Results Of Operational Modal Analysis Research Articles

Impact of sound-insulated joints in the dynamic behavior of Cross-Laminated Timber structures

Cross-Laminated Timber (CLT) has gained an increasing success and widespread diffusion among wood products. Due to the novelty of the components and the dry-assembling techniques, certain aspects of its behavior and performance remain unexplored; among these, the use of sound-insulated joints and brackets, along with their possible drawbacks on mechanical performance.In this paper the authors discuss the results of dynamic identification (ID) tests carried out on twin full-size CLT mockups (two-story), one of which was furnished with sound-insulation details based on an elastomeric interlayer. Ambient (white noise and random excitations) and forced vibration (mass shaker) tests were carried out to identify modal parameters.The paper mainly aims at: i) assessing potential differences in the overall dynamic behavior of the two mockups, likely due to the introduction of acoustic insulation, ii) detecting possible drawbacks of insulation layers on mechanical performances, iii) increasing the knowledge about dynamics of CLT structures.The results of Operational Modal Analysis (OMA) are presented and the variations in the dynamics between the mockups are discussed. The construction of finite element (FE) models and their calibration – with joint properties variations – are then presented. The outcomes revealed a drop of modal frequencies values (at least 20 %) upon the introduction of elastomeric insulation, whilst no significant alteration was observed in the mode shapes. Model updating based on the FE models demonstrated that the dynamic response and the frequency variations were mostly associated with wall-to-wall and wall-to-diaphragms joints, and hold-down connections.

Model-assisted clustering for automated operational modal analysis of partially continuous multi-span bridges

Long multi-span bridges represent a broad section of the civil roadway infrastructure. Despite being so common, their condition-based maintenance through vibration-based Structural Health Monitoring (SHM) has been scarcely investigated in the literature. The dynamic identification of such structures through Operational Modal Analysis (OMA) is especially challenging due to of their quasi-periodic nature and the common existence of weak inter-span coupling. Even when designed following an isostatic scheme, there always exists a certain degree of coupling between spans due to the continuity of the deck, the pavement and imperfect expansion joints. Hence, the modal poles of the spans typically appear as dense clusters with closely spaced frequencies and mode shapes with similar wavelengths, which significantly hinders the identification of physical poles through stabilization diagrams. In this light, this paper proposes a model-based machine learning approach to conduct and interpret the OMA results of partially continuous multi-span bridges. The proposed method is a hierarchical clustering approach that leverages on the analytical solution of the vertical free vibration response of multi-span girders with weak inter-span rotational coupling, allowing the estimation of the modal features of any bridge configuration ranging from simply supported to perfectly continuous conditions. Detailed parametric analyses and discussions are presented to appraise the correlation between the inter-span rotational coupling and the clustering of the modal poles of multi-span bridges, as well as the influence of damage conditions of varying severity and extension. On this basis, a model-based cut-off distance threshold for hierarchical clustering of stable poles is proposed to assist the automation of the global OMA of multi-span bridges. The developed formulation is tested in a real-world in-operation seven-spans reinforced concrete girder bridge, the Trigno V Bridge in Italy.

Advances in intelligent long-term vibration-based structural health-monitoring systems for bridges

The true realization of the benefits of vibration based structural health monitoring (VBSHM) in real-world applications is acquired through long-term continuous monitoring so that one can attain a detailed grasp of the behavior of the monitored structure. The challenges in long-term continuous VBSHM include: the large volume of accumulated monitoring data; the effective extraction of engineering information amid the influences of noise and uncertainties embedded in the monitoring data; maintaining continuity and consistency in the long-term monitoring data considering that the system and instrumentation may change due to sensor failure or renewal due to advances in sensing technologies. To meet these challenges, this paper presents recent research that has resulted in the development of a framework and specialized signal processing and data analytic tools for long-term continuous VBSHM suitable for real-world monitoring applications of structures in the field. These include efficient tools for large scale intelligent data processing and analysis, management of monitoring database and extracted information relevant to the structural health of the monitored structure. The novel Automated In-Line Full Space Identification (AI-FSI) method is presented to address the needs and challenges associated with long-term continuous VBSHM, such as the automation of all data processing and analysis operations including modal parameter estimations and mode tracking, and the need of minimizing the measurement and computational uncertainties and variability in the operational modal analysis results. A smart self-diagnostic system for the monitoring of the health of the data collection sensors and monitoring system has also been developed that will allow the consistent use of the monitoring data of different sensor configurations and era in the monitoring project. Examples on the efficiency of analyzing the monitoring data collected over 20 years from the Confederation Bridge monitoring project in Atlantic Canada by using the developed novel framework and data analytic tools are presented.

Optimal sensor placement and model updating applied to the operational modal analysis of a nonuniform wind turbine tower

Abstract. Test planning is a crucial step in the operational modal analysis (OMA) of wind turbines (WT), and it is an essential part of choosing the best positions for installing the sensors of the structures. On the other hand, updating the finite element model (FEM) with the OMA results implies a better prediction of the real structure's dynamic and vibrational behavior. This paper aims to show how the OMA of a nonuniform and two-section wind turbine tower can be performed more effectively, using the required test planning and optimal sensor placement. Then, accordingly, the OMA is used in operating and parked conditions to find the objective bending mode characteristics. Moreover, the updating of the applicable FEM of the multi-sectional wind turbine tower will be described. The tailor-made genetic algorithm (GA) is used to find the MEMS (micro electro-mechanical system) sensors' optimal positions of the WT under study. The OMA was performed and the acquired data analyzed using the stochastic subspace identification (SSI) method. Based on the OMA results, the FEM is updated by applying the sensitivity method. The results show that a tailor-made GA is a practical and quick approach to finding the optimal position of the sensors to obtain the best results for the objective modes of the WT. The OMA results, under operating and parked conditions, prove some modal characteristics of WTs. Based on the sensitivity analysis and engineering judgment, the modulus of elasticity was selected as a parameter for updating. Finally, we found that the updated FEM had less than 1 % error compared to the obtained frequencies from the test.

Dynamic analysis of Sitar: A comparative study of operational and experimental modal analysis

Dynamic analysis of Sitar, an Indian string instrument, is important for better understanding of the instrument behavior during performance. Sitar has complex geometry, and most of its components have anisotropic material properties, which generate a lot of challenges in performing numerical modal analysis. Considering this, an experimental approach of operational modal analysis (OMA) is performed on Sitar to extract its natural frequencies using the Stochastic Subspace Identification method. Hammer or shaker excitation required for conventional experimental modal analysis (EMA) has huge limitations of using harder hammer tips and high magnitude force as the instrument is delicate. However, to validate OMA results, EMA is performed with extreme care using an instrumented hammer with soft tip and with a very low excitation force. PolyMAX algorithm is used in EMA. It is observed that most of the correlated OMA and EMA modes lie in the audible frequency range. The maximum absolute percent error observed for these frequencies is 2.14%. All the modes obtained in OMA are significant as the string excitation simulate close to the real-life performing situation. Most of these modes map to musical note frequencies. Considering the detrimental effect of excitation required for EMA, OMA is a recommended method for extracting modal characteristics of Sitar.

Operational modal analysis and finite element model updating of a 230 m tall tower

Dynamic response levels are critical for tall and slender civil structures. Studying the dynamic behavior of large civil structures with finite element modeling techniques requires detailed and accurate modeling of structural geometry, material properties, member fixities, connection types, and accompanying assumptions. Still, the finite element model results are approximations that could be away from representing the actual structural behavior. Structures are dynamically tested at their operational conditions to validate the finite element model results. This paper presents Operational Modal Analysis (OMA) and finite element model updating of a tall structure located in the West Bay area of Doha (Qatar). The structure is a reinforced concrete building with shear wall cores situated towards the center of the building plan, which was constructed between 2012 and 2016. With 53 stories above the ground and two stories below ground, the 230 m (755 ft) tall building is being used for residential and hotel purposes. For the finite element model updating and calibration tasks presented in this paper, the authors intentionally introduced drastic model changes for the first two model updates so that the results from the first two attempts guide how to proceed with a more reasonable update for the third calibration of the finite element model. While this is a non-standard technique that represents a specific condition where the initial attempts on the finite element model are very crude approximations, it is a systematized demonstration of how to operate when the structural parameters are sparse or uncertain for modeling purposes. While in theory, the finite element model updates can always be fine-tuned in a way to further decrease the error between the measured and predicted OMA results, in this paper, the authors predominantly focused on the presentation of three finite element model updates to demonstrate the way they have improved the modal assurance criteria plots and lowered the average absolute errors by visiting two drastic and then one moderate finite element model updates. The material presented here in this paper is arguably the first published work on large-scale dynamic testing of a civil structure in the State of Qatar.

Structural dynamic analysis of a musical instrument: Tibetan singing bowl

Operational Modal Analysis (OMA) of Tibetan singing bowl is performed to extract natural frequencies and mode shapes without measuring excitation data. It is kept free on a rigid surface, which is a common way of playing this musical instrument. OMA results are validated using Experimental Modal Analysis (EMA) and Numerical Methods using FEA. Numerical simulations using ANSYS® software establishes a benchmark for EMA results. The input and response data for 144 response points are collected using instrumented hammer and accelerometer, connected to a four-channel FFT analyser. A self-generated MATLAB® code processes the response signals for EMA and OMA. For natural frequencies, the absolute error lies within 6%, except for the first mode. For mode shapes, the Modal Assurance Criteria (MAC) value is more than 70%, except for the fourth mode. Thus, OMA is the best available method compared to the EMA and Numerical method using FEA for structural analysis under actual performance conditions.

Case study: The Harbor Bridge in Bratislava

AbstractBratislava Harbor Bridge is an important part of Slovak highway and railway network. The authorities of the bridge (Railways of the Slovak Republic and National Motorway Company) cannot afford to close it. In the case of closure, traffic in Bratislava would be paralyzed, as up to 100,000 vehicles pass the bridge daily. Because of that, only dynamic tests during operation are possible, but the mentioned companies do not possess a sophisticated measurement system for structural health monitoring purposes. Therefore, a preliminary proposal of a measuring system has been prepared, in accordance with the fact that the bridge is one of the most complex structures in Slovakia. An initial and subsequent validated and verified FEM model have been used for numerical calculations. Time history analysis has also been used for verification and validation of the FEM model. This article also presents a comparison of current results of operational modal analysis and results of numerical analysis. It includes the recommendations for the fib Model Code 2020 (MC2020) as well.

Stress Response and Damage Characteristics of Local Members of a Structure due to Tunnel Blasting Vibrations Based on the High-Order Local Modal Analysis

Damage characteristics and dynamic stress response of aging masonry structures for blast-induced ground motion were performed using high-order local modal analysis method. A complete investigation of damage types and locations of aging masonry buildings due to tunnel blasting vibration were performed by on-site survey. A typical 2-storey aging masonry building located above a tunnel was selected for dynamic response analysis. The experimental dynamic characteristics of the structure were determined by using the operational modal analysis (OMA) method. Finite element models for the masonry structures were updated by modifying material parameters based on OMA results. The first five natural frequencies of the updated finite element models ranged from 8.80–24.99 Hz, and the first five modes were global modes. The sixth to twentieth natural frequencies ranged from 26.10–36.34 Hz, and the sixth to twentieth modes were local modes whose deformation was greater than the global deformation. Since the principal frequencies of the tunnel blast vibration were mostly higher than the natural global modes’ frequencies and were much closer to the natural frequencies of local members, local members experienced more intensive vibrations compared to the main body structure. The principal compressive stress (PCS) and principal tensile stress (PTS) of local members were several times greater than that of the main body structure. Therefore, local members of the masonry building suffered most from the tunnel blasting vibration. Corners due to stress concentration, the contact area between brick and concrete, local members, and precast floor seams are prone to damage during tunnel blasting. With the vibration velocity increasing, the PCS and PTS of local members gradually increase. But, the PTS ratio of local members decreases with the increase of peak particle velocities. The dynamic response analysis result and the damage locations using high-order local modal analysis method are in accordance with the damage found at the site.

Full structural dynamic response from ambient vibration of Giotto's bell tower in Firenze (Italy), using modal analysis and seismic interferometry

The preservation of historical buildings mainly depends on our capacity to monitor their ageing and quickly detect potential damage. We show how the dynamic characteristics of a building (i.e., frequency, shapes mode and seismic wave velocity) can be automatically identified and tracked in real-time by combining modal analysis with seismic interferometry. The assessment of these characteristics is performed on Giotto's bell tower in Firenze, Italy, by installing a temporary network of seismic sensors. The natural frequencies and modal shapes of the structure are calculated by enhanced frequency domain decomposition (EFDD). The impulse response functions estimated by following a waveform deconvolution approach are analysed to investigate the seismic wave dispersion in Giotto's bell tower. We show that Giotto's bell tower is a dispersive structure affected by a bending deformation, with a weak contribution from soil-system interaction. Combining the results of operational modal analysis (OMA) and seismic interferometry from the same ambient noise recording provides a unique and complete dynamic response of important monuments such as Giotto's bell tower.

Frequencies and damping ratios of bridges through Operational Modal Analysis using smartphones

Seismic Force Assessment in structural design of bridges requires a good estimation of the fundamental period. Estimated periods, using current methodologies in seismic provisions, are approximations that may significantly differ from the real periods. These differences are related to assumptions and simplifications made in the model development. Through Operational Modal Analysis (OMA) it is possible to estimate operational frequencies of a bridge and provide evidence about its current dynamic behavior. In this paper, vertical operational frequencies of 451 structures, 285 pedestrian bridges and 166 vehicular bridges, and horizontal operational frequencies of 111 structures, 101 pedestrian bridges, 9 vehicular bridges and 1 railway bridge of the transportation network of Santiago de Cali, Colombia, were estimated using ambient vibration tests through OMA. Data were obtained using smartphones and processed using NExT-ERA and SSI techniques. For a sample of thirteen bridges, the results were compared with OMA results using seismic sensors. The results show that the smartphones used are appropriate as instrumentation systems for modal identification of flexible structures. The experimental frequencies identified using both time-domain techniques are similar among them and to equations proposed in technical literature. The damping ratios associated to the identified operational frequencies do not show relation with the geometric characteristics of the bridges or a clear difference between materials. Equations relating fundamental frequencies with structural characteristics of the bridges are presented.

Operational modal analysis and fatigue life estimation of a chisel plow arm under soil-induced random excitations

Chisel plow is an important implement for primary tillage when the soil is dry and firm. In this research, fatigue life of a prototype of chisel plow arm under soil induced random excitation is analyzed. For this purpose, first, the field test is carried out on the chisel plow arm under soil-induced random excitations and operational modal analysis of the chisel plow arm is performed using stochastic subspace identification approach. In addition, a finite element model is constructed and successfully validated by stochastic subspace identification results. Then, the validated finite element model and power spectral density results of the measured random responses are used to achieve a process of the Mises stress in the arm needed for fatigue analysis. Finally, the fatigue life estimation of chisel plow arm is carried out in frequency domain using Wirsching-Light and Dirlik methods for different values of geometrical variables. The results of operational modal analysis demonstrate the possibility of identifying modal model of chisel plow arm in its operating condition on a basis of output-only data and the results of fatigue life estimation show that Wirsching-Light method provides a marginally safer prediction for chisel plow.

Effects of Structural Parameters on Seismic Behaviour of Historical Masonry Minaret

The October 23, 2011 (Mw = 7.2) and November 9, 2011 (Mw = 5.6) earthquakes increased the damage in the minaret of Van Ulu Mosque, an important historical masonry structure built with solid bricks in Eastern Turkey, resulting in significant shear cracks. It was found that since the door and window openings are not symmetrically placed, they result in unsymmetrical stiffness distribution. The contribution of staircase and the core on stiffness is ignorable but its effect on the mass is significant. The pulpit with chamfered corner results in unsymmetrical transverse displacements. Brace wall improves the stiffness however contributes to the unsymmetrical behaviour considerably. The reason for the diagonal cracks can be attributed to the unsymmetrical brace wall and the chamfered pulpit but the effect of brace wall is more pronounced. After introducing the cracks, a new model was created and calibrated according to the results of Operational Modal Analysis. Diagonal cracks were found to be likely to develop under earthquake loading. Drifts are observed to increase significantly upon the introduction of the cracks.

A complete methodology for identifying dynamics of heavy machine tool through operational modal analysis

Heavy machine tool work under such high-load conditions that chatter vibrations are prone to occur, which significantly diminishes the machining efficiency and quality. Stability lobe diagrams are commonly used to select appropriate spindle speed and axial depth of cut to get rid of chatter and maximize the material removal rate. However, this needs precise identification of the dynamics of the entire machine tool structure, especially in the low-frequency range. Operational modal analysis has been the proven technique for estimating dynamic characteristics of machine tool structures in operation conditions. In this article, a complete methodology was presented for employing operational modal analysis for heavy machine tool in machining conditions. A random cutting exciting method originally presented by Minis is modified which generates pseudorandom impulse force to excite a heavy vertical lathe structure. And the excitation signal of random cutting force was modeled to analyze the effect of cutting parameters on energy and frequency band of the excitation. One operational modal analysis method, the pLSCF (referred to as PolyMAX) method, was employed to estimate modal parameters during machining. It was also observed in chatter tests that the operational modal analysis results are more accurate than the traditional impact test results in characterizing the dynamics of machine tool structure in machining.

Operational Modal Analysis of Weir on Odra River in Poland

The paper presents the results of Operational Modal Analysis (OMA) of a weir on Odra river in Poland. The immediate reason for the dynamic analysis was the occurrence of clearly perceivable vibration of the weir in time of the high water level and big water discharge of the Odra river. The modal analysis was performed experimentally, using OMA. The experimental research and measurements were performed when the level of water in river was rather low. The research will be continued when the high water level in river occurs in nearest future.

Applications of Operational Modal Analysis to a Single-Phase Distribution Transformer

Previous studies have shown that a transformer's modal characteristics can be identified by its internal electromagnetic excitations and are related to structural damage. These may be useful for diagnosing the causes of failure modes in power transformers during normal operation. Operational modal analysis (OMA) appears to be a suitable tool for this purpose. In this letter, the feasibility of OMA in identifying transformer modal parameters is discussed with a special focus on the mechanisms of natural excitation. OMA is also applied to a 10-kVA transformer with and without artificially designed structural damage. Good correlation is obtained between the results of OMA and those of experimental modal analysis.

Online Simultaneous Reconstruction of Wind Load and Structural Responses—Theory and Application to Canton Tower

AbstractThe actual wind load information is helpful in evaluating the health status of high‐rise structures. However, as a type of distributed load, the wind load is very difficult to be measured directly. A possible solution is to reconstruct it from the structural response measurements. This is often an ill‐posed inverse problem. In this article, such ill posedness is solved by using a stable input estimator. With the help of the proposed application‐oriented algorithm selection guidance, a type of state and input estimator is formulated. This type of estimator is designed based on the Kalman filter scheme, and is capable of estimating the unknown inputs and the system states within one sampling time. This actually facilitates the online simultaneous reconstruction of the wind load and the structural responses. The 600 m tall Canton Tower is situated in a typhoon active area, and a structural health monitoring system has already been integrated onto this tower. These two points make the Canton Tower an ideal test bed for validating the above illustrated online reconstruction strategy for the wind load and the structural responses. An operational modal analysis (OMA) is first performed to identify the modal properties of the Canton Tower under the Typhoon Nanmadol in 2011. Then a reduced‐order finite element model (FEM) of the Canton Tower is updated according the OMA results. Finally, the equivalent fluctuating lateral loads and moments, which act on the nodes of the reduced‐order FEM are reconstructed using the acceleration measurements recorded during Tyhoon Kai‐tak in 2012. The reconstruction results are validated by comparing the simultaneously reconstructed structural acceleration with the corresponding sensor measurements. The mean component of the loads and moments are calculated using the real‐time wind speed measurements and the available aerodynamic force coefficients. It is noted here that the focus of this article is not to develop a totally new theory, but rather to explore the application of a state and input estimator in the foreground to a practical complex structure.

New Operational Modal Analysis Method of Spindle System Based on Multiple Pulse Excitation

Modal parameters of the spindle system is an important basis for the prediction of processing stability and parameters optimization. Aiming at the difficulty in the implementation of effective incentive for the spindle system, a new operational modal analysis(OMA) method based on multiple pulse excitation is brought forward, pulse excitations are respectively applied on the tool, toolholder and spindle, response and cross-correlation functions of vibration signals are achieved and modal parameters are identified through the poly-reference least square complex exponential(pLSCE) method. False modes derived from the recognition process is removed to improve the accuracy of modal parameters identification by the modal assurance criterion method. Comparative analysis shows that the maximum relative error of the natural frequency and damping ratio is 4.57% and 11.02% respectively and the OMA results are in good agreement with the hammer modal test results. Results indicate the modal parameters of spindle system can be accurately identified by the cross-correlation functions of response signals under multiple pulse excitation, the work provide a new method for OMA of spindle system.

Impacts of Epistemic Uncertainty in Operational Modal Analysis

AbstractField experimentation on constructed systems demands consideration of many mechanisms of epistemic and aleatory uncertainties as well as human errors and subjectivity. This is especially true in operational modal analysis (OMA) applications that aim to identify the dynamic properties of a structure. Although statistics and probability theory are sufficient for quantifying aleatory uncertainty and bounding the resulting errors in OMA results, there is much debate as to whether the same tools may also be used to quantify epistemic uncertainty. This study explored a framework for better understanding the distinctions and impacts of these two types of uncertainties in OMA and how human errors and subjectivity may be classified. A physical laboratory model was designed to simulate four key sources of epistemic uncertainty that represented the primary test variables: structural complexity (changing boundary conditions, nonlinearity), ambient excitation characteristics (magnitude, directionality, and ban...

Evaluation of blast effects on reinforced concrete buildings considering Operational Modal Analysis results

Seismic protection of buildings under risk can be achieved by increasing the knowledge about the behaviour of existing structures. Operational Modal Analysis is a powerful tool used for this purpose all over the world. It provides the dynamic characteristics of structures under operational conditions or some particular environmental issues such as blasts and earthquakes. The main objective of this study is to evaluate blast effects on a reinforced concrete (RC) building considering experimentally determined dynamic characteristics. The study consists of three phases: the measurement of vibration characteristics of blasting, the theoretical modal analysis of the inspected building, and experimental verification of dynamic characteristics using modal testing. The vibration characteristics of blasting are measured around the inspected building on hard soil using a geophone set. The initial analytical model of the building is constructed according to the in-situ investigation on building. The theoretical modal analysis results are verified by carrying out modal testing on the RC building. The Operational Modal Analysis method is used for the extraction of the dynamic characteristics of the building, and blast vibrations are taken into account as environmental vibrations. The effects of blasting on the reinforced concrete building are introduced by assessing the vibration of blasting with the dynamic characteristics of the building.