Showing papers on "Modal testing published in 2022"

Modal analysis and tension estimation of stay cables using noncontact vision‐based motion magnification method

Abstract: This paper describes a study on modal analysis and tension estimation of stay cables by vision‐based measurement under ambient condition. Microvibration of stay cable is captured by video camera, and the amplitude is amplified using phase‐based video motion magnification method. From the sequences of cable images, spatial displacements of the cable are extracted via discretized centroid searching method. Modal parameters of the cable, namely, natural frequency, damping ratio, and mode shape, are identified by dynamic mode decomposition method from cable displacement responses. Furthermore, tension of the stay cable is estimated based on the identified natural frequencies by minimizing an error function of an approximate relationship between frequency and tension iteratively. The technique is verified in the laboratory‐scale experiments and implemented to full‐scale measurement of medium size and long‐span cable‐stayed bridges. To compare the accuracy and efficacy of the vision‐based method, noncontact vibration measurement using laser Doppler vibrometer was also conducted. The results demonstrate that the proposed vision‐based vibration measurement techniques can estimate modal parameters and tension of the stay cables accurately in a noncontact and distant measurement under ambient condition. The proposed method offers an alternative of effective and accurate vibration measurement of stay cables using only motion of the cables recorded by video camera.

Identification of boundary conditions of railway bridges using artificial neural networks

Abstract: Abstract This article presents a study that aims to identify the boundary conditions of a railway bridge using system identification and artificial neural networks. Vibrations generated by three different train types recorded during a 24-h long measurement campaign is used to identify the modal frequencies and mode shapes of a single-span 50 m long railway bridge. Frequency Domain Decomposition and Stochastic Subspace Identification with Covariance methods were used to identify the modal properties from the recorded vibrations and the effect of the used Operational Modal Analysis on the identified modal properties was evaluated. An initial finite-element (FE) model based on the design drawings was not able to replicate the observed dynamic behavior of the bridge. Using a sensitivity analysis, the key parameters of the finite-element model that impact the vibration frequencies of the bridge was determined. 300 finite-element models were created by changing the values of these key parameters within their effective range and were used to identify the relationship between these parameters and the vibration frequencies using Artificial neural networks (ANNs). Leveraging this relationship, the values of the FE model parameters that minimizes the error between the measured and computed frequencies was determined. As a result, the mean error between the computed and the identified vibration frequencies was reduced from 27.3% for the initial model to 3.0% for the updated model. The study indicates that boundary conditions are among the most influential parameter on the dynamic behavior of bridges and can deviate significantly from the simplistic models generally used in the FE models.

Bridge modal identification based on successive variational mode decomposition using a moving test vehicle

Abstract: Bridge modal identification using an instrumented test vehicle as a moving sensor is promising but challenging. A key factor is to extract bridge dynamic components from vehicle responses measured when the bridge is operating. A new method based on an advanced adaptive signal decomposition technique, the successive variational mode decomposition (SVMD), has been developed to estimate the bridge modal parameters from the dynamic responses of a passing test vehicle. When bridge-related dynamic components are extracted from the decomposition, the natural excitation technique and/or random-decrement technique based fitting methods are used to estimate the modal frequencies and damping ratios of the bridge. Effects of measurement noise, moving speed and vehicle properties on the decomposition are investigated numerically. The superiority of SVMD in the decomposition is verified by comparing to another adaptive decomposition technique, the singular spectrum decomposition. The results of the proposed method confirm that the bridge modal frequencies can be identified from bridge related components with high accuracy, while damping ratio is more sensitive to the random operational load. Finally, the feasibility of the proposed method for bridge monitoring using a moving test vehicle is further verified by an in-situ experimental test on a cable-stayed bridge. The components related to the bridge dynamic responses are successfully extracted from vehicle responses.

Modal analysis of an arch dam combining ambient vibration measurements, advanced fluid‐element method and modified engineering approach

Abstract: Modal analysis, aiming at estimating modal characteristics such as natural frequencies and mode shapes, is fundamental for studying the dynamic behaviours of a structure. This paper presents a modal analysis of an arch dam using three different techniques. The reference one is based on the statistical analysis of the ambient vibration data collected from the dam crest. The two other approaches are both numerical but with different methods (fluid‐element and Westergaard) for the modelling of the reservoir and its interactions with the dam. By applying the three techniques to the studied dam and comparing their results, it is demonstrated that: (1) analysing the ambient vibration data through an operational modal analysis method is able to extract the dam modal characteristics; (2) the fluid‐element method is effective for arch dams since the first 10 natural frequencies can be accurately predicted once the material parameters are calibrated on the first three modes; and (3) the Westergaard method, a technique with only additional masses, produces significantly under‐estimated frequencies for the first few modes if same parameters are used as the fluid‐element method; the underestimation can be corrected for several modes by using a higher stiffness parameter but the required value is unrealistic for the case study. Furthermore, a modified Westergaard method is introduced in this paper by using a reduced added‐mass coefficient. This method, once the coefficient is calibrated on the 1st mode, is able to well predict the partially coupled modes as illustrated with the case study of the Saint‐Guérin dam.

An Explicit Solution of Modal-Damping Ratios for Higher Modes of a Cable with an External Damper

Abstract: Design of external dampers for stay cables requires repetitive evaluation of damping ratios for target modes of cables under various damper parameters and installation positions, and explic...

Modal vibration decomposition method and its application on multi-mode vibration control of high-speed railway car bodies

Abstract: The vibration of a railway car body is a superposition of the vibrations of its various modes. It is typically easy to obtain the physical vibration of the car body using sensors in an in situ or a simulated test vehicle. However, it is difficult to determine the modal vibration of the body and its contribution. There are no effective multi-mode vibration control methods for the car bodies. This study proposes a modal vibration decomposition method (MVDM) based on singular value decomposition (SVD) and least squares fitting (LSF). Accordingly, the physical vibration of a railway car body is decomposed into modal vibrations. A method for calculating the modal contribution factor (MCF) is presented, and the dominant flexible modes of the car body are determined and considered the target for the vibration control method. Several pieces of equipment are considered as dynamic vibration absorbers (DVAs) to control the multi-mode vibration of the car body using the dynamic vibration absorption theory and determine the installation parameters of the individual equipment. Finally, the effectiveness of vibration control is verified through dynamic simulations. The results demonstrate the effective decomposition of the physical vibration of the car body into various modal vibrations using the MVDM. This provides accurate data for the MCF calculation and determination of the flexible modes of the car body. The proposed method reduces the vibration of the target modes and improves the ride quality of the railway vehicle. At the optimal damping ratio, the vibration of the DVA-based equipment itself is acceptable. This allows for multi-mode vibration control without requiring extensive modification to the car body structure or suspension system parameters of the vehicle.

An Explicit Solution of Modal-Damping Ratios for Higher Modes of a Cable with an External Damper

Abstract: Design of external dampers for stay cables requires repetitive evaluation of damping ratios for target modes of cables under various damper parameters and installation positions, and explicit formulas with high accuracy to estimate the modal-damping ratios for a cable-damper system are helpful in designing external dampers. This study deals with the evaluation of the modal damping of cables with an external viscous damper close to the anchorage. A considerably accurate and explicit formula for calculating the modal-damping ratios of the cable-damper system is derived by evaluating the imaginary and real part of the complex frequency equation. It is demonstrated that the formula is able to predict the modal-damping ratios of the cable-damper system both for lower and higher modes. Based on the proposed formula, the effects of the modal order and installation position on the modal-damping ratio of the cable-damper system are studied. A closed-form solution for the optimal damping coefficient is proposed and the maximum attainable modal damping is also obtained.

Multi-Variant Modal Analysis Approach for Large Industrial Machine

Abstract: Power generation technologies are essential for modern economies. Modal Analysis (MA) is advanced but well-established method for monitoring of structural integrity of critical assets, including power ones. Apart from classical MA, the Operational Modal Analysis approach is widely used in the study of dynamic properties of technical objects. The principal reasons are its advantages over the classical approach, such as the lack of necessity to apply the excitation force to the object and isolate it from other excitation sources. However, for industrial facilities, the operational excitation rarely takes the form of white noise. Especially in the case of rotating machines, the presence of rotational speed harmonics in the response signals causes problems with the correct identification of the modal model. The article presents a hybrid approach where combination of results of two Operational Modal Analyses and Experimental Modal Analysis is performed to improve the models’ quality. The proposed approach was tested on data obtained from a 215 MW turbogenerator operating in one of Polish power plants. With the proposed approach it was possible to diagnose the machine’s excessive vibration level correctly.

Modal Identification Technologies for High-Rise Buildings Under Non-Stationary Excitations

Abstract: For high-rise buildings subjected to typhoon or earthquake actions, the excitations are likely non-stationary, which inevitably violates the stationary assumption that is generally adopted by conventional modal identification methods. Under such a condition, conventional modal identification methods may not be applicable, and their performances need to be evaluated. To this end, the performances of seven widely used modal identification algorithms for structural modal estimation under non-stationary excitations are evaluated via a numerical simulation study. Then, three methods with good performance are selected to evaluate their applicability and accuracy for field measurements involving non-stationary excitations. From the comparative study, a time–frequency domain method with the best performance among these seven methods is employed to investigate the structural dynamic properties of a 509-m-tall skyscraper under typhoon and earthquake excitations. Based on the reliable modal estimates, the relationships between the modal properties with response amplitude and environmental temperature are presented and discussed. This paper aims to identify the effective methods for accurate estimation of structural modal parameters based on non-stationary structural responses and investigate the structural dynamic properties of high-rise buildings under typhoon and earthquake excitations.

Piezoelectric sensor-based 3D modal analysis under shaker derived random excitations

Abstract: Modal analysis based vibration monitoring has been extensively adopted for health assessment and continuous monitoring of structures. Conventional accelerometer derived displacement based modal parameters are generally used for evaluation but with a high operational cost, fragility issues and bandwidth limitations. Strain based modal parameters have been increasingly explored for monitoring purposes owing to their high sensitivity towards any perturbation in the structural property. Piezoelectric sensors are highly sensitive, low cost, smart material based dynamic strain sensors. However, they have been sparsely investigated for their efficiency in obtaining modal parameters, specially under random excitations. This study examines the ceramic based lead-zirconate-titanate (PZT) piezoelectric sensors for their applicability in modal analysis based vibration monitoring under shaker driven random excitations. PZT patches and accelerometers were used to record the measurements of a scaled down model of pedestrian under random excitations from a mechanical shaker. Polyreference Least-Squares Complex Exponential (p-LSCE) system identification algorithm was adopted to obtain the modal parameters from both the sensors. PZT patches were equally effective as accelerometers in capturing the modal parameters namely frequency, damping and mode shapes. First ten modes under consideration were obtained by PZT patches, with an error of under one percent with accelerometers, from both the system identification techniques. Damping ratios obtained from both the sensors were in good agreement with each other. Mode shapes form the either of sensors were in excellent correlation with each other with modal assurance criteria (MAC) values higher than threshold value of 0.75.