How do you measure and monitor vibration in structures and soils?

1 Vibration sources and effects

Vibration can be divided into two categories: free and forced. Free vibration occurs when a system is disturbed from its equilibrium position and then left to oscillate without any external forces. Forced vibration occurs when a system is subjected to a periodic or random external force, such as wind, traffic, or machinery. The amplitude and frequency of vibration depend on the characteristics of the system, such as mass, stiffness, damping, and geometry, as well as the characteristics of the external force, such as magnitude, direction, and duration. Vibration can have several consequences on structures and soils, including resonance when the frequency of the external force matches the natural frequency of the system; fatigue when repeated cycles of stress and strain cause weakening and cracking of materials; settlement when vibration causes soil particles to rearrange and consolidate; and liquefaction when vibration causes water-saturated soil to lose its strength. All these effects can lead to possible damage or failure of foundations and structures.

2 Vibration measurement and monitoring

Vibration measurement and monitoring involve the use of sensors, data acquisition systems, and analysis software to record and process the vibration signals from structures and soils. Sensors convert physical motion or deformation of the system into electrical signals, such as voltage or current. Data acquisition systems collect, store, and transmit the signals from the sensors to a computer or a cloud server. Analysis software processes the signals and extracts useful information, such as amplitude, frequency, phase, mode shape, damping ratio, and response spectrum. Common sensors used for vibration measurement and monitoring include accelerometers for measuring acceleration (proportional to the force acting on it), displacement transducers for measuring displacement or distance from a reference point, strain gauges for measuring strain or change in length (proportional to the stress applied to it), and piezometers for measuring pore water pressure in soils (related to the degree of saturation and consolidation).

3 Vibration measurement methods

Vibration measurement methods can be divided into two categories: direct and indirect. Direct methods measure the vibration of the system itself, while indirect methods measure the vibration of the surrounding medium, such as air or water. Direct methods provide more accurate and detailed information, but require more installation and maintenance costs. Indirect methods provide less invasive and more economical solutions, but require more calibration and interpretation efforts. Common vibration measurement methods include modal testing, which uses a known force to identify the natural frequencies, mode shapes, and damping ratios of the system; ambient vibration testing, which uses natural sources of vibration to excite the system and measure its response; ground vibration testing, which uses an artificial source of vibration to generate waves in the ground; and seismic testing, which uses seismic waves from natural or artificial sources to investigate the dynamic properties and behavior of soils and structures.

4 Vibration monitoring applications

Vibration monitoring applications can be divided into three categories: diagnostic, prognostic, and preventive. Diagnostic applications detect and locate existing defects or damages in structures and soils, using vibration signatures or patterns. Prognostic applications predict and assess the future performance or deterioration of structures and soils, using vibration trends or models. Meanwhile, preventive applications control or mitigate the adverse effects of vibration on structures and soils, using vibration feedback or isolation techniques. Some of the common vibration monitoring applications include structural health monitoring, soil-structure interaction, vibration serviceability, and vibration mitigation. Structural health monitoring is a diagnostic and prognostic application which evaluates the condition and functionality of structures like bridges, buildings, dams, and pipelines. Soil-structure interaction is a diagnostic and prognostic application that examines the dynamic interaction between soils and structures such as foundations, retaining walls, and tunnels. Vibration serviceability is a preventive application that ensures the comfort of occupants in structures like offices, hospitals, and schools. Lastly, vibration mitigation is a preventive application that designs solutions to reduce or eliminate the undesirable effects of vibration on structures and soils such as dampers, isolators, and barriers.

5 Vibration measurement and monitoring challenges

Vibration measurement and monitoring in civil engineering is a complex and uncertain phenomenon, with high installation and maintenance costs. This requires multidisciplinary knowledge, robust and reliable data acquisition techniques, as well as cost-effective and sustainable solutions to effectively measure and monitor vibration.

6 Here’s what else to consider

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