Dynamics of Civil Structures, Volume 2

This paper presents the results obtained from the ambient vibration measurements done on a 20 story tall building, with reinforced concrete core, located in Vancouver, British Columbia, Canada. The experiment reveals the dynamic characteristics of the investigated building by advanced system identification methods using enhanced signal processing techniques and the fundamentals of frequency domain decomposition. The results include the natural frequencies and the mode shapes of the building obtained from the ambient vibration measurements. The dynamic characteristics of interest in this study are the lateral and torsional natural frequencies and the corresponding mode shapes. A total of 11 modes of vibration, up to the fourth translational and torsional modes, were successfully identified. This paper uses the Enhanced Frequency Domain Decomposition (EFDD) and the Curve-fit Frequency Domain Decomposition (CFDD) methods to identify the modes and utilizes the Frequency Domain Operating Deflection Shapes (FDODS) technique for modal validation. The experimental results were then compared to the analytical estimations from the ETABS models of the building created at the time of the structural design phase and model validation and calibration is performed.

The dynamic characteristics of a structure after the completion of its construction phase can be determined using ambient vibration measurement techniques. A newly constructed 30 stories tall reinforced concrete building in Burnaby, British Columbia, Canada, is tested using this technique. The experimental results reveal the dynamic characteristics of the investigated building including the building rocking and sliding at the foundation level. The method used is the advanced system identification using enhanced signal processing techniques based on the fundamentals of frequency domain decomposition. The paper expands on the technique, from the basics of the test setups to the details of the analysis.

A total of 15 natural periods of vibration are successfully identified including the translational and torsional modes (up to mode 5). The modal estimation is performed using the techniques such as the Enhanced Frequency Domain Decomposition (EFDD), the Curve-fit Frequency Domain Decomposition (CFDD), and the Frequency Domain Operating Deflection Shapes (FDODS); and the outcomes are compared against each other. The modes found from the analytical ETABS models of the building at the structural design phase are then compared and calibrated against the obtained experimental results.

This paper presents the results of ambient vibration tests performed on the 4-storey reinforced concrete Health Sciences Parkade at the University of British Columbia in Vancouver, Canada. Two tests were conducted in March 2014 under different operational conditions: for the first test, a detailed ambient vibration test, was conducted during normal operating conditions while in the second test, the data was acquired when the Parkade was almost empty, but only at a few locations.

The comparison between the results of the two tests shows that the frequency values are quite similar, even if the levels of vibration during the first test were much higher than those from the almost empty case. The vertical component of vibration was found to be predominant motion, showing a strong amplification of the vibrations due to traffic. The results also show that the “added” mass due to the presence of the cars did not significantly affect the modal frequencies.

The Frequency Domain Decomposition (FDD) and the Stochastic Subspace Identification (SSI) were used to estimate the dynamic parameters of the structure. A total of six modes were identified within the frequency range of 2–8 Hz, and all showed a significant 3D response. The natural frequencies and mode shapes from each technique have been compared and the similarities and differences are further discussed in the paper.

Over the last decade, Blind Source Separation (BSS) has evolved as a powerful tool for system identification of flexible structures. Several numerical and experimental studies have been proposed to demonstrate its effectiveness in dealing with noisy full-scale data. The author has recently developed a suite of methods that enhance the capabilities of BSS in addressing issues associated with decentralized implementation, autonomous processing, nonstationary excitations, and the presence of narrowband excitations in ambient vibration measurements. The basic idea of the algorithms proposed by the author is to cast the problem of identification within the framework of underdetermined BSS invoking sparsifying transforms. The resulting partial mode shape coefficients are combined to yield complete modal information. The transformations are undertaken using Stationary Wavelet Packet Transform (SWPT), yielding a sparse representation in the wavelet domain. Both numerical and experimental studies demonstrate the potential of these methods. The speaker will introduce this suite of methods and some examples where these methods have successfully been applied.

Non-contact measurement of the response of vibrating structures may be achieved using several different methods including the use of video cameras that offer flexibility in use and advantage in terms of cost. Videos can provide valuable qualitative information to an informed person, but quantitative measurements obtained using computer vision techniques are essential for structural assessment. Motion Magnification in videos refers to a collection of techniques that amplify small motions in videos in specified bands of frequencies for visualization, which can also be used to determine displacements of distinct edges of structures being measured. We will present recent developments in motion magnification for the modal identification of structures. A new algorithm based on the Riesz transform has been developed allowing for real-time application of motion magnification to normal-speed videos with similar quality to the previous computationally intensive phase-based algorithm. Displacement signals are extracted from strong edges in the video as a basis for the data necessary for modal identification. Methodologies for output-only modal analysis applicable to the large number of signals and short length signals are demonstrated on example videos of vibrating structures.

An increasing number of structures, such as pedestrian bridges, are affected by excessive vibration when they are dynamically excited. The development of new materials and improvements in guidelines have given rise to designs with longer spans and slender structural elements, which in turn cause such structures to be more susceptible to vibration problems. However, serviceability guidelines do not address the changes of the dynamic properties of pedestrian bridges nor do they accurately predict their dynamic response. In order to improve the structural analysis and design, a new computational tool is developed using an intercommunication platform between MATLAB and SAP2000 application programming interface (API) to consider different human walking models, including user-defined models, directly on the footbridge to take into account human-structure interaction effects. Details of the application are presented in this paper and results of the analysis for some walking models and their effects on the dynamic parameters of the bridge are given.

The interaction between human and structure can produce significant dynamic effects. This has been demonstrated in several occasions including the closing of the Millennium bridge in London shortly after being open to traffic. Models based on springs, dampers and lumped masses have been widely accepted by the scientific community to model the human in human-structure interaction (HSI) problems. Recently, models of the human body based on control theory have been proposed. This paper provides a comparison between two traditional models using spring, dampers and lumped masses and those using control theory. The models are updated in a probabilistic sense using Bayesian inference. The experimental data used for the comparison is obtained from a laboratory test structure specially designed for HSI studies.

Pedestrians may cause vibrations in footbridges and these vibrations may potentially be annoying. This calls for predictions of footbridge vibration levels and the paper considers a stochastic approach to modeling the action of pedestrians assuming walking parameters such as step frequency, pedestrian mass, dynamic load factor, etc. to be random variables. By this approach a probability distribution function of bridge response is calculated. The paper explores how sensitive estimates of probability distribution functions of bridge response are to some of the decisions to be made when modelling the footbridge and when describing the action of the pedestrians (such as for instance the number of load harmonics). Focus is on estimating vertical structural response to single person loading.

Already on the opening day of the Lardal Bridge in 2001 were large lateral vibrations observed. These excessive vibrations were seen as relatively dense and continuous flow of people was crossing the bridge. This type of observation has later been made for several other bridges, old as well as new. These observations have initiated an extensive investigation program quantifying structural properties such as natural frequencies, mode shapes and damping and their influence on the pedestrian induced vibration. Thus, the pedestrian lateral load phenomenon has equally been thoroughly investigated. As part of the pedestrian load investigation at the Lardal Bridge were a large number of time series with different sized groups of people recoded. This paper aims to re-analyze these data with the emphasis on the energy transfer between modes. It is clear from these response recordings that the lateral displacement is often not initially initiated, rather vertical or torsion motions. This is also true for several crossings of larger groups of pedestrians. This raises some interesting questions of how and when the pedestrian induced energy transfers between modes. From short time Fourier transforms (STFT) it can be seen that energy transfers after certain levels of response are reached. Interestingly, the horizontal mode, a reversed pendulum motion, includes a small vertical component with twice the frequency of the horizontal mode. The investigation explores the system sensitivities to group size, initially triggered response modes and the pacing frequencies of the pedestrian motion.

Many civil engineering structures are occupied by humans, and often humans are considered as a static load in calculations. However, active humans on structures can cause structural vibrations. Passive humans might also be present on that structure and they do change the structural system (such as structural damping and therefore also structural vibration levels). The paper addresses this subject and explores implications of having passive humans present on the structure. In experimental tests with a laboratory floor it is examined to which degree the posture of humans passively sitting on the floor influences the damping added to the floor. A numerical case study explores how passive humans may influence vibration levels of a floor.

The monitoring for measuring structural behavior has been advanced. Recently, many researchers have studied on the structural health monitoring using the GNSS (Global Navigation Satellite System). This paper presents temperature behavior for a Gwang-An bridge which is three span suspension bridge in Korea. The behavior of a middle span have analyzed on ambient temperature change. As the time passed from January to June in 2013, the vertical displacement was decreased and the temperature was increased more and more at the middle span. And the correlation analysis was performed between the temperature and the vertical displacement using the thermometer and GNSS. Also monthly changes of the temperature and natural frequency had been measured. And then the correlation analysis was performed between the temperature and natural frequency. As a result of the evaluation regarding thermal effect at the middle span, relationship between the temperature and natural frequency seemed to have trend of inverse proportion.

This paper studies the maximum total load effects of short span portal frame railway bridges when traversed by high-speed trains. It is generally assumed that in single span bridges the maximum stresses, displacements and accelerations occur at the mid-span section. However, this in not necessarily correct and the maximum might be located in a wide area around the mid-span. This study aims to quantify the underestimation of the mid-span assumption when calculating maximum load effects. A numerically validated 2D Vehicle-Bridge Interaction model is used to analyze the stresses, displacements and accelerations that develop during the passage of high-speed trains. These load effects are studied along the full length of the structure and compared to the maximum obtained at the mid-span section. Particular attention is given to the resonant speeds near the operational speeds of high-speed railways. The results show that significantly higher load effects can be expected. The presented study is the preliminary work for deciding on the optimum configuration of empirical field tests.

The Hardanger Bridge is the longest suspension Bridge in Norway and among the top 10 longest suspension bridges in the world. A comprehensive monitoring system was installed after it was completed in August 2013. The monitoring system is designed to provide data that can be used to verify the numerical methods used to predict wind induced dynamic response of slender bridges located in complex terrain. The monitoring system is outlined in this paper together with preliminary analysis of the accuracy of the model used to describe the self-excited forces acting on the bridge deck. Extensive wind tunnel testing was performed in the design of the Hardanger Bridge to achieve an excellent aerodynamic behaviour of the cross-section of the bridge deck. The experimental results of the aerodynamic derivatives that describe the self-excited forces have been combined with a finite element model of the bridge to predict the in-wind natural frequencies and damping ratios of the combined structure and flow system. The numerical predictions have been compared to results obtained from measured data using data-driven and covariance-driven stochastic subspace identification. It is concluded that the model for the self-excited forces provides in-wind frequencies and damping ratios that corresponds well to the observations from measured data.

The Norwegian Public Roads Administration is currently planning a ferry-free Coastal Highway Route E39. Floating bridges represent feasible options in this project with already two long span floating bridges in function, i.e. the Bergsøysund and Nordhordaland Bridges. In connection with this project, one of the main objectives is to quantify the accuracy of the numerical methods used to predict dynamic behaviour of floating bridges. An extensive monitoring system is installed to measure structural response as well as environmental actions from wind and waves on an existing floating bridge: the Bergsøysund Bridge. These measurements are used to estimate the modal system properties of the structure. The system identification is performed using a parametric time-domain Stochastic Subspace Identification method as well as the Frequency Domain Decomposition method. Challenges of system identification for highly damped structural systems, such as a floating bridge, are especially emphasized. The results are also compared with numerical predictions from a two part combined linear frequency-domain model set-up. The first part consists of a hydrodynamic model, including wave excitation as well as fluid-structure interaction, and relies on linearized potential theory. The results from this are thereafter introduced into a finite element model, for a complete structural dynamic analysis.

The practical and inexpensive nature of ambient vibration testing has contributed to this approach being widely used by researchers and practitioners for identifying the dynamic characteristics of a wide range of laboratory and full-scale structures. The dynamic characteristics identified from such testing can be used for validating and calibrating analytical models, for quantifying and evaluating actual operating or performance characteristics, or for identifying damage and structural health monitoring. Although ambient vibration testing has been used extensively, it remains subject to a number of assumptions and limitations that are not always readily apparent in the results and that can reduce the reliability and utility of these results for many structures and applications. The authors believe that a pseudo ambient vibration testing approach using controlled excitation from a spatially distributed network of low-cost excitation devices could provide more consistent and reliable dynamic characterizations while maintaining many of the desirable attributes of ambient vibration testing. The writers conducted a study of the proposed testing approach using a large-scale laboratory model. The dynamic properties of the structure were identified by ambient vibration testing and by different implementations of the proposed pseudo ambient vibration testing approach. The results obtained from the testing are compared and discussed.

The E. Torroja Bridge is a steel-composite bridge which combines inverted steel arch trusses with a concrete deck. This bridge crosses the Guadalquivir River in the small town of Posadas, 30 km far from Córdoba, Spain. It was E. Torroja, the renowned civil engineer who designed and built it until its inauguration in 1951 with an original deck 7 m width. Nevertheless, in 1995 it was remodeled by the author’s grandson, José Antonio Torroja, who raised the width of the deck to 11 m and added two new inverted steel arches beside the original ones. In order to assess the structural health condition of the bridge, ambient vibration tests were carried out in June 2014.

The assessment procedures include full-scale ambient vibration testing, modal identification from ambient vibration responses, finite element modelling and dynamic-based identification of the uncertain structural parameters. All the changes experienced by the structure suppose a high level of uncertainty, which affects material’s properties and structural schemes in relation to deterioration processes and the structural modifications. Hence, a rigorous updating process in the finite element model was necessary to approach the experimental data with the numeric calculations.

Dynamic load testing is an important part of the acceptance process for new bridges in Italy. This paper is an overview of a part of a field-testing program carried out to investigate the dynamic properties of the five main new viaducts along the Brescia-Milano highway (BreBeMi) before their operation. Among them, the focus of the paper is on the Muzza Bridge and the VX1 Bridge: they are examples of continuous multi-girder composite structures. VX01 Bridge has a total length of 112 m with three continuous spans while Muzza spans approximately 80 m with a significant skew angle. Structural analysis was performed with the commercial FE software named Midas Gen. Modal parameters were obtained from experimental testing and were then employed in the calibration of the numerical models. The experimental evaluation of the performances of bridges proves very advantageous since it provides a benchmark for the validation of the numerical simulations, which often exhibit an inherent uncertainty. The presence of simplifications and assumptions in the numerical analysis may lead to results that don’t accurately predict the service life conditions of the bridge. In this case study, a comparative discussion of experimental results and numerical predictions is carried out with reference to the two different, seismic isolated, highway bridges mentioned above, both of which were tested using both environmental excitation and forcing: a large set of data was thus collected and an extensive model tuning activity could be carried out, allowing a thorough sensitivity analysis of a number of modelling parameters. The effects of different assumptions used when modelling some peculiar features of composite bridges, such as diaphragms, stiffeners, skew angle, expansion joints, rubber bearings etc., on the prediction of the dynamic properties of the composite viaducts, were investigated. At the same time, a comparison between the experimental results provided by ambient and forced vibration test results was carried out, based on their effectiveness in providing a reliable and useful benchmark for model tuning. Some conclusive suggestions based on the case study are finally addressed to structural engineers needing to set up an efficient procedure to perform similar tests and computer analyses.

The problem of automatic damage detection in civil structures is complex and requires a system that can interpret sensor data into meaningful information. We apply our recently developed switching Bayesian model for dependency analysis to the problems of damage detection, localization, and classification. The model relies on a state-space approach that accounts for noisy measurement processes and missing data. In addition, the model can infer statistical temporal dependency among measurement locations signifying the potential flow of information within the structure. A Gibbs sampling algorithm is used to simultaneously infer the latent states, the parameters of state dynamics, the dependence graph, as well as the changes in behavior. By employing a fully Bayesian approach, we are able to characterize uncertainty in these variables via their posterior distribution and answer questions probabilistically, such as “What is the probability that damage has occurred?” and “Given that damage has occurred, what is the most likely damage scenario?”. We use experimental test data from two laboratory structures: a simple cantilever beam and a more complex 3-story, 2-bay structure to demonstrate the methodology.

Accurate structural damage identification calls for dense sensor networks, which are becoming more feasible as the price of electronic sensing systems reduces. To transmit and process data from all nodes of a dense network would be an onerous task which creates a BIG DATA problem; therefore scalable algorithms are needed so that decision on the current state of the structure can be made based on efficient data processing. In this paper, an iterative spatial compressive sensing scheme for damage existence identification and localization will be introduced. At each iteration, a subset of sensors is selected for data transmission and relevant information will be extracted at central station for damage existence identification/localization. This information will also provide useful guidance in future selection of sensing locations. The devised algorithm is applied to identify damage in a simulated gusset plate.

Crack formation in a vibrating cantilever beam has been identified with the in situ nondestructive health monitoring Nonlinear Model Tracking (NMT) technique. The nonlinear cubic stiffness parameter has been chosen as the system’s dominant nonlinearity and has been tracked until catastrophic failure using a Continuous Time based system identification. The use of a nonlinear model allows for a range of healthy but complex system dynamics, such as changing natural frequency, which indicates a change in system health in traditional linear system health monitoring. Previous research has shown that significant change in the nonlinear parameter indicates a transition from a healthy to unhealthy system. The purpose of this study is to improve the robustness of the NMT method by investigating new data processing techniques. Numerical integration, regression fit with linear FRF, and strain gage sensors were introduced. The results of these new techniques were then compared with results from previous techniques to highlight effectiveness in determining change in a system’s health.

This paper presents an approach for determining three-dimensional global displacement (for arbitrarily-sized deformations) of thin rod- or tether-like structures from a limited set of scalar strain measurements. The approach is rooted in exploiting a reference frame that is materially adapted, i.e., it moves with the cross section. Local linearization of the frame evolution equations is shown to yield local solutions that may be assimilated into a global solution via continuity relationships. The solution is shown to be robust to potential singularities from vanishing bending and twisting angle derivatives and from vanishing measured strain, and the approach includes strain resulting from pure neutral axis extension (such as due to thermal loads). Validation of the approach is performed through comparison with finite element simulations. The average root mean square reconstruction error of 0.01–1 % of the total length, for reasonable sensor counts. Analysis of error due to extraneous noise sources and boundary condition uncertainty shows how error scales with those effects.

In-service condition assessment of large civil infrastructure systems has remained a particularly challenging area of research in the fields of nondestructive evaluation and structural health monitoring (SHM). Extensions of theoretically-based and laboratory verified vibration-based methods for assessing damage have been investigated experimentally on full-scale structures within several studies offering mixed conclusions. This paper introduces a recent experimental test program conducted on a full-scale bridge beam subjected to prescribed damage to the tension reinforcement. Details of the experimental testing program and vibration testing of the full-scale bridge beam both prior to and after damage to tension reinforcement are presented. System identification is applied to compare estimates of the natural frequencies, relative damping factors, and mode shapes obtained in the as-built state against those obtained after cutting over half of the tension reinforcement strands in the beam. A data-driven damage detection algorithm previously applied to detect damage in a full-scale bridge is also explored for application to the current dataset.

Vibration-based damage detection research aims to develop efficient algorithms to identify structural damage from monitoring data. One of the main categories of such algorithms is data-driven techniques which extract features from measured signals, and identify the damage by evaluating the significance of potential changes in these features. This paper presents application of several data-driven damage identification methodologies on a multivariate simulated data set. First, general regression models are applied to data collected through clusters of sensors and damage sensitive features are extracted. For systems with linear topology, it is shown that substructural regression modeling can also be performed on time- and frequency-domain transforms of the measured signals to estimate local stiffness of the structure as damage features. Subsequently, change detection techniques are utilized to statistically determine the significance of changes in the extracted features in order to distinguish between assignable changes as a result of damage and common changes due to environmental factors. Finally, a toolsuite is developed to facilitate application of the developed algorithms and improve the damage identification performance through incorporation of various combinations of regression models, damage features and statistical tests.

A novel superconducting magnetic levitation transportation systems has been proposed at University of L’Aquila, Italy. The bogie floats due to a passive, self-balancing interaction between high temperature superconducting skaters on board and permanent magnets on the track, in all phases of motion, zero speed included. A scaled superconducting skater has been statically tested measuring the repulsive-attractive magnetic forces varying, in a controlled way, the distance between the skater and the track. A non linear hysteretic characteristic curve has been identified averaging a set of suitable measures. In a first step, considering the thinness of the hysteretic cycles, the characteristic curve has been simplified in a non linearly elastic one. On the same time the equivalent tangent stiffness of such a curve has been identified, knowing the geometry and the mass characteristics of the bogie, by an experimental modal analysis conducted in operational conditions. A companion numerical model of the system has been introduced to forecast the working conditions with particular attention to dynamic behavior.

Although international railways have seen a massive increase in high speed rail there are still large amounts of older existing infrastructure designed for completely different criteria. The current supply systems of old electric railway lines, called soft catenary systems, are characterized by their design towards an optimal quasi-static behaviour. To increase the speed it is important to explore possible limiting factors, i.e. to identify the dynamic consequences and limitations. This paper explores a newly developed sensor system. Several sensors are placed over approximately 150 m to capture the dynamic behaviour. This is then used to create a base line for future monitoring as well as for assessing the possibilities of increased speed. For soft contact lines it is important to control maximum uplift at the pole support and the dynamic behaviour. The stiffness of the system changes between poles as well as along the section depending especially on track geometry; this makes it equally important to assess several other points between pole supports. Excessive vibrations can produce loss of contact rendering arching, increased wear and disrupted power supply. In the present paper acceleration time series were used to predict maximum vertical displacement, train speed, to assess the dynamic behaviour and to quantify modal parameters.

One challenge in the finite element model (FEM) updating of a physical system is to estimate the values of the uncertain model variables. For large systems with multiple parameters this requires simultaneous and efficient sampling from multiple a prior unknown distributions. A further complication is that the sampling method is constrained to search within physically realistic parameter bounds. To this end, Markov Chain Monte Carlo (MCMC) techniques are popular methods for sampling from such complex distributions. MCMC family algorithms have previously been proposed for FEM updating. Another approach to FEM updating is to generate multiple random models of a system and let these models evolve over time. Using concepts from evolution theory this evolution process can be designed to converge to a globally optimal model for the system at hand. A number of evolution-based methods for FEM updating have previously been proposed. In this paper, an Evolutionary based Markov chain Monte Carlo (EMCMC) algorithm is proposed to update finite element models. This algorithm combines the ideas of Genetic Algorithms, Simulated Annealing, and Markov Chain Monte Carlo techniques. The EMCMC is global optimisation algorithm where genetic operators such as mutation and crossover are used to design the Markov chain to obtain samples. In this paper, the feasibility, efficiency and accuracy of the EMCMC method is tested on the updating of a real structure.

Structural identification of civil infrastructures, using measured modal properties, remains a promising research field with many applications in performance-based civil engineering and structural health monitoring. In particular, either computationally swift or direct methods for identifying structural models from partially described and incomplete modal parameter estimates are of foremost interest to facilitate near real-time and reliable structural performance assessment and diagnostics. This paper proposes modeling structural systems as Constraint Satisfaction Problems (CSPs) for structural identification to solve for uncertain parameters in structural models. Consistent with measurement data, modal parameter estimates are treated as truncated both in terms of the number of modes measured and the number of measured degrees of freedom relative to the analytical model, which yields a challenging nonlinear inverse eigenvalue problem. Using nonlinear constraints and parameter bounds, the Constraint Programming approach is demonstrated to be capable of properly reconstructing estimates of both uncertain structural parameters and unmeasured modal parameters for a truss model with only a limited number of measured degrees of freedom.

This paper describes the model updating techniques utilized for a nine-story concrete core wall building located on the University of British Columbia campus in Vancouver, Canada. Constructed in 1963, in a region of high seismic risk, the tower is slated for retrofit in the near future. The first five floors of the tower are connected to an adjacent, recently retrofitted five-story building. A structural model was created in finite element software using the original design documents. The dynamic properties of the structure were then determined experimentally through an ambient vibration test. Model updating was implemented to better match the model predictions to experimental results. The model updating targeted many parameters for specific structural elements to arrive at a strong correlation with the experimental results. A total of seven natural frequencies were matched.

The study of vibrations in beams has been largely addressed by authors and researchers. However, relatively few researchers have considered the case of unknown boundary conditions, as usually it is reasonable to assume the classical cases such as simply supported, clamped or free. Indeed, there are a wide variety of boundary-condition configurations, each one representing a whole different problem with its own modal characteristics. A method for updating experimental beam models to specifically address the issue of unknown boundary conditions is proposed; this methodology takes advantage of vector comparison techniques such as the modal assurance criterion based on the Cauchy-Schwartz inequality to determine the degree of linear relationship between two mode shapes systematically and iteratively until an acceptable parametric match is found. This paper includes the phases of numerical study and experimental validation. A brief introduction with some relevant previous publications is presented, before explaining the methodology derivations and considerations in the numerical study section. A section devoted to demonstrate the methodology with an experimental example is presented in the Experimental validation section, and finally some conclusions and future work.

Prediction of coordinated dynamic loads induced by groups and crowds of people remains one of the most significant problems faced by designers of grandstands in entertaining venues, such as stadia and concert halls. Available guidance portrays humans as deterministic robot-like force generators moving at a single frequency with either perfect synchronisation or with random phases. Humans are not robots, and natural variability and imperfect synchronisation of individuals point to a random approach for crowd loading.

This research aims to tackle this challenging topic by studying, measuring and quantifying coordination between force signals measured from 15 individuals jumping to a selection of popular pop and rock songs with different dominant beats. The results show a lack of strong synchronisation pattern between individuals in a group at all given songs and rhythms. However, there is a moderate level of synchronisation at songs with predominant beats in the range 2–3 Hz.

It is well known that of people standing or sitting on a structure can change the dynamic behavior of the structure itself. Particularly, when a significant number of people are occupying a structure, high variations of non-dimensional damping ratios and natural frequencies are often experienced. The extent of these changes depends not only on the number of persons, but also on the properties of the empty structure and on people position and postures.

This work analyses the effects of the presence of people on a grandstand of the San Siro stadium during some football matches and concerts. These effects are analyzed in terms of changes in modal parameters and amplitudes of vibrations. The impact of the number of people present on the structure on its dynamic behavior is also analyzed. The first results of an analytical approach, based on a FE model of the stand and a lumped parameter model of the people, to foresee the effect of people presence on stadia grandstands, is proposed in the end of the paper. The obtained results show that if an accurate model of the structure and detailed description of the stand occupancy are available the predicted results are in good agreement with the experimental evidence.

This work presents an analysis of the measured vibration levels during different events on two different grandstands of the Milano stadium (Giuseppe Meazza). The stands vibration is nowadays measured 24 h a day all year long, providing a huge quantity of data coming from ambient vibration, football matches and live rock concerts. A part of these data, the one gathered during different events, is here analyzed according to international standards and recognized national guidance in order to assess the stadium serviceability against people induced vibrations during different events and to critically face the limits imposed by the standards.

The analysis also puts into evidence how the different dynamic properties of the selected stands affect the measured vibration levels and how concerts and football matches provide a totally different excitation source.

Predictions of footbridge or long-span floor vibrations induced by pedestrian crowds can often prove inaccurate. One of the main deficiencies of the methods used for predicting these vibrations is the lack of consideration or erroneous representation of human-structure interaction (HSI). In this paper, the results from a series of footbridge tests designed to observe and then model HSI are presented. A laboratory footbridge was excited to three predetermined vibration amplitudes by an actuator, with and without the presence of pedestrians. In the tests with pedestrians, 4, 7 and 10 pedestrians were asked to walk repeatedly across the footbridge. Frequency response functions (FRF) of the footbridge with and without pedestrians were extracted from test data. To account for the HSI, pedestrians on the bridge were modeled as a spring-mass-damper (SMD) system attached to the footbridge. The mass, damping and stiffness of a single pedestrian were calculated by fitting the FRF obtained from the tests. It was found that the SMD models of pedestrians could adequately model HSI between a structure and its walking occupants. Furthermore, the stiffness and damping of the SMD model of a single pedestrian were found to be close to half of a standing person with two bent legs.

Slender footbridges are often highly susceptible to human-induced vibrations, due to their low stiffness, damping and modal mass. Predicting the dynamic response of these civil engineering structures under crowd-induced loading has therefore become an important aspect of the structural design. The excitation of groups of pedestrians and crowds is generally modelled using moving loads but also the changes in dynamic characteristics due to human-structure interaction are found to significantly affect the footbridge response. The present contribution investigates the influence of the presence of the pedestrians onto the dynamic characteristics of the occupied structure by means of an extensive experimental study on a footbridge in laboratory conditions. The analysis shows that the natural frequencies slightly reduce due to the additional mass but more significant is the observed increase in structural damping. Similar observations are made on a in situ footbridge. This interaction is simulated using a coupled human-structure model in which the human occupants are represented by simple biomechanical models.

This paper proposes an iterative identification approach to extract the stiffness, damping and coefficients of biological force of a spring-mass-damper (SMD) model for human beings in walking. Gait experiment records from 73 test subjects were used for the identification. The three-dimensional motion capture technology was adopted in the walking tests. Thirty-nine reflective markers were attached to each test subject during the test and the trajectories of each marker were monitored by motion capture system. The displacement, velocity and acceleration of center-of-mass of a subject in each test case were then obtained by the system. Assume that the biological force can be expressed by Fourier series, the parameters, including stiffness and damping of SMD model and coefficients of biological force, are identified by the following two steps. Step 1, initial guesses of damping ratio and natural frequency of SMD are introduced into the equation of motion to identify the coefficients of the first several orders of Fourier series and a new stiffness parameter. Step 2, acceleration resonance assumption is adopted to determine a new damping ratio parameter. Replace the conjectured/identified values in the previous step with the new values and repeat the above two steps until presumed convergence criteria is satisfied. The identified mean value of damping ratio and natural frequency of SMD slightly increase with the increase of walking frequency. The identified damping ratios are found larger than published values for people in standing posture.

Producing simulated test data from models of mechanical structures is often important for validation of analysis methods, for example parameter identification methods. Whereas investigations of test procedures and identification methods for operational modal analysis (OMA) are widely reported based on simulations, little attention is often given to the actual procedures to produce simulated response signals. For example the influence of the excitation strategy in a simulation on the following identified properties. In the present work different excitation strategies are applied to simulate responses of a modal model of a Plexiglas plate. OMA parameter identification is performed on the simulated responses and the results are compared to see the influence of chosen excitation strategy. Furthermore OMA results from a lab measurement of the real physical Plexiglas plate are presented and compared to the simulation results.

Operational Modal Analysis (OMA) techniques provide in most cases reasonably accurate estimates of structural frequencies and mode shapes. In contrast though, they are known to often produce uncertain structural damping estimates, which is mainly due to inherent random and/or bias errors. In this paper a comparison is made of the effectiveness of two existing OMA techniques in providing accurate damping estimates for random stationary loading, varying levels of signal noise, number of added measurement channels and level of structural damping. The investigation is focusing on the two frequency domain techniques, the Frequency Domain Decomposition (FDD) and the Frequency Domain Polyreference (FDPR). The response of a two degree-of-freedom (2DOF) system is numerically established with specified modal parameters subjected to white noise loading. The system identification is evaluated with well separated and closely spaced modes. Finally, the results of the numerical study are presented, in which the error of the structural damping estimates obtained by each OMA technique is shown for a range of damping levels. From this, it is clear that there are notable differences in accuracy between the different techniques.

In cases of reducing the number of modes in an operating response – for instance when using band pass filtering – often time domain identification techniques suffer from a tendency to over fitting. As a consequence, it might be useful to decrease the number of measured DOF’s to moderate the number of modes in the modal model. In these cases the dimension of the identified mode shapes is diminished accordingly and therefore it is preferable to have a way to expand back to the full set of DOF’s so that the estimated mode shapes can be animated in detail. In the present paper it is considered to use the correlation matrix for the filtered response including all measured DOF’s as a basis for the mode shape expansion. This matrix contains the normal modes that can easily be extracted from the column space of the correlation matrix. In this investigation the main focus has not been on the reduction problem, so engineering judgment has been used to secure a reasonable choice of reduced channels. The technique is illustrated on an OMA case where the modes of the tail part of a Panther helicopter is estimated during different flight conditions.

Expo 2015 is about to begin and the theme will be

Feeding the Planet, Energy for Life

. This exhibition will be focused on the nutrition issue but it is also important because of its economic benefits for the country, mainly for the development of the infrastructures necessary to support the event. For instance, one of the most strategic construction site is the BreBeMi highway connecting the cities of Brescia, Bergamo and Milan, which will host the exhibition.

This paper will present a comparative discussion about a BreBeMi flyover, crossing the Oglio river, which was deeply studied by means of operational modal analysis, experimental modal analysis and finite element simulations. The bridge is made by hollow core prestressed elements with unbounded tendons, supported by hollow core circular piers. Friction pendulum isolators are used at the supports to disconnect the deck from the piers as for seismic actions. The FE model was tuned by the comparison of the eigenvalues and eigenvectors computed by a linear modal analysis and the data obtained by the experiments. The results show the good correlation between the model and the real structure in terms of frequencies and mode shapes. Moreover this bridge is a remarkable test case to compare benefits and drawbacks of using operational and/or experimental modal analysis.

Over the past years the use of Operational Modal Analysis (OMA) for Structural health monitoring has become more and more widespread. Such a methodology would also be relevant to wind farm owners that want to monitor the integrity of their turbines’ foundations.

However, harmonic components, originating from the rotor and periodic aerodynamic forces, are present within the measured vibrations. The harmonics violate the set of assumptions of common OMA techniques and as a result, these techniques potentially fail. This paper explores the different techniques presented in the literature to handle these harmonics. The techniques are first presented and later applied on data of an offshore wind turbine in the Belgian North Sea.

This paper describes the experimental modal identification techniques used to identify the dynamic properties of a nine-story concrete core wall building located on the University of British Columbia campus in Vancouver, Canada. Constructed in 1963, in a region of high seismic risk, the tower is slated for retrofit in the near future. In order to identify the dynamic properties of the structure, an ambient vibration test was performed using three instrument setups, synchronized via GPS, and positioned strategically throughout the building. Frequency domain decomposition and stochastic subspace identification methods were used to determine and validate the dynamic characteristics of the tower. The natural modes of vibration, frequencies, and damping ratios are presented. The results indicate that the structure exhibits rocking in the foundations as well as significant interaction with the adjacent structure.

In certain buildings such as synchrotrons and large ground telescopes, strict requirements are stated regarding the vibration levels. Both external and internal vibration sources, for example traffic and indoor water pumps, can have an appreciable effect on the vibration levels in the buildings. The synchrotron research facility MAX IV, which is currently under construction in Sweden, serves as an example case for the analyses. In MAX IV, several water-pipe systems used for cooling purposes will be placed near vibration sensitive equipment. These systems will transmit vibrations, into structural parts of the facility, which could exceed the vibration requirements. Structural modifications of pipe systems are investigated here by means of the finite element method in order to reduce vibration levels in the building. The finite element model employed includes a water-pipe system, adjacent building parts and the underlying soil. The use of fluid-structure interaction is investigated as well as the use of component mode synthesis. This paper focuses on the numerical procedure used as well as the effects on transmitted vibrations by different placements of the pipe supports.

This study aims to evaluate the seismic performance of steel frames upgraded with shape memory alloy (SMA)-based self-centering viscous dampers. The proposed Superelastic Viscous Damper (SVD) relies on SMA elements for re-centering capability and employs viscoelastic (VE) damper that consists of two layers of a high damped (HD) blended butyl elastomer compound to augment its energy dissipation capacity. First, experimental tests are conducted to characterize behavior of SMA elements and VE damper and to assess the influence of various parameters such as displacement amplitude and loading frequency on their mechanical response. A prototype of the SVD is designed and fabricated. Then, an analytical model of a four-story steel special moment frame building with the installed SVDs is developed to determine the dynamic response of the structure. The incremental dynamic analysis is used to evaluate the behavior of controlled and uncontrolled buildings under 18 different ground motion records. The analytical results indicate that the buildings upgraded with the proposed passive control device effectively mitigate the peak interstory drifts and residual story drifts.

Cables are structural elements designed to bear tensile forces and experience vibration problems due to their slenderness and low mass. In the field of civil engineering, they are mostly used in bridges where the vibrations are mainly induced by wind, rain, traffic and earthquakes. This paper proposes the use of a tuned-inerter-damper (TID) system, mounted on cables to suppress unwanted vibrations. These are to be attached transversally to the cable, in the vicinity of the support, connected between the deck and the cable. The potential advantage of using a TID system consists in the high apparent mass that can be produced by the inerter. Our analysis showed that the modal damping ratio obtained is much higher than in the case of traditional dampers or tuned mass dampers, leading to an improved overall response. An optimal tuning methodology is also discussed. Numerical results are shown with a cable subjected to both free and forced vibrations and the TID performance is improved when compared with equivalent dampers.

In order to reduce the vibration transmission in multi-storey wood buildings, it is common to insert viscoelastic elastomer materials between parts of the buildings. The studies presented here investigate to which extent different design choices for the elastomer layers affect the isolation of low-frequency vibrations (0–100 Hz). A finite element model of two storeys of a multi-storey wood building, involving blocks of elastomer material in between the storeys, was used to perform numerical investigations. Parametric studies were carried out, considering different properties of the elastomer material and different placements of the elastomer blocks. Considering the transmission from the floor of the upper storey to the underlying ceiling, the material properties of the elastomer material were found to affect the vibration levels appreciably. A too stiff elastomer material can result in an amplification of the vibration levels in the ceiling for certain frequencies, whilst a less stiff material, in general, reduces the vibration transmission. The placement of the elastomer blocks was varied by shifting the position of the blocks while maintaining their centre-to-centre distance, resulting in a small effect on the vibration levels.

In recent years, active control of flexible structures has been studied extensively. The motivation for continual studies with this approach is that the vibration performance of flexible structures can be improved significantly via control. For example, the performance of civil engineering floor structures, which the present research work is based on, is increasingly being governed by meeting permissible vibration serviceability limits depending upon their respective usages, and this can usually be enhanced via active control. This then offers designers increased flexibility to realise more lightweight, longer span and open-plan floor layouts that are in tune with the advancements in material and design technologies as well as meeting the challenges for reduced carbon footprint of new constructions.

The work presented here focuses on active control of human-induced vibrations in floor structures using dynamic compensators. These are formulated from reduced order plant models and vary in complexity depending on the number of plant modes of vibration used for their respective designs. It is demonstrated that there are increased options offered by higher dynamic compensator orders with respect to realising various vibration mitigation performance objectives: for example, the isolation and targeting of specific vibration modes. These compensators are found to possess desirable stability margins and are much less sensitive to disturbances at lower frequencies in comparison with direct velocity feedback (DVF). A study of the robustness of the dynamic compensators designed here to changes in structural properties, for example, that would arise under human-structure interaction is also presented. It is found that the performance of dynamic compensator performance can be sensitive to changes in structural dynamic properties as compared with a direct velocity feedback scheme, as seen in the closed-loop stability properties, which is not so obvious from a study of the disturbance rejection properties.

Tuned liquid column gas damper (TLCGD) show excellent vibration absorbing capabilities appropriate for applications in wind- and earthquake engineering. However, in the early regime of strong motion seismic excitation or to counteract strong wind gusts the performance of the passive device can be increased substantially by active elements obtained from adding a pressurized gas supply with input–output valves to the sealed ends of the TLCGD. To prove the working principle of active TLCGD several small scale laboratory experiments have been performed with single and multiple degree of freedom host structures. To obtain a desired dynamic behavior, a conventional feedback control law is used to compute small active pressure inputs to the TLCGD. The experiments have proven that the active device is able to substantially reduce the dynamic system response in a broad frequency range. In fact, dangerous structural resonances of lightly damped structures can be avoided even if the passive absorber is not tuned perfectly. For multiple degree of freedom host structures a suitable control enables a single active TLCGD to counteract several modes of vibrations thereby avoiding the need to install numerous passive devices.

This article considers the dynamic analysis and semiactive vibration control on a building-like structure, excited on its base through an external force generated by an electromechanical shaker providing harmonic and seismic motion at the base of the overall structure. The mathematical model of the overall system is obtained using Euler-Lagrange methods, which is validated by means of experimental modal analysis techniques. In fact, the external force excites the first three (lateral) vibration modes of the building-like structure. Therefore, to suppress and/or attenuate the undesirable vibrations on the structure, it is proposed a semiactive vibration control scheme considering a Magneto-Rheological damper directly coupled between the base and the first floor. The hysteretic behavior of the Magneto-Rheological damper is modeled by means of the polynomial approach proposed by Choi-Lee-Park. Finally, a Multi Positive Position Feedback controller combined with Sliding-Mode Control techniques is synthesized, using as output the position provided by an accelerometer collocated on the first floor. Some experimental results are presented to show the dynamic performance of the overall building-like structure.

Vibration-sensitive equipment such as high-resolution imaging devices requires stable environments for optimal performance. It is often desirable to locate this equipment on ground, either at grade level or in basement areas, away from any significant sources of vibration. During planning, the site is surveyed to evaluate existing vibration conditions and to assess proposed equipment locations. This assessment informs the development of the structural scheme. If no significant changes to the site vibration environment are anticipated, the data from the site survey form the basis of design. However, in situations where significant changes to the ground vibration environment are anticipated, additional, more complex studies are often required.

This paper presents a case study involving the design of a vibration-sensitive research facility. Late in the development of the design the team was made aware of plans for a future light rail line directly adjacent to the building. A detailed evaluation of future rail vibration impacts was conducted using a combination of testing and simulation techniques to predict future site vibration levels and establish feasibility of colocation of sensitive equipment. The results from the assessment were used to finalize equipment selections, and to establish the final details of the structural design of the facility.

Mechanical equipment and occupant footfalls are often the most critical sources of floor vibration on the elevated floors of buildings. Achieving stringent vibration criteria on these floors requires sufficiently stiff and massive floor structures to effectively resist the forces exerted by mechanical equipment and user traffic. The difficulty for engineers in modelling these buildings can be exacerbated in structures that are very old and/or of unusual construction. In this paper, two case studies are presented of modelling such structures in order to predict vibrations in sensitive areas. The first structure dates to 1925, and is of massive concrete construction. A new 1 megawatt emergency generator is being installed directly above a floor containing vibration sensitive computer equipment. The challenges associated with the uncertainty of computer modelling of the dynamic properties of the historic structure are explored. The second structure is a large private residence, which has long span floors constructed of concrete, engineered wood joists and steel beams. Excessive vibrations due to footfall activity resulted in cracking of the concrete topping. The challenges associated with modelling this unique structure are explored. Field testing of both structures to examine and verify the accuracies of the assumed model parameters is presented.

Ground-borne vibration from road and rail sources is often a critical consideration for the functionality of research and healthcare facilities. Sensitive equipment can be isolated individually, but when improved vibration performance of the overall structure is required vibration must be mitigated before it enters the structure. To accomplish this, more comprehensive analysis methods must be considered. In the current case study a 12-storey hospital building containing microsurgical and surgical suites is under construction 15 m from a busy commuter and freight rail line. A neighbouring building located at a similar distance from the rail line was measured to have clearly perceptible vibration levels on elevated floors. In order to determine the vibration mitigation required for the new building, an extensive measurement plan was conducted to determine vertical and lateral soil propagation characteristics, a foundation model was constructed to determine soil-structure interaction properties and a finite element model of the building structure was constructed to determine vibration propagation to all floors in the building. The study shows how careful analysis and testing can lead to informed evaluation of various mitigation strategies to combat a very serious problem.

On days with high wind speeds, it was observed that the pool-deck fence of an oceanfront condominium experiences large amplitude vibration, which can cause welds holding pickets to fail, resulting in impact noise. This work investigates the cause of the wind-induced vibration of the fence and presents a method to mitigate it. The fence was modelled using commercial finite element (FE) software, which was validated by modal testing. The FE model was used as the base for simulating various design modifications to the fence. After review of results from wind pressure measurements, modal testing, and FE model simulation, the primary mechanisms generating the vibration were identified to be vortex-induced vibration and buffeting. Due to design constraints, there are limited modification options. Increasing the geometric stiffness of the fence was selected as the mitigation technique and a modified design was proposed for the fence. The modified fence was installed and tested, and results indicated that vibration occurred less frequently. On-site monitoring of the fence by the owner indicated reduced vibration.

Although a scanning electron microscope (SEM) has very low tolerance to being disturbed by vibration, it requires a chiller for operation. Isolating the SEM from its associated chiller vibration takes careful consideration. This paper presents a case study of the performance of several slab-on-grade configurations. These configurations were specifically constructed to support various water chiller units and other service equipment as well as various vibration sensitive microscopes in a high performance research facility. In this study, the slabs are subjected to shaker-induced harmonic loading similar to that of a water chiller unit used to cool a SEM. The actual performance will be discussed in the context of generic design criteria for sensitive equipment and the SEM manufacturer-specified design criteria.

Beam is one of the most commonly used object to verify the vibration theory since its analytic solutions have been well derived. For a double layer beam case, however, there are difficulties to estimate dynamic behavior of the layered beam with the derived analytic solutions for a single layer beam. This study is regarding investigations of dynamic characteristic changes of double layer beams with respect to various boundary conditions.

Automatic selection of modal parameters from stability diagrams is a requirement for automatic SHM systems and damage assessment. In the present article we describe and apply a methodology of an automatic modal selection based on data recorded continuously for 5 years in a building structure located in Chile. The building has been subjected to more than 1,700 earthquakes. Environmental conditions and noisy signals are present on the data and identified modal parameters. The methodology considers soft and hard discriminants between real and spurious modes. Evaluation of the methodologies is done based on the effectiveness of the discriminants and the computational cost of its evaluation. Results indicate that the parameters should be selected based on the number and type of sensors, noise level on the data, modal properties and damage characteristics of the structure.

A new approach of cable dynamics is presented in this paper. It is based on the exact expression of tension coming from continuum mechanics, while the previous elastic models of cables in open literature considered an approximation of small strain which reduced the cable to a spring. The equations of a mass suspended to an elastic cable are derived on the basis of this new formulation, and numerically calculated. A comparison with the classical approach is presented.