Smartphone App To Assess Structural Integrity Of Bridges: Study

There could be a smartphone app to tell if San Francisco’s Golden Gate Bridge, or any other bridge for that matter, is holding up well, according to a study. The new study shows that cell phones placed in vehicles, equipped with special software, can collect useful structural integrity data while crossing bridges. By doing so, they could become a less expensive alternative to arrays of sensors attached to bridges themselves.

“The core finding is that information about the structural health of bridges can be extracted from smartphone-collected accelerometer data,” says Carlo Ratti, co-author of the study. The research was conducted, in part, on the Golden Gate Bridge itself. The study, involving researchers from the Massachusetts Institute of Technology (MIT), USA, showed that mobile devices can capture the same type of information about bridge vibrations that static sensors compile.

The researchers also estimate that depending on the age of a road bridge, mobile phone monitoring could add 15 percent to 30 percent more years to the structure’s lifespan. “These results suggest that massive and inexpensive data sets collected by smartphones could play an important role in monitoring the health of existing transportation infrastructure,” the authors write in their new paper, published in Nature Communications Engineering.

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Bridges naturally vibrate, and to study the essential “modal frequencies” of those vibrations in many directions, engineers typically place sensors, such as accelerometers, on bridges themselves. Changes in the modal frequencies over time can indicate changes in the structural integrity of a bridge.

To conduct the study, the researchers developed an Android-based mobile phone application to collect accelerometer data when the devices were placed in vehicles passing over the bridge. They could then see how well that data matched a data log from sensors on bridges themselves, to see if the cell phone method worked.

“In our work, we designed a methodology to extract modal vibration frequencies from noise data collected by smartphones,” said lead researcher Paolo Santi. “As data from multiple trips over a bridge are recorded, noise generated by engine, suspension and traffic vibrations, (and) asphalt, tend to cancel out, while the underlying dominant frequencies emerge.”

In the case of the Golden Gate Bridge, the researchers drove over the bridge 102 times with their devices running, and the team used 72 trips by Uber drivers also with activated phones, the study said. The team then compared the resulting data with that of a group of 240 sensors that were placed on the Golden Gate Bridge for three months.

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The result, according to the study, was that the data of the telephones converged with that of the sensors of the bridge; for 10 special types of low-frequency vibrations engineers measure on the bridge, there was a close match, and in five cases, there was no difference between the methods at all. “We were able to show that many of these frequencies correspond very precisely to the prominent modal frequencies of the bridge,” says Santi.

However, only one percent of all bridges in the United States are suspension bridges. About 41 percent are much smaller concrete span bridges. So, the researchers also examined how well their method would perform in that environment. To do this, they studied a bridge in Ciampino, Italy, comparing 280 vehicle trips over the bridge with six sensors that were placed on the bridge for seven months.

Here, too, the researchers were encouraged by the findings, although they found up to a 2.3 percent divergence between methods for certain modal frequencies across all 280 trips, and a 5.5 percent divergence over a smaller sample. This suggests that a larger number of trips could provide more useful data. “Our initial results suggest that only a modest amount of trips over the course of a few weeks is sufficient to obtain useful information about bridge modal frequencies,” says Santi.

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Looking at the method as a whole, Professor at MIT, Markus Buehler observes, “Vibrational signatures are emerging as a powerful tool to assess properties of large and complex systems, ranging from viral properties of pathogens to structural integrity of bridges as shown in this study,” said Buehler.

“It’s a universal signal found widely in the natural and built environment that we’re just beginning to explore as a diagnostic and reproductive tool in engineering,” Buehler said. As Ratti acknowledges, there are ways to refine and expand the research, including accounting for the effects of the smartphone mount in the vehicle, the influence of the vehicle type on the data, and more.

“We still have work to do, but we believe that our approach could be scaled up easily – up to the level of an entire country,” Ratti said. “It may not reach the accuracy that can be obtained using fixed sensors installed on a bridge, but it could become a very interesting early warning system. Small anomalies could then suggest when to do further analyses,” said Ratti.

(With PTI inputs)

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