Engineering: Adding sensors and other devices to bridges, tunnels and buildings can turn them into “smart structures” capable of sensing and, in some cases, even responding to problems

The Economist - ON AUGUST 1st 2007, as commuters were driving home from work on the I-35W Mississippi River Bridge near Minneapolis, it abruptly collapsed. Thirteen people died and over a hundred were injured. The bridge had opened in 1967 and had not been scheduled for replacement until 2020. What had gone wrong? In 2008 the National Transport Safety Board concluded that extra concrete, which had been added to the bridge over the years as the level of traffic increased, had helped cause the collapse. After an inspection in 2005 engineers had classified the bridge as “structurally deficient”, and repairs were planned. But many other bridges in the area were thought to be in an even worse condition, so the work was not prioritised, and the true state of the bridge became apparent only when it failed, with tragic results.

ON AUGUST 1st 2007, as commuters were driving home from work on the I-35W Mississippi River Bridge near Minneapolis, it abruptly collapsed. Thirteen people died and over a hundred were injured. The bridge had opened in 1967 and had not been scheduled for replacement until 2020. What had gone wrong? In 2008 the National Transport Safety Board concluded that extra concrete, which had been added to the bridge over the years as the level of traffic increased, had helped cause the collapse. After an inspection in 2005 engineers had classified the bridge as “structurally deficient”, and repairs were planned. But many other bridges in the area were thought to be in an even worse condition, so the work was not prioritised, and the true state of the bridge became apparent only when it failed, with tragic results.

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As well as doing data processing, smart sensors can also manage their own behaviour more intelligently to save energy, only switching on their radios occasionally to transmit a batch of data, for example. The extra energy required to do the data processing is outweighed by the energy saved by switching off the radio. “Constantly transmitting data across a network uses up a lot of power,” says Jennifer Rice, a civil engineer who specialises in sensors at Texas Tech University. While working at the University of Illinois, Dr Rice and a team of colleagues devised a smart-sensor system to monitor the Jindo Bridge in South Korea that has 113 nodes, each with six sensors, and uses clever programming to control the resulting flood of data while also minimising energy consumption.

The sensors were installed in the summer of 2009 to monitor tension in the bridge’s suspension cables, wind loading and deck vibrations. Each of the nodes is programmed to stay in a low-power “sleep” state most of the time, but to wake up for a split second, every ten seconds, to look out for any signals from the gateway computer. “We have nodes that take turns staying awake for longer periods, called sentry nodes, and if their sensors detect vibrations or winds that are above a certain threshold, they send a signal to the gateway that tells it to send a wake-up call to all the sleeping nodes,” explains Dr Rice. If a sleeping node detects a signal from the gateway during its split-second awake, it wakes up fully, activates its sensors, analyses the incoming data and then transmits information that meets specific predefined criteria.

All this is expected to enable the sensors and nodes to operate for years without needing battery replacements, says Dr Rice. In addition, many of the nodes are powered by small wind turbines and solar cells, which may be able to power them indefinitely. The team is monitoring the bridge closely to see just how effective these systems are. The sensor network on the Jindo Bridge is the largest of its kind for a civil-infrastructure project, and represents the state of the art: the central computer can even send text messages to engineers about the bridge’s status.

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