Smartphones could be used to monitor the safety of bridges much more quickly and cheaply than currently possible, providing engineers with data they can use to fix the structures before they become dangerously unstable.

Usually, bridges’ state of repair is monitored in one of two ways: either engineers visually inspect them for cracks and faults, or sensors collect data about their vibrations and movements. But a new method developed by researchers at West Point Military Academy and other universities avoids the need for either by collecting accelerometer data from smartphones in cars as they drive over the bridges. 

In tests that involved driving across the Golden Gate Bridge in San Francisco and a reinforced concrete bridge in Italy, they found that just two smartphones could provide data of similar accuracy to 240 stationary sensors. The phones pick up on naturally occurring vibrations from the bridges, allowing researchers to monitor their structural changes over time. Their research is described in a study published in Communications Engineering today.

The researchers estimate that monitoring this sort of smartphone data throughout a bridge’s life could extend the longevity of the structure by 30%, simply by helping maintenance crews to make more timely repairs. 

Making sure bridges are well maintained is vital, as demonstrated just a few days ago when a bridge collapsed in Gujarat, India, killing 135 people. It’s a problem in other countries too. Although bridges in the US are required by law to be visually inspected every two years, that doesn’t eliminate catastrophes like the collapse of the I-35W bridge in Minneapolis in 2007, which killed 13 people and injured 145. 

Maintaining bridges is expensive. There are 600,000 of them in the US, and the organizations that own and maintain them can easily pay $50,000 for sensor equipment alone, with further costs incurred by the need to maintain them and analyze the data they generate. Smartphones are a much cheaper option.

However, work still needs to be done to make this technique a reality, says Ahmet Emin Aktan, a professor of civil, architectural, and environmental engineering at Drexel University, who was not involved in the study. He believes it’ll be a long time before the technique becomes widely adopted. 

Aktan expects visual inspections to remain the primary method of monitoring bridges for the next 10 to 20 years, because both sensors and smartphones can produce data that’s harder to interpret than what engineers see with their own eyes. Even something as ordinary as weather or variations in traffic load can affect the way structures behave and move, which can then affect the data. For example, they become stiffer in colder weather.

But eventually, he says, it’s likely that the industry will want to use a combination of that visual observation with the data collected from smartphones.