Washington, November 6
There may be a smartphone app to see 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 safety data while crossing bridges. In doing so, it could become a less expensive alternative to groups of sensors connected to the same bridges.
“The key finding is that information on the structural health of the bridges can be extracted from accelerometer data collected by the smartphone,” says Carlo Ratti, co-author of the study.
The search was conducted, in part, on the Golden Gate Bridge itself. The study, which involved researchers from the Massachusetts Institute of Technology (MIT) in the US, showed that mobile devices can capture the same kind of information about bridge vibrations that stationary sensors collect.
The researchers also estimate that, depending on the age of the road bridge, mobile monitoring could add an additional 15 percent to 30 percent of years to the life of the structure.
“These findings suggest that large and inexpensive data sets collected by smartphones can play an important role in monitoring the health of existing transportation infrastructure,” the authors write in their new research paper, published in Nature Communications Engineering.
Bridges vibrate naturally, and to study the basic “typical frequencies” of those vibrations in many directions, engineers typically place sensors, such as accelerometers, on the bridges themselves.
Changes in modular frequencies over time may indicate changes in the structural integrity of the bridge.
To conduct the study, the researchers developed an Android mobile app to collect accelerometer data when devices are placed in vehicles passing over the bridge.
They can then see how well this data matches the data log by sensors on the bridges themselves, to see if the mobile method is working.
“In our work, we designed a methodology to extract typical vibration frequencies from noisy data collected from smartphones,” said lead researcher Paolo Santi.
“As data is recorded from multiple trips across the bridge, noise from the engine, suspension, and traffic (and) asphalt vibrations tend to cancel out, while the underlying dominant frequencies appear.”
In the case of the Golden Gate Bridge, the researchers drove the bridge 102 times with their devices turned on, and the team used 72 rides for Uber drivers with also enabled phones, the study said.
The team then compared the resulting data with an array of 240 sensors that were placed on the Golden Gate Bridge over a period of three months.
The result, according to the study, was that the data from the phones converged with that from the bridge sensors; For ten specific types of low-frequency vibrations measured by the engineers on the bridge, there was a close match, and in five cases, there was no discrepancy between the methods at all.
“We were able to show that many of these frequencies correspond very precisely to typical bridge frequencies,” Santi says.
However, only one percent of all bridges in the United States are suspension bridges. About 41 percent are much smaller concrete bridges. Therefore, the researchers also examined how well their method works in this setting.
To do this, they studied a bridge in Ciampino, Italy, and compared 280 car trips across the bridge to six sensors that had been placed on the bridge over a period of seven months.
Here, too, the results encouraged the researchers, although they found a 2.3% divergence between routes of given frequencies on all 280 flights, and 5.5% divergence for a smaller sample. This suggests that a larger volume of trips can yield more useful data.
“Our preliminary results indicate that only a modest number of trips over a few weeks is sufficient to obtain useful information about the typical frequencies of the bridge,” says Santi.
Looking at the method as a whole, MIT professor Markus Buehler notes that “vibration signatures are emerging as a powerful tool for assessing properties of large and complex systems, ranging from the viral properties of pathogens to the structural integrity of bridges as demonstrated in this study,” said Buehler.
“It’s a universal signal that is so widespread in the natural and built environment that we are just beginning to explore it as a diagnostic and generative tool in engineering,” Boehler said.
As Ratti acknowledges, there are ways to improve and extend the search, including calculating the effects of a smartphone mounting in a car, the effect of car type on data, and more.
“We still have work to do, but we believe our approach can easily be scaled up – all the way to the level of an entire country,” Ratti said.
“It may not be as accurate as one can get with bridge-mounted sensors, but it could become an interesting early warning system. Small anomalies could then indicate when further analyses are needed,” Ratti said.
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