## Vibration Based Damage Detection of Bridges

by Nikolay Donets – April 4, 2012 · 3 min read

Vibration Based Damage Detection **VBDD** of bridges is a field of Structural Health Monitoring **SHM**, which aims to detect damage in bridges using dynamic response measurements.

A bridge is a key component of a transportation system, which provides the connection between two locations. A bridge damage will affect the traffic operation and result in the loss of the economy and human life. Maintenance of bridges is necessary to guarantee the safe operation. And **VBDD** of bridges is an important component of the bridge health monitoring system, which aims to detect damage in bridges using dynamic response measurements.

The basic principle of the vibration based damage detection of bridges is that the damage in bridges will cause the change of the dynamic properties, such as the modal frequency, modal damping, modal shape and modal strain energy. The modal frequency and modal damping are the dynamic properties of bridges in the time domain, and the modal shape is the dynamic property of bridges in the spatial domain. Therefore, the damage in bridges can be detected using the change of the them.

The modal frequency is the most common dynamic property of bridges.

*A structure will vibrate at a certain natural frequency when it is subjected to an excitation at that frequency. Natural frequency is the frequency at which a structure will vibrate when it is not subjected to damping.*

The modal frequency method is simple and convenient, and can be easily applied to the damage detection of the in-service bridges. However, the modal frequency is not sensitive to the local damage of bridges, and the damage detection using the modal frequency is only suitable for the global damage of bridges.

The modal damping is another important dynamic property of bridges.

*Damping is the dissipation of energy that occurs when a structure is subjected to vibration. The amount of damping in a structure can be represented by the damping ratio.*

The modal damping is sensitive to the damage of bridges, and the damage detection using the modal damping is more sensitive than the modal frequency method. However, the modal damping is not easy to be obtained, and the damage detection using the modal damping is not widely used.

The modal shape is another dynamic property of bridges in the spatial domain.

*A mode shape is a plot of the amplitude of vibration at each measurement point at a particular natural frequency. A mode shape is a plot of the vibration pattern of a structure at a particular natural frequency.*

The modal shape is sensitive to the local damage of bridges, and the damage detection using the modal shape is suitable for the local damage of bridges.

As of now, the main approach for **VBDD** of bridges is Operational Modal Analysis as it does not require a set up of special experiment and excitation of a structure in comparison to Experimental Modal Analysis.