The twin towers of the City Link Bridge are a new feature on Melbourne's landscape. Dr Len Koss and his team have conducted wind tests to see how they cope in the exposed Docklands environment.
By David Bruce
Two 25-tonne blocks of steel and concrete sit on each of the twin towers that stand either side of Melbourne's new City Link Bridge over the Yarra River at Docklands. Each block rests on three large metal balls contained in cones - but motorists will not see them when the bridge opens later this year.
The bridge has been constructed by Baulderstone Hornibrook Engineering (BHE) as part of the $500 million Western Link of the Melbourne City Link project. The structure was designed for BHE by Hyder/CMP.
When they drive through the City Link acoustic tube over Melbourne's Flemington area, motorists may not realise that two years earlier there had been a balsa wood replica of the structure, 200 times smaller than the real thing, that was subjected to ferocious wind gusts.
The winds gusts, created inside Monash University's wind tunnel at Clayton, simulated the south-westerlies that blow uninterrupted from Bass Strait over the flat lands of western Victoria before hitting the high-rise buildings and bridges of Melbourne's central business district and docklands.
As the world's structures get taller and more elaborate in design, the science behind the sturdiness of large objects is being advanced by researchers such as those in Monash's Faculty of Engineering.
The ball and cone structures proposed by BHE for the City Link Bridge are not the work of an urban artist but an energy-absorbing device configured to reduce the sway of the towers in the exposed Docklands environment. Today, wind dampers are a common feature on tall structures such as bridges and office blocks. They are also of special interest of Dr Len Koss, an associate professor in the Department of Mechanical Engineering, who assisted in the selection of the dampers.
"Most tall or exposed structures need something to reduce the vibrations caused by wind. Although the ball and cone damper has been used before in the US, this is the first time it has been used in Australia," Dr Koss said.
During the construction phase of the City Link Bridge, BHE commissioned a wind-speed monitoring program for the towers. Dr Koss and his team are using anemometers placed high on the towers. "Some of the wind modelling can't be done perfectly in the wind tunnel, so we have to do it with the real thing," he said.
As much as the general public may feel uncomfortable with the idea, Dr Koss says, all large structures are designed to move in the wind. Buildings sway, bridges rock, and towers, like those used for City Link, may move sideways by up to a metre and still be considered well-designed, solid structures.
Whether the movement is beyond the acceptable standard is an issue dealt with at Monash's wind tunnel by Professor Bill Melbourne, Dr John Holmes and Dr John Cheung, also from the Department of Mechanical Engineering. The engineers are also involved in determining whether the special sound-proofed walls made for freeways are effective in withstanding repeated wind gusts.
Monash has two wind tunnels on its Clayton campus. The 450-kilowatt tunnel has been used to test structures for more than 25 years. A dusty model of the West Gate Bridge hangs by its entrance. The newer and larger one-megawatt tunnel is one of the largest in the world, with a working section 12 metres wide and four metres high.
The wind tunnel was originally designed by Professor Melbourne to study the problems associated with the dispersion of pollutants, such as effluent from power station chimneys. It is now also used for aeroelastic modelling of large bridges, buildings and stadiums, for testing the aerodynamics of vehicles, and for testing full-sized objects such as communications antennas.
Monash has a skilled team of model makers to construct the complex scale versions of the real thing. The corridors of the faculty are adorned with miniature structures from around Australia and Southeast Asia - the Docklands Stadium and the MCG, the Homebush Aquatic Centre in Sydney, the Adelaide velodrome, the Dubai Leisure Complex, Auckland's Skytower, the Macau Tower, the Batang Baram Bridge in Malaysia, and many others. And awaiting the call to service for the testing of proposed high-rise towers are 1:400 models of the entire CBDs of Melbourne, Sydney and Brisbane.
The towers and the acoustic tube of City Link can now be added to the list of landmark structures tested to their limits at the Monash wind tunnel.
From a balsa wood model to the real thing - 200 times larger - the City Link acoustic tube began its life in the Monash wind tunnel under the gaze of, from left, Professor Bill Melbourne, Dr John Cheung and Dr John Holmes.