Sydney Airport's High-Tech Bridge

Sydney airport's new upper level access bridge typifies modern infrastructure projects which brook no compromise in aesthetics, function or the operation of an existing facility. Instead, the project itself made the compromises, drawing on high-tech CAD systems and time honoured trade skills to satisfy the brief.

Completed early in 1999 as part of a massive re-make of the Mascot facilities, the bridge provides access to the upstairs departure levels created by both Qantas and Ansett in their new terminals. It helps address the fact that Australia's busiest commercial airport occupies the smallest land area of any Australian capital city airport. The new arrangement gives Sydney the two-level format seen at airports from Tullamarine to Los Angeles. By effectively doubling the kerbside area available for departures and arrivals, it greatly enhances the terminal's ability to handle the 22 million passengers who pass through every year.

It also adds an eye-catching focus to the upgraded facility, with its two cable-stayed bridges, each with elegant twin towers of box-section steel. Sleekness is its hallmark, with the decking structure fully clad in a Symonite composite aluminium skin, and details such as the cable ends designed to please the eye even at quite close quarters.

The use of steel for the towers and deck structure addressed several key considerations. Severely limited working space and site access called for prefabricated sections, and the light weight of steel allowed these to be erected using a single 200 tonne crane, eliminating the need for heavier equipment. Taking full advantage of steel's ease of transportation and handling, the contractors were able to finish on time and on budget while satisfying strict architectural objectives.

The bridge provides almost a kilometre of elevated roadway, effectively duplicating the U-shaped road already provided to the airline facilities. Whereas the ground level road has three traffic lanes, the upper level has two lanes on the straight sections and three around the curve. This is intended to cope with projected ground traffic volumes until at least the year 2012.

There are two separate cable-stayed bridges, each 195 metres long, located in front of the Ansett and Qantas terminals. Completing the 985 metre total length, the approach spans total 505 metres and the up-and-down ramps are each 45 metres long. The cable-stayed bridges provide visual interest and maximise column-free access at ground level for arrivals. Reinforced concrete columns support a complex structure of fabricated steel I-beams composite with a reinforced concrete deck.

Concept and design of the structure was by McMillan Britton & Kell (MBK) and construction by Transfield Constructions, with strict design parameters laid down by the client, Sydney Airport Corporation Limited (formerly the Federal Airports Corporation). Fabrication was by Lifese Engineering, whose past projects include the Capitol Theatre and St Marys Cathedral reconstructions. Lifese worked to digital shop drawings produced by Transfield Technologies, and pre-assembled every element of the structure at its own yards before sending it for erection on site.

Aesthetics

Although the basic function of the bridge was straightforward - to provide upper level vehicular access to departure lounges and parking stations - the details were complex. Because of the airport's role as a gateway to the Sydney Olympics, it had to be aesthetically impressive, and it had to be finished before the Games got under way. Complicating matters further, the bridge design had to be finalised before the new Ansett terminal had reached concept design stage.

Design parameters included the need to minimise deck width, to maximise natural light to the ground level, and working around myriad underground services including the planned New Southern Railway tunnel and station. Above all, there had to be no disruption to the airport's operations, with no option to close the facility or divert traffic elsewhere. All construction had to be done during the airport curfew hours of 11 pm to 6 am.

The basic design incorporates two separate cable-stayed spans on opposite arms of a U-shape, with complex curves in both plan and elevation. Elegantly simple to the casual observer, its clean skin hides a maze of individually designed structural elements which were put in place using a single 200 tonne Manitowoc crane.

Despite the complex requirements, the job was completed on time and on budget. A key factor was Transfield Technologies' application of Computer Aided Design (CAD) to integrate the aesthetic and practical requirements without having to standardise the individual structural components. In the end, scarcely any two structural elements were identical, but the fabrication was greatly simplified by computerised cutting based on the CAD drawings.

Transferring the high-tech CAD inputs into steel called for traditional fabrication skills rather than new technology. The fabricator ensured all the pieces would fit before they left the workshops for reliable assembly when brought together on site.

Lightweight towers

Most site connections of steel components were by high strength bolting, although the box sections of the four towers were welded. Each tower was prefabricated in four sections and then welded together on site into a single, large, lightweight tower which was erected the same night.

Height limitations for structures at the airport determined the 37.5 metre height of the towers, which comprise of box sections 800 milimetre thick and varying in width from 1,200 milimetres at the base to 1000 milimetres at the top. Tension rod ties are used at the knee joints and the plate is Grade 350L15 XLERPLATE®*, generally 28 metres thick with 40 milimetre flanges in the lower vertical sections.

A significant feature of the pre-cast concrete supports is the positioning of piers to minimise ground level obstruction. Headstocks typically extend beyond the edges of the deck to provide for the three traffic lanes below while achieving the narrow two-lane upper deck to allow the light through. Almost every headstock has a different support arrangement, usually to accommodate the ground level road, but the design achieves a consistent section size and appearance.

Minimising light obstruction also influences the cable-stayed sections, where the connection points for the cables sit outside the width of the carriageway. Special compound cross girders extend beyond the edge of the deck to the outboard attachment point. The cables are housed in tubes which feature spiral ribbing to avoid wind or rain induced cable vibration.

Aesthetically, one of the most significant features is the aluminium cladding of the entire structure to give a clean appearance to the sides and lower surface. This allowed the design of the structural steel to focus on simplicity, economy and constructability rather than appearance. Specially designed lighting also adds to the finished appearance, both above and below the deck and on the towers, where coloured lights form a visual feature at night. Lighting also pays special attention to light levels in transition areas from shade to full sun.

Extensive consultation

Design and construction of such a complex project involved extensive consultation. Initial design parameters took account of diverse stakeholders including the airlines, private vehicles and pedestrians, taxi, bus operators, rental car companies and police. Formal meetings and paperwork were a key success factor. During the design phase there were regular coordination meetings between the engineers, the client and various consultants. Construction was similarly subject to shift coordination meetings, aided by a computer scheduling package.

Several original solutions were devised for various aspects of this project, often with appearance as well as function in mind. Most conspicuous is the cladding of the entire deck structure with composite aluminium sheeting, which encloses the structural steel and hides stormwater, fire hydrants, water supply and electrical services. Details include the curved section of the side cladding and the use of open guard rails to enhance both the sleekness and the amount of light allowed through to ground level. Deck lighting is integrated into the railings.

All up, the project incorporates about 1,700 tonnes of structural steel in the towers, deck and other structural elements. Lifese fabricated the I-girders mainly from Grade 350L15 XLERPLATE®*plate, using submerged arc welding throughout. The deck itself uses a Transfield-developed variation on established permanent steel formwork, with inverted LYSAGHT BONDEK® sheeting as the base element.

Mill orders

The varied and non-standard nature of most sections required the fabricators to place mill orders with BlueScope Steel according to both the design and very rigid time requirements. Working from the CAD drawings and files supplied by Transfield, Lifese worked around the clock for much of the construction period. Special care was taken with welding to avoid deflections caused by differential heating, particularly with curves and cambers, and all sections were pre-assembled and dismantled at the fabricator's yard to ensure accurate assembly.

All I-girders were painted with 125 microns of solvent-based inorganic zinc silicate (IZS), and the towers were finished with an additional two coats of brilliant white polyurethane paint, each of 125 microns.

With so many constraints and compromises to be met, the ultimate achievement was the completion of the A$44 million project on time and on budget. Already a landmark at the airport, the bridge will reach its full aesthetic and functional potential as the final elements of the refurbishment are completed in time for the influx of Olympic visitors.

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