An innovative solution designed jointly by specialist formwork and falsework engineers from RMD Australia, Leighton Contractors Pty Limited and consulting engineers Hawkins Engineering & Construction has helped to ensure that the $42 million Narrows Bridge Duplication Project in Perth, Western Australia has been completed successfully.  The bridge, which spans the Swan River, is a near replica of the existing Narrows Bridge completed in 1959 and provides a dedicated two-lane bus transitway, plus an extra traffic lane in each direction.  The existing bridge carries three lanes in each direction and a reversible peak lane, which is shared with buses.

The new bridge, which is on one of the city's busiest commuter routes, is 338 metres long, 28 metres wide and has been built 6-metres from the existing bridge.  It consists of five spans, the spans and pier locations being a mirror image of the original bridge.  The main navigation span is approximately 100 metres, while the other spans are 50 metres and 70 metres. 
 
incremental launch technique whereby sections of the bridge were cast on the south side of the Swan River and were then hydraulically launched, utilising Teflon slip pads and temporary piers, to the northern side.  This technique is particularly suitable for bridges that have a constant profile throughout their length.  It also ensured that waterway use was unhindered while obviating the need for complicated deck support falsework. 
 
The curved and variable surfaces of the arched profile did not allow for the more conventional technique of launching from the underside of the structure.  Instead Leighton Contractors opted to launch the deck from the underside of the top flange of the girder – the structure that supports the bridge

The bridge deck was constructed in two halves, side-by-side, each 14 metres wide, to allow for a more efficient and productive use of the available resources.  Once launching was completed, a 1200mm stitch was constructed that joined the two halves together.  The abutments and piers consist of concrete walls, columns and pier caps that are supported on steel piles.
 
A number of RMD formwork and falsework systems were used and, although components were reused in a number of configurations, all were standard, readily available RMD stock items.  This ensured that an economic solution was implemented, enabling the contractor to hire the equipment for the 18-month contract.

The bridge deck was constructed in two halves, side-by-side, each 14 metres wide, to allow for a more efficient and productive use of the available resources.  Once launching was completed, a 1200mm stitch was constructed that joined the two halves together.  The abutments and piers consist of concrete walls, columns and pier caps that are supported on steel piles.
 
A number of RMD formwork and falsework systems were used and, although components were reused in a number of configurations, all were standard, readily available RMD stock items.  This ensured that an economic solution was implemented, enabling the contractor to hire the equipment for the 18-month contract.
 
Among the systems used, RMD supplied extensive quantities of its Megashor high load capacity modular propping system.  This proved to be a particularly appropriate choice of system as its 270mm leg increments facilitated the easy breaking down of the Megashor tables
 
that differed for each segment. This versatility also allowed easy adjustment of the girder trolleys.  Other RMD systems used on the Narrows Bridge Duplication project included Super Slim Soldiers – the backbone of the RMD range - RMD Alform beams and Rapidshor lightweight shoring, together with push-pull props, turnbuckles and accessories.
 
The government client for the project – which also involved refurbishing the existing bridge – was Mains Roads Western Australia.

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