Issue Three 2012 - Issue #17
Taking the high load
Story by James Hutson
The West Gate Bridge spans Melbourne’s Yarra River, linking the inner city and western suburbs. It carries more than 160,000 vehicles per day, including numerous heavy trucks. When the bridge opened in 1978 it carried only 40,000. Worsening congestion put strain on the bridge and commuters. This major arterial road needed an upgrade, and Swinburne played a major part in delivering a solution.
The solution accepted by Vic Roads was appealingly simple. “Widening” of the bridge would occur “in lane” only. Emergency lanes would be absorbed
and lines redrawn to create five commuter lanes in each direction.
Sinclair Knight Mertz (SKM) were the consulting engineers for the West Gate Bridge Strengthening Alliance on the concrete sections of the bridge. Their engineers analysed the increased stresses and strains which increased traffic loads would likely inflict.
The bridge would certainly need strengthening; in fact, it would become the largest retrofitting project of its kind in the world.
SKM bridge engineer Grahme Williams approached Professor of Structural Engineering at Swinburne, Riadh Al-Mahaidi, whose research focuses on retro-fitting structures with advanced composite materials.
As Williams explains, the collaboration that followed was invaluable. “Typically in construction we go for the quickest and cheapest method, which often relies on standard methodologies. It’s rare we get projects of this magnitude, with this much scope for potential savings. By spending a little bit of money upfront on this research program we were able to save millions of dollars in implementation down the road.”
“Grahme and I looked at the efficiencies in the proposed design together,” Professor Al-Mahaidi says. “We discussed what options we had and if design guidelines would allow us to use alternative techniques. And they did, if we could prove the efficiency of another system experimentally.”
Traditionally bridges are strengthened by reinforcing them to resist strains by glueing steel plates or jacketing sections with additional concrete that act in the same way as a splint or putting a cast on a broken limb. But over the past two decades engineers have been investigating alternative bracing materials like carbon-fibre reinforced polymer (CFRP). CFRP is a strong, lightweight fabric of interlocking carbon threads with up to 10 times the strength of steel, twice the stiffness, yet only one-seventh the weight.
Furthermore, it is very durable, with none of the corrosion problems experienced with steel and concrete.
Prefabricated carbon-fibre laminate beams can be fixed with epoxy to structures like external ribs. In the case of the West Gate Bridge, however, only around 20 per cent of the CFRP’s strength would have been harnessed using these standard design guideline approaches.
Anchors of cloth
“We developed an anchorage system that is added to the carbon-fibre laminates but also uses carbon-fibre material. This anchoring system increased the efficiency of the fibres by up to 260 per cent. What this really meant was that we reduced the overall amount of fibre we needed to use,” Professor Al-Mahaidi says.
The anchorage system is simple and cheap. A 25 centimetre-wide strip of carbon-fibre fabric runs across the end of all the carbon-fibre beams, like a line of super-strong sticky tape.
The fabric anchor is a different weave so the strength-bearing threads run in two directions. It anchors the laminates and spreads their load to surrounding concrete to increase the overall strength of the system.
The strength, delicacy, ease and versatility of the CFRP laminate and fabric system recently took Professor Al-Mahaidi to Karbala city in Iraq. The system was used in the repair of Al-Abbas ibn Ali shrine masonry dome, which was damaged by artillery and tank fire in 1991.
Mimicking a bridge
At Monash University and then at Swinburne, Williams, Professor Al-Mahaidi and his team tested possible anchoring solutions to the point of failure using concrete blocks to mimic bridge sections and the position of areas prone to delamination (stress fractures).
“During tests we monitored the blocks using surface sensors to measure the level of stress and strain, and used photogrammetry, two cameras continually recording any surface deformation,” says Professor Al-Mahaidi.
“In addition, computer simulation gave us a deeper understanding of what was happening within these zones. These computer models also correlated with the physical evidence from the lab testing.”
This work was commissioned by The West Gate Bridge Strengthening Alliance comprising SKM, VicRoads, John Holland and Flint & Neill, with funding from the federal and Victorian governments.
Strengthening the curriculum
The scale of the West Gate Bridge strengthening project, the novelty of the solution and importance of these maintenance processes have created a body of knowledge Professor Al-Mahaidi feels is worth codifying and sharing.
“The research over the past 10 years has encouraged us to introduce a new unit of study to the curriculum, which is the first of its kind in Australia: ‘Strengthening and monitoring of structures’.”
The unit relates many findings from the West Gate Bridge and is suitable for fourth-year and masters engineering students.
West Gate Bridge stats
15 November 1978
Number of Lanes:
38km of carbon-fibre laminate, 12,000m2of carbon fibre fabric, 400,000 bolts and 1600 tonnes of steel fabricated into 80,000 pieces.