'Bridge doctor' keeps city's structural marvels in shape

PUBLISHED : Sunday, 08 July, 2012, 12:00am
UPDATED : Sunday, 08 July, 2012, 12:00am


The Tsing Ma Bridge, with its long span, lofty towers and imposing cables, may look like an unshakeable engineering marvel standing proudly in Hong Kong.

But it does move: it swings several metres when typhoons hit and lengthens in summer by up to two metres. For safety reasons, its every move is recorded by an engineering professor, known as the bridge's 'doctor'.

'Bridges are like human beings. Like you, like me,' said Xu Youlin, the 59-year-old head of Polytechnic University's civil and structural engineering department.

'You may not need to go to hospital now, but when you're about my age, you will have to visit it someday. Bridges are the same. They have to see doctors,' he said.

Since the 1.4 kilometre bridge opened in 1997, Xu's structural-health monitoring system, which uses powerful sensors, has been used to conduct safety checks. The system comprises more than 350 sensors fitted across the bridge, which measure everything from tarmac temperature and strains in structural parts to wind speed. These factors determine whether a bridge can withstand frequent typhoons as well as wear and tear.

'Some people work too hard, and so do bridges - we call it fatigue as well,' Xu explained.

For his 'outstanding contributions to the theory and practice of the aerodynamics and health monitoring of long-span cable supported bridges', Xu was last week given the 2012 Robert H. Scanlan Medal, a prestigious award given out by the American Society of Civil Engineers.

The medal is presented annually to 'a person who has made significant contributions in the field of engineering mechanics', and Xu is the second Chinese scholar to receive it in its 10-year history.

Xu and his team devised the safety system for the Stonecutters Bridge connecting Tsing Yi and Stonecutters Island - which is now part of Kowloon due to reclamation.

The sensor-based inspection system is also used to monitor the health of several skyscrapers in Hong Kong and on the mainland.

'It is my team's effort. It is impossible to do it on my own,' Xu said. 'We started with Tsing Ma, and as our department grew, we undertook some projects in China, like the Canton Tower and the yet-to-be-finished Shanghai Tower.'

Canton Tower, standing 612 metres tall, was briefly the world's tallest skyscraper before being eclipsed by the 634-metre-tall Tokyo Skytree last year.

The Shanghai Tower, meanwhile, is slated to be the tallest building in China and the second-tallest in the world when construction is completed in 2014.

Much like bridges, skyscrapers can sway like trees under the force of horizontal winds. Sturdy design can prevent the building's occupants from feeling these swings.

'Before, people just built the tall buildings and bridges [then] used them without taking care of them,' Xu said. 'Without knowing an old bridge's health, how do you know if it is still safe to use? How do you know to what extent, for instance, damage is done?'

For the Tsing Ma Bridge, which connects Tsing Yi and Ma Wan islands, the sensors also measure any movement of the bridge decks and towers, along with the deflection and rotation of the kilometres-long suspension cables, which carry the weight of the deck on which cars and trains pass.

The sensors serve as an early-warning system, alerting experts about any threat to the bridge's structural integrity. The sensors provide essential data that makes it easier for the Highways Department to monitor safety from a government building in Tsing Yi, and alert it if repairs are needed.

The deck weighs around 49,000 tonnes and its two main cables each carry a load of 53,000 tonnes. 'We monitor many, many things. It's like human beings who have fever. If you overstress it, then the material will break,' Xu said.

'Some deformations [such as dents, bends or cracks] cannot be too large. If they are too large, vehicles and trains cannot go across the bridge.'

While the bridge can swing when the wind is strong, 'we can't feel it because when the wind is that strong, the bridge will be closed', Xu said. '[Authorities] cannot let people take the risk.'

His team is now working to develop software that can simulate the impact of extreme events on the Stonecutters Bridge.

The programme will virtually stress-test the bridge against hurricane-force winds, ground motion akin to earthquakes, a ship colliding with bridge pylons, and the combined force of heavy traffic and fluctuating winds.

The results will be released next year and will help improve maintenance plans.

The next step will be merging the structural-health monitoring and vibration-control systems to create one that, Xu dreams, can operate and conduct repairs on its own.