Traditionally, slowly growing damage has been prevented by e.g. periodic visual inspection and maintenance of the structures. In case of sudden events like earthquakes, additional visual inspections have been carried out.

For years it has been recognized that using response vibration measurements to predict when inspections are required can dramatically reduce the required inspections and thus make better use of the inspection engineers as well as reducing the overall maintenance costs. The use of response vibration measurements is known as vibration-based Structural Health Monitoring.

Vibration-based Structural Health Monitoring normally makes use of permanently installed sensors to monitor the behaviour of the structure over time. In many cases the sensors are accelerometers but also geophones, strain gauges or Fiber Bragg Grating (FBG) are used. Most importantly is that the same type of sensors is used in the same locations and directions all the time.

In earthquake prone areas the use of e.g. Forced Balanced Accelerometers (FBA) with a large dynamic range is often used along with a 24-bit data acquisition system. This allow measuring the weak (ambient) motion as well as the strong motion during shaking. The ability to obtain low-noise measurements event in case of weak motion allow the use damage detection methods in between the earthquake events.

In areas where not prone to earthquakes or other strong motion event, accelerometers of the MEMS type can also be used. They typically have less dynamic range and less signal-to-noise ratio which can be compensated by longer measurement durations.

During strong motion events the type of damage detection analyses usually performed is Interstorey Drift Analysis of structural element such as columns. In addition, several values are calculated from the event measurements to characterize the ground motion, such as:

• Peak Ground responses of acceleration, velocity and displacement, PGA, PGV, PGD
• Spectral values (SA, SV, SD)
• Instrumental intensity using the Mercalli intensity scale
• Arias Intensity, Normalized Arias Intensity, Significant Duration, Mean Period

Normally, operational modal analysis and damage detection is not performed on the actual event records due to the short records and the fact that the structure is subject to strong shaking that will trigger non-linear and hysteretic behaviour.

Operational modal analysis and damage detection is on the other hand preferred tools for analysing the recorded measurements obtained before and after an extreme event in the ambient regime. These forms of weak motion analysis are therefore complementary to the strong motion analysis, and the use of weak motion analysis there maximizes the investment of the monitoring system instrumentation installed.