Steel Builds Gateways To Melbourne Docklands

Two new bridges serving Melbourne's Docklands development exemplify the versatility of modern steel bridge construction in meeting the needs of complex sites. The bridges provide pedestrian and vehicle links from the CBD to the Docklands precinct, spanning the Spencer Street rail yards and providing access to the Colonial Stadium and Spencer Street station.

Both bridges were built by Transfield Powercor Consortium (TPC). Although they differ in detail, they illustrate the extent to which steel composite construction has become a natural choice for such projects. In both cases, steel I-beams are married with prefabricated decking to meet the challenge of building new structures without disrupting existing services. A significant difference is the choice of span-by-span placement of braced girder pairs by crane for the road bridge, as against launching of the pedestrian bridge from one end. 

La Trobe Street Bridge spans 264 metres over the rail yards, past Colonial Stadium to Docklands Esplanade (formerly Footscray Road). The A$40 million bridge is 28.97 metres wide with two lanes in each direction, two tram tracks in the middle and a 3 metre footpath on each side.

As with the pedestrian bridge, no two spans are the same. Spans vary from 19.4 to 50.1 metres, and the skew of the piers similarly varies between 12 degrees and 27 degrees.

Bourke Street Bridge, built at a cost of A$20 million, incorporates an innovative Gateway feature at the Spencer Street end, with glass towers and cascading water. Other visual highlights include bright red, curved steel ribs run along both sides, with a weather protection canopy on one side, and a red steel tower about halfway across. It is estimated that after an event at the Stadium, up to 39,000 people will traverse the bridge.

Comprising composite steel I-girders of constant depth, this bridge features a main spine continuous over eight spans which vary between 17 and 38.7 metres for a total length of 205 metres. Completing the structure are a West Ramp of four continuous 28 metre spans, and a two-span curved section over North-South Road.

Concept design

Both projects were influenced by relatively large and differing spans, access constraints, the service utilities to be bridged and restricted construction time.

Railway track possession for the road bridge was on weekends only; for the pedestrian bridge it was between 1.00am and 5.00am.

Transfield worked closely with design consultants Sinclair Knight Merz (SKM), and the site complexities tended to dictate the use of steel for the structure of both bridges. Faced with a maze of obstacles - including the rail tracks, signal boxes, communication and electrical cables - steel provided the flexibility required.

The road bridge substructure comprises concrete piers and abutments on driven concrete and steel piles, along with bored piles. The superstructure comprises 11 composite steel I-girders supported on elastomeric bearings and carrying a reinforced concrete deck across seven continuous spans.

Transport

Fabrication for this bridge was by Transfield Seven Hills, which transported the girders from Sydney in braced pairs. Transverse cross bracing is used at eight metre spacings, and a horizontal anti-racking brace stabilised the girder pairs during transport and erection. Fabricated using BlueScope Steel Grade 350L15 XLERPLATE®*, the girders have a constant depth of 1800 milimetres, top flange thickness of 20 militres and bottom flange thickness varying up to 40 milimetres. Web thickness is 20 milimetres, and flange-to-web welds consist of 8 milimetre fillet welds. Surface protection is a single coat of 75 to 100 microns of inorganic zinc silicate.

In selecting the method of construction, Transfield Construction considered span-by-span placement to be faster and more predictable than other options. Girders were erected in lengths of up to 45 metres, and were spliced using High Strength Friction Grip (HSFG) bolts.

Transfloor

On completion of the steelwork erection, Transfloor precast decking panels were installed. The decking, as supplied by Fabcon for both bridges, was precast in sections 60 milimetre thick and 2.5 metre wide. It extends between the girders with lateral cantilevers of up to 900 milimetre and was placed in three segments, each continuous across two or more steel girders. This system provided an instant safe working platform for subsequent work.

Pedestrian bridge

For the pedestrian bridge, fabrication was by Raven Industries of Victoria, again using BlueScope Steel Grade 350L15 XLERPLATE®*. The I-girders are 900 milimetres deep, and were delivered to the site in braced pairs and placed on falsework at pier height level. Girder splices were then butt welded and areas near the welded joints were painted before placing the precast deck formwork.

 

Site restrictions were even more challenging than for the road bridge, as the bridge spans electrified cables and railway pedestrian platforms and has little room for cranes. As a result, the 205 metre main spine was launched from the southern end with the completed deck. The front end of the launched bridge comprised 40 metres of undecked steel I-girders with a small launching nose.

 

The launching operation was designed by GHD Consultants and implemented by Structural Systems using a twin linear winch system anchored at Pier 10. Temporary launching bearings were PolystoneTM pads with plastic "feed through" sheets for friction values of around 6.0%. Once the deck sections were launched, other work could be carried out during normal working hours.

Curved spans over North-South Road, connecting the main spine to the West Ramp, use a framework of steel girders supported by a 100 tonne steel box cross-head measuring 1500 milimetre deep, 1200 milimetre wide and 47 metres long. This was fabricated off site and erected on temporary supports using two cranes. Steel I-girders were then bolted to the cross-head to bridge over the road before placing the precast decking.

Next to be installed was the 31 metre steel tower, from which cables help support the cross-head cantilevers. These cables consist of 2no, 56 milimetre diameter CT Stress Bars anchored on the tower top and connected to the extremities of the cross-head cantilevers. Two more cables run at right angles to the support cables and connect to outriggers to brace the tower.

Gateway

A different form of composite construction was chosen for the Gateway at the Spencer Street end of the bridge. Designed by Wood Marsh Architects, it incorporates red glass-clad towers, a fountain and neon light display, complementing the red canopy ribs and tower of the bridge and the bright metal balustrades. Railway station platforms are reached via lifts, escalators and stairs.

Construction of the ramps comprises steel beams and LYSAGHT BONDEK® formwork topped by a concrete slab. This allowed maximum speed and flexibility, as the LYSAGHT BONDEK® can be handled and installed manually and can be easily cut on site to fit any geometry.

Overall, the Docklands project exemplifies the relative ease and safety with which modern steel bridges can be constructed, and the variety of construction options available. The use of structural steel, combined with the Transfloor and LYSAGHT BONDEK® decking, enabled fast, safe and economical construction of the deck slabs with complex geometry in a difficult site.

As a bonus, the ease of transporting steel girders over hundreds of kilometres allowed for keen competition by interstate fabricators and minimised the risk of delays.

* From 1 August 2002, BlueScope Steel Plate Products are known as XLERPLATE®