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Optical Detection of Modal Frequencies of Structures

Description: 

The experimental assessment of the modal frequencies of engineering structures is a key-step in several seismological applications, such as providing the entry point in seismic response spectra, the calibration and validation of dynamic models, the evaluation of the effect of interventions/retrofitting in the dynamic behavior of existing structures, and more.

Accelerometers are still the preferred motion transducers in this type of application by the engineering community, while the seismological community prefers seismometric transducers (geophones), due to their larger sensitivity at low frequencies and intrinsic lower noise, which makes them more suitable for weak motion analysis and soil applications.

In recent times sensors measuring displacements (based on electromagnetic wave source-to-target travel times or on interferometric approaches) have also started to be used. These have the advantage of being contactless and capable of exploring a larger portion of the structure from a single acquisition point. This is however achieved at the expense of much larger amounts of data to process. Assessing the direction of motion with these methods is more difficult than with the standard acceleration/velocity transducers. These methods also require stable air conditions to not impair the displacement/distance measurements.

Differently from the measurement principles listed above, here we focus on optical (based on visible radiation) approaches, applicable with standard photo cameras, and explore their potential in the assessment of modal frequencies of structures. Optical approaches have been used to study mechanical vibrations, particularly in laboratory experiments, where several variables can be kept under control. Here we apply them to detect the modal frequencies of standard structures under operative conditions. We will explore different workflows, discuss their performance and possible improvements and show that dynamic characterizations of large vibrating structures can also be achieved by means of standard videocameras.

Session: Geophysics in a Changing World: Monitoring Applications from Seismology and Beyond - I

Type: Oral

Date: 4/15/2025

Presentation Time: 05:30 PM (local time)

Presenting Author: Silvia

Student Presenter: No

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