Dynamics of Civil Structures, Volume 4

A procedure for the automatic modal identification based on Narrow-Band algorithms is presented in this paper. The Lagrange interpolation polynomial is firstly utilized to find the modal peaks automatically. Then the modal assurance criterion values between adjacent modal vectors are calculated to decide the fitting band automatically. The procedure can be applied to Narrow-Band algorithms both for experimental and operational modal analysis. The result of automatic identification of real life testing and experimental data is finally demonstrated.

In this paper, a method to determine the effective homogenous beam parameters for a stranded cable is presented. There is not yet a predictive model for quantifying the structural impact of cable harnesses on space flight structures, and towards this goal, the authors aim to predict cable damping and resonance behavior. Cables can be modeled as shear beams, but the shear beam model assumes a homogenous, isotropic material, which a stranded cable is not. Thus, the cable-beam model requires calculation of effectively homogenous properties, including density, area, bending stiffness, and modulus of rigidity to predict the natural frequencies of the cable. Through a combination of measurement and correction factors, upper and lower bounds for effective cable properties are calculated and shown to be effective in a cable-beam model for natural frequency prediction.

Non-linear normal modes (NNMs) can be considered as a non-linear analogon to the description of linear systems with linear normal modes (LNMs). The definition of NNMs can be found in Vakakis (Normal modes and localization in nonlinear systems, Wiley, New York, 1996). Small systems with a low number of degrees of freedom and non-linear couplings (cubic springs) are investigated here. With increasing energy in the system the progressive non-linearity leads to a hardening effect. One typical dynamical property of non-linear systems is the frequency-energy dependency of the resulting oscillations. A good graphic illustration is to plot such a dependency in a so called frequency-energy plot (FEP). A NNM branch can be calculated by a numerical continuation method with starting at low energy level in a quasi linear regime and increasing the energy and reducing the period of the oscillation iteratively. Thereby a branch is a family of NNM oscillations with qualitatively equal motion properties (Peeters et al., Mech Syst Signal Process 23:195–216, 2009). In non-linear systems internal resonances and other phenomena can occur. Several tongues can bifurcate from a NNM branch. Therefore ordinary continuation methods fail at such bifurcation points. Here a predictor-corrector-method is used and different corrector algorithms are discussed for the branch continuation.

The use of tuned-mass-dampers (TMD) as structural vibration suppressors has been discussed widely over several decades and many parameter selection strategies exist for minimising the displacement of the host structure. Normally these strategies work best when the resonant frequency of the TMD is closely tuned to that of the structural mode that is being targeted. This can be an issue for structures with significant live loads such as slender bridges with heavy traffic. For this type of structure nonlinear or semi-active retunable TMDs have been proposed. In this paper we consider replacing the damper in the TMD with an electrical generator device. In its simplest form this device could be a motor/generator with a resistive load such that the velocity- force relationship is approximately proportional hence mimicking a viscous damper. Here we consider using a voice-coil linear actuator connected to an impedance emulator, which is capable of harvesting, rather than dissipating, some of the vibrational energy. We discuss how this harvested power can then be used to modify the resistive loading in real-time and hence allow a wider bandwidth of operation. The work present both numerical and experimental results and shows some viable strategies for the control and the design of the device.

There is a need in the future for maintaining and increasing oil and gas production in the Danish North Sea. Related to this are studies for exploring the potential for extending the lifetime of offshore platforms by implementation of Structural Monitoring Systems (SMS). The project, which this paper is based on, uses an expansion technique as a first step in the sequence of assessing the actual lifetime of a platform. Mode shapes and natural frequencies are estimated using operational modal analysis. The mode shapes are then expanded by expressing each experimental mode shape as an optimal linear combination of selected modes from a finite element model. The offshore platform, Valdemar, which is fully instrumented with accelerometers, GPS, strain gauges and wave radars, is chosen as a case study. Results show that the measured response can be expanded with high precision, which provides valuable information when assessing the actual lifetime of the platform. Also it is shown that the expansion technique can be used for assessment of measurement uncertainties.

It is often difficult to conduct modal testing and response measurements during the construction of civil structures. This is a result of both the time requirements and accessibility, both of which limit the opportunities available for testing. Clean measurement data can sometimes be difficult to obtain due to disturbances on site from other construction activities. This paper presents the results from a unique opportunity to conduct full modal tests on a lightweight office floor during renovation for adaptive re-use. Originally a warehouse facility, the building renovation involved adaptation to executive office spaces. This included renovation of a lightweight mezzanine floor area that would house senior management staff. The author was provided the opportunity to measure the floor in a bare (unfinished) state, followed by partial and complete fit-out conditions. Footfall response measurements were conducted along two distinct walking paths during each phase so that the variations in floor response could be quantified at key locations. The modal and footfall response tests were correlated to finite element models of the floor for evaluation of finite element and footfall force modeling techniques. The results from the study provided insight on finite element modeling techniques and the effects of finishes and other non-structural elements on the dynamic characteristics of the floor.

With the increasing use of instruments that require micro- and nano-level precision, the consideration of stringent vibration criteria in the design of the facilities that contain these instruments has also increased. This paper presents a case study of the performance of a slab on grade constructed in a high performance research facility that houses various pieces of sensitive equipment such as an electron beam lithography system, scanning electron microscopes and transmission electron microscopes. The investigation explores the dynamic properties of a slab-on-grade specifically designed and constructed to support the sensitive equipment. Analytical techniques to predict the dynamic behavior are discussed and compared to actual performance with respect the generic design criterion and the equipment manufacturer-specified design criteria.

Prestressed concrete has received increased attention as a structural system for blast resistance and protection. However, prestressed concrete members not intentionally designed to resist blast loads, as well as connections to such members, may be especially susceptible to blast initiated damage. Full-scale testing of a prestressed double-tee joist roof in an industrial building was conducted both prior to and after internal detonation of a modestly sized explosive charge located approximately six feet below the base elevation of the double-tee joists. The testing included experimental modal analysis of the roof using a pair of long stroke electrodynamic shakers and a distributed network of 60 accelerometers. A large set of modal parameter estimates are extracted from the measurement data using a combined stochastic-deterministic subspace identification algorithm. Comparisons are made to a numerical model of the roof, developed using properties obtained from supplemental nondestructive evaluation and local historic design handbooks, to assess the plausibility of the modes. Differences in the natural frequencies and mode shapes are highlighted to qualitatively draw conclusions on plausible damage to the roof system alongside physical observations from the site.

This paper evaluates the structural behavior of a nineteenth century historical construction of rammed earth built in Lima (Perú). The dynamic response of the building was experimentally analyzed from Operational Modal Analysis tests. A FE model was built in order to approach the experimental behavior. Manual calibration of material properties and interaction with surrounding buildings were considered in order to achieve this goal. Due to the complexity of the structure, the level of damage and possible non-linear effects, only the first two modes were considered to be clear enough for calibration purposes. The building is a three story construction made of adobe masonry and a traditional composite material made of wood, cane and mud known as “quincha”. Experimental and numerical results evidenced the different behavior of the quincha and adobe walls in terms of flexibility. Moreover, loss of connectivity or stiffness between floors and walls is observed from the relative displacements between different parts of the building. The paper contributes to the application of vibration based structural health monitoring techniques to earthen historical constructions.

Dynamic characteristics of large civil infrastructures have been monitored for safe operation and efficient maintenance of the structures. To measure vibration data, the conventional system uses cables which cause very expensive costs and inconvenience for installation. Therefore, various wireless sensor nodes have been developed to replace the conventional wired system. However, there remain lots of issues to be resolved such as power supply, data loss, data security, etc. In this study, a smart distributed sensor node (SDSN) was incorporated to measure vibration data of large civil infrastructures. The SDSN is basically a timely synchronized one-channel data acquisition system. It consists of local time clock with high accuracy and SD memory card for local data storage. It is designed for temporal measurement, not long-term monitoring, since it can only operate for several hours using embedded batteries. Several applications for large civil infrastructures subjected to harsh environment were carried out. Estimated dynamic characteristics of the structures showed close results of numerical analysis.

The paper describes a vibration issue experienced on a surgical microscope and presents the results of a series of dynamic testing to resolve the issue. The microscope is located in an operating room (OR) on the fourth floor of a 10-story, steel framed, inpatient hospital building constructed in 2011. The fifth floor of the building is a mechanical space. On multiple occasions, neurosurgical cases have been disrupted due to the vibrations of the microscope. The microscope manufacturer did not provide any specific vibration limits. Generic vibration criteria available in the literature are provided for floors—with no reference to the eye vibrations of a microscope—and limit neurosurgery floor vibrations to 1,000 micro-in/s (mips) in RMS. An independent vibration criterion for the eye (3,500 mips) has been developed over the series of tests by comparing the subjective perception/tolerance of the OR personnel against the measured data. The largest vibration levels measured at the eye reached 10,000 mips at 21 Hz and coincided with the motor speed of a condenser water pump operating at 1,260 RPM on the fifth floor. Comparisons of floor and eye vibrations in the OR indicate that microscope amplifies the floor vibrations three to four times at 21 Hz.

Human interaction with structural systems is not totally understood, leading to either overdesign or extreme vibrations. Different numerical models have been proposed to represent this interaction. Most of these models use masses, springs and dampers to model the human. One could argue that the interaction between structural system and human is more complicated, specially, that the human might react with the structure even though he or she is still at a particular location. This paper proposes the use of a close loop controller system to model the human–structure interaction. The model is updated in a probabilistic sense using experimental data collected in the laboratory.

The objective of this paper is to localise damage in a single or multiple state at early stages of development based on the principles of symbolic dynamics. Symbolic Time Series Analysis (STSA) of noise-contaminated responses is used for feature extraction to detect and localise a gradually evolving deterioration in the structure according to the changes in the statistical behaviour of symbol sequences. The method consists of four primary steps: (1) generating the time series data by a set of measurements over time at evenly spaced locations along the structure; (2) creating the symbol space to generate symbol sequences based on the wavelet transformed version of time series data; (3) developing the symbol probability vectors to achieve anomaly measures; (4) localising damage based on any sudden variation in anomaly measure of two adjacent locations. The method was applied to a clamped–clamped beam subjected to random excitation in presence of 5 % white noise to examine the efficiency and limitations of the method. Simulation results under various damage conditions confirmed the efficiency of the proposed approach for localisation of gradually evolving deterioration in the structure, however, for the future work the method needs to be verified by experimental data.

Data driven SHM methodologies take raw signals obtained from sensor networks, and process them to obtain features representative of the condition of the structure. New measurements are then compared with baselines to detect damage. Because damage-sensitive features also exhibit variation due to environmental and operational changes, these comparisons are not always straightforward and an automated, probabilistic approach is necessary, particularly for large-scale sensor networks. In this paper an automated novelty detection methodology based on one-class support vector machines (OCSVM) is proposed and tested on an instrumented experimental steel frame structure. OCSVMs are an advanced machine learning method which can classify new data points based only on data from one class. This enables training of a classifier for damage detection based only on information from a baseline structure. OCSVMs can suffer from over-fitting, a problem which is usually ameliorated by cross-validation. In the absence of any data from the damaged state cross-validation is not possible. In this paper the over-fitting problem is combated by the use of three different recently proposed parameter selection heuristics. These strategies are tested for various damage scenarios of the laboratory structure and the results compared.

On structures two types of humans may be present. (1) Active humans and (2) passive humans (sitting or standing on the structure). The active humans can excite the structure and cause structural vibrations. The mass of the passive humans will interact with the structure and basically it changes the structural system (modal characteristics and structural vibration levels). The paper addresses this subject and explores the implications of having passive humans present on the structure.

Vibration serviceability is an established design criterion for assembly-type structures to limit the vibration induced such that the structure’s occupants are not disturbed by such movement. Most current design guidance recommends the use of the dynamic properties of the empty structure in the dynamic response prediction. This guidance neglects the effects of human–structure interaction where the dynamic properties of the occupied structure are affected by the occupants. Experimental results from seated occupants on a cantilevered laboratory test structure are compared with analytical results obtained by modeling the occupants as an attached spring–mass–damper system. The cantilevered test structure was designed with variable supports to allow varied configurations with different natural frequencies. The experimental study includes various seated postures and group sizes on the test structure with various frequency configurations. The analytical modeling is performed using the modal parameters recommended by the Joint Working Group for passive occupants. A comparison of the resulting modal parameters of the occupied structure is presented and the results are discussed with respect to the estimation of the dynamic response of a structure for vibration serviceability assessment.

Footfalls are often the most critical source of vibration on elevated floors. Advanced modeling techniques consider walker weight and cadence, harmonics/subharmonics of footfall loading, dynamic loading factors (DLFs), and a resonant build-up factor. Design guides have traditionally treated the pedestrian weight, walking frequency, and DLFs as deterministic parameters; however, recent research has suggested that a stochastic approach is more appropriate. These stochastic models use Monte Carlo simulations to develop probability distributions of the vibration response from thousands of combinations of walking speeds, weights, and harmonics/subharmonics. Unfortunately, this methodology drastically increases computation time, significantly limiting its applicability. In this study, the sensitivity of the floor response to the distribution of possible input variable combinations is considered. Response predictions are developed using finite element models of two actual buildings, with 50 modes of vibration used for each floor. The goal is to determine the variables which most strongly influence the amplitude of the floor response. Using this information, only the important variables can be modeled in future simulations, which will dramatically decrease computational time. These results can be used to develop stochastic models that are both robust and practical.

Due to favourable mechanical and physical properties, and the potential to provide a resilient and low-carbon infrastructure, fibre-reinforced polymer (FRP) material has increasingly been used for construction of highway and pedestrian bridges. Relative low mass, low damping and low stiffness make these bridges sensitive to dynamic excitation, which may lead to discomfort of human occupants and larger dynamic amplification of stress and deformation than is encountered in structures made of traditional structural materials. Consequently, design might be governed by a vibration serviceability state. Lack of data on vibration performance of FRP structures and non-existence of a state-of-the-art vibration serviceability design guideline means that current practice is conservative, often meaning only short-span FRP bridge solutions are executed. To fully exploit the benefits of using FRP material and to extend its use beyond current practice requires a better understanding of dynamic behaviour.

The objective of this paper is to study vibration performance of bridges made with FRP components. Two FRP footbridges having main spans of 15.6 and 63.0 m are used as case studies, and vibration behaviour is critically evaluated against steel/concrete structures of comparable span lengths. Both dynamic properties of the FRP and non-FRP bridges, as well as the vibration response under dynamic excitation by pedestrians, are reported. It is shown that the FRP footbridges could exhibit one order of magnitude larger vibration response under nominally the same dynamic loads. This finding highlights the need for timely research on the in-service vibration performance of FRP bridges with the aim of developing design guidance tailored for this newest structural material.

With the new Signature Engineering Building at Virginia Tech over 250 accelerometers and other sensors (temperature, wind, etc.) are being installed to capture building data in real-time. This program allows the study of myriad topics associated with building design and operation from infancy through the useful life of the structure. Topics include structural health monitoring, building occupancy patterns for improving sustainable development, and studies on floor vibrations and human motion, among many other topics. This paper presents initial information on corridor instrumentation, including pilot data taken from four accelerometers mounted in the fourth floor corridor. Accelerometer data was collected for the case from someone walking and then running down the corridor. Initial observations are presented from the pilot data to demonstrate the type and form of data that can be obtained from this setup. The results from this study demonstrate consistency with foot impact trends seen in literature, and confirm the feasibility of the system to be used in future studies.

There is a lack of reliable models of dynamic loading of civil engineering structures, such as footbridges, floors and grandstands, due to active groups and crowds of people. The key reason for this is the lack of knowledge on coordination of body motion between multiple individuals walking, running, jumping and bouncing in groups of various sizes. This paper presents a potential of the previously published video-based motion tracking method to measure kinematics of multiple individuals in groups. Unlike traditional motion tracking methods, such as maker-based optical systems, this new method is inexpensive, does not require any preparation of test subjects (e.g. attaching tracking markers) and is not constrained to artificial laboratory environment. Moreover, it provides motion data of the similar high quality.

Real-time hybrid substructuring (RTHS) is a relatively new method of vibration testing that enables system-level characterization of highly complex, rate-dependent, or nonlinear physical components. Its predecessors are pseudo-dynamic testing in civil earthquake engineering and hardware-in-the-loop testing in mechanical and aerospace engineering. RTHS allows a coupled dynamic system to be partitioned into separate physical and numerical components or substructures. Feedback control using servo-hydraulic actuation is then used to interface the physical and numerical substructures together in real-time as a closed-loop hybrid test. This paper describes the mathematical framework for RTHS of marine structural systems, including the details of the control architecture for coupling physical and numerical substructures together and techniques for modeling the effects of fluid loading in real-time. This paper also presents a modification to a model-based feedforward–feedback actuator tracking controller using a minimum-phase inverse of the modeled actuator dynamics for the feedforward compensator. RTHS is applied to a notional marine structural system comprised of a one-dimensional (1D) fluid-loaded mass–spring system. Initial experimental results from uniaxial RTHS testing at the University of Connecticut Structures Research Laboratory are also presented.

This paper concerns damage identification of a bridge located in Luxembourg. Vibration responses were captured from measurable and adjustable harmonic swept sine excitation and hammer impact. Different analysis methods were applied to the data measured from the structure showing interesting results. However, some difficulties arise, especially due to environmental influences (temperature and soil-behaviour variations) which overlay the structural changes caused by damage. These environmental effects are investigated in detail in this work. First, the modal parameters are identified from the response data. In the next step, they are statistically collected and processed through Principal Component Analysis (PCA) and Kernel PCA. Damage indexes are based on outlier analysis.

The paper tackles the dynamic identification and the damage detection carried out by a spectral-based method on the well-known Z24 bridge, a three-span pre-stressed concrete bridge located in Switzerland. Before being destroyed, the bridge was progressively damaged and tested in the framework of the Brite Euram project SIMCES. Starting from this benchmark, the presented spectral-based identification technique is validated and the usefulness of this method as a non-destructive tool able to catch the dynamic behavior of a structure and locate the damage is widely discussed. Firstly, a FE model of the bridge was built and calibrated in order to analyze its response to different excitation types (ramp force, triangular pulse, shaker and random vibrations) and several damage scenarios. Secondly, aiming at identifying both the modal parameters and the damage of the bridge, the spectral-based method is applied making use of the power spectral matrix decomposition. Finally, a proper index is defined and applied to this case-study in order to locate the damage.

The dynamic behavior of structures with breathing cracks forced by harmonic excitation is characterized by the appearance of sub and super-harmonics in the response even in presence of cracks with small depth. Since the amplitude of these harmonics depends on the position and the depth of the crack, an identification technique is developed based on such a dependency. The main advantage of the proposed method relies on the use of different modes of the structure, each sensitive to the damage position in its peculiar way. In this paper the identification is tested against structure of increasing complexity to evaluate the applicability of the method to engineering applications. In particular, a robustness analysis is carried out for each test case to assess the influence of measuring noise on the damage identification.

Hybrid testing is an emergent technology encompassing a variety of contemporary methods such as hardware-in-the-loop, pseudo-dynamic, and real-time dynamic testing. A system to be tested is split into two or more subsystems, with some of the components represented by numerical models and the remainder being comprised of real, physical hardware. Forces and displacements are transmitted between subsystems via actuators and sensors. This paper is concerned with the challenges that emerge when trying to accommodate real-time simulations with highly nonlinear force characteristics in the physical substructure. A brief review of hybrid testing considerations is provided, including actuation hardware, controllers, and numerical time-integration schemes. An new approach is then proposed which unites novel methods in the numerical model, the integration scheme and the actuator control to achieve high performance levels independent of the physical component being tested, specifically for the case of highly nonlinear components and relatively coarse timesteps in the numerical substructure. Simulation results are provided to substantiate the projected benefits.

In the field of ground vehicles durability testing, the interaction between the structure being tested and the testing facility is a critical issue. This is particularly true when testing massive structures (e.g. engine components). The reason is that due to the design and manufacturing limitations, the frequencies of the testing facility often overlap, at least partially, with those of the test specimen. This paper documents the development of a new facility for durability testing which addresses the above interaction issue. The key issues affecting the table performance include: (1) the table should not vibrate into resonance with the input signal, (2) the table should have high stiffness with moderate mass and (3) the motion of the table should not be in the unwanted degrees of freedom. These issues have been addressed while designing the table. This paper presents the innovative guidelines for designing the table platform, its assembly and the investigations to provide insights into its response characteristics.

Uncertainty quantification metrics provide a quantitative measure of the agreement between predictions and observations. These metrics not only significantly influence the outcomes of model calibration but also provide a means of determining the desired level of fidelity for model validation. This manuscript evaluates the influence of these uncertainty quantification metrics on model validation focusing on Euclidian distance, i.e. the absolute geometric distance between two points; and Mahalanobis distance, i.e. the weighted distance between a point and a population that considers the correlations and Bhattacharyya distance, i.e. the weighted distance between two populations that considers the correlations. Discussions are provided on the use of these three metrics when comparing model predictions against observations in the context of model calibration and validation. These metrics are implemented and examined via a model validation method based on Monte Carlo and stochastic test-analysis correlation techniques. A finite element model of a frame structure with a set of uncertain parameters is provided in the simulated example to demonstrate these ideas.

This study is focused on evaluating the performance of different approaches for nonlinear finite element (FE) model calibration of a seven-story shear wall structure based on its response during an earthquake. The seismic response of the structure is simulated numerically using a refined FE model of a specimen that was tested on the UCSD-NEES shake table in 2006. A simplified FE model of the structure with fewer modeling parameters is calibrated to match the ‘measured’ data. In this model, nonlinearity is defined by hysteretic models at fiber elements to represent the behavior of reinforced concrete walls. Calibration is performed by minimizing different types of residuals between measured/identified and model predicted response features (correlation features). Three types of correlation features used in this study consist of (1) time-varying modal parameters identified at different amplitudes of response, (2) response time-histories such as displacement and acceleration, and (3) a combination of response time histories and modal parameters. Accuracy of the updated models is studied by comparing the frequency and time responses of the calibrated models and the exact ones. The updated models are further simplified in the form of state-space nonlinear models suitable for real-time system identification. The parameters of the state-space models are finally identified by means of the Unscented Kalman filter (UKF).

Structural identification has continued to develop into a versatile tool for developing high fidelity analytical models of large civil structures that accurately reflect the measured in-service response. The results of successful structural identification have been applied to validate the performance of innovative systems and improve assessments of response analysis for operational and extreme loads. Furthermore, the developing field of vibration-based damage detection has sought to employ structural identification for long-term performance monitoring and condition assessment of aged structures. Overwhelmingly, the finite element method has served as the analytical framework for such models. However, alternative physics engines, such as the Applied Element Method, offer distinct advantages over the finite element method both with respect to the computational considerations in the identification process and with respect to the use of the calibrated model for assessment of structural response to extreme loads. A general framework for structural identification with applied elements is discussed, and advantages are contrasted with traditional finite element approaches. A case study application, a prestressed concrete double-tee joist roof tested in a full-scale building, is presented to demonstrate the approach and emphasize these advantages.

It is well known that the mode shapes are not mass normalized in the operational modal analysis process. In the last years several equations have been proposed to scale de mode shapes using the mass change method, which consists of modifying the dynamic behavior of the structure attaching some masses at positions where the mode shapes are known. The modal masses are then estimated using the modal parameters of both the unperturbed and the perturbed systems together with the mass change matrix corresponding to the applied perturbations.

In this paper it is proposed a new technique to scale the mode shapes in operational modal analysis which consists of combining the mass matrix of a finite element model and the experimental mode shapes identified by operational modal analysis. It is proven that, if a reasonable correlation exists between the experimental and the numerical systems, the results provided by this method are of the same order of accuracy as the results of the mass change method.

The design of modern day offshore wind turbines (OWTs) relies on numerical models, which are used for simulating the dynamic behavior in different operational and environmental conditions. From these results one can estimate ultimate and fatigue loads, which are needed for determining the design life of the turbines. The dynamic behavior, and thus the lifetime, of the turbines are influenced for a large part by its structural properties, such as the natural frequencies and damping ratios. Hence, it is important to obtain accurate estimates of these modal properties. For this purpose Operational Modal Analysis (OMA) techniques are used to estimate the modal domain of the OWT. As, for instance, the loads in the structure and the damping ratios are inversely related, higher damping values will results in lower loads, and hence in more optimized and less costly support structures. In this paper the data-driven Stochastic Subspace Identification (SSI) method is used to evaluate the modal domain of the OWT by using output-only measurements obtained from an installed 3.6 MW offshore wind turbine. In order to better satisfy the OMA assumptions of having a Linear Time-Invariant (LTI) system and white noise uncorrelated input, the analyses are performed in case of idling turbines, thereby avoiding the effect of rotational harmonic components, changing system properties due to yawing and pitching actions, as well as strong aerodynamic nonlinearities. In this paper we focuss on the first four global eigenfrequencies that were found and the associated damping ratios. Even though a sensor mix of several strain gauges and a single bi-directional accelerometer are available, the best results were obtained by only using the accelerometer on the nacelle.

Laminated glass beams can be considered sandwich elements consisting of two or more glass sheets and one or more interlayers such as polyvinyl butyral (PVB). The response of these elements present two borderline cases: (1) the monolithic limit, which appears at low temperatures and high frequencies and (2) the layered limit, which appears at high temperatures and low frequencies. In this work, the modal parameters of several laminated glass beams were estimated by operational modal analysis in the temperature range 10–45 °C. The results show that the natural frequencies decrease and the damping ratios increase with increasing temperature whereas there are not significant changes in the mode shapes. The experimental results are compared with those determined using the analytical models of Mead and Markus and Ross, Kerwin and Ungar.

This paper presents an application of a novel data collection method: mobile sensing. Mobile sensor networks can provide extensive information similar to dense fixed sensor networks while conserving the ease of smaller networks. However, mobile sensing data is expected to have missing observations in time and space, leaving data matrices incompatible with common identification techniques. STRIDE is an algorithm implemented for modal identification using this class of sensor data, which includes missing observations. Although mobile sensing devices are not widely available and large-scale mobile sensors networks have yet to be implemented, pseudo mobile sensing data is extracted from a dense sensor network using a simulated mobile sensor network. In this paper, ambient vibrations of Golden Gate Bridge are considered and pseudo (simulated) mobile sensing data are populated from a subset that shares the paths of simulated mobile sensors. The paper provides promising results to encourage the implementation of large-scale mobile sensor networks in future SHM endeavors.

The ability to identify the dynamic properties of offshore wind turbines allows validating and updating numerical tools, which can be used to enhance the design. At the same time, these dynamic parameters can serve as a basis to continuously monitor the integrity of the machine. However, modal identification of turbines in operating conditions still poses some major issues, in particular in removing the rotor harmonic components, which are polluting the measured signals.

This paper will present and discuss some recent developments for removing harmonic components from operational wind turbine data. The possibility to track the evolution of specific modes is compared against classical techniques such as Time Synchronous Averaging and Cepstrum, which show limitations due to rotational speed fluctuations, amplitude modulation of the harmonic components and the interaction between the harmonics and the aerodynamic loads. The methodologies are firstly presented and then applied to real data of an offshore wind turbine installed in the Belgian North Sea. The ability to identify more accurately the modal parameters will allow improving the correlation with the varying environmental conditions and provide additional input data to validate numerical models.

This paper will evaluate different automated operational modal analysis techniques for the continuous monitoring of offshore wind turbines. The experimental data has been obtained during a long-term monitoring campaign on an offshore wind turbine in the Belgian North Sea. State-of-the art operational modal analysis techniques and the use of appropriate vibration measurement equipment can provide accurate estimates of natural frequencies, damping ratios and mode shapes of offshore wind turbines. To allow a proper continuous monitoring the methods have been automated and their reliability improved. The advanced modal analysis tools, which will be used, include the poly-reference Least Squares Complex Frequency-domain estimator (pLSCF), commercially known as PolyMAX, the polyreference maximum likelihood estimator (pMLE), and the frequency-domain subspace identification (FSSI) technique. The robustness of these estimators with respect to a possible change in the implementation options that could be defined by the user (e.g. type of polynomial coefficients used, parameter constraint used…) will be investigated. In order to improve the automation of the techniques, an alternative representation for the stabilization charts as well as robust cluster algorithms will be presented.

Recently, with the continuous development of materials, designs, and construction technologies, the construction of long-span bridges has been increasing. Especially, for long-span bridges using cables, it is important to monitor the safety of the bridges continuously by measuring the tension of the cables under construction and in traffic use. Among the various methods for evaluating the tension of cables, the vibration method, which estimates the tension using shape conditions and natural frequencies measured from the cables, is widely used at the moment as it is more economical and convenient than the direct method, which measures the stress of cables using a load cell. Accelerometers, as one of the conventional sensors being used to monitor tension of existing cable-supported bridges, may not easy to install at cables and require the considerable cabling work to facilitate a direct connection between each sensor and DAQ logger. For this reason, a technique using digital image processing, which is one of non-contact sensing systems, may be needed to monitor the tension of the cable. Therefore, in this study, for the monitoring of cables, a vision-based monitoring system using the image processing technique was developed, which can estimate the dynamic characteristics of many cables using a single system. The developed system was applied to a cable-stayed bridge in traffic use, and to verify the possibility of the long-term monitoring of cables, the recognition rate of cables was investigated using images acquired under various weather conditions.

All structures exhibit some form of damping, but despite a large literature on the damping, it still remains one of the least well-understood aspects of general vibration analysis. The synthesis of damping in structural systems and machines is extremely important if a model is to be used in predicting vibration levels, transient responses, transmissibility, decay times or other characteristics in design and analysis that are dominated by energy dissipation. In this paper, a new structural damping identification method is proposed. The proposed structural damping identification is a direct method and requires prior knowledge of accurate mass and stiffness matrices. The proposed method doesn’t require initial damping estimates. The effectiveness of the proposed structural damping identification method is demonstrated by numerical and experimental studies. Firstly, a numerical study is performed using lumped mass system. The numerical study is followed by a case involving actual measured data of cantilever beam structure. The results have shown that the proposed structural damping identification method can be used to derive accurate model of the system. This is illustrated by matching of the complex FRFs obtained from the analytically damped model with that of experimental data.

The increasing speed of railway traffic sets not only higher safety and serviceability requirements to the rolling stock but also to the civil engineering infrastructure. An important criterion that has to be satisfied for bridges is, that resonances between moving axle loads and the structure’s natural frequencies should be avoided. This has usually to be proven by numerical investigations. However, it has been observed, that the respective proves can often not be satisfied by common engineering models. Furthermore, dynamic tests have shown that in many cases the calculated and identified modal parameters of existing structures deviate considerably.The presented study investigates several numerical models of a typical railway bridge, the calculated dynamic properties assuming common assumptions for the model parameters and the sensitivity of specific model parameters to the numerical output. Furthermore, a series of dynamic tests is described. The parameters of the numerical models are calibrated with respect to the experimental results. To validate the numerical models, the simulated and measured structural responses due to a train passage are compared.

After the construction of a bridge, different problem could arise demanding for an evaluation of the structural performances. In case of a complex structure, the use of a numerical model without an experimental validation is strongly discouraged.

Accordingly to the previous considerations, a correct roadmap is described in this paper, by analyzing the dynamical characteristics of a complex bridge whose skew deck is supported by 18 cables suspended by a twin-arches frame. Because of the skew configuration, the two upstream and downstream arches are not symmetric. The bridge is 110 m long and 14.80 m wide. The cross-section consists of two longitudinal steel girders surmounted by a reinforced concrete slab. The deck is suspended from two steel arches having a circular cross section and a maximum sag of 28 m. The suspending cables are anchored to the steel girders and to the arches.

Dynamic tests were conducted in operational conditions to estimate the dynamic properties. Up to the first 20 vibration modes were identified. Additional impulsive tests were carried out to identify the frequencies of the cables. The numerical part of the investigation involved the determination of a FE model and the application of model updating procedures in order to enhance the correlation between experimental and numerical results.

Structural health monitoring is fundamental to improve safety of critical structures, such as bridges. Most of health monitoring techniques is based on the variation of modal parameters along the lifetime of the structure. However experimental identification of modal parameters of bridges represents a difficult task. Ambient vibration tests were proved to provide a reliable estimation of modal parameters. However when the bridge is still close to the traffic and excitation is only provided by wind buffeting, signal-to-noise ratio may be unfavorable, making identification of modal parameters challenging. Forced vibration tests represent an alternative. Compared with ambient vibration tests, this approach presents the advantage of providing well defined input excitations, which can be optimized to enhance the response of the vibration modes of interest. The drawback is that the bridge must be closed to the traffic and that in the case of large and flexible bridges (suspended and cable-stayed bridges) with natural frequencies of predominant modes in the range 0–1 Hz, it is challenging to provide controlled excitation for a significant level of response. In this paper, the effectiveness to induce bridge vibrations by means of a heavy vehicle running on a series of cleats is discussed. On the purpose, a numerical model for truck–bridge interaction was developed. Results of the implemented model are compared with experimental data collected during an experimental campaign carried out on the Adige bridge. During the tests both truck and bridge were instrumented with accelerometers to measure excitation and bridge response.

The Oglio flyover is one of three major flyovers belonging to the new BreBeMi highway in Northern Italy. The flyover consists of two separate ways, each of which made of 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 piles as for seismic actions. The finite element model implemented for the dynamic analyses of the Oglio flyover was developed using the commercial structural analysis software MidasGen. The 3D model of the whole deck was implemented using a refined mesh of shell elements to reproduce the longitudinal variations in the geometry of the cross section, the mass distribution, the fraction of dead loads and finishing actually in place during the dynamic tests in the most accurate possible way. Friction pendulum isolators were modelled implementing into the software the actual elasto-plastic behaviour curve provided by the producers, in order to take into account the different actual lateral stiffness of each isolator, depending on its vertical load. A linear modal analysis was carried out in order to obtain the numerical eigenvalues and eigenvectors, to be compared with the experimental values, derived from the operational modal analysis carried out on the flyover. The comparison in terms of frequencies and mode shapes was successful, yielding very good correlation for a significant number of modes. A model tuning procedure is currently being carried out in order to obtain a fully validated benchmark numerical model to be used for predictive purposes.

Many researchers are working to develop more rational, robust and timely condition evaluation approaches for the nation’s inventory of aging and deteriorating bridges. Dynamic characterizations of bridges using forced or ambient vibration testing techniques have received much attention in this regard since the measurements can effectively capture and describe the actual in situ properties and behavior of the structure. The dynamic excitation is an important consideration for bridge vibration testing applications. It is challenging and expensive to adequately excite a full-scale bridge for forced vibration testing using one or two traditional dynamic shakers, and ambient vibration testing requires assumptions related to the nature of the unmeasured dynamic excitation that cannot be easily validated. The writers have been evaluating a new approach for multiple-input, multiple-output forced vibration testing of short to medium span bridges that employs a spatially distributed network of low-cost exciters. Forced vibration testing of a large-scale, laboratory model structure was performed using the prototype multi-shaker excitation system to identify the optimal dynamic excitation signals. Deterministic multisine signals and stochastic burst random signals with various durations and force levels were evaluated with the system. The resulting system identifications and the merits of the different excitation signals are presented and compared.

The Eureka-Samoa Channel Bridge is a 20-span pre-stressed concrete I-girder bridge, built in 1971 with a 225 ft navigable span. It is the North-eastern-most part of three bridges collectively known as the Samoa Bridge, which carry two lanes of California State Route 255 between Eureka and the Samoa Peninsula. In June 2013, a detailed ambient vibration test was performed on the bridge. This was done using a set of nine GPS-timed velocity/acceleration recorders. Along the length of the 2,500 ft structure, every pier and every midspan on the west side of the bridge was measured; on the east side every midspan was measured. In addition, the navigable span was measured in detail on both sides; span 5 and 16 were measured with four points along the length on each side, and a measurement was taken on either side of each expansion joint. During the testing, one lane was closed at a time allowing for the test team to have access to the outside of the lanes; traffic crossed with a lead vehicle each time at 25 mph. This created a unique situation of no-vehicle excitation followed by a single line of traffic, moving in one direction, at a uniform speed. This paper presents the results of the ambient vibration tests.

The Painter St Overpass is a continuous concrete box girder bridge with two-spans over Highway 101 in Rio Dell, California. The bridge has a 17-channel permanent monitoring system that has recorded several earthquakes including the 1992 Petrolia Magnitude 7.1, with a measured ground acceleration of 0.5 g at the site. An ambient vibration test was performed in 1993, which showed that the dynamic properties of the bridge are very sensitive to the level of shaking, in particular at the abutments. In June 2013, another detailed ambient vibration test was performed on the bridge. Both of the tests captured several points along both edges of the bridge deck. The largest earthquake since the 1993 ambient test was the 2010 Ferndale Magnitude 6.5, with ground accelerations of 0.36 g. This paper presents the results of the 2013 ambient vibration test and compares to a reanalysis of the 1993 test using the latest available tools.

We investigate the effects of steady aerodynamic loads on stability and natural frequencies of long-span suspension bridges through a simplified analytical model. The single (central) span suspension bridge model is considered, and the linearized integro-differential equations describing the flexural-torsional deformations of the bridge deck-girder are adopted as starting point. Thus, taking into account the second-order effects induced by a constant transverse wind in the bridge equations of motion, we derive a generalized eigenvalue problem in which all configurations intermediate between those of pure lateral-torsional buckling, pure torsional divergence, and pure free vibrations can be investigated. We show that the natural frequencies of a suspended deck-girder depend upon the mean (quasi-static) wind loading. As a consequence, the input parameters to the aeroelastic stability analysis result affected by that dependence, suggesting the possibility of modifying the dynamic stability analysis in order to take into account the mentioned influence. Based on this fact, possible implications for the flutter analysis of long-span suspension bridges are discussed.

This paper discusses the seismic response control of a building by connecting it to another adjacent building. Coupled buildings have demonstrated to be an attractive method to mitigate excessive dynamic responses. This paper studies the behavior of two structures connected by passive dampers and also considering local feedback control systems installed in each structure, an hybrid strategy. These feedback control systems are designed and operated independently using active devices with limited actuation capacity like force actuators. The governing differential equations of motion of the coupled system are derived and solved for relative displacement. The advantages and disadvantages of each control strategy are examined in order to determine the more appropriate one. Based on the present research results, general conclusions and recommendations are given for further studies.

A goal of the structural health monitoring (SHM) community has been to endow structures with a nervous system not unlike that present in biologic organisms. Typically in SHM, data from sensors is collected then analyzed using statistical classifiers and/or compared to results from physics based models. Despite successes using these techniques, they are not, in general, adaptable to unforeseen loading and/or ambient conditions. Past studies have shown that, in some cases, humans can outperform computers in classification under such conditions. In this work, we are interested in testing an interface between SHM data and humans using a vibro-tactile device. Research from the neuroscience community suggests this interface may be possible by leveraging the phenomena of sensory substitution. To test the hypothesis, an excitation was provided to a three-story structure, and the output acceleration response for each floor was collected. The structure could be modified with bumpers to exhibit non-linear dynamic responses on any combination of its three floors. The human subject wore a vibro-tactile glove excited by the acceleration data and reported which bumpers they believed were engaged based upon the stimulation of the actuators. The performance of human subjects to identify the cause of non-linearity in structures was compared with results achieved using a support vector machine classifier.

In recent decades, the application of experimental modal identification tests in civil engineering structures is gaining interest for various purposes such as model calibration, quality control while the construction process, damage detection, and structural health monitoring of new buildings and bridges. The use of these tests in archaeological earthen sites is a novel area of application. The paper presents the results of operational modal analysis tests carried out on an archaeological site in Peru as part of an extensive research for assessing its structural vulnerability. The case study is related to the tests on one of the remaining stone masonry walls of Chokepukio archaeological site, which dates back to the twelfth century. The paper shows a brief summary of the historical condition of the site, the details of the tests carried out, the experimental data processing results, as well as the Finite Element model updating process using manual and automatic routines.

Transmitted vibrations from water-pipe systems, involving supports standing on a concrete slab and the underlying ground, are investigated here. The effects of varying parameters of the supports, such as location, their elastic modulus and their cross-section, on the level of vibrations transmitted by the water-pipe system are analysed in order to minimise vibrations transmitted to sensitive parts in a facility. A finite element model involving use of both finite and infinite elements was employed for analyses in the frequency domain where account was taken of both fluid-structure interaction and soil-structure interaction. It was concluded from the analyses performed that the parameters of the supports had an appreciable effect on the vibration level in the facility.

This paper deals with the structural and dynamic analysis of a building-like structure consisting of a three-story building with a PZT embedded actuator on one of the main columns. The mechanical structure is perturbed using an electromagnetic shaker, which provides forces with a wide range of excitation frequencies. The mechanical structure is modeled using Euler–Lagrange and experimental modal analysis techniques. For attenuation of the overall system response is implemented an active vibration control scheme via the PZT actuator, using both Positive Position Feedback (PPF) and Sliding-Mode Control in order, to simultaneously reduce the dominant modes and improve the overall closed-loop system response. Some experimental results are included to validate and illustrate the effectiveness of the overall system performance.

In this study a new simple active control algorithm is proposed for controlling the seismic responses of elastic three-dimensional structures. This simple proposed control algorithm (PC) is based on minimizing the time dependent performance index, which can be defined as the total energy input of the structure. To investigate the efficiency of this method, a 3-dimensional tier building is considered. This structure is tested under a real earthquake excitation. Emphasis has been placed on comparison of the proposed algorithm with classical linear optimal control. With reference to the results, the proposed simple control algorithm is very effective in suppressing the uncontrolled structural vibrations. The numerical results also demonstrate that with very little control energy consumption, the reduction in the responses is similar in amount to that obtained with classical linear optimal control. The resulting closed-loop control algorithm doesn’t require the future knowledge of earthquakes, and it also doesn’t require the solution of the nonlinear matrix Riccati equation, which would increase the computer solution time.

Excessive vibration levels were measured at a newly installed Motor and Pump Assembly. Vibration measurements were taken at selected points under normal operational running speed conditions, and a dominant amplitude at first order running speed was observed in all the measured frequency domain signals. A Bump test revealed a natural frequency of the system very close to the running speed. Dynamic Vibration Absorbers are very effective to reduce vibration levels at resonance conditions. Therefore a tunable Absorber was designed to be tuned in at the forced frequency. This paper describes the detailed design analysis of the Absorber. The equations of motion were formulated with a two-degree-of-freedom approach. A single-degree-of-freedom approach was first used to characterize the equivalent vertical shaking force to provide an average measured vibration amplitude. By using the magnitude of this shaking force, the equations of motion for the two-degree-of-freedom system were solved with numerical integration to determine the responses at the Assembly and also at the Absorber with computer simulations. The two natural frequencies were also computed. The design was experimentally verified and the predicted and measured natural frequencies corresponded well. The Absorber proved to very effectively reduce the Assembly’s vibration levels.

This paper introduces a novel type of passive control system designed to suppress unwanted vibrations in civil engineering structures subjected to base and lateral excitation. The new system configuration, inspired by traditional tuned mass dampers (TMD) where the mass element has been replaced with an inerter is presented. An inerter is a two-terminal flywheel device with the capacity to generate high apparent mass and it was initially developed for Formula 1 racing cars suspension systems. An analytical tuning procedure for inerter-based systems has been developed. This is inspired by traditional tuning rules for damped vibration absorbers. The inerter-based system performance is assessed in comparison to TMDs. It is shown that the new control system suppresses the response of all modes, which constitutes an advantage with respect to TMDs. Moreover, our analysis shows that the new system is most effective when located at ground storey level, which is advantageous for its installation. A multiple-degree-of-freedom structure is analysed numerically to verify our theoretical findings. This has been subjected to a range of excitation inputs, including wind and earthquake loads and its performance was similar or superior to that of TMDs, making the new device an attractive vibration-suppression method.

Tuned Mass Dampers (TMDs) are devices that oppose the motion of a floor which has been excited by occupant footfalls. They have been demonstrated to be effective when considered during the design process or in mitigation situations. If designed and implemented properly, they achieve three goals: (1) maintain structural motion levels below commonly accepted criteria, (2) optimize the size and configuration of the structural system in order to provide more useable space in a building, and (3) reduce the cost of construction due to fewer and/or smaller structural elements. Traditionally, TMDs have only been used to control perceptible and excessive motions from wind loading and crowd movement. As such, they have not been used in laboratory and other sensitive spaces due to equipment criteria which specify vibration levels far below perceptibility. With the advent of a new dashpot damper design, it is now possible to use TMDs to control imperceptible motions as required in sensitive occupancies. This paper presents the results of controlled testing of these new dashpot dampers and a full-scale implementation of TMDs with this component in a long-span structure.

In the structural design of high-rise buildings the dynamic analysis represents one of the most onerous stages of the process. As a matter of fact, for this building typology, the horizontal actions are a predominant factor in the evaluation of the structural stability. Consequently, due to the complexity of the issue, advanced methods of analysis are needed. In most cases the designers rely on finite element (FE) methods, which analyse the structure through a very high number of degrees of freedom. Nevertheless, this entails a lengthy and time consuming process, which may result inappropriate, especially during the early phases of evolution of the concept. On the contrary, analytical formulations, based on simplifying hypotheses, may represent a sufficiently accurate tool for the evaluation of the structural parameters governing the dynamic response of the building. In the present paper an analytical approach for the dynamic behaviour of high-rise buildings is proposed. The method considers those vertical members usually employed to stiffen horizontally a building, such as shear-walls, frames and braced frames, internal cores coupled with external tubes. The benefits provided by the formulation are mainly a faster data preparation as well as a highly reduced running time, if compared to that of FE models.

Although linear modal analysis has proved itself to be the method of choice for the analysis of linear dynamic structures, extension to nonlinear structures has proved to be a problem. A number of competing viewpoints on nonlinear modal analysis have emerged, each of which preserves a subset of the properties of the original linear theory. From the geometrical point of view, one can argue that the invariant manifold approach of Shaw and Pierre is the most natural generalisation. However, the Shaw–Pierre approach is rather demanding technically, depending as it does on the construction of a polynomial mapping between spaces, which maps physical coordinates into invariant manifolds spanned by independent subsets of variables. The objective of the current paper is to demonstrate a data-based approach to the Shaw–Pierre method which exploits the idea of independence to optimise the parametric form of the mapping. The approach can also be regarded as a generalisation of the Principal Orthogonal Decomposition (POD).