2.1.2. Damping

Although damping can also be used for SHM system to monitor the health condition of structures [14,77–81], it is less frequently observed in practice than frequencies and mode shapes since it is more difficult to measure. Frizzarin et al. [77] analyzed the damping by using ambient vibration data to detect damage without baseline, and demonstrated the proposed method by a large-scale concrete bridge model with seismic damage. Mustafa et al. [78] introduced an energy based damping evaluate approach to evaluate the health condition of a truss bridge by numerical simulations. Cao et al. [79] compared damping based damage detection methods by using reinforced concrete structures and fiber reinforced composites, and clarified the factors that influenced the capability of damping on damage detection. Recently, Liu et al. [14] proposed a novel complex eigen-parameter identification method to evaluate the stiffness reduction and damping defect simultaneously on a non-classically damped shear building.

Ideally, the damping change due to local damage can be observed because the cracks may increase the frictions between interfaces. However, the measurement is vulnerable to noise, especially for structures subject to ambient environmental vibrations, so that the change of damping due to local damage is concealed by the measurement error. On the other hand, the damping model is difficult to select or construct whereas which is important in identification of damping. Classical Rayleigh damping which is a combination of mass and stiffness is frequently adopted in practice because it is the simplest damping model. However, it cannot be applied to many structures; therefore, some more advanced damping models have been proposed. It should be noted that for different types of structure, different damping models should be considered. Moreover, damping is a global property for a structure, similar to frequency, so damping itself can hardly be used to identify the location of local damages.
