Deciding when it’s safe for a building’s residents to move back in after an earthquake is a major challenge for civil engineers. The new technique, which works by measuring a building’s ambient vibrations, promises to improve the accuracy of these assessments.
“We took systems that are already used to measure the condition of bridges, and applied them to the assessment of buildings damaged by an earthquake,” said Pierino Lestuzzi, one of the researchers on the project.
Critically, he added, the method doesn’t require knowledge of the pre-earthquake state of the building to perform t the assessment.
Engineers currently perform this diagnosis using a visual assessment carried out according to an appraisal form developed by Italian researchers. This approach proved highly effective after two major earthquakes in central Italy in 2009 and 2016. But even though a visual assessment is essential for spotting the damage caused to a building, it takes a long time to complete – some 2–3 hours per building – and is fairly complicated and subjective. And it does not eliminate the uncertainty about whether a building could withstand aftershocks.
With the new method, engineers record a building’s ambient vibrations (such as those created by wind or by human activity, like road traffic) with a portable seismograph; this involves placing three or four sensors at different points in the building and measuring the vibrations for half an hour.
The recordings are then processed to separate out signals resulting from changes in the building’s structure from signals due to weather conditions, ambient noise or the building’s age. The results are entered into a computer model in order to predict the building’s ability to withstand another earthquake. The group estimates that their model’s predictions are 50–100 per cent accurate. Engineers can then combine these quantitative data with the results of their visual assessment.
The group performed its research by studying the demolition of the Villa Marguerite building on the EPFL campus and analysing the results of experiments carried out by EPFL’s Earthquake Engineering and Structural Dynamics Laboratory (EESD) on a pilot building constructed on a shake table.
A detailed study on the work is published in the journal Soil Dynamics and Earthquake Engineering.
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