Mount Henry Bridge

The Mount Henry Bridge carries the Kwinana Freeway and Mandurah railway line over the Canning River in Perth, approximately 10 km south of the Perth central business district. At 688 m in length including abutments, it is the longest road bridge in Western Australia. It spans the river between the Mount Henry Peninsula and the suburb of Mount Pleasant.

History
The bridge was constructed as part of the extension of the Kwinana Freeway carried out in the late 1970s and early 1980s. Construction began 31 May 1979, and was completed by 24 April 1982, with the bridge officially opened 9 May 1982. The construction contractor was Clough, and the project manager for Main Roads WA was Geoff Smith.

Between 2004 and 2006, the Mount Henry Bridge was widened and strengthened to accommodate the tracks of the Perth Southern Suburbs Railway. The widening was carried out by adding an independent structure, which overlaps the original bridge. The new bridge opened to traffic on 22 January 2006.

Structural details
The original bridge is made of post-tensioned concrete. It has nine spans with a total length of 660 m, or 688 m including abutments. The deck width is 28.8 m. In cross section it is a double box-section, with the upper deck (including cantilevers) carrying traffic, and cantilevers at the bottom of the box carrying shared pedestrian and bicycle paths. The bridge was constructed segmentally, using a balanced cantilever construction method. The original bridge carried three lanes of traffic in each direction, separated by a central concrete median barrier, with two shared pedestrian and bicycle paths on cantilevers at the lower level.

Widening and strengthening
To accommodate the new Southern Suburbs Railway (now the Mandurah railway line), the bridge was widened and strengthened between 2004 and 2006. Under the new arrangement, the original bridge carries three southbound traffic lanes and the two railway tracks. The new structure carries the three northbound traffic lanes.

Because of the aesthetic requirements, load limitations and limited site space at the approaches, the new bridge overlaps the original in cross section. The upper east side cantilever sits above the upper west side cantilever of the original structure, giving a total effective deck width of 40 m. The foundations of the new bridge are independent of the original, with each pier comprising two columns tapering from bottom to top.

The new bridge was constructed by incremental launching. Contractors were Leighton Constructions. The design team comprised Wyche Consultants, GHD and Coffey Geosciences.