Composite Bridge A CAD-CAM Work of Art

In Italy bridges are sometimes called opere d'arte - works of art. In Australia, the translation of a new composite construction bridge into a work of art has been made possible by sophisticated CAD-CAM technology. Roberts Road bridge over the Hume Highway and railway at Chullora in Sydney will have its chance to impress an international audience as a main access route to the Homebush Olympic site.

If judged a work of art, it will be due to the high-tech marrying of form and function by the New South Wales government designer and architect, RTA Technology. RTA used computer aided design to make its complex design both buildable and aesthetically pleasing.

Construction challenges included the need to build the bridge above operating rail tracks and roads while maintaining access across the bridge itself. It also had to be erected from a single craning point and the end result had to be pleasing to the eye.

The result is a classic modern composite bridge design, a sinuous, lightweight structure which would have defied traditional construction techniques. Its superstructure comprises four steel trough girders supporting a reinforced concrete deck across six continuous spans of 25 to 40 metres. The fully enclosed girders combine low profile, precision engineering, light weight and low maintenance.

This bridge is built on a 240 metre vertical curve, while its horizontal alignment comprises reverse curves of 240 metres radius at each end with a straight horizontal mid-section. Carriageway widths vary from 8.6 metres to 9.8 metres between kerbs, and the median skews across the centreline. Piers and earth abutments are constructed with varying skews to suit the intersection layout, railway tracks, utilities and existing structures.

Computer aided design and fabrication were crucial in addressing this complex geometry. The steel distributor used computerised shop drawings to cut the steel plates, and the fabricator assembled the girders using jigs, saving both time and cost while ensuring precision in the pre-assembled steel work.

The curved steel trough girders strictly follow the alignment of the structure, despite the varying deck width, thus enhancing its appearance. In recognition of the area's rail heritage, with the nearby Chullora workshops and Enfield marshalling yards, the girders echo the green-and-yellow livery of the historic 3801 steam locomotive.

The railway theme is also reflected in the bridge piers, modelled on steam engine crankshafts, and in the use of locomotive wheels and axles in landscaping.

As befits a showpiece of computer-driven design, a defining element is the shallow superstructure. The trough girders are approximately 1400 milimetre deep with a 2250 milimetre wide bottom flange of varying thicknesses ranging between 16 and 25 milimetres. They are stiffened longitudinally with 200 x 20 milimetre plates. Top flanges are typically 500 to 600 milimetre wide and the vertical webs are 16 milimetre thick throughout. Steel is Grade 350 XLERPLATE®*and 350L15 XLERPLATE®* to AS 3678 for the top flanges and longitudinal stiffeners.

These girders offer benefits both during construction and over the life of the bridge. Erected at night to minimise disruption to trains and traffic, they were delivered pre-painted in segments of up to 40 metres in length and erected span by span from the single craning point on the northern abutment. The segments were then butt welded together and the welds ground smooth and painted. In the longer term the elegant lines promise cost savings because the absence of nooks and crannies simplifies maintenance and eliminates incidental problems such as bird nests.

This project exemplifies innovative use of steel to meet the challenge of a difficult site. Completion ahead of time and under budget established a new benchmark for cost-effective fabrication, one which would scarcely have been possible with the technology of even a decade ago.

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