of low-friction ducts showed that post-tensioning was possible without such anchorages. Information used in the analysis was gathered from the U.S. and Europe, and checked by independent experts.

The bridge was completed in October 2001 on budget at a cost of $8.5 million. The engineers estimate that the approach taken saved up to 20% in the costs of transporting and erecting the girders, and through the elimination of the premiums for long end spans. That saving, together with the bid difference between the steel and concrete tenders, reduced costs by approximately $2 million.

Name of project: McLean Bridge, SH 864 Oldman River Crossing

Other key players: Getkate Construction (general contractor), Con-Force Structures (girder prefabrication)