Enhanced Frequency Domain Decomposition Research Articles

Structural Health Assessment of Kretek II Bridge using Enhanced Frequency Domain Decomposition

The development of infrastructure is growing rapidly in Indonesia. Kretek II bridge is one of the infrastructures built in the country. Dynamics aspects are one of the very important aspects used to validate structural analysis model or being linked to Structural Health Monitoring (SHM) of the bridge. Dynamics properties such as natural frequency, damping ratio, and mode shape also referred as modal parameters. The objective of this research is to determine the modal parameters of Kretek II bridge using Enhanced Frequency Domain Decomposition (EFDD) which is commonly used to extract modal parameters from the acceleration data recorded. Compared to the conventional method, EFDD is very practical and robust in structural health assessment because of its user-friendly uses in the ARTeMIS Modal software. To make sure that the results from EFDD are accurate, numerical modeling is necessary to validate it. This research was conducted using dynamics load test results as data for the modal extraction with EFDD method assisted by ARTeMIS Modal software. Dynamics load test was performed by placing 8 accelerometers in the span and exciting it with impact load from dropped truck on ramp. Modal parameters from the EFDD results are then compared to the numerical modeling results. The first two modes of the EFDD and numerical modeling results consecutively are 3.09 Hz, 3.745 Hz and 3.06 Hz, 3.49 Hz. The EFDD and numerical modeling results are both in agreement with both results having similar mode shape on their first two mode and low error percentage with only 0.89% and 7.17% respectively.

Modal Identification of Train Passenger Seats Based on Dynamic Tests and Output-Only Techniques

Railways are one of the most efficient and widely used mass transportation systems for mid-range distances, also being pointed out as the best strategy to reach European Union decarbonisation goals. However, to increase railways attractiveness, it is necessary to improve the quality of the ride, namely its comfort, by decreasing the vibration at the passenger level. This article describes the experimental vibration modal identification of train seats based on a dedicated set of dynamic tests performed on Alfa Pendular and Intercity trains. This work uses two output-only modal identification techniques: the transmissibility functions and the Enhanced Frequency Domain Decomposition (EFDD) method. The last method allows us to clearly distinguish the seat structural movements, particularly the ones related to torsion and bending of the seat frame, from the local vertical foam vibrations. The natural frequencies and mode shapes are validated by matching the results derived from the transmissibility functions and EFDD method. The identified modal parameters are particularly relevant to characterise the vibration transmissibility provided by the foams (local transmissibility) and the vibration transmissibility derived from the metallic seat frame (global transmissibility).

Modal parameter identification and seismic assessment of historical timber structures under near-fault and far-fault ground motions

Timber, one of the oldest structural materials, is an essential material in the construction of traditional structures due to its undeniable advantages. However, wood is sensitive to environmental effects, and hence, historical timber structures experience various degradations and destructions during their service life. This highlights the necessity of investigating the structural responses of such structures using experimental techniques and calibrated finite element models. This paper presents vibration-based system identification of two historical timber structures and an in-depth comparison of the effects of near-fault and far-fault ground motions on seismic responses. Ambient vibration tests were carried out using the operational modal analysis method. Enhanced Frequency Domain Decomposition and Stochastic Subspace Identification methods were used for experimental modal parameters estimation. Also, the initial finite element models were calibrated considering the ambient vibration test results to correctly assess the seismic responses of the selected structures under near-fault and far-fault ground motions. The distributions of maximum amplitudes (displacements, internal forces, and absolute accelerations) under earthquake records with near-fault and far-fault are comparatively presented. All numerical results indicated that both near-fault ground motions and their far-fault counterparts played major roles in terms of the seismic responses of selected historical timber structures. In addition, these results illustrate that both had the potential to increase displacements and, internal forces at load-bearing elements, and leading to notable structural responses and destructive effects.

Vibration Characteristics of Corn Combine Harvester with the Time-Varying Mass System under Non-Stationary Random Vibration

In field harvesting conditions, the non-stationary random vibration characteristics of the harvester are rarely considered, and the results of vibration frequency calculated by different time–frequency transformation methods are different. In this paper, the harvester’s vibration characteristics under the time-varying mass were studied, and the correlation between vibration frequency and modal frequency was analyzed. Firstly, under the conditions of time-varying mass (field harvesting conditions) and non-time-varying mass (empty running condition), the non-stationarity characteristics of vibration signals at 16 measurement points of a combined corn harvester frame were studied. Then, fast Fourier transform (FFT), short-time Fourier transform (STFT), and continuous wavelet transform (CWT) were used to calculate the vibration frequency distribution characteristics of the corn harvester. Finally, based on the EFDD (enhanced frequency domain decomposition) algorithm, the correlation between the primary vibration frequency and the operating mode frequency is studied. The results show that the mean, variance, and maximum difference of the vibration amplitude under harvesting conditions (mass time-varying system) are 0.10, 26.5, and 1.0, respectively, at different harvesting periods (0~10 s, 10~20 s, 20~30 s). The harvesting conditions’ vibration signals conform to the characteristics of non-stationary randomness. The FFT algorithm is used to obtain more dense vibration frequencies, while the frequencies based on STFT and CWT algorithms are sparse. The correlation between the FFT method and the EFDD algorithm is 0.98, and the correlation between the STFT, CWT, and the EFDD algorithm is 0.99 and 0.98. Therefore, the primary frequency of the STFT methods is closer to the modal frequency. Our research laid the foundation for further study and application of mass time-varying combined harvester system non-stationary random vibration modal frequency identification and vibration control.

Dynamic Response and Seismic Vulnerabilities of the Historic Tophane-i Amire: Dynamic and Seismic Analysis of a Historic Masonry Building

This study deals with the dynamic characterization of the historic Tophane-i Amire building, located in Istanbul. The domed structure was constructed in the 18th century and is known as the first industrial building in the Ottoman times. It was an arsenal building used for the production of cannons and cannon balls. An ambient vibration survey was conducted with the Enhanced Frequency Domain Decomposition method to identify the dynamic characteristics of the structure. Later, a finite element model of the structure was constructed and numerical dynamic identification was pursued. The obtained experimental and numerical results have been compared and the finite element model of the structure has been tuned to obtain a validated tool to represent the real dynamic behavior of the structure. While the tuned model of the structure is intended to contribute to understanding its structural behavior and evaluate the mechanical properties of masonry, it has also been used for the evaluation of its seismic vulnerabilities. For this purpose, the structure has been analyzed under maximum considered earthquake response spectrum, and stress concentration regions have been determined. The results also serve as data for future structural health monitoring studies related to the structure.

Structural health monitoring and modal parameters identification of Mezcala Bridge

Since 2016 the Instituto Mexicano del Transporte (Mexican Transport Institute) has continuously instrumented and performed the structural monitoring of the Mezcala Bridge, which is a cable-stayed bridge located in the State of Guerrero, Mexico. In order to identify early damage, several monitoring strategies based on changes in the bridge modal parameters have been implemented. To establish the limits of the parameters, such as the natural frequencies, damping ratios, and mode shapes, it is necessary to characterize the bridge dynamic behavior through monitoring information from the last 3 years, in order to analyze statistically every parameter and simulate different real damage scenarios. Modal parameters are calculated and evaluated every week or after accidents, earthquakes, or strong winds. Applying the proposed methodology based on the Enhanced Frequency Domain Decomposition (EFDD), the modal parameters of the Mezcala Bridge were efficiently estimated and monitored and subsequently, they could be used for finite element model development and calibration in order to evaluate the structural integrity of the bridge.

Monitor wind characteristics and wind‐induced responses of a skyscraper during two typhoons

SummarySkyscrapers are wind‐sensitive structures due to their large slenderness ratio and good flexibility. Understanding the wind characteristics and wind‐induced response from the actual data provides first‐hand information for wind resistance design. This paper reports the wind properties and dynamic performance of a 632 m high‐rise building in Shanghai during the passage of typhoons Jongdari and Rumbbia. Field measurements obtained from the structural health monitoring system include acceleration and wind speed. The data analysis of turbulence intensity, turbulence integral length, and gust factor are compared and discussed. For the identification of dynamic characteristics, the stochastic subspace method and enhanced frequency domain decomposition method are employed to identify the modal parameters. Besides, the comfort assessment using peak value and root mean square of accelerations is compared with different standards. Studying the wind characteristics during the passage of typhoons and their impact on high‐rise buildings in a metropolis is expected to gain an insight into the dynamic mechanism of high‐rise buildings and collect a database for wind resistance design in typhoon‐prone regions.

Multiple Tests for Dynamic Identification of a Reinforced Concrete Multi-Span Arch Bridge

This paper presents the results of an experimental dynamic campaign carried out on a reinforced concrete multi-span arch bridge. Five expeditious ambient vibration tests were conducted separately on five spans (one test in each span) of the bridge using only six piezoelectric uniaxial accelerometers. Modal parameters were identified through the well-known Enhanced Frequency Domain Decomposition (EFDD) procedure developed using Matlab R2021b software. At the same time, a finite element model was accurately implemented through a commercial software (Midas Civil) to evaluate the main modal features. A manual model update was successively pursued varying the elastic modulus of the reinforced concrete to make the identified and numerical modes as close as possible. A complete and suitable instrumentation to perform global experimental dynamic tests is not always available. Recursive/Multiple tests have different advantages: handy, easily executable, and could provide a more robust identification thanks to a statical characterization. The paper aims to highlight the peculiarities of recursive/multiple dynamic tests on multi-span arch bridges. The procedure also provides useful suggestions for designing a permanent and continuous vibration-based monitoring system.

Operational modal analysis of a catamaran using a limited set of accelerometers

ABSTRACT Experimental modal analysis tests were performed during the catamaran’s normal operation and using only 06 accelerometers on the main deck. This work focuses on estimating and comparing modal parameters using experimental (time and frequency domain techniques) and numerical analyses. Ten (10) experimental natural frequencies and (incomplete) mode shapes were estimated, in the range of 4–50 Hz, using Balance Realisation, Frequency-Spatial Domain Decomposition and Enhanced Frequency Domain Decomposition techniques. As only 06 accelerometers had been used for modal testing, a finite element (FE) model was used to augment the spatial resolution of the experimental mode shapes. Experimental and numerical natural frequencies showed good correlation with only the 3rd bending mode shape exhibiting 13.5% deviation. It was concluded that the experimental techniques can: (i) identify natural frequencies in the presence of harmonic components and, (ii) estimate mode shapes using a limited set of accelerometers, aided by a FE model.

Experimental Study on the Natural Dynamic Characteristics of Steel-Framed Modular Structures

Corner-supported modular structures are made of repetitive prefurnished, prefinished modular units, which are fabricated in a factory and transported to the site of a building to form a permanent building block. The modular units are then tied together through the use of so-called inter-modular connections, or inter-connections, which form a different configuration at joints compared to conventional steel structures. The presence of inter-connections in these structures, in addition to beam-to-column connections or intra-connections, may change their dynamic characteristics, including natural frequencies, mode shapes, and damping ratios. This paper aims to investigate the dynamic characteristics of a modular building through the use of operational modal analysis (OMA). A half-scaled three-storey modular structure, designed and instrumented with highly sensitive accelerometers, was experimentally tested under pure and randomly generated vibrations. The time history of the response acceleration of the structure was recorded using a data acquisition system. Different output-only techniques of OMA, based on both frequency and time domains, were employed to analyse the recorded response acceleration of the structure and extract the natural frequencies, mode shapes, and damping ratios. These techniques are peak picking (PP), enhanced frequency-domain decomposition (EFDD), and stochastic subspace identification (SSI). The outcomes in this paper can be used for further research on the development of an experimental formula for the design of multistorey modular buildings against lateral loads.

Determination to the period of a school building before and after its retrofitting using ambient vibration records

Determining the dynamic parameters of the structures allows us to obtain a substantial amount of information about the state of the building. Recently, the determination of these parameters were performed through Structural Health Monitoring Systems (SHMSs), which are non-destructive methods. In scope of this study, the period values prior to and following the retrofitting of a school building, which are among the dynamic parameters, were examined. The Operational Modal Analysis (OMA) method, which is based on the principle of measuring the responses of ambient effects in the structure in the experimental study, has been utilized. For this purpose, response records taken from the building under the effect of ambient vibration were used. Before and after retrofitting, the records of the building under the effect of ambient vibration were taken from the same locations. 3 sensors were used to record ambient vibrations. The data from these sensors were transferred to a computer by using a data logger. In order to measure and evaluate the responses Enhanced Frequency Domain Decomposition Method (EFDDM) in the frequency domain and Stochastic Subspace Identification Method (SSIM) in the time domain were used. Following the analysis, a comparison of how the period values had changed was conducted.

Analytical, numerical, and experimental determination of dynamic characteristics of prestressed concrete girders

To determine the dynamic characteristics of bridges with prestressed girders, their natural frequencies and mode shapes used to construct the superstructure must be known. However, there is no agreement among scientists on how prestress force affects the dynamic characteristics of a precast prestressed girder. The purpose of this paper is to obtain the dynamic characteristics of prestressed concrete girders through analytically, numerically, and operational modal testing. For this purpose, one of a typical precast I-girder with 1.2 m height and 27.45 m effective span length is selected as a numerical application. The three-dimensional (3D) finite element model (FEM) of the girder is modeled by SAP2000. Experimental measurements of the girder were conducted at the construction site by the operational modal testing method. For experimental measurements, ten uniaxial seismic accelerometers were mounted at the top flange of the PSC girder in the x- and z-direction. The vibrations that occur from the movement of trucks and cranes at the construction site and the impact of the hammer were measured by these accelerometers as acceleration. The measured signal was collected at the data bank and then sent to the computer equipped with Operational Modal Analysis software which used Enhanced Frequency Domain Decomposition and Stochastic Subspace Identification techniques. The dynamic behaviors of girder were derived from analysis performed by this software. At the end of the study, the dynamic characteristic was obtained by the analytical prediction, numerical and experimental were compared with each other. It is seen that natural frequencies and mode shapes obtained from theoretical prediction, numerical and operational modal testing are not too far apart.

Structural Identification of a 90 m High Minaret of a Landmark Structure under Ambient Vibrations

This paper presents the operational modal analysis of a 90-m-high RC minaret of an iconic mosque considered as a landmark of the city. The minaret was monitored for three days with 11 tri-axial MEMS accelerometers. The purpose of the study was to observe the behavior, develop a representative finite element (FE) model, and establish baseline data for health monitoring studies. The modal properties were extracted using three operational modal analysis techniques (OMA): Enhanced Frequency Domain Decomposition (EFDD), Stochastic Subspace Identification (SSI), and Natural Excitation Technique with Eigensystem Realization Algorithm (NExT-ERA). The first 10 identified modes were below 7 Hz. Eight modes out of the ten were bending-dominant, while the remaining two were torsion-dominant. A FE model was also developed in ETABS to ascertain and compare the response of the structure with the identified results. From the FE model, the modes corresponding to the first ten identified modes were considered for comparison with the identified frequencies from ambient monitoring. The maximum 7.71% error was observed between the experimental and numerical frequencies. The error was minimized by using the manual updating the material properties and adding the weight of nonstructural elements. The variation of identified modal frequencies with ambient temperature was observed to be linearly dependent to a reasonable degree. A general trend of decreasing identified frequencies was observed with the rise in temperature.

Calibration of the numerical model of a freight railway vehicle based on experimental modal parameters

The simulation of the dynamic behavior of the train-track system is strongly dependent on the accuracy of the numerical models of the train and track subsystems. The use of calibrated numerical models of the railway vehicles, based on experimental data, enhances their ability to correctly reproduce the dynamic responses of the train under operational conditions. In this scope, studies involving the experimental calibration of freight wagon models are still scarce. This article aims to fill this gap by presenting an efficient methodology for the calibration of a numerical model of a freight railway wagon based on experimental modal parameters. A dynamic test was performed during the unloading operation of the train, adopting a dedicated approach which does not interfere with its tight operational schedule. From data collected during the dynamic test, five natural frequencies and mode shapes associated with rigid-body and flexural movements of the wagon platform were identified through the Enhanced Frequency-Domain Decomposition (EFDD) method. A detailed 3D finite-element (FE) model of the loaded freight wagon was developed, requiring precise knowledge of the vehicle design details which, in most situations, are difficult to obtain due to confidentiality reasons of the manufacturers. The model calibration was performed through an iterative method based on a genetic algorithm and allowed to obtain optimal values for seven numerical parameters related to the suspension’s stiffnesses and mass distribution. The stability of the parameters considering different initial populations demonstrated the robustness of the optimization algorithm. The average error of the natural frequencies decreased from 8.5% before calibration to 3.2% after calibration, and the average MAC values improved from 0.911 to 0.950, revealing a significant improvement of the initial numerical model.

High resolution operational modal analysis of Sant’Agata del Mugello in light of its building history

The building history of a cultural heritage building and the after-effects induced by extreme loads such as earthquakes have a durable impact on its modal parameters. This article aims to discuss the peculiarities of some modal parameters extracted from ambient vibration measurements in light of the complex history of Sant’Agata del Mugello. The Romanesque church located in the Mugello basin (Tuscany, Italy) suffered from many earthquakes, at least from 1542. In this context, we carried out two dense ambient vibration campaigns in March and June 2019. We extracted each mode’s natural frequency, mode shape, and damping using the Enhanced Frequency Domain Decomposition. We identified ten modes. Some modes highlight structural singularities related to the building history of the church: rocking of the base of the bell tower; phase opposition between the bell tower and the nave; bending of the walls of the nave. Since the instrumentation benefited from a vast number of recording positions (83), we tested the impact of the sensor placement strategy on the resolution of the identified modal parameters in the specific case of Sant’Agata del Mugello. The resolution of the higher-order modes decreases strongly in case of degraded sensor placement strategy impacting the dynamic analysis of the church.

INVESTIGATION OF EFFECT OF USING NANO COATING ON WOODEN SHEDS ON DYNAMIC PARAMETERS

In this article, the dynamic parameters (frequencies, mode shapes, damping ratios) of the uncoated wooden shed and the coated by silicon dioxide are compared using the operational modal analysis method. Ambient excitation was provided from micro tremor ambient vibration data on ground level. Enhanced frequency domain decomposition (EFDD) was used for output. Very best correlation was found between mode shapes. Nano-SiO2 gel applied to the entire outer surface of the red oak shed has an average of 14.54% difference in frequency values and 13.53% in damping ratios, proving that nanomaterials can be used to increase internal rigidity in wooden slabs. High adherence of silicon dioxide to wooden surfaces was observed as another important result of this study.

System Identification of a Volumetric Steel Modular Frame Using Experimental and Numerical Vibration Analysis

This paper presents the findings of an experimental and numerical study on the system identification of modular steel frames in a laboratory environment. The project is a part of a broad study about the structural response of modular steel buildings. For this purpose, an experimental program is conducted on a full-scale volumetric prefabricated steel module, which is designed based on Australian standards. The module is fixed on a strong floor, instrumented with sensitive accelerometers, and its ambient and free vibration responses are captured and recorded. The output-only algorithms of operational modal analysis procedure, that is, direct fast Fourier transform, enhanced frequency domain decomposition, and stochastic subspace identification, are employed for extracting the main dynamic properties of the module, including natural frequencies, mode shapes, and modal damping ratios. Then the numerical finite-element models of the module are generated in micro and macro levels with continuum solid and beam elements, respectively. The numerical models are verified with the results of the experimental program and the discrepancies are discussed. The findings of this study are be used as a basis for research on the fundamental behavior of steel module assemblies and their interaction for the design of multistory modular buildings.

Model Updating of a Freight Wagon Based on Dynamic Tests under Different Loading Scenarios

This article presents an efficient methodology for the calibration of a numerical model of a Sgnss freight railway wagon based on experimental modal parameters, namely natural frequencies and mode shapes. Dynamic tests were performed for two distinct static loading configurations, tare weight and current operational overload, under demanding test conditions, particularly during an unloading operation of the train and without disturbing its tight operational schedule. These conditions impose restrictions to the tests, especially regarding the test duration, sensor positioning and system excitation. The experimental setups involve the use of several high-sensitivity accelerometers strategically distributed along with the vehicle platform and bogies in the vertical direction. The modal identification was performed with the application of the enhanced frequency-domain decomposition (EFDD) method, allowing the estimation of 10 natural frequencies and mode shapes associated with structural movements of the wagon platform, which in some cases are coupled with rigid body movements. A detailed 3D FE model of the freight wagon was developed including the platform, bogies, wheelsets, primary suspensions and wheel–rail interface. The model calibration was performed sequentially, first with the unloaded wagon model and then with the loaded wagon model, resorting to an iterative method based on a genetic algorithm. The calibration process allowed the obtainment of the optimal values of eight numerical parameters, including a double estimation of the vertical stiffness of the primary suspensions under the unloaded and loaded static configurations. The results demonstrate that the primary suspensions present an elastic/almost elastic behaviour. The comparison of experimental and numerical responses before and after calibration revealed significant improvements in the numerical models and a very good correlation between the experimental and numerical responses after calibration.

An update on techniques to assess normal-mode behavior of rock arches by ambient vibrations

Abstract. Natural rock arches are rare and beautiful geologic landforms with important cultural value. As such, their management requires periodic assessment of structural integrity to understand environmental and anthropogenic influences on arch stability. Measurements of passive seismic vibrations represent a rapid and non-invasive technique to describe the dynamic properties of natural arches, including resonant frequencies, modal damping ratios, and mode shapes, which can be monitored over time for structural health assessment. However, commonly applied spectral analysis tools are often limited in their ability to resolve characteristics of closely spaced or complex higher-order modes. Therefore, we investigate two techniques well-established in the field of civil engineering through application to a set of natural arches previously characterized using polarization analysis and spectral peak-picking techniques. Results from enhanced frequency domain decomposition and parametric covariance-driven stochastic subspace identification modal analyses showed generally good agreement with spectral peak-picking and frequency-dependent polarization analyses. However, we show that these advanced techniques offer the capability to resolve closely spaced modes including their corresponding modal damping ratios. In addition, due to preservation of phase information, enhanced frequency domain decomposition allows for direct and convenient three-dimensional visualization of mode shapes. These techniques provide detailed characterization of dynamic parameters, which can be monitored to detect structural changes indicating damage and failure, and in addition have the potential to improve numerical models used for arch stability assessment. Results of our study encourage broad adoption and application of these advanced modal analysis techniques for dynamic analysis of a wide range of geological features.

Beam mode characterization by applying operational modal analysis to neutron detectors data

Operational Modal Analysis techniques are widely used in civil, aerospace and industrial applications. There are several methods and one of the most utilized is Enhanced Frequency Domain Decomposition (EFDD). In the nuclear sector, part of this method was applied to analyze simulated neutron detectors data with promising results in the framework of CORTEX project. Given the possibilities of this methodology, in this work, EFDD was fully implemented to characterize the Beam Mode frequency region in plant data from two KWU reactors. The results show that closely spaced resonances can be separated and that three modes of vibration are found. The mode shapes obtained for each resonance give information on the physical origin of the vibration and with them it was proved that the dominant resonance in the Beam Mode region is not connected to a pendular motion.