Narrows Bridge: A critical connection

Perth’s Narrows Bridge provides a vital connection between the city’s northern and southern suburbs. At the time of its opening it was the largest precast prestressed concrete bridge in the world.Perth’s progression to the bustling metropolis it is today is a long story, one that has strong roots in the region’s role as a major centre of resource industries.

As the city expanded, so did its population and the requirement for more reliable transport infrastructure.

During the first half of the 20th century, the Causeway was one of the primary routes for crossing the Swan River and connecting the city’s northern and southern suburbs.

By the 1950s, the state government recognised the need for an additional river crossing. Traffic surveys undertaken by the Western Australian Main Roads department at the time indicated that traffic on the existing Causeway had more than doubled in five years, prompting additional investigation into building a bridge at the Narrows.

The idea for the bridge tied into the wider plan for the city and region, and the decision was made to build the Narrows Bridge.

Maunsell and Partners were appointed consulting engineers to prepare schematic plans for the bridge. Following detailed design, the construction contract was awarded to Christiani and Nielsen, in association with J.O. Clough and Son in March 1957.

The bridge design included six lanes with footpaths on both sides. The prestressed concrete girder structure would consist of five spans, each varying in length between 48 and 96 metres, and an overall length of 336 metres.

One particular challenge the project team encountered was the complex engineering conditions. Beds of sand with clay intrusions with thicknesses between 10 and 20 metres were prevalent across the site and sat upon shale beds roughly 40 to 50 metres deep.

The contractor chose large diameter hollow closed-ended steel piles to support the bridge piers. The first piling commenced in late 1957.

The columns for the four internal piers (16 in total) were constructed in reinforced concrete to provide support between piling and deck.

The deck comprises individual, 15-tonne, precast concrete deck units, which were cast and assembled on the Mill Point foreshore.

The overall structure was built as five individual sections consisting of the two main cantilevers with a central suspended span and two end-suspended spans. Each of the sections were individually prestressed using a hydraulic jack and joined by high tensile continuity cables passing through heavy concrete blocks in the adjoining sections.

The Narrows Bridge was officially opened on 13 November 1959.

At the time of its opening it was also the largest precast prestressed concrete bridge in the world. Even with this vital structure now in place, Perth continued to grow and the volume of traffic traversing increased exponentially.

As traffic volumes grew on the Kwinana/Mitchell Freeway, more pressure was put on the Narrows Bridge. Increasing congestion on the crossing during the 1970s led to the bridge being widened to seven lanes.

By the late 1990s, the volume of traffic using the bridge had increased again. Instead of widening the existing structure, a separate second road bridge alongside the original structure was approved for construction. The new bridge would run parallel to the original and carry six-lanes of traffic. The second bridge was competed and open in February 2001.

A third bridge was also constructed in the mid 2000s in the six-metre gap between the existing road bridges to carry trains across the bridge. It was open and operational by December 2007.

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