Operational Modal Testing Research Articles

Implementation of phase controlled impact device for enhancement of frequency response function in operational modal testing

This paper studies how to enhance frequency response function (FRF) estimation in the presence of harmonic disturbances during operational modal testing. A novel technique which utilizes impact-synchronous time averaging (ISTA) called impact-synchronous modal analysis (ISMA) was introduced where modal analysis can be performed in the presence of ambient forces. The phase angle information of the harmonic signal at the impact events is shown to be a key factor in enhancing the effectiveness of this technique. However, lack of knowledge and control of impact with respect to the phase angle of the disturbances using conventional impact hammer in ISMA has limited the effectiveness and practicality of this novel technique. A portable and automated phase controlled impact device is introduced in the effort to eliminate non-synchronous components with minimal possible impacts applied. This device makes use of the feeding phase angle information of responses from the cyclic load back to the device and imparts the impact at the correct time/phase which is always non-synchronous with respect to the phase of response from cyclic load. A reduced number of averages thereby expedite the overall modal testing procedure, an improved of FRF estimation and a good correlation of modal extraction data with benchmark data shown in this study has highlighted the advantages of ISTA using the proposed device.

Experimental static and dynamic tests on a large-scale free-form Voronoi grid shell mock-up in comparison with finite-element method results

Grid shells supporting transparent or opaque panels are largely used to cover long-spanned spaces because of their lightness, the easy setup, and economy. This paper presents the results of experimental static and dynamic investigations carried out on a large-scale free-form grid shell mock-up, whose geometry descended from an innovative Voronoi polygonal pattern. Accompanying finite-element method (FEM) simulations followed. To these purposes, a four-step procedure was adopted: (1) a perfect FEM model was analyzed; (2) using the modal shapes scaled by measuring the mock-up, a deformed unloaded geometry was built, which took into account the defects caused by the assembly phase; (3) experimental static tests were executed by affixing weights to the mock-up, and a simplified representative FEM model was calibrated, choosing the nodes stiffness and the material properties as parameters; and (4) modal identification was performed through operational modal analysis and impulsive tests, and then, a simplified FEM dynamical model was calibrated. Due to the high deformability of the mock-up, only a symmetric load case configuration was adopted.

Scaling of mode shapes from operational modal analysis using harmonic forces

This paper presents a new method for scaling mode shapes obtained by means of operational modal analysis. The method is capable of scaling mode shapes on any structure, also structures with closely coupled modes, and the method can be used in the presence of ambient vibration from traffic or wind loads, etc. Harmonic excitation can be relatively easily accomplished by using general-purpose actuators, also for force levels necessary for driving large structures such as bridges and highrise buildings. The signal processing necessary for mode shape scaling by the proposed method is simple and the method can easily be implemented in most measurement systems capable of generating a sine wave output. The tests necessary to scale the modes are short compared to typical operational modal analysis test time. The proposed method is thus easy to apply and inexpensive relative to some other methods for scaling mode shapes that are available in literature. Although it is not necessary per se, we propose to excite the structure at, or close to, the eigenfrequencies of the modes to be scaled, since this provides better signal-to-noise ratio in the response sensors, thus permitting the use of smaller actuators. An extensive experimental activity on a real structure was carried out and the results reported demonstrate the feasibility and accuracy of the proposed method. Since the method utilizes harmonic excitation for the mode shape scaling, we propose to call the method OMAH.

Experimental and numerical serviceability assessment of a steel suspension footbridge

The paper summarizes the main results of the serviceability assessment of a steel suspension footbridge, performed through dynamic testing and numerical simulations.The experimental part of the study involved both operational modal testing and measurement of the structural response under the crossing of different groups of pedestrians. The footbridge exhibited quite complex dynamic characteristics (i.e. two couples of closely spaced modes and five modes in the frequency range 1.9–3.0Hz) and the maximum accelerations induced by pedestrians and joggers turned out to be in the range of discomfort. An accurate FE model, based on the design drawings, was then developed and a general procedure is proposed to tackle the crucial issue of assigning the design tension forces to the suspension elements.Since the comparison between numerical and experimental results generally shows a good agreement, the model is adopted to perform a numerical assessment of vibration serviceability according to the European guideline HiVoSS. A minor shortcoming of HiVoSS and the unusual relevance of the 2nd harmonic of pedestrian-induced load are highlighted for the investigated footbridge.

Structural and seismic monitoring of the “Cardarelli” Hospital in Campobasso

Recent earthquakes occurred in Italy and throughout the world have once again emphasized the critical role of health facilities for post-earthquake emergency management. Due to earthquake, several hospitals (L’Aquila 2009, Amatrice 2016) have lost their functionality because of damage to structural as well as non-structural members, equipment and installations.The primary role of hospitals after hazardous events requires the development of specific analysis and monitoring strategies aimed at quickly assessing their health conditions. Turning a hospital into a "Smart Health Facility" (SHF) allows remote assessment of the structural health; moreover, it can effectively support the mitigation of administrative and organizational vulnerability by acting on preparedness of the medical staff and supporting management and maintenance of structural as well as non-structural subsystems over time. Continuous monitoring of health and performance of hospitals can support the formulation of disaster mitigation plans and the definition of investment priorities to ensure the overall safety.In the present paper, some results of an on-going monitoring project for the main hospital in Campobasso (Southern Italy) are presented, focusing the attention on the response of the structure in operational conditions and after seismic events. Moreover, the results of experimental as well as operational modal analysis tests on a drug dispenser are discussed in order to assess their applicative perspectives for qualification of medical equipment in view of their seismic vulnerability assessment as non-structural elements.

Significant improvement in structural features, mechanical and physical properties of a novel CAR processed Al foam by nano-SiCp addition

A brief outline was prepared to study the effect of nano-SiCp reinforcement on the structural features, mechanical and physical properties of the continual annealing and roll-bonding (CAR) processed aluminum foam. Nanoindentation, drop weight impact and operational modal tests were applied to evaluate mechanical and physical properties of the nanocomposite foam. Scanning electron microscopy and elemental distribution images were utilized to analyze the results. Findings showed that the application of nano-SiCp results in matrix strengthening and remarkable improvements in dynamic compressive properties (strength increase of 48%), energy absorption (35% increase) and damping behavior (77% increase in damping ratio) of the nanocomposite foam.

Investigations on the structural behaviour of archaeological heritage in Peru: From survey to seismic assessment

The conservation of archaeological heritage is of major importance for preserving the scientific, ethnographic and artistic values of past cultures. Once archaeological sites are exposed after being buried for centuries, they are subjected to natural hazards, which should be studied with up-to-date techniques. Moreover, conservation works are primarily focused on aesthetic aspects or on solving localized problems. In earthquake-prone areas, it is of extreme importance to carry out structural analysis studies for assessing the actual behaviour of archaeological constructions, and for proposing adequate intervention measures. This paper presents an extensive study on structural behaviour of archaeological building remains in Peru, based on in-situ non-destructive testing as well as on numerical approaches. The case of the Chokepukio Archaeological Site is presented, which was built between 1000 and 1450 AD in the Pre-Columbian era, with a mixed masonry of stone units and earthen mortar. The paper begins with a comprehensive description of the historical, architectural and structural aspects of the archaeological site. The possibility of applying operational modal analysis tests is then explored with reference to a representative wall of Chokepukio. The results of the experimental field campaign are used to develop calibrated finite element models of the wall, and to indirectly estimate mechanical characteristics of the masonry. Basing on the investigations performed, potential failure mechanisms are identified for the wall and validated by pushover analysis. Finally, the mechanisms are evaluated through kinematic limit analysis, to proceed with the seismic assessment.

Finite-Element Model Calibration of Historical Masonry Domes Using Operational Modal Testings

AbstractThis paper aims to determine the dynamic characteristics, such as natural frequencies, mode shapes and damping ratios, of two different domed structures, namely Hamza Pasa Mausoleum and Kavak Meydan Fountain, built in the sixteenth century by experimental methods. The experimental dynamic characteristics are used to develop initial analytical models and update these models. Analytical models of the structures are constituted using finite element theoretical software, and dynamic characteristics are determined analytically. The experimental measurements are carried out by operational modal testing under environmental loads, which are wind and traffic loads, and the dynamic characteristics are identified experimentally. Vibration responses are collected from the dome’s outer surface for Hamza Pasa Mausoleum and the dome pulley’s inner surface for Kavak Meydan Fountain. Measurement time, frequency span, and effective mode number are determined by considering pretest measurement results. At the end of...

A two-stage fast Bayesian spectral density approach for ambient modal analysis. Part II: Mode shape assembly and case studies

A two-stage fast Bayesian spectral density approach based on a novel variable separation technique for ambient modal analysis was formulated in the companion paper. In full-scale operational modal tests covering a number of locations, the dofs of interest are usually measured or processed separately in individual setups so that a set of local mode shapes are obtained. The difficulty on how to assemble these local mode shapes to form overall mode shapes is a problem not addressed in the companion paper that needs to be resolved properly. This study presents a theory to assemble the local mode shapes using the Bayesian statistical framework so that the data quality of different clusters can be accounted for automatically. The optimal global mode shape can be obtained by a fast iterative scheme, while the associated uncertainties can be derived analytically. The theory described in Part I and II of this work is applied to modal identification using synthetic data and field data measured from two laboratory models equipped with wireless sensors. Successful validation of the proposed method demonstrates the potential for Bayesian approaches to accommodate multiple uncertainties for ambient modal analysis.

Monitoring the dynamic characteristics of an urban bridge before, during and after widening

The dynamic characteristics of a multi-span pre-stressed concrete urban bridge before, during and after widening operations have been studied. The widening operations have been carried out by adding two new bridges on the either sides of the old bridge. The decks of the new bridges have been structurally separated from the old one by two longitudinal joints on the either side of the old bridge. The dynamic characteristics have been extracted from operational modal tests in three phases during different stages of the widening operations. The dynamic interaction between the new parts and the old part of the bridge has also been investigated. The results show that after the completion of the widening works, and when the whole bridge was placed in service condition, this interaction affects the natural frequencies of almost all the vibration modes of the bridge. It was also found that the longitudinal joints were unable to effectively isolate the new parts from the old part. Finally, in order to achieve a consistent behaviour in the first bending mode of the new and old bridges, a geometrical index has been introduced. This index includes both the effects of stiffness and mass properties of the bridge deck. In widening projects, where the span lengths, material properties and support conditions of the two bridges are taken the same, adopting equal geometrical indices leads to almost equal values of the natural frequencies and similar mode shapes in the first bending mode. The proposed index can be used in the initial design stage for such projects.

Frequency Response of Laminated Glass Elements: Analytical Modeling and Effective Thickness

Laminated glass elements are sandwich structures where the glass presents linear-elastic behavior, whereas the polymer interlayer is, in general, a linear-viscoelastic material. Several analytical models have been proposed since the 1950s to determine the response of laminated glass elements to both frequency and thermal conditions. In this paper, it is proved that Ross, Kerwin, and Ungar's model can be considered as a particular case of the Mead and Markus model when the exponential decay rate per unit length is neglected. The predictions of these models are compared with those obtained from operational modal tests carried out on a laminated glass beam at different temperatures. Finally, a new effective thickness for the dynamic behavior of laminated glass beams, which allows the determination of the dynamic response using a simple monolithic elastic model, is proposed.

Operational modal testing of historic structures at different levels of excitation

The paper focuses on the dynamic tests of two historic structures, performed in operational conditions at different levels of ambient excitation. The tests were aimed at checking the invariance of the dynamic characteristics (i.e. natural frequencies and mode shapes) with increasing excitation amplitude.A masonry bell tower, dating back to the XVI century, is firstly considered and the modal parameters of key vibration modes were identified from the data collected in two ambient vibration tests: in the first test, the excitation was only provided by micro-tremors and wind, whereas in the second test a higher level of excitation was induced by swinging of bells.The second structure is a well-known iron arch bridge, erected in 1889 and still used as a combined road and rail bridge. Within the dynamic tests performed on the historic infrastructure since June 2009 to address a Structural Health Monitoring program, the bridge response at selected points was continuously recorded for several hours, in order to check the variation of the dynamic characteristics under different traffic conditions.

Vibration Response Analysis and Experiment on 2.0 MW Wind Turbine Gearbox

In order to get a reasonable working condition, using lumped parameter method and (Finite element method)FEM, a coupling dynamic model of the wind turbine gearbox is established. Analyze the natural characteristics and dynamic response in different conditions, and experiment study using operational modal test is carried out. Comparing the experimental data with the FEM result, vibration of the gear box in the rated power condition is the most stable, while vibration becomes violent when overload, it matches well with the other one, it is verified that the model is reasonable. This paper provides theory support for wind turbine gearbox design and analyze.

Operational Modal Test of an Arch Bridge

A modal test was carried out on a 120m span concrete filled steel tubular arch bridge under operational conditions. The moving traffic load is considered as uncorrelated white noises at various points of the road surface and the test is of the type of multi-input multi-output (MIMO). The modal parameters are identified by Global Rational Fraction Polynomials (GRFP) method, a frequency domain method. In order to taking the mobile nature of the traffic load into consideration, a multi-input NExT method is utilized to retrieve the frequency response function needed by GRFP. A proof of the NExT method in the case of multi-input case is also provided in this paper.

Operational Modal Test of an Arch Bridge

A modal test was carried out on a 120m span concrete filled steel tubular arch bridge under operational conditions. The moving traffic load is considered as uncorrelated white noises at various points of the road surface and the test is of the type of multi-input multi-output (MIMO). The modal parameters are identified by Global Rational Fraction Polynomials (GRFP) method, a frequency domain method. In order to taking the mobile nature of the traffic load into consideration, a multi-input NExT method is utilized to retrieve the frequency response function needed by GRFP. A proof of the NExT method in the case of multi-input case is also provided in this paper.

Operational modal testing and FE model tuning of a cable-stayed bridge

The paper presents the results obtained from the application of different output-only modal identification techniques to ambient response data collected in two dynamic tests of a cable-stayed bridge and the subsequent finite element model correlation analysis. The first test, performed by using a traditional data acquisition system with servo-accelerometers, was aimed at investigating the vertical dynamic characteristics of the bridge. In the second test, an innovative radar vibrometer was used for non-contact measurement of deflection time series of the forestays and the identification of local natural frequencies of the stay cables. In the theoretical study, vibration modes were determined using a 3D Finite Element model of the bridge and the information obtained from the field tests, combined with a classic system identification technique, provided a linear elastic model, accurately fitting the modal parameters of the bridge in its present condition.

Traditional and operational modal testing of masonry vaults

This paper discusses the practical and technical differences between the usual form of experimental modal analysis (EMA) and operational modal analysis (OMA), as applied to Gothic-style masonry vaulted structures. In situ vibration measurements were collected from the masonry fan vaults of the National Cathedral in Washington, DC, USA. Instrumented impact hammer excitation was used during the experimental traditional modal analysis (TMA) tests, while excitation by peal bells, carillon bells, organ, an orchestra and chorus, and ambient noise were exploited for OMA. The acoustic inputs of the first four excitation types were measured using a microphone and were found to approximate a uniform acoustic excitation across the frequency bandwidth of interest during the carillon and the orchestra and chorus excitations. For both TMA and OMA, system identification was completed by the least-squares complex exponential method. The natural frequencies were found to be consistent for all excitation sources and modal test techniques. Certain natural frequencies found by TMA, however, were omitted from the sequence of frequencies as determined in the OMA. This paper illustrates that OMA is a suitable alternative to global vibration analysis of a masonry building, especially when there are concerns about the use of a hammer or shaker for this purpose.

Seismic Response of a Historical Masonry Minaret using a Finite Element Model Updated with Operational Modal Testing

This paper presents the earthquake response of a historical masonry minaret after a finite element model updating was undertaken using the information from full scale ambient vibration testing. The Iskenderpaşa historical masonry minaret dating back to the 16th century with a height of 21m located in the city center of Trabzon, Turkey is selected as an application. Analytical modal analysis is performed on the 3D finite element model of the minaret considering field survey and engineering judgments to obtain the analytical frequencies and mode shapes. The field ambient vibration tests on the minaret under natural excitations such as wind loading and human movement are conducted. The Peak Picking and the Stochastic Subspace Identification techniques are used to extract the modal parameters from the ambient vibration test. A good correlation was found among the modal parameters identified from the two techniques. The finite element model of the minaret is updated to minimize the differences between analytically and experimentally estimated modal properties by changing some uncertain modeling parameters such as material properties and boundary conditions. The analytical model of the minaret after finite element model updating is analyzed using the 1992 Erzincan earthquake record, which occurred near the area, to determine the earthquake behavior of the minaret. At the end of the study, maximum differences in the natural frequencies are reduced on average from 27% to 5% and a good agreement is found between analytical and experimental natural frequencies and mode shapes by model updating. Also, it is seen from the earthquake analysis that the displacements increase along the height of the minaret and the maximum and minimum principal stresses occur at the region of the transition segment and the cylindrical body.

On the estimate of the FRFs from operational data

In this paper, the effects of different mass loadings required for the estimation of the frequency response functions, FRFs, from data gained by the emerging technique of operational modal testing, is proposed. This technique allows the evaluation of the natural frequencies, mode shapes and damping ratios from operational data achieved from a first session of tests, then the scaling factors are derived from a further experimental investigation. The approach is based on the sensitivity of the eigenproperties to structural modifications, such as the mass and stiffness distribution. It is shown that the generalized modal parameters could be derived by the measurements of the natural frequency shifts due to a controlled mass variation in the structure, assuming negligible changes in the mode shapes. Such generalized modal parameters are finally used to estimate the FRFs. This mode shape scaling technique, together with the investigation of the effects of the mass positioning on the uncertainties in the estimates of the scaling factors will be experimentally investigated on simple aerospace structures.

Operational Modal Analysis of Aluminum Beams

Natural frequencies obtained by modal analysis are important to engineers interested in predicting the dynamic behavior of structures. Traditional modal analysis involves impact testing or shaker testing, where response signal and input force are measured to obtain the transfer function. However, for large structures, input excitation force measurement may be difficult, if not impossible. Large structures may be subjected to ambient excitation; operational modal analysis (OMA), also known as output-only modal analysis, has been used for extracting modal parameters of these types of structures. The main advantage of operational modal analysis is that no artificial excitation is needed, and the analysis is based on measurements of only the output data of the system. Operational modal analysis tests are performed under the actual operating conditions of the system without any change of boundary conditions; the tests use the ambient loads as input and thus do not interfere with the normal functioning of the system. In this study, six aluminum beams of different configurations (beams with and without cuts of various lengths) were used for conducting experiments. Results based on impact test, shaker test, and operational modal analysis are presented.