Contents I. Introduction
Determining a structural response to the occurrence of natural phenomena (earthquake, landslide, etc.) is a primary goal for risk assessment activities (e.g., [1]–[4]). A suitable monitoring allows the identification of the dynamic characteristics and the estimation of their possible changes over time as a result of structural degradation [5]–[7]. This is an important tool to evaluate the integrity of large infrastructure exposed to intense operational demands for long periods [8]. The best method to determine the dynamic parameters is of course to record earthquakes inside permanently monitored building, but this is a costly option, used in a limited number of case studies [9]. It is not feasible to extend this technique to a very large number of buildings, thus preventing the knowledge of the real behavior of the kind of houses where most of the population lives. A possible alternative to permanent monitoring is to perform forced vibration studies either with vibrodines or the snap-back approach (see [10] and references therein), but again, these detailed approaches can be used only for a limited number of selected buildings. Boutin and Hans [11] and Gallipoli et al. [12] demonstrated that impacts, forced vibration, and ambient noise yield the same values for main period estimations; moreover Ditommaso et al. [13], [14] show that, when a structure exhibits a linear behavior, it is possible to retrieve the same results by mean different techniques, also using impulsive actions such as explosions.
