Ambient Vibration Tests Research Articles

An efficient approach to model updating for a multispan railway bridge using orthogonal diagonalization combined with improved particle swarm optimization

In this paper, a novel approach to model updating for a large-scale railway bridge using orthogonal diagonalization (OD) coupled with an improved particle swarm optimization (IPSO) is proposed. Particle swarm optimization (PSO) is a well-known and widely applied evolutionary algorithm. However, as other evolutionary algorithms (EAs), PSO has two main drawbacks that may reduce its capability to tackle optimization issues. A fundamental shortcoming of PSO is premature convergence. On the other hand, since PSO employs all populations to seek the best solution through iterations, it is very time-consuming. This makes PSO as well as EAs difficult to apply for optimization problems of large-scale structural models. In order to overcome those drawbacks, we propose coupling OD with IPSO (ODIPSO). OD is applied to arrange the position of particles and to select only particles with the best solution for next iterations, which helps to reduce the computational cost dramatically. There are several significant features of ODIPSO: (1) IPSO is employed to tackle the problem of premature convergence of PSO; (2) only one guide is used to update the velocity of particles instead of utilizing both guides, consisting of the local best and the global best; and (3) in each iteration, only the velocity and the position of the best particles are updated. In order to assess the effectiveness of the proposed approach, a large-scale railway bridge calibrated on the field is employed. This paper also introduces the use of wireless triaxial sensors (replacing classical wired systems) to obtain structural dynamic characteristics. The appearance of wireless triaxial transducers increases significantly the freedom in designing an ambient vibration test. The results show that ODIPSO not only outperforms PSO, IPSO and OD combined with PSO (ODPSO) in terms of accuracy, but also dramatically reduces the computational time compared to PSO and IPSO.

Vibration properties monitoring and uncertainty quantification of precast and cast‐in‐situ shear wall buildings using a Bayesian framework

Precast buildings have been becoming popular in China in these 5 years since they have many merits such as effectiveness, safety, high quality, and environmental friendliness. In some cities of China, it is required to make the proportion of precast buildings account for 30% in newly built construction areas. Following the rapid development, their vibration behaviors are attracting more attention since it may be quite different from the cast-in-situ building. To investigate the difference, this paper studies the modal properties of two kinds of buildings, i.e., new-type precast building and cast-in-situ building. They were built in the same area, while the structural forms are also similar. The main difference is the construction manner. Ambient vibration tests of these two structures were conducted respectively. Since the number of sensors is limited, the locations of interest were measured by a multiple-setup technique. The difficulty encountered during the field test was discussed. A frequency domain Bayesian approach was used to conduct operational modal analysis. Both the most probable value and posterior uncertainties of modal parameters were studied, including translational, torsional, and wrapping modes. The difference of the modal parameters of the two kinds of buildings are compared and discussed. Finite element models were also built to perform a comparison. It is seen that although the structural forms and appearances of the two buildings are similar, some obvious differences were observed regarding the dynamic characteristics, which provide valuable information for popularizing and future assessment of the new-type precast structure.

A new full‐scale testbed for structural health monitoring and soil–structure interaction studies: Kunming 48‐story office building in Yunnan province, China

This paper introduces a new and unique full-scale testbed site for structural health monitoring and soil–structure system identification studies being developed in southwest China. The site is a 48-story skyscraper with an extended four-level basement, supported by piles, located in Kunming, the capital of Yunnan Province. Located in the diffused zone of collision of the Indian and Eurasian tectonic plates, Yunnan is one of the most active seismic areas in mainland China. The final sensor deployment will consist of 43 triaxial accelerometers (129 channels) and one weather station. The accelerometer array comprises (a) a structural array of 25 accelerometers installed at 10 levels aboveground, (b) a basement array of 14 accelerometers distributed in the first and fourth basements, and (c) two borehole arrays installed close to the basement perimeter wall, each with one accelerometers at the surface and another one at 50-m depth, which is the depth reached by the piles. With such dense instrumentation of structure, basement, and pile foundation, this site will be the first permanently instrumented full-scale testbed to enable identification of a soil-foundation–basement-structure system and validation of many assumptions commonly made in the prediction of the soil–structure interaction effects. A high-performance wired local area network has been installed in the building, featuring a Precision Time Protocol-enabled time synchronization and real-time remote access over the Internet. The site will be fully operational in late spring of 2020. Results of preliminary system identification of the structure from ambient vibration test data are presented.

Empirical formulation for estimating the fundamental frequency of historical stone mosques with masonry domes

SummaryHistorical stone mosques with masonry dome are complex structures due to their different structural elements, material properties, boundary conditions, connections, etc. It is very difficult to develop the numerical models representing the real behaviour of these structures. Therefore, their numerical models should be checked with ambient vibration tests for the reliable safety evaluations before the restoration applications. However, it is known that the ambient vibration tests of historical mosques are very difficult and costly and need permissions from the owners and relevant government agencies. The paper aims to develop an empirical formulation for estimating the fundamental frequency of historical stone mosques with masonry dome to check the initial finite element models. Nine stone mosques with masonry dome in different dimensions in the eastern Black Sea Region of Turkey were selected as examples for this purpose. By performing ambient vibration tests of the selected mosques, experimental natural frequencies, mode shapes, and damping ratios have been determined and evaluated together. An empirical formulation for the fundamental frequency and coefficients for the second and third frequencies of stone mosques with masonry dome based on vibration data are obtained by using the statistical regression analysis and verified with the control data. The comparative results confirm that the proposed formulation has a reliable predictive performance.

System identification from ambient vibration measurements using output-only modal identification techniques

Frequency domain decomposition (FDD) method and Random decrement technique are proposed to analyze ambient vibration data. An ambient vibration test was conducted on an 18-story reinforced concrete building, and the proposed methods were applied to estimate modal parameters. The estimated natural frequencies by the FDD method and the RD technique are in good agreement, and comparisons for damping values estimated by the EFDD method and the RD technique show a 72-246 percent difference. The results imply that the proposed methods can accurately evaluate structural dynamic properties from ambient vibration data in a non-destructive manner.

Modalidentifícation of a building structure by frequency domain decomposition

Frequency domain decomposition (FDD) and Enhanced frequency domain decomposition(EFDD) methods were adopted to estimate modal parameters of an18-story reinforced concrete building. Ambient vibration test was conducted on the 18-story reinforced concrete building and the two modal identification methods above were applied. The results show that FDD and EFDD methods can estimate modal parameters accurately and effectively from ambient vibration data.

Near-fault vertical ground motion effects on the response of balanced cantilever bridges

Post-tensioned balanced cantilever reinforced concrete (RC) bridges allow the economic spanning of wide spans using fewer columns than in more traditional bridges. More and more post-tensioned balanced cantilever bridges are constructed every year all round the world. The aim of this study was to investigate numerically the effects of near-fault vertical ground motions (VGMs) on the seismic behaviour of segmentally constructed, balanced cantilever RC bridges. Two long bridges constructed in Artvin, Turkey – the three-span Budan Bridge and the two-span Sengan Bridge – were selected for the application. Three-dimensional finite-element models (FEMs) of the bridges were created to obtain their dynamic characteristics such as natural frequencies and mode shapes numerically. The frequencies calculated from the FEMs of the bridges were then verified using experimental frequencies measured from ambient vibration tests. Acceleration records of near-fault earthquakes (1992 Erzincan (M = 6·69), 1999 Kocaeli (M = 7·51), 1999 Düzce (M = 7·14) and 1989 Loma Prieta (M = 6·93)) were selected for the seismic analyses. The seismic behaviours of the verified FEMs of the bridges with two and three spans were determined by considering horizontal, vertical and combined horizontal and vertical components of the earthquakes. It was found that near-fault VGM significantly affects the seismic response of segmentally constructed, balanced cantilever bridges.

Lessons from Structural Analysis of a Great Gothic Cathedral: Canterbury Cathedral as a Case Study

ABSTRACT Damage in Gothic cathedrals can occur due to single extreme events or long-term processes, often associated with large deformations and cracking. This paper presents the damage survey and structural assessment of a UNESCO world heritage site and one of the oldest, most visited and most known Christian temples in the UK: Canterbury Cathedral, in Kent. Inspection and damage survey showed repetitive crack patterns on the vault’s intrados, with more severity in the south aisle, including outward rotating movement and cracking in the south flying buttresses. Ambient vibration tests were carried out to identify the structure modal properties. An FE model of a typical transversal section of the nave and lateral aisles was prepared, and calibrated based on the tests. Nonlinear static analyses were performed, considering the main parameters of influence to the structure, as the construction process, the infill volume, the presence of lateral thrusts from the nave’s roof and differential settlements. A validated FE model, presenting sufficient correlation with the existing damage was used to determine the safety factors for lateral and vertical loading, which according to the local hazard are considered adequate. A comparison with limit analysis, based on the static approach, was also carried out.

Optimal Sensor Placement for Laminated Composite and Steel Cantilever Beams by the Effective Independence Method

In modal testing, the quality of vibration signals and clarity of peak points to extract the natural frequencies, corresponding mode shapes and damping ratio depend on the number and location of the sensors. High measurement costs are required for structural identification and long-term structural health monitoring in large structures, which require a high number of sensors. Therefore, the minimum number of sensors should be placed at appropriate locations on the system during experimental measurements to ensure both information of sufficient quality and cost reductions. The aim of this study is to perform cost-efficient non-destructive modal tests for a laminated composite and steel cantilever beams using an optimal sensor placement approach. Finite element models of the sample beams are constituted in ANSYS® software to determine the initial candidate set of sensor locations. Then, ambient vibration tests are conducted. Based on the experimental modal amplitudes and mode shapes, optimal sensor locations are determined using the effective independence method, and measurements are repeated. The study shows that there is good agreement between the natural frequencies and mode shapes obtained from the initial measurements and those obtained using the limited number of sensors.

Ambient vibration test and modelling of historical timber mosques after restoration

This paper reports on a study to determine the structural dynamic characteristics, such as natural frequencies, mode shapes and damping ratios, of timber mosques using ambient vibration tests after restoration. Kuşluca historical timber mosque in the Sürmene district of Trabzon, Turkey is selected as a case study. The mosque was built in the early twentieth century with two floors. Restoration projects were prepared in 2008 and the mosque was re-opened in 2011. A finite-element model of the mosque was constituted based on restoration drawings, and modal analysis was performed using orthotropic material properties. A literature review was used to determine the initial dynamic characteristics, such as natural frequencies, mode shapes and damping ratios. Non-destructive experimental measurements were conducted after construction to validate the numerical results using in situ tests. The first three natural frequencies were obtained numerically and experimentally. There was a close agreement between mode shapes, but 30% difference in natural frequencies. To minimise the differences, the finite-element model was updated using manual and automated updating procedures, changing uncertain parameters such as material properties, semi-rigid element connections, boundary conditions and section properties.

Bayesian inference for parameters estimation using experimental data

A finite element (FE) model is developed for a curved cable-stayed footbridge located in Terni (Umbria Region, Central Italy) which accounts for uncertainties in geometry, material properties, and boundary conditions as well as limited knowledge on the behavior of connections and other components. Ambient vibration tests (AVTs) are carried out to identify the main dynamic parameters which are used for model updating in the Bayesian framework. Sensitivity analysis is performed to identify the main mechanical parameters affecting natural frequencies and mode shapes to be used as updating parameters. Finally, the posterior probability distributions of the selected updating parameters is estimated and used to assess the accuracy of the FE-based model. The importance of using a proper informative reference data set in the updating framework is assessed using different observations together with the importance of reliable surrogate models able to reduce the computational costs related to the whole framework.

Modal identification of Bridge 44 of the Carajás Railroad and numerical modeling using the finite element method

Abstract Regular use and the effects of time can affect the behavior of a structure. Over time, problems such as the occurrence of small fissures, oxidation of steel elements, and excessive displacements at some points may arise in a structure. In this context, the monitoring of structures through experimental tests has gained more importance, because it allows for the identification of the dynamic characteristics (natural frequencies, mode shapes, and damping rate) of structures. The dynamic characteristics can be obtained through forced vibration tests, which are based on measuring the response of a structure subjected to an excitation of known magnitude, or through tests in which only the structural response is measured, such as free vibration and ambient vibration tests. The present study aims to identify the modal parameters of bridge 44 of the Carajás Railroad, using experimental data obtained on site by monitoring the vibration caused by a group of people jumping, and it compares them with the results obtained through numerical modeling performed using the finite element method, developed in CSiBridge. The modal parameters were obtained using the commercial software ARTeMIS Modal, and stochastic subspace identification was used for modal identification.

Output-only damage diagnosis for plan-symmetric buildings with asymmetric damage using modal flexibility-based deflections

Effective methods for vibration-based damage diagnosis in building structures are based on the estimation of structural deflections from identified modal flexibility (MF) matrices. However, most of the existing methods and previous studies that use MF-based deflections were developed by considering the case of buildings that can be modeled as planar structures (e.g. plan-symmetric buildings) both in the baseline and possibly damaged states. In an attempt to fill this research gap, a deflection-based method for output-only damage diagnosis in building structures tested under ambient vibrations is proposed in this paper. The method is applicable for simple rectangular plan-symmetric “box type” 3D building structures which may experience asymmetric damage, and it can be used for detecting, localizing and quantifying the damage. One of the main differences between the proposed and the existing methods is related to the inspection loads adopted to estimate the MF-based deflections. The inspection loads of the proposed method are more complex than the uniform translational loads adopted in the existing methods. Such proposed loads in fact can be assembled only after having estimated the position of the center of stiffness for each story of the structure. According to the proposed approach, the locations of the centers of stiffness are extracted from the experimentally-derived modal flexibility matrices, before performing the damage diagnosis process. The effectiveness and validity of the proposed method was demonstrated both through numerical analyses and by executing experimental ambient vibration tests on a frame building. The results of the proposed method were also compared with the existing approach which was developed for planar structures. This comparison (made both through numerical and experimental analyses) demonstrated that only the proposed approach can be used to perform a correct damage diagnosis process in plan-symmetric buildings which may experience asymmetric damage, especially when considering the damage quantification problem.

Fem Model Calibration of Experimental Environmental Vibration Tests on Two Churches Hit by L’Aquila Earthquake

ABSTRACT The paper deals with the seismic investigation of two masonry churches, namely Visitazione and San Giuliano, which are located in Poggio Picenze, a small town about 10 km far from L’Aquila. The seismic response assessment of the churches, which were damaged by the severe seismic event occurred in Italy on 2009 April 6th, has been performed by means of experimental dynamic tests and numerical analyses. Firstly, an experimental campaign based on Ambient Vibration Tests (AVTs), which are non-destructive tests especially applied to historical and monumental buildings for evaluating their modal response, has been carried out on the selected constructions in the framework of a cooperation activity between the Ss. Cyril and Methodius University of Skopje and the University of Naples “Federico II”. Such an experimental activity has allowed to obtain the dynamic properties of the examined structures, namely natural frequencies, vibration mode shapes and damping coefficients. The experimental results have been subsequently used to implement a numerical FEM model of the churches in the ABAQUS non-linear analysis software environment. Dynamic linear FEM analyses have been conducted in order to calibrate the experimental results in order to simulate as well as possible the real structural behaviour of inspected churches. Finally, the performed analyses have been of a fundamental importance for both detecting damages into the constructions and programming effective retrofitting interventions.

An innovative continuous Bayesian model updating method for base-isolated RC buildings using vibration monitoring data

Base isolation is the most widespread passive control strategy for seismic protection of low- and medium-rise Reinforced Concrete (RC) buildings. The excellent performance of base-isolated structures just after construction is well recognized but their seismic behavior after a few operation years can be significantly modified, due to isolators’ aging. In this perspective, the availability of long-term vibration data allows to eventually assess the evolution of degradation and plan maintenance or replacement actions. This paper presents an innovative continuous Bayesian model updating method specific for base isolated buildings. In the proposed method, the Bayesian functions of the isolation system are periodically updated to account for daily measured data. The results of a comprehensive study on a base isolated three-stories RC school building, located in Bojano (Italy), are presented. The investigation comprises Ambient Vibration Testing (AVT), modal identification, installation of a continuous monitoring system and calibration of a Finite Element Model (FEM) of the isolated structure. In order to reduce the computational cost of the model updating procedure, a surrogate model is constructed and calibrated using the complete FEM of the structure through the Response Surface Methodology (RSM). The advantage of having long-term monitoring data on the results of the Bayesian model updating for base isolated structures is highlighted.

Dynamic monitoring of bridges during static load tests: influence of the dynamics of trucks on the modal parameters of the bridge

This paper presents results of an extensive experimental campaign carried out during the construction phases of a base-isolated bridge across the Potenza river in central Italy. The structure is constituted by a half-through steel arch bridge composed by two coupled weathering steel tubular arches sustained by thirty couples of hangers and by two approaching viaducts. The bridge has a continuous twin-girder steel–concrete composite deck. During construction, impact load tests were carried out on hangers as well as on other tie elements to estimate their tension force from the relevant resonance frequencies. During the proof test, relative displacements occurred between the upper and lower plates of the seismic isolators are measured for different loading conditions. In addition, ambient vibration tests are performed during the final static load tests to investigate the bridge dynamics subjected to different loading conditions. Comparisons of test results with the numerical predicted values contributed to evaluate the consistency between the design and the real structure at different construction stages. Dynamic identification tests contributed to identify the significance of the truck–bridge interaction in the interpretation of the bridge dynamics evaluated experimentally.

Case study: Post-earthquake model updating of a heritage pagoda masonry temple using AEM and FEM

This paper presents a linear applied element method (AEM) modeling approach for investigating the effects of the Mw 7.8 2015 Gorkha Nepal earthquake on a historical Pagoda temple. The impact of the earthquake is also investigated using linear FEM models and the results from both modeling approaches are compared. The numerical models are developed and calibrated using pre- and post-earthquake data collected from ambient vibration testing, lidar scans and data processing, and visual damage assessment. The numerical results indicate that the linear AEM models are as reliable as the finite element method (FEM) models to predict the modal response of the case study temple. In addition, the AEM models can potentially be further developed to estimate the damage from future earthquakes through nonlinear analyses. The present study not only informs the present health status of the masonry temple, but it also estimates how acceleration at the base of the temple is propagated through the height of the temple through comparison of floor response spectra, which could provide valuable insights into the force distribution over the height of the temple for the design of future retrofits.

Image‐based back analysis for tension estimation of suspension bridge hanger cables

In suspension bridges, hanger cables are the main load-supporting members. The tension of the hanger cables of a suspension bridge is a very important parameter for assessing the integrity and safety of the bridge. In general, indirect methods are used to measure the tension of the hanger cables of a suspension bridge in use. A representative indirect method is the vibration method, which extracts modal frequencies from the cables' responses and then measures the cable tension using the cables' geometric conditions and the modal frequencies. In this study, ambient vibration tests were conducted on a suspension bridge in use to verify the validity of the image-based back analysis method, which can estimate the tension of remote hanger cables using the modal frequencies as a parameter. The tension estimated through back analysis, which was conducted to minimize the difference between the modal frequencies calculated using finite element analysis of the hanger cables and the measured modal frequencies, was compared with that measured using the vibration method. It was confirmed that reliable tension estimation is possible even with low-order modal frequencies when the image-based back analysis method is used.

Identification of Structural Matrices of Shear Buildings Using Ambient Vibration Tests with Incomplete Measurements

System identification methods (SIMs) are powerful tools for structural health monitoring. SIMs are used either for identification of modal parameters or for extraction of structural characteristic matrices. The current study aims to identify the real matrices of a structure using structural dynamics theory and stochastic subspace identification based on realization theory. The study focuses on extraction of the condensed mass and stiffness matrices of shear buildings by means of ambient excitation responses for limited structural degrees of freedom. Realization theory states that there are minimal realizations for appropriate identification of the main real-system matrices. The present study shows that, by using values smaller than the minimal realization, condensed structural matrices can be correctly identified. This is accomplished by accurate estimation of the full real-system order. It is shown that successive repetitions of this approach can lead to complete structural matrices. The practical application of this procedure is examined using an experimental model and an analytical six-story shear model. Analysis revealed that, even in the presence of noise, this method can accurately identify structural matrices in all cases with high precision.

Earthquake-induced damage localization in an historic masonry tower through long-term dynamic monitoring and FE model calibration

This paper presents an enhanced version and the validation of a recently proposed methodology for earthquake-induced damage detection and localization in masonry towers by using long-term vibration monitoring data. The proposed enhanced method is based on continuous operational modal analysis through stochastic subspace identification, dynamic multiple linear regressive analysis to remove the effects of changing environmental conditions and finite element (FE) model updating. Any anomalous frequency deviation from normal conditions resulting from an earthquake triggers the damage localization process. This task is performed by solving an inverse FE model calibration problem, where equivalent elastic properties of macrostructural elements are identified by minimizing an objective function considering experimentally identified and numerically predicted damage-induced decays in natural frequencies and changes in eigenvector components. To minimize the computational effort of this calibration procedure, a quadratic surrogate model is constructed using a tuned numerical FE model of the structure. The methodology is validated through application to the “Sciri Tower”, an historic civic masonry tower located in Perugia, Italy, that has been continuously monitored by the authors for more than 1 year. The validation is carried out by using simulated damage scenarios and a set of real far-field earthquake data and is based on a FE model of the tower including surrounding buildings, calibrated on the basis of the measured data from ambient vibration tests. The results demonstrate that the proposed procedure is capable of correctly detecting and localizing earthquake-induced damages.