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Modal parameter evaluation is widely used for structural health monitoring, damage detection, and numerical model calibration. This study benchmarks two methods for modal identification under controlled laboratory conditions using operational modal analysis to compare dynamic parameters from ambient vibrations and shaker tests. A 2‐storey 3D spatial steel frame equipped with 3‐axial accelerometers is used as a case study. The identified dynamic parameters are benchmarked against a preliminary finite element model by computing Auto‐MAC and MAC matrices. It can be concluded that the preliminary FEM approach performs better in the prediction of modes shapes than natural frequencies. It was found that analysing the structural response to both ambient and artificially induced excitations provide a more complete understanding of a structure's dynamic characteristics. An additional mode shape was identified from the artificially induced excitation that was not previously captured from the ambient excitation. [ABSTRACT FROM AUTHOR]

This paper presents the experimental analysis on the "Municipal Headquarters" of Palagianello Town. This is a strategic building with a high complexity at planimetric and altimetric level, so it is interesting the evaluation of its structural dynamic performance in case of seismic emergency management. To this aim, environmental vibrations have been acquired in situ and accelerometers have been positioned in well-defined points. The data were analyzed with an easy and accurate process that includes the Operational Modal Analysis (OMA), to identify the natural frequencies, the mode shapes and the damping ratios with non-destructive testings. Subsequently, the experimental results were compared with those of two numerical models (with and without infill walls) defined by means of the finite element method (FEM); this allowed for better calibration of the model and to arrive at more realistic conclusions about the behavior of the structure. The paper discusses the influence of the stiffness contribution of masonry infills on the dynamic properties of the building. Moreover, it shows that the adoption of ad hoc chosen locations of the sensors could influence the accuracy of the experimental results, especially for structures characterized by irregularities. [ABSTRACT FROM AUTHOR]

This paper presents the experimental analysis on the “Municipal Headquarters” of Palagianello Town. This is a strategic building with a high complexity at planimetric and altimetric level, so it is interesting the evaluation of its structural dynamic performance in case of seismic emergency management. To this aim, environmental vibrations have been acquired in situ and accelerometers have been positioned in well-defined points. The data were analyzed with an easy and accurate process that includes the Operational Modal Analysis (OMA), to identify the natural frequencies, the mode shapes and the damping ratios with non-destructive testings. Subsequently, the experimental results were compared with those of two numerical models (with and without infill walls) defined by means of the finite element method (FEM); this allowed for better calibration of the model and to arrive at more realistic conclusions about the behavior of the structure. The paper discusses the influence of the stiffness contribution of masonry infills on the dynamic properties of the building. Moreover, it shows that the adoption of ad hoc chosen locations of the sensors could influence the accuracy of the experimental results, especially for structures characterized by irregularities.

Operational modal analysis (OMA) methods are frequently applied for obtaining information on structural dynamic parameters, such as natural frequencies and mode shapes. While synchronization between acceleration response data sets used in OMA is not a prerequisite for estimating natural frequencies, synchronization discrepancies may lead to non-negligible errors in the estimation of mode shapes. Synchronization discrepancies are predominantly associated with wireless structural health monitoring systems, where each wireless sensor node functions as a separate data acquisition unit (DAQ), and clock synchronization is therefore not intrinsic. However, synchronization discrepancies may also occur in cable-based systems, particularly when more than one DAQ is used. In this paper, a synchronization method for detecting and correcting synchronization-induced errors in OMA is proposed. Unlike existing approaches on removing synchronization-induced errors at the output stage, the method proposed herein yields synchronized structural response data at an intermediate stage of OMA. Specifically, time lags between acceleration response data sets are detected based on estimates of the mode shapes obtained either from preliminary structural analysis or from engineering judgment assuming classical damping. The applicability of the proposed method is verified through simulations of a multi-degree-of-freedom oscillator and validated through ambient vibration field tests on a pedestrian overpass bridge. [ABSTRACT FROM AUTHOR]

This paper presents the experimental application of a new method for seismic vulnerability assessment of buildings recently introduced in literature, the SMAV (Seismic Model Ambient Vibration) methodology with reference to their operational limit state. The importance of this kind of evaluation arises from the civil protection necessity that some buildings, considered strategic for seismic emergency management, should retain their functionality also after a destructive earthquake. They do not suffer such damage as to compromise the operation within a framework of assessment of the overall capacity of the urban system. To this end, for the characterization of their operational vulnerability, a Structural Operational Index (IOPS) has been considered. In particular, the dynamic environmental vibrations of the two considered strategic buildings, the fire station and the town hall building of a small town in the South of Italy, have been monitored by positioning accelerometers in well-defined points. These measurements were processed through modern Operational Modal Analysis techniques (OMA) in order to identify natural frequencies and modal shapes. Once these parameters have been determined, the structural operational efficiency index of the buildings has been determined evaluating the seismic vulnerability of the strategic structures analyzed. his study aimed to develop a model to accurately predict the acceleration of structural systems during an earthquake [ABSTRACT FROM AUTHOR]

The preservation of concrete dams is a key issue for researchers and practitioners in dam engineering because of the important role played by these infrastructures in the sustainability of our society. Since most of existing concrete dams were designed without considering their dynamic behaviour, monitoring their structural health is fundamental in achieving proper safety levels. Structural Health Monitoring systems based on ambient vibrations are thus crucial. However, the high computational burden related to numerical models and the numerous uncertainties affecting the results have so far prevented structural health monitoring systems for concrete dams from being developed. This study presents a framework for the dynamic structural health monitoring of concrete gravity dams in the Bayesian setting. The proposed approach has a relatively low computational burden, and detects damage and reduces uncertainties in predicting the structural behaviour of dams, thus improving the reliability of the structural health monitoring system itself. The application of the proposed procedure to an Italian concrete gravity dam demonstrates its feasibility in real cases. [ABSTRACT FROM AUTHOR]

This paper reports the results of an experimental campaign carried out on prestressed concrete beams (PCB), equipped both with bonded and unbonded strands, with the aim to employ Operation Modal Analysis (OMA) techniques to detect early warning signs (i.e., variation of natural frequencies and/or relevant mode shapes) associated to the onset of structural damages. With respect to similar past experimental studies, the main novelties presented hereafter concern the application of OMA to the following situations: (1) a PCB equipped with bonded strands and affected by a "almost pure" shear-cracks pattern; (2) a PCB equipped with unbonded strands and affected by an almost negligible (i.e., reversible) flexural damage. Among the outcomes of this study, it was found that, especially in the second case, the assessment of variations in the PCB natural fundamental frequencies, is not always an effective index of damage. On the contrary, a qualitative comparison between the undamaged and damaged identified mode shapes can be helpful even for the spatial location of the damage. Eventually, despite the diverging opinions in the reference literature, it is found that OMA technique is not a suitable tool to assess the potential influence of variations of the precompression load on the PCB fundamental frequencies, unless this variation is so high to cause cracking. • Operation Modal Analysis (OMA) applied to prestressed concrete beams (PCBs). • Beam-1 equipped with bonded strands and affected by almost pure shear-cracks. • Beam-2 equipped with unbonded strands and affected by negligible (i.e., reversible) flexural cracks. • Surprisingly, variations in the PCB natural fundamental frequencies are not always an effective index of damage. • On the contrary, a qualitative comparison between the undamaged and damaged identified mode shapes can be helpful even for the spatial location of the damage. • OMA technique is not a suitable tool to verify the potential influence of variations of the actual strands' tensile load on PCB fundamental frequencies. [ABSTRACT FROM AUTHOR]

In this paper, a non-destructive technique based on the monitoring of the environmental vibrations of two strategic buildings by positioning accelerometers in well-defined points was used for fixing their dynamic behavior. The accelerometers measurements were elaborated through Operational Modal Analysis (OMA) techniques, in order to identify natural frequencies, damping coefficients, and modal shapes of the structure. Once these parameters have been determined, a numerical model calibrated on the identified frequencies and verified on the corresponding mode shapes was created for each building. The structural operational efficiency index of the buildings was determined by using the Seismic Model Ambient Vibration (SMAV) methodology, which allows us to evaluate their seismic vulnerability. The results obtained from the experimental analysis (on three different tests for each analyzed building) concern the frequencies and the modal shapes of the structure. They have been compared to the results of the finite element model, with a very small error, indicating a good quality of the analysis and also the possibility of using directly well-tuned models for verifying the structural operating indices.

• Complex modal parameter identification under ambient excitation is studied. • A Bayesian strategy is adopted to find the uncertainty in identified parameters. • Objective function gradient is employed to make the optimization efficient. • Phased optimization technique is prescribed for better convergence. • An experimental study is included to investigate the applicability of the method. In this work, we present a Bayesian time–domain method for identifying the complex modal parameters of a linear dynamic system from ambient vibration data. Complex modal parameter identification is valid for both classically and non-classically damped systems, in contrast to real modal parameter identification, which is exclusively applicable to classically damped systems. A Bayesian method is adopted here which makes it possible to determine the joint posterior Probability Density Function (PDF) of the modal parameters for given measured data and modeling assumptions. For sufficient data, it is seen that the modal parameters are globally identifiable, and the posterior PDF is approximated by a Gaussian distribution with mean at the optimal parameters that maximize the posterior PDF. The uncertainty in the identified modal parameters is given by the covariance matrix of the posterior PDF. Analytical expressions are derived for computing the gradient for efficient optimization and the Hessian for determining the covariance matrix. Further, it is shown that the assumption of classical damping during analysis results in poor prediction of damping ratios in the case of non-classically damped systems. The application of the proposed approach is illustrated by means of simulated examples and an experimental study. [ABSTRACT FROM AUTHOR]

The challenges of accurately identifying the dynamic characteristics of bridge structures from ambient vibration responses persist because, unlike in the ideal laboratory environment, elevated levels of noise in data are unavoidable at any in-situ testing site and may have detrimental effects on the modal parameter identification process and results. This is especially true for vibration tests conducted under weak ambient excitation sources resulting in poorer signal-to-noise ratios (SNRs). This paper presents an investigation into the feasibility and reliability of modal identification using operational modal analysis (OMA) techniques under such weak excitation circumstances and with responses measured by inexpensive stand-alone accelerometers/data recorders. An eleven-span concrete motorway off-ramp bridge, closed to traffic, was excited only by ground vibrations generated by traffic on the motorway passing underneath the bridge as well as on nearby motorway on- and off-ramps, weak winds, and possible micro tremors. A high spatial resolution of measuring points on the bridge deck was used to collect vibration responses. Three output-only modal parameter identification algorithms were utilised to extract the modal properties, namely the peak picking (PP), the frequency domain decomposition (FDD) and the data driven stochastic subspace identification (SSI) method. Nine lateral and three vertical modal frequencies below 10 Hz could be identified despite the weakness of the environmental excitations and noise in sensors. The identified experimental natural frequencies were stable, damping ratios, however, had a marked scatter. A comparison with the results of a numerical modal analysis using a finite element model revealed, however, that several higher order vertical modes were missing from the experimental results altogether, and some of the OMA methods missed the fundamental lateral mode. Overall the PP method was the most successful in finding the largest number of frequencies but the SSI method yielded the highest quality mode shapes. The SSI method is, however, computationally more expensive that the remaining two methods. For quick, preliminary results, the PP and FDD methods can still be useful and detailed analyses could use SSI and FDD. Overall, the study argues that output-only system identification can provide useful quantitative insights into the modal properties of stiff bridges even under weak environmental excitations, or poorer SNRs, but its limitations need to be acknowledged. [ABSTRACT FROM AUTHOR]

In this study, the dynamic behavior of a model building for different damage cases is studied based on the changes in the natural frequencies and mode shapes. The damage is assumed to occur on the walls and at the column-ends of the first story, because they are generally affected by earthquakes, explosions, etc. A 1/2-scale three-story reinforced-concrete (RC) building model is constructed in the laboratory. Different damage cases are applied to the model gradually by using mechanical cutting equipment. The wall damage is assumed to reach the interior of the walls and the column-end damage is created by removing the cover of columns. The ambient vibration tests are performed by using the operational modal analysis method under unknown random impact loads. The accelerometers are placed on the columns in the transverse and longitudinal directions to get the responses of the model in these directions. It is observed that the natural frequencies, corresponding mode shapes, and modal damping ratios of the building model vary depending on the damage location and damage extent. The natural frequencies show a decreasing trend with damage, and for the last damage case the natural frequencies are 50 % lower than for the undamaged case. In addition, the mode shapes are affected considerably by wall damage. [ABSTRACT FROM AUTHOR]

In this paper, a non-destructive technique based on the monitoring of the environmental vibrations of two strategic buildings by positioning accelerometers in well-defined points was used for fixing their dynamic behavior. The accelerometers measurements were elaborated through Operational Modal Analysis (OMA) techniques, in order to identify natural frequencies, damping coefficients, and modal shapes of the structure. Once these parameters have been determined, a numerical model calibrated on the identified frequencies and verified on the corresponding mode shapes was created for each building. The structural operational efficiency index of the buildings was determined by using the Seismic Model Ambient Vibration (SMAV) methodology, which allows us to evaluate their seismic vulnerability. The results obtained from the experimental analysis (on three different tests for each analyzed building) concern the frequencies and the modal shapes of the structure. They have been compared to the results of the finite element model, with a very small error, indicating a good quality of the analysis and also the possibility of using directly well-tuned models for verifying the structural operating indices. [ABSTRACT FROM AUTHOR]