Operational Modal Analysis Techniques Research Articles

Structural health monitoring of the retrofitting process, characterization and reliability analysis of a masonry heritage construction

In the last decades, heritage constructions (HC) had been rehabilitated and inserted in functional areas of cities, therefore, the necessity for keeping them structurally safe and functional is high. Therefore, the employment of structural health monitoring (SHM) and operational modal analysis (OMA) techniques can be worthy alternatives for structural characterization and safety management. However, in the literature few cases of SHM of HC are reported, and guidelines or recommendations specifically for safety assessment of HC is an effort still undergoing. Thus, the present work describes the strategies for SHM and structural characterization of a Portuguese stone heritage construction, namely Santo Antonio Church and introduces a new approach for heritage assessment so-called Reliability Analysis Based on Vibrational Measures (RABVIM). The results showed that SHM allowed to guarantee the efficiency level of the retrofitting measures carried out, while OMA provided useful information on the structure modal parameters, under operational loading actuation, apart from the data for reliability assessment. Finally, this work demonstrates that RABVIM can be an interesting and useful tool to support the safety assessment of HC.

Damage Detection on Timber Floors’ Supports through Dynamic Analysis

ABSTRACTThe dynamic changes in a traditional layout of a short-span timber floor due to deterioration on supports were investigated experimentally and numerically. The floor was evaluated for its normal situation (undamaged) and for simulated situations of total support degradation. The experimental dynamic parameters were obtained by an operational modal analysis technique and comprised the undamped frequencies, damping ratios, as well the mode shapes. A finite-element model was then developed in a software package and validated with experimental results. The comparison of numerical and experimental results revealed that damage location at supports is easily detectable by a qualitative comparison of mode shapes with the ones of the undamaged situation.

A procedure for in situ wind load reconstruction from structural response only based on field testing data

The field application of a proposed procedure for the wind load identification is presented. The wind loads is inversely reconstructed from measured structural response in time domain, using an augmented impulse response matrix. The inherent noise amplification, arising from the ill-conditioning associated with the inverse problem, is resolved by means of Tikhonov regularization scheme in conjunction with two techniques for optimal regularization parameter estimation. To increase the accuracy along with the availability of the measured response only at a limited number of sensor locations, the problem is projected onto the modal coordinates. The structural modal parameters are obtained by an operational modal analysis technique. The case study of this paper is a 9.1m (30ft) tall guyed mast. Numerical simulation was implemented by finite element modeling of the mast and a realistic two dimensional multivariate fluctuating wind speeds, to verify the experimental results by analogy. The results are provided in time and frequency domain. Comparison of the experimental results with the numerical simulation, where actual loads are available, confirm the capability of the proposed method. Based on the existing analogy, the reconstructed wind load in higher modes, derived from different regularization parameter estimation techniques, can also be validated.

Evaluation of damping estimates by automated Operational Modal Analysis for offshore wind turbine tower vibrations

Abstract Reliable predictions of the lifetime of offshore wind turbine structures are influenced by the limited knowledge concerning the inherent level of damping during downtime. Error measures and an automated procedure for covariance driven Operational Modal Analysis (OMA) techniques has been proposed with a particular focus on damping estimation of wind turbine towers. In the design of offshore structures the estimates of damping are crucial for tuning of the numerical model. The errors of damping estimates are evaluated from simulated tower response of an aeroelastic model of an 8 MW offshore wind turbine. In order to obtain algorithmic independent answers, three identification techniques are compared: Eigensystem Realization Algorithm (ERA), covariance driven Stochastic Subspace Identification (COV-SSI) and the Enhanced Frequency Domain Decomposition (EFDD). Discrepancies between automated identification techniques are discussed and illustrated with respect to signal noise, measurement time, vibration amplitudes and stationarity of the ambient response. The best bias-variance error trade-off of damping estimates is obtained by the COV-SSI. The proposed automated procedure is validated by real vibration measurements of an offshore wind turbine in non-operating conditions from a 24-h monitoring period.

Operational Modal Analysis and Rational Finite-Element Model Selection for Ten High-Rise Buildings based on On-Site Ambient Vibration Measurements

The frequencies and mode shapes of high-rise buildings obtained from on-site ambient vibration testing and operational modal analysis (OMA) are very important for dynamic structural analysis and seismic design. This paper introduces a high-rise building ambient vibration test project in Laibin. Some results obtained from full-scale measurements of the dynamic behavior of 10 high-rise buildings are described. Different pre- and postprocessing techniques were used for ambient vibration signal analysis, from which the modal parameters were obtained using three OMA techniques. By rationally analyzing and modeling the stiffness of the infill walls, six finite element (FE) models were built in PKPM and SAP2000 to estimate the analytical modal information. The influences of the infill wall mass and stiffness on the dynamic properties of a high-rise building are further discussed. According to the identified and calculated results, all three modes emerged in each modal dense region in the frequency domain of high-rise buildings. Finally, based on 25 proposed empirical equations, the fundamental periods of 10 buildings are calculated and summarized.

Operational modal analysis for the characterization of ancient water towers in Pompeii

In the framework of an investigation campaign carried out in June 2015 by the authors on four ancient water towers (10–20 BC) in the archaeological site of Pompeii, modal analysis and output-only identification techniques were employed to extract the dynamic properties in order to assess structural vulnerabilities and support numerical model updating.The four investigated towers (selected among the fourteen present within the archaeological site) are free-standing structures at least 6 m tall, belonging to the Castellum Aquae, i.e. the ancient aqueducts system of the city. During the Roman Age, until the destruction of Pompeii due to the volcanic eruption in 79 AD, water towers provided fresh water to houses, palaces and villas. This particular type of structures are classified as no. 1, 2, 3 and 4 by archaeological literature: no. 1 and 4 are made of soft stone masonry (tuff, limestone), while no. 2 and 3 are composed by brickwork masonry.The paper reports the outcomes of ambient vibration tests performed on four towers in terms of extracted modal parameters using various operational modal analysis techniques. Obtained data are then used to study numerically the soil-structure interaction problem and implement model updating procedures.

One-year monitoring of a reinforced concrete school building: Evolution of dynamic behavior during retrofitting works

This work illustrates the results of a one-year structural monitoring campaign of a Reinforced Concrete (RC) school building subjected to a substantial retrofitting works. Through Operational Modal Analysis (OMA) techniques, it has been possible to follow the dynamic evolution of the structure, using Finite Element Method (FEM) model refinement as a valuable tool for non-invasive structural assessment. The long-term Ambient Vibration Survey (AVS) highlighted the increasing of building performance in terms of natural frequencies following the retrofitting works. This relevant data collection can be extremely helpful in calibrating of other similar retrofitting interventions and provides useful guidelines that can assist in the planning of Structural Health Monitoring (SHM) applications.

Calibration of the dynamic behaviour of incomplete structures in archeological sites: the case of Villa Diomede portico in Pompeii

This paper reports the research activities carried out on Villa Diomede in Pompeii, built during the “Pre-Roman period” (i.e. the 3rd century BC) and discovered between 1771 and 1774 during the archaeological excavations. It is one of the greatest private buildings of Pompeii and it is located on the western corner of the modern archeological site. Three levels compose the building: the ground floor, the lower quadriportico with a square plan and a series of colonnades on the four sides around the inner garden and the cryptoportico. Villa Diomede was damaged by the strong earthquake occurred in AD 63 that caused the collapse of the western pillars of the quadriportico and later damaged after the big eruption of Vesuvius in AD 79. In June 2015 a series of nondestructive tests (NDT) were carried out by the authors in order to obtain information on the state of conservation of the building and to assess its structural behavior. Direct and tomographic sonic pulse velocity tests, ground penetrating radar, endoscopies and operational modal analysis were performed on the remaining structural elements on the two levels of the Villa. The present paper reports the main outcomes and findings of ambient vibration tests implemented to extract the modal parameters in terms of eigenfrequencies, mode shapes and damping ratios. Operational modal analysis and output-only identification techniques were applied to single stone pillars of the quadriportico structure and then to the entire square colonnade of Villa Diomede. Results are then used to study the soil-structure interaction at a local level and extend the gained information for the numerical calibration of the whole structure. Thanks to this methodology a detailed model updating procedure of the quadriportico was performed to develop reliable numerical models for the implementation of advance structural and seismic analysis of this “incomplete” archaeological structure.

Wind Turbine vibration based SHM system: influence of the sensors layout and noise

With the goal of reducing the investment in monitoring equipment and simultaneously keep an adequate accuracy in structural condition assessment, this paper studies the results that can be obtained with different strategies of instrumentation of a 2.0 MW onshore wind turbine. The data processing strategy is based on the continuous tracking of the wind turbine main vibration modes using Operational Modal Analysis techniques combined with algorithms that deal with the particularities of operation of the wind turbines and that permit an online automated identification. A database of continuously collected acceleration time series during one year is used to evaluate the efficiency on the monitoring system on the detection of simulated damage scenarios, considering variations on the number of sensors, on their distribution in the wind turbine tower and on their noise level.

An Investigation of the Seismic Behaviour of an Ancient Masonry Bastion Using Non-Destructive and Numerical Methods

Determining the structural behavior of masonry structures is a challenge due to their lack of homogeneity. The seismic behavior of masonry structures is especially complex. The aim of this study was to examine the structural behavior of Zaganos Bastion using both experimental and numerical methods. The Operational Modal Analysis technique, including the Enhanced Frequency Domain Decomposition Method, and the Stochastic Subspace Identification Method were used to illustrate experimentally the dynamic characteristic of the bastion. A finite element model was developed using ANSYS software in order that the dynamic characteristics of the bastion, including natural frequencies and mode shapes, could be calculated numerically. Seismic analysis was carried out using the 1999 Kocaeli earthquake ground motion record to determine the linear and nonlinear seismic behavior of the bastion. The Turkish Earthquake Code and its general technical specifications were used to evaluate the seismic results. The results show that the maximum and minimum principal stresses exerted on the masonry components exceeded the code requirements at some points, but in general the requirements for the stresses were satisfied.

STRUCTURAL HEALTH MONITORING OF ROTATING BLADES ON HELICOPTERS

The paper discusses the structural health monitoring of rotating blades on helicopters (RBH) based on the application of Modal Analysis. The study discussed in this paper includes the experimental validation of state-of-the-art techniques for the on-line measurement of dynamic signals of helicopter rotating units, optimization of the sensor type for rotating unit measurement, analysis of the practical applicability of modal analysis techniques for condition-based monitoring of rotating structures and estimation of the efficiency of the experimental system for the identification of practical damages of blades. The research was conducted using helicopter blade models operating within an experimental test bench. The capabilities of diagnostic technique application to main rotor gears and bearings are also presented. Conclusions are made about the successful analysis of the operational modal analysis technique applicability for the structural health monitoring of a rotating blade, and its effectiveness for damage identification. Two strategies of the RBH repair – with a continuously condition-based monitoring with the proposed technology and one without such monitoring, were discussed. The Markov chain reliability models for each strategy were analyzed and the reliability improvement factor for the proposed monitoring technology in comparison with a traditional one was evaluated. It is shown that the reliability improvement factor is more effective for the proposed method as compared to the traditional one.

Photogrammetry in Documentation and Ambient Vibration Test of Historical Masonry Minarets

This study was conducted to investigate modal parameters (e.g. mode shapes and modal frequencies) of a masonry-type historical minaret by performing numerical and experimental analyses. The Operational Modal Analysis technique was used for the experimental study to obtain modal parameters of a historical masonry minaret. For this purpose, the Buyuk Mosque located in Samsun, Turkey, was chosen for the experimental analysis. The photogrammetry method was used to document the minaret and to create a finite element (FE) model. The Frequency Domain Decomposition (FDD) method was utilised to experimentally identify the natural frequencies and mode shapes. ANSYS software was used to carry out 3D FE modelling of the historical masonry minaret and to analytically determine the natural frequencies and mode shapes of the minaret. Furthermore, the FE model of the minaret was calibrated according to the experimental results using the Response Surfacebased FE model calibration technique to obtain more accurate results of the modal parameters of the structure. The results of the experimental, initial, and calibrated FE model were compared. Significant differences were noted when comparing the results of the experimental and analytical studies to the initial conditions. Model calibration techniques are necessary to obtain a more reasonable FE model.

Non-destructive testing of an ancient Masonry Bastion

Abstract Historical masonry structures have seismic vulnerability and most damages and demolishes arise from the seismic actions like earthquakes. In this paper, the structural behavior of Zaganos Bastion is examined with experimental and numerical methods. The operational modal analysis technique is used to illustrate the dynamic characteristics of the bastion experimentally. Finite element model is developed by ANSYS software and dynamic characteristics of the bastion, such as natural frequencies and mode shapes are calculated numerically. Furthermore, time history seismic analysis is carried out. The results show that the ambient vibration measurements are enough to identify the structural response of the bastion. Maximum differences between the natural frequencies are attained as 26%. To minimize these differences, finite element model of the bastion should be updated by using some uncertain parameters. The principal stresses are satisfied in general, but the maximum tensile and compression stresses values exceed the allowable code limits at some points of the masonry components.

Comparison of in-Flight Measured and Computed Aeroelastic Damping: Modal Identification Procedures and Modeling Approaches

The Operational Modal Analysis technique is a methodology very often applied for the identification of dynamic systems when the input signal is unknown. The applied methodology is based on a technique to estimate the Frequency Response Functions and extract the modal parameters using only the structural dynamic response data, without assuming the knowledge of the excitation forces. Such approach is an adequate way for measuring the aircraft aeroelastic response due to random input, like atmospheric turbulence. The in-flight structural response has been measured by accelerometers distributed along the aircraft wings, fuselage and empennages. The Enhanced Frequency Domain Decomposition technique was chosen to identify the airframe dynamic parameters. This technique is based on the hypothesis that the system is randomly excited with a broadband spectrum with almost constant power spectral density. The system identification procedure is based on the Single Value Decomposition of the power spectral densities of system output signals, estimated by the usual Fast Fourier Transform method. This procedure has been applied to different flight conditions to evaluate the modal parameters and the aeroelastic stability trends of the airframe under investigation. The experimental results obtained by this methodology were compared with the predicted results supplied by aeroelastic numerical models in order to check the consistency of the proposed output-only methodology. The objective of this paper is to compare in-flight measured aeroelastic damping against the corresponding parameters computed from numerical aeroelastic models. Different aerodynamic modeling approaches should be investigated such as the use of source panel body models, cruciform and flat plate projection. As a result of this investigation it is expected the choice of the better aeroelastic modeling and Operational Modal Analysis techniques to be included in a standard aeroelastic certification process.

Structural identification with incomplete output‐only data and independence of measured information for shear‐type systems

SummaryStructural identification is the inverse problem of estimating physical parameters of a structural system from its vibration response measurements. Incomplete instrumentation and ambient vibration testing generally result in incomplete and arbitrarily normalized measured modal information, often leading to an ill‐conditioned inverse problem and non‐unique identification results. The identifiability of any parameter set of interest depends on the amount of independent available information. In this paper, we consider the identifiability of the mass and stiffness parameters of shear‐type systems in output‐only situations with incomplete instrumentation. A mode shape expansion‐cum‐mass normalization approach is presented to obtain the complete mass normalized mode shape matrix, starting from the incomplete non‐normalized modes identified using any operational modal analysis technique. An analysis is presented to determine the minimum independent information carried by any given sensor set‐up. This is used to determine the minimum necessary number and location of sensors from the point of view of minimum necessary information for identification. The different theoretical discussions are illustrated using numerical simulations and shake table experiments. It is shown that the proposed identification algorithm is able to obtain reliably accurate physical parameter estimates under the constraints of minimal instrumentation, minimal a priori information, and unmeasured input. The sensor placement rules can be used in experiment design to determine the necessary number and location of sensors on the monitored system. John Wiley & Sons, Ltd.

Structural Health Monitoring challenges on the 10-MW offshore wind turbine model

The real-time structural damage detection on large slender structures has one of its main application on offshore Horizontal Axis Wind Turbines (HAWT). The renewable energy market is continuously pushing the wind turbine sizes and performances. This is the reason why nowadays offshore wind turbines concepts are going toward a 10 MW reference wind turbine model. The aim of the work is to perform operational analyses on the 10-MW reference wind turbine finite element model using an aeroelastic code in order to obtain long-time-low- cost simulations. The aeroelastic code allows simulating the damages in several ways: by reducing the edgewise/flapwise blades stiffness, by adding lumped masses or considering a progressive mass addiction (i.e. ice on the blades). The damage detection is then performed by means of Operational Modal Analysis (OMA) techniques. Virtual accelerometers are placed in order to simulate real measurements and to estimate the modal parameters. The feasibility of a robust damage detection on the model has been performed on the HAWT model in parked conditions. The situation is much more complicated in case of operating wind turbines because the time periodicity of the structure need to be taken into account. Several algorithms have been implemented and tested in the simulation environment. They are needed in order to carry on a damage detection simulation campaign and develop a feasible real-time damage detection method. In addition to these algorithms, harmonic removal tools are needed in order to dispose of the harmonics due to the rotation.

Operational modal analysis of flow-induced vibration of nuclear fuel rods in a turbulent axial flow

Flow-induced vibration of nuclear reactor fuel pins can result in mechanical noise and lead to failure of the reactor's fuel assembly. This problem can be exacerbated in the new generation of liquid heavy metal fast reactors that use a much denser and more viscous coolant in the reactor core. An investigation of the flow-induced vibration in these particular conditions is therefore essential. In this paper, we describe an analysis technique to evaluate flow-induced vibration of nuclear reactor fuel pins subjected to a turbulent axial flow of heavy metal. We deal with a single fuel pin mockup designed for the lead–bismuth eutectic (LBE) cooled MYRRHA reactor which is subjected to similar flow conditions as in the reactor core. Our analysis is based on operational modal analysis (OMA) techniques. We show that the accuracy and precision of our OMA technique is higher compared to traditional methods and that it allows evaluating the evolution of modal parameters in operational conditions. We also demonstrate the possible onset of a fluid-elastic instability by tracking the modal parameters with increasing flow velocity.

Modal Identification Using OMA Techniques: Nonlinearity Effect

This paper is focused on an assessment of the state of the art of operational modal analysis (OMA) methodologies in estimating modal parameters from output responses of nonlinear structures. By means of the Volterra series, the nonlinear structure excited by random excitation is modeled as best linear approximation plus a term representing nonlinear distortions. As the nonlinear distortions are of stochastic nature and thus indistinguishable from the measurement noise, a protocol based on the use of the random phase multisine is proposed to reveal the accuracy and robustness of the linear OMA technique in the presence of the system nonlinearity. Several frequency- and time-domain based OMA techniques are examined for the modal identification of simulated and real nonlinear mechanical systems. Theoretical analyses are also provided to understand how the system nonlinearity degrades the performance of the OMA algorithms.

Experimental Technology of Operational Pipeline Condition Monitoring

This work presents a summary of the research study of operational modal analysis (OMA) application for condition monitoring of operating pipelines. Special focus is on the topicality of OMA for definition of the dynamic features of the pipeline (frequencies and mode shapes) in operation. The research was conducted using two operating laboratory models imitated a part of the operating pipeline. The results of finite-element modeling, identification of pipe natural modes and its modification under the influence of virtual failure are discussed. The work considers the results of experimental research of dynamic behavior of the operating pipe models using one of OMA techniques and comparing dynamic properties with the modeled data. The study results demonstrate sensitivity of modal shape parameters to modification of operating pipeline technical state. It is resumed about ability of operating pipeline condition monitoring by measuring dynamic deformations of the operating pipe and OMA techniques application for dynamic properties extraction.

Structural health monitoring of offshore wind turbines using automated operational modal analysis

This article will present and discuss the approach and the first results of a long-term dynamic monitoring campaign on an offshore wind turbine in the Belgian North Sea. It focuses on the vibration levels and modal parameters of the fundamental modes of the support structure. These parameters are crucial to minimize the operation and maintenance costs and to extend the lifetime of offshore wind turbine structure and mechanical systems. In order to perform a proper continuous monitoring during operation, a fast and reliable solution, applicable on an industrial scale, has been developed. It will be shown that the use of appropriate vibration measurement equipment together with state-of-the art operational modal analysis techniques can provide accurate estimates of natural frequencies, damping ratios, and mode shapes of offshore wind turbines. The identification methods have been automated and their reliability has been improved, so that the system can track small changes in the dynamic behavior of offshore wind turbines. The advanced modal analysis tools used in this application include the poly-reference least squares complex frequency-domain estimator, commercially known as PolyMAX, and the covariance-driven stochastic subspace identification method. The implemented processing strategy will be demonstrated on data continuously collected during 2 weeks, while the wind turbine was idling or parked.