Abstract
Ambient vibration modal identification, also known as Operational Modal Analysis, aims to identify the modal properties of a structure based on vibration data collected when the structure is under its operating conditions, i.e., no initial excitation or known artificial excitation. This procedure for testing and/or monitoring historic buildings, is particularly attractive for civil engineers concerned with the safety of complex historic structures. However, since the external force is not recorded, the identification methods have to be more sophisticated and based on stochastic mechanics. In this context, this contribution will introduce an innovative ambient identification method based on applying the Hilbert Transform, to obtain the analytical representation of the system response in terms of the correlation function. In particular, it is worth stressing that the analytical signal is a complex representation of a time domain signal: the real part is the time domain signal itself, while the imaginary part is its Hilbert transform. A 3DOF numerical example will be presented to show the accuracy of the proposed procedure, and comparisons with data from other methods assess the reliability of the approach. Finally, the identification method will be extended to the real case study of the Chiaramonte Palace, a historic building located in Palermo and known as “Steri”.
Highlights
Most of the literature concerning dynamic identification deals with the estimation of the modal parameters of a structure starting from the measurement of both the dynamic input and structural response signals.In the past, the dynamic identification of the modal characteristics of buildings was generally based on force vibration tests involving impact tests or other complex setups, applying several types of input exciters directly in-situ
A novel identification procedure based on ambient vibration data, denoted as Analytical Signalbased method (ASM) has been developed
When a multi degree of freedom (MDOF) system is considered, the structural responses are characterized by all the structural modes and modal parameters cannot be extracted directly
Summary
Most of the literature concerning dynamic identification deals with the estimation of the modal parameters (frequencies, damping coefficients and mode shapes) of a structure starting from the measurement of both the dynamic input and structural response signals. The basic idea of OMA hypothesises that the excitation source, due to natural or operative loadings, yields an input force which can be modeled as a white Gaussian noise [5, 8] In this case, modes can be estimated from the amplitude of their peaks at the correspondent main frequencies of the system [10]. The main disadvantage of some of these methods seems to be the tendency to yield non-conservative damping estimates with noisy data [3] and to encounter problems in distinguishing structural modes from spurious or noise modes On this base, the present study proposes an identification technique combining a proper mode decomposition algorithm with the application of the Hilbert Transform (HT) [17,18,19,20] to the output response data. The presented case study concerns the Chiaramonte Palace, a rare and precious example of Sicilian fourteenth-century architecture
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