Rocco Ditommaso

Interferometric Analysis of Strong Ground Motion for Structural Health Monitoring: The Example of the L’Aquila, Italy, Seismic Sequence of 2009

Abstract Structural health monitoring (SHM) aims to improve knowledge of the safety and maintainability of civil structures. The usage of recording systems exploiting wireless communication technology is particularly suitable for SHM, especially for rapid response following earthquakes. In this study, both of these issues are combined, and we report on the application of seismic interferometry to SHM using a dataset of seven earthquakes collected using a novel wireless system of accelerometers during the L’Aquila, Italy, seismic sequence in 2009. We show that interferometric analysis allows the estimation of the shear-wave velocity of seismic phases propagating throughout a structure, and, most important for SHM purposes, allows the monitoring of the velocity variations during the aftershock sequence. Moreover, innovatively we apply the S transform to the building response functions retrieved by interferometry to estimate the fundamental resonance frequency and the quality factor Q .

Analysis of non-stationary structural systems by using a band-variable filter

One of the main tools used to study the dynamic response of structural systems is certainly the Fourier Transform. This tool is very useful and reliable to investigating the response of a stationary system, i.e. a generic system that does not changes its characteristics over time. Conversely, the Fourier Transform is no longer reliable if the main goal is to study the evolution of the dynamic response of a system whose features rapidly vary with time. To this regard, several mathematical tools were developed to analyze time-variable dynamic responses. Soil and buildings, subject to transient forcing such as an earthquake, may change their characteristics over time with the initiation of nonlinear phenomena. This paper proposes a new methodology to approach the study of non-stationary response of soil and buildings: a band-variable filter based on S-Transform. In fact, with the possibility of changing the bandwidth of each filtering window over time, it becomes possible to extract from a generic record only the response of the system focusing on the variation of individual modes of vibration. Practically, it is possible to extract from a generic non-stationary signal only the phase of interest. The paper starts from examples and applications on synthetic signals, then examines possible applications to the study of the non-stationary response of soil and buildings. The last application focuses on the possibility to evaluate the mode shapes over time for both numerical and scaled model subjected to strong motion inputs.

Monitoring the structural dynamic response of a masonry tower: comparing classical and time-frequency analyses

The monitoring of the evolution of structural dynamic response under transient loads must be carried out to understand the physical behaviour of building subjected to earthquake ground motion, as well as to calibrate numerical models simulating their dynamic behaviour. Fourier analysis is one of the most used tools for estimating the dynamic characteristics of a system. However, the intrinsic assumption of stationarity of the signal imposes severe limitations upon its application to transient earthquake signals or when the dynamic characteristics of systems change over time (e.g., when the frequency of vibration of a structure decreases due to damage). Some of these limitations could be overcome by using the Short Time Fourier Transform (STFT). However, the width of the moving window adopted for the analysis has to be fixed as a function of the minimum frequency of interest, using the best compromise between resolution in both the time and frequency domains. Several other techniques for time-frequency analysis of seismic signals recorded in buildings have been recently proposed. These techniques are more suitable than the STFT for the application described above, although they also present drawbacks that should be taken into account while interpreting the results. In this study, we characterize the dynamic behaviour of the Falkenhof Tower (Potsdam, Germany) while forced by ambient noise and vibrations produced by an explosion. We compare the results obtained by standard frequency domain analysis with those derived by different time-frequency methods. In particular, the results obtained by the standard Transfer Function method, Horizontal to Vertical Spectral Ratio (HVSR), Short Time Fourier Transform (STFT), Empirical Mode Decomposition (EMD) and S-Transform are discussed while most of the techniques provide similar results, the EMD analyses suffer some problems derived from the mode mixing in most of the Intrinsic Mode Functions (IMFs).

Dynamic Survey of the Musmeci Bridge by Joint Application of Ground-Based Microwave Radar Interferometry and Ambient Noise Standard Spectral Ratio Techniques

This letter aims at analyzing the potentialities of the ground-based microwave radar interferometry technique for the dynamic characterization of civil infrastructures. This technique has been applied to estimate the fundamental dynamic parameters of the reinforced concrete Musmeci bridge in Basilicata region (southern Italy). The results have been validated by the comparison with the ones obtained applying consolidated techniques using data from accelerometers and tromometers. The good agreement obtained could suggest the joint application of such techniques as a new technological approach to set up a non-invasive and non-destructive evaluation procedure for structural health monitoring of infrastructures.

Damage detection on framed structures: modal curvature evaluation using Stockwell Transform under seismic excitation

The key parameters for damage detection and localization are eigenfrequencies, related equivalent viscous damping factors and mode shapes. The classical approach is based on the evaluation of these structural parameters before and after a seismic event, but by using a modern approach based on time-frequency transformations it is possible to quantify these parameters throughout the ground shaking phase. In particular with the use of the S-Transform, it is possible to follow the temporal evolution of the structural dynamics parameters before, during and after an earthquake. In this paper, a methodology for damage localization on framed structures subjected to strong motion earthquakes is proposed based on monitoring the modal curvature variation in the natural frequency of a structure. Two examples of application are described to illustrate the technique: Computer simulation of the nonlinear response of a model, and several laboratory (shaking table) tests performed at the University of Basilicata (Italy). Damage detected using the proposed approach and damage revealed via visual inspections in the tests are compared.

Electromagnetic Sensing Techniques for Non-Destructive Diagnosis of Civil Engineering Structures

Health Assessment Methods (HAM) and Structural Health Monitoring (SHM) aim to improve the standard of knowledge regarding the safety and maintenance of structures and infrastructure acquiring information about geometrical, mechanical and dynamical characteristics of structures. In earthquake-prone areas, this activity has the double aim of assessing the buildings structural integrity and extracting information regarding their response during a seismic event in order to define appropriate activities for risk mitigation. A number of factors afflict buildings and infrastructure safety in seismic areas:  Outdated codes of practice: a significant number of highly urbanized areas are present globally, where a high percentage of structures have been designed and erected considering only gravity loading.  The age of the structures and the real in-situ performance of construction material significantly affect their overall behaviour.  Structural deficiencies such as poor material qualities and/or degradation of structural materials (rust, spalling etc.), inadequate construction detailing, low levels of ductility, brittle collapse mechanisms. The seismic assessment of structures is performed in terms of the estimation of the earthquake intensity that would lead to a certain damage condition and/or collapse. The assessment of the seismic vulnerability of existing buildings is generally based on the knowledge of building characteristics and through a complex analysis of the possible collapse mechanisms in order to identify the most probable failure for the given structure (as example: Ansari, 2005; Douglas, 2007; Moustafa et al., 2010). The methodological approach for the evaluation of a structure resistance is represented in Figure 1 where structural knowledge obtained through a series of test assessments is needed in order to define vulnerability and thus design suitable retrofit strategies.

Damage Assessment on Framed Structures Through Regression Models Retrieved by Nonlinear Numerical Analyses

Nowadays, Structural Health Monitoring is certainly a topic of great interest both in the research and in the professional field as evidenced by the large number of applications and systems installed all over the world. A permanent monitoring system could be installed in order to identify a possible damage occurred on framed structures after moderate/destructive earthquake. Furthermore, it could be an useful tool for evaluating the service condition and the remaining service life of a structure. Most of simplified methods for structural damage detection are based on the evaluation of the dynamic characteristics evolution associated to the fundamental mode of vibration of a monitored structure. Particularly, the variation of parameters such as eigenfrequencies, equivalent viscous damping factors and modal and/or operational mode shapes has been evaluated and analyzed in order to identify structural damage. Methods based on frequency variation can be applied to detect damage, but they are not able to localize structural damage. Instead, to this aims, methods based on the evolution of modal shapes and/or of their derivatives such as mode curvatures could be more effective. Aim of this work is the improvement of an existing method for damage localization on framed structures based on the evaluation of the mode curvature change associated to the fundamental mode of vibration during an earthquake. The approach is based on the use of a nonlinear filter, the band-variable filter, able to extract the nonlinear response of each mode of vibration. The paper focuses also on the possibility to quantify the damage occurred on the monitored structure by considering the correlation among maximum value of mode curvature variation and the maximum inter-story drift. The regression model between these two parameters has been defined through nonlinear numerical models for different reinforced concrete framed structures designed for gravity loads only, with and without the presence of infill panels.

Experimental damage localization in a full-scale 7 story benchmark building under seismic excitation

In this paper two methods of damage localization previously proposed by the authors are combined to smooth the possible drawbacks and boost the advantages each of them. The Modal Interpolation Method (IM), recently proposed, is based on a damage feature defined in terms of the loss of smoothness (that is local increases of curvature) of the modal shapes induced by a local reduction of stiffness. Herein the combination of the IM with the Curvature Evolution Methods (CEM) is proposed. The CEM is based on the use of a Band-Variable Filter able to extract from recorded responses the nonlinear response of one mode of vibration enabling the detection of possible changes of a properly defined damage feature, during a single earthquake. In the CEM the modal curvature is assumed as damage feature. The combination of the two methods CEM and IM is carried out using the Band-Variable Filter to extract the nonlinear response of the structure and assuming as a damage feature the variation of the interpolation error computed at different times during the strong motion. The validation of the combined approach, named Interpolation Evolution Method (IEM), is carried out on a full scale experimental benchmark tested on the UCSD-NEES shake table.

ANALYSES OF THE SEISMIC ACTIONS RECORDED DURING THE 2016 CENTRAL ITALY SEISMIC SEQUENCE: OBSERVED VS CODE PROVISION VALUES

On August 24, 2016 at 03.36 a.m. (Italian time), a Mw 6.0 earthquake struck an extensive portion of the Central Italy. The epicentre was located among the municipalities of Accumoli (province of Rieti), Amatrice (province of Rieti) and Arquata del Tronto (province of Ascoli Piceno). After this first seismic event, from the academic point of view, several technical and scientific activities started with the aim to carry out preliminary evaluations on the characteristics of the seismic sequence, in order to interpret the possible effects of soil amplification in the areas affected by the earthquake and the damages observed on structures. The activities are continuing after the Mw 5.9 earthquake on October 26, 2016 and the Mw 6.5 earthquake on October 30, 2016. The strong motion recorded signals have been analysed in terms of peaks (PGA, PGV and PGD) and integral seismic parameters starting from data recorded by the closest stations to the epicentres. Several comparisons between the elastic response spectra retrieved from accelerometric recordings and the elastic demand spectra provided by the Italian seismic code (NTC 2008) have also been performed. 3923 Available online at www.eccomasproceedia.org Eccomas Proceedia COMPDYN (2017) 3923-3936