Showing papers in "Structural Control & Health Monitoring in 2013"

Recent research and applications of GPS-based monitoring technology for high-rise structures

Abstract: SUMMARY Monitoring the response of structures, especially tall buildings, under severe loading conditions is an important requirement for the validation of their design and construction, as well as being a maintenance concern. This paper presents a review of current research and development activities (since 1995) in the field of high-rise structure health monitoring using the Global Positioning System (GPS). The GPS monitoring technology and its accurate assessment method are firstly briefly described. Then, the progresses on monitoring the displacement of the high-rise structure caused by the ambient effects including wind, thermal variation, and earthquake-induced responses are discussed in details. Following that, the states of the art of augmenting the GPS monitoring technology are reviewed. Finally, existing problems and promising research efforts in the GPS-based health monitoring are given. Copyright © 2012 John Wiley & Sons, Ltd.

Field monitoring and numerical analysis of Tsing Ma suspension bridge temperature behavior

Abstract: SUMMARY It is important to take into account the effect of temperature in assessing the structural condition of bridges. However, very few quantitative studies have examined the temperature behavior of large-scale bridges because of their large size and complicated configuration. This paper, for the first time, investigates the temperature distribution and associated responses of a long-span suspension bridge—the 2132-m-long Tsing Ma Bridge—through a combination of numerical analysis and field monitoring. With appropriate assumptions, fine finite element models of a deck plate, section frame, and bridge tower are constructed to facilitate thermal analysis. With ambient temperature measurements and a solar radiation model, the time-dependent temperature distribution within each of these components is calculated through transient heat transfer analysis. The numerical results are verified by comparing them with field monitoring data on temperature distribution and variation at different times and in different seasons. The temperature data are then input into the structural model of the whole bridge to obtain the displacement and strain responses of various bridge components, with a good level of agreement being achieved between the bridge responses and the monitoring data. This exercise verifies both the accuracy of the analytical method employed and the effectiveness of the monitoring system installed on the bridge. The study shows that integrating numerical analysis with field monitoring data provides for a thorough understanding of the temperature behavior of long-span bridges. Copyright © 2012 John Wiley & Sons, Ltd.

Structural health monitoring of the Tamar suspension bridge

Abstract: This paper presents experiences and lessons from the structural health monitoring practice on the Tamar Bridge in Plymouth, UK, a 335-m span suspension bridge opened in 1961. After 40 years of operations, the bridge was strengthened and widened in 2001 to meet a European Union Directive to carry heavy goods vehicles up to 40 tonnes by a process in which additional stay cables and cantilever decks were added and the composite deck was replaced with a lightweight orthotropic steel deck. At that time, a structural monitoring system comprising wind, temperature, cable tension and deck level sensors was installed to monitor the bridge behaviour during and after the upgrading. In 2006 and 2009, respectively, a dynamic response monitoring system with real-time modal parameter identification and a robotic total station were added to provide a more complete picture of the bridge behaviour, and in 2006 a one-day ambient vibration survey of the bridge was carried out to characterize low-frequency vibration modes of the suspended structure. Practical aspects of the instrumentation, data processing and data management are discussed, and some key response observations are presented. The bridge is a surprisingly complex structure with a number of inter-linked load–response mechanisms evident, all of which have to be characterized as part of a long-term structural health monitoring exercise. Structural temperature leading to thermal expansion of the deck, main cables and additional stays is a major factor on global deformation, whereas vehicle loading and wind are usually secondary factors. Dynamic response levels and modal parameters show apparently complex relationships among themselves and with the quasi-static load and response. As well as the challenges of fusing and managing data from three distinct but parallel monitoring systems, there is a significant challenge in interpreting the load and response data firstly to diagnose the normal service behaviour and secondly to identify performance anomalies. Copyright © 2012 John Wiley & Sons, Ltd.

Recent perspectives in dynamic testing and monitoring of bridges

Abstract: SUMMARY The paper reviews some of the most recent aspects involved in the dynamic testing and continuous monitoring of bridges. This includes a discussion of testing techniques, instrumentation, modal identification and damage detection. On the basis of their experience, the authors described several case studies in which some of the most recent developments have been used to accomplish different purposes: the Millau Viaduct, in France, the Pedro e Ines footbridge and the Infante D. Henrique Bridge, in Portugal, and the Grande Ravine Viaduct, at the Reunion Island. Copyright © 2012 John Wiley & Sons, Ltd.

Damage classification in structural health monitoring using principal component analysis and self-organizing maps

Abstract: SUMMARY Damage classification is an important issue within SHM going beyond the purely damage detection. This paper proposes a data-driven statistical approach for damage classification, which is constructed over a distributed piezoelectric active sensor network for excitation and measurement of vibrational structural responses. At different phases, a single piezoelectric transducer is used as actuator, and the others are used as sensors. An initial baseline model for each phase for the healthy structure is built by applying PCA to the data collected in several experiments. In addition, same experiments are performed with the structure in different states (damaged or not), and the dynamic responses are projected into the different baseline PCA models for each actuator. Some of these projections and damage indices are used as input features for a self-organizing map, which is properly trained and validated to build a pattern baseline model. This baseline is further used as a reference for blind diagnosis tests of structures. Both training/validation and diagnosis modes are experimentally assessed using an aluminum plate instrumented with four piezoelectric transducers. Damages are simulated by adding mass at different positions. Results show that all these damages are successfully classified both in the baseline pattern model and in further diagnosis tests. Copyright © 2012 John Wiley & Sons, Ltd.

Active tuned mass dampers for control of in‐plane vibrations of wind turbine blades

Abstract: This paper investigates the use of active tuned mass dampers (ATMDs) for the mitigation of in-plane vibrations in rotating wind turbine blades. The rotating wind turbine blades with tower interaction represent time-varying dynamical systems with periodically varying mass, stiffness, and damping matrices. The aim of this paper is to determine whether ATMDs could be used to reduce in-plane blade vibrations in wind turbines with better performance than compared with their passive counterparts. A Euler–Lagrangian wind turbine mathematical model based on energy formulation was developed for this purpose, which considers the structural dynamics of the system and the interaction between in-plane and out-of-plane vibrations. Also, the interaction between the blades and the tower including the tuned mass dampers is considered. The wind turbine with tuned mass dampers was subjected to gravity, centrifugal, and turbulent aerodynamic loadings. Investigations show promising results for the use of ATMDs in the vibration control of wind turbine blades. Copyright © 2013 John Wiley & Sons, Ltd.

Monitoring of long‐term static deformation data of Fei‐Tsui arch dam using artificial neural network‐based approaches

Abstract: SUMMARY The objective of this paper is to develop methods for extracting trends from long-term static deformation data of a dam and try to set an early warning threshold level on the basis of the results of analyses. The static deformation of a dam is mainly influenced by the water pressure (or water level) of the dam and the temperature distribution of the dam body. The relationship among the static deformation, the water level, and the temperature distribution of the dam body is complex and unknown; therefore, it can be approximated by static neural networks. Although the static deformation almost has no change during a very short time, it changes with time for long-term continuous observation. Therefore, long-term static deformation can be approximated dynamically using dynamic neural networks. Moreover, static deformation data is rich, but information is poor. Linear and nonlinear principal component analyses are particularly well suited to deal with this kind of problem. With these reasons, different approaches are applied to extract features of the long-term daily based static deformations of the Fei-Tsui arch dam (Taiwan). The methods include the static neural network, the dynamic neural network, principal component analysis, and nonlinear principal component analysis. Discussion of these methods is made. By using these methods, the residual deformation between the estimated and the recorded data are generated, and through statistical analysis, the threshold level of the static deformation of a dam can be determined on the basis of the normality assumption of the residual deformation. Copyright © 2011 John Wiley & Sons, Ltd.

Particle filter scheme with mutation for the estimation of time-invariant parameters in structural health monitoring applications

Abstract: SUMMARY In this study, a novel method is presented for non-linear, non-Gaussian online state and parameter identification, developed for use in structural health monitoring (SHM) problems. The algorithm consists of a particle filter (PF) that combines the use of the standard PF with mutation operators. The algorithm aims at alleviating the sample impoverishment problem, which is a well-known limitation of the standard PF, yielding it inefficient for demanding non-linear identification problems. To overcome this hurdle, we introduce here an alternative approach, influenced by the principles of evolutionary computation. After the standard PF steps are performed to a point where the sample diversity drops below some threshold, the unfit particles are replaced by either the fittest particles or the current weighted estimate of the state. Next, the time-invariant components of the particles are mutated under some mutation probability, and the new sample is then propagated to the next time step. This process is well suited for joint state and parameter estimation problems, as is usually the case in SHM techniques. As a result, the loss of diversity associated with the standard PF is overcome, and the new PF with mutation is shown to outperform the standard PF and the unscented Kalman filter for the case of high process noise. The method is validated through an established benchmark problem found in the literature, lying outside of the structural identification concept, and a previously referenced 3DOF structural system with hysteresis elaborating the SHM aspect. Copyright © 2012 John Wiley & Sons, Ltd.

Feasibility of displacement monitoring using low-cost GPS receivers

Abstract: SUMMARY Many of the available SHM approaches neither readily support displacement monitoring nor work in concert with one another to take advantage of displacement-based SHM for various long-period structures. Although survey-quality GPS technology offers the possibility of measuring such displacements with sub-centimeter precision, the associated cost is too high to allow for routine deployment. Low-cost GPS chips commonly found in mobile phones and automobile navigation equipment are attractive in terms of size, cost, and power consumption; however, the displacement accuracy of these GPS chips is on the order of several meters, which is insufficient for SHM applications. Inspired by sensory information processing strategies of weakly electric fish, this paper investigates the potential for using dense arrays of relatively low-precision GPS sensors to achieve high-precision displacement estimates. Results show that dynamic response resolution as low as 20–30 cm can be achieved and that the resolution improves with the number of sensors used. Copyright © 2012 John Wiley & Sons, Ltd.

Bayesian methodology for diagnosis uncertainty quantification and health monitoring

Abstract: SUMMARY This paper develops a Bayesian approach for the continuous quantification and updating of uncertainty in structural health monitoring. The uncertainty in each of the three steps of damage diagnosis—detection, localization, and quantification—is considered. Bayesian hypothesis testing is used for damage detection, thus facilitating easy quantification and updating of the uncertainty in damage detection. Qualitative damage signatures derived from the model are used for rapid damage localization; when the damage signatures fail to localize the damage uniquely, the uncertainty in damage localization is quantified using the principle of likelihood. Damage quantification is done through the method of maximum likelihood, and the uncertainty in damage quantification is estimated through Bayesian inference. The uncertainty in each of the three steps is continuously updated with the acquisition of more measurements. The overall uncertainty in diagnosis is also calculated, using the concept of total probability. The proposed methods are illustrated using two types of example problems—structural frame and a hydraulic actuation system. Copyright © 2011 John Wiley & Sons, Ltd.

Response-only modal identification of structures using limited sensors

Abstract: SUMMARY Herein, we propose a method based on the existing second-order blind identification of underdetermined mixtures technique for identifying the modal characteristics—namely, natural frequencies, damping ratio, and real-valued partial mode shapes of all contributing modes—of structures with a limited number of sensors from recorded free/ambient vibration data. In the second-order blind identification approach, second-order statistics of recorded signals are used to recover modal coordinates and mode shapes. Conventional versions of this approach require the number of sensors to be equal to or greater than the number of active modes. In the present study, we first employ a parallel factor technique to decompose the covariance tensor into rank-one tensors so that the partial mode shapes at the recording locations (sensors) can be estimated. The mode shape matrix identified in this manner is not square, which precludes the use of a simple inversion to extract the modal coordinates. As such, the natural frequencies are identified from the recovered modal coordinates' Autocovariances. The damping ratios are extracted using a least-squares technique from modal free vibrations, as they are not directly recoverable because of the inherent smearing produced by windowing processes. Finally, a Bayesian model updating approach is employed to complete the partial mode shapes—that is, to extract the mode shapes at the DOFs without sensors. We use simulated and physical data for verifying and validating this new identification method, and explore optimal sensor distribution in multistory structures for a given (limited) number of sensors. Copyright © 2012 John Wiley & Sons, Ltd.

On the monitoring of historic Anime Sante church damaged by earthquake in L'Aquila

Abstract: The paper shows the activity of a 2-year static and dynamic monitoring activity to check the structural response and the actual level of damage of a historic church hit by earthquake in L'Aquila (Italy) in 2009. Structural health monitoring was carried out with accelerometers and transducers that recorded time histories of the main earthquake-induced ground motions and ambient vibrations. The paper focuses particularly on residual performance and assessment of the ancient church after six earthquake response data set—because of the seismic swarm—via a modal identification of output-only systems by using frequency domain decomposition. In this way, fundamental frequency, mode shapes, damping ratios and displacements can be estimated without knowing the input that has excited the system. The dynamic monitoring provides also a way to check possible drops of tension in the cables and relaxation in the FRP belts used for safety. Efficiency and reliability of modal identification of basilica-type churches, via the dynamic response of macro-elements only in comparison with the whole analysis, are finally proposed. Copyright © 2012 John Wiley & Sons, Ltd.

TinyOS-based real-time wireless data acquisition framework for structural health monitoring and control

Abstract: SUMMARY Wireless smart sensor networks have become an attractive alternative to traditional wired sensor systems to reduce implementation costs of structural health monitoring systems. The onboard sensing, computation, and communication capabilities of smart wireless sensors have been successfully leveraged in numerous monitoring applications. However, the current data acquisition schemes, which completely acquire data remotely prior to processing, limit the applications of wireless smart sensors (e.g., for real-time visualization of the structural response). Although real-time data acquisition strategies have been explored, challenges of implementing high-throughput real-time data acquisition over larger network sizes still remain because of operating system limitations, tight timing requirements, sharing of transmission bandwidth, and unreliable wireless radio communication. This paper presents the implementation of real-time wireless data acquisition on the Imote2 platform. The challenges presented by hardware and software limitations are addressed in the application design. The framework is then expanded for high-throughput applications that necessitate larger networks sizes with higher sampling rates. Two approaches are implemented and evaluated on the basis of network size, associated sampling rate, and data delivery reliability. Ultimately, the communication and processing protocol allows for near-real-time sensing of 108 channels across 27 nodes with minimal data loss. Copyright © 2012 John Wiley & Sons, Ltd.

Design, performance test and analysis on magnetorheological damper for earthquake mitigation

Abstract: SUMMARY As a semi-active control device, magnetorheological (MR) dampers have been paid more attention because of their high controllability, fast response and low power requirement When MR dampers are used for vibration mitigation, some challenge topics must be taken into account, such as design method, performance study and intelligent control algorithm In this paper, a detailed design process of MR damper involving the geometry design and magnetic circuit design is carried out, and a multistage shear-valve mode MR damper is designed and manufactured Then the MR damper is tested to investigate the influence of control current, displacement amplitude and excitation frequency on the damper's mechanical behavior and energy dissipation performance At the same time, the design target values are compared with experimental results Comparison results show that the proposed design method holds promise in designing and optimizing the MR damper Finally, a modified Sigmoid model is proposed Comparison results between the experimental data and the numerical data indicate that the modified Sigmoid model can accurately describe the behaviors of the MR damper Copyright © 2012 John Wiley & Sons, Ltd

Damage detection in an experimental bridge model using Hilbert–Huang transform of transient vibrations

Abstract: SUMMARY This paper presents the health monitoring of an experimental bridge model using Hilbert–Huang transform of transient vibration data. The Hilbert–Huang transform involves decomposition of vibration data into ‘intrinsic mode functions’ through the process of empirical mode decomposition. The Hilbert transform of intrinsic mode functions yields magnitude and frequency of oscillations as a function of time, which is called the Hilbert spectrum. Marginal Hilbert spectrum is obtained by integration of the Hilbert spectrum over time. A 4 × 1 m single span-bridge instrumented with 10 wireless sensor nodes is used for the study. Three levels of damage are introduced by removing bolts connecting the midspan floor beam with one of the girders. Bridge vertical accelerations are measured as a wheel and axle is gently rolled across its length (called one ‘run’) to simulate passing vehicles. The Hilbert spectrum for multiple runs, joint time–frequency analysis for individual runs, marginal Hilbert spectrum, and instantaneous phase were examined for the baseline (healthy) bridge and the three damage cases. The results demonstrate that the method can detect and locate damage under transient vibration loads. Copyright © 2011 John Wiley & Sons, Ltd.

Implementation of passive and active vibration control on an in-service footbridge

Abstract: SUMMARY The current trend toward lighter and slender pedestrian structures, with new aesthetic requirements and high-performance materials, has resulted in structures with increased susceptibility to vibration. Notable vibrations under human-induced excitations might appear, and the vibration serviceability requirements might not be accomplished. The Valladolid Science Museum Footbridge (Spain) is an example of a lively structure that might achieve excessive vertical acceleration under walking or running excitation. The control of excessive footbridge vibrations via passive and active devices is dealt with in this work. More specifically, this paper is concerned with the design and experimental implementation of a passive tuned mass damper (TMD) and an active mass damper (AMD) to mitigate human-induced vibrations on this in-service footbridge. The TMD, with a mass ratio of 1%, is designed by a numerical method based on H∞ controllers. The AMD consists of a proof-mass actuator, with a mass ratio of approximately 0.2%, controlled by a strategy based on acceleration feedback with a phase-lag network. The performance of both devices has been assessed. Copyright © 2011 John Wiley & Sons, Ltd.

Optimal placement of active control devices and sensors in frame structures using multi‐objective genetic algorithms

Abstract: SUMMARY Active control efficiency is highly dependent on the control algorithm and device types as well as the locations of the devices and sensors in a building. A gene manipulation, multi-objective genetic algorithm is proposed to optimize the placement of active devices and sensors in frame structures to reduce active control cost and increase the structural control strategy's effectiveness. Gene manipulation uses engineering judgment to modify the encoded variable information defining the number of devices and sensors per floor in selected Pareto-optimal front individuals. The proposed methodology evolves Pareto-optimal layouts that minimize the number of devices/sensors used while also minimizing the building interstory drift for a 20-story steel-frame building under earthquake loading. The results indicate that the number and location of the devices and sensors in the layouts obtained strongly depends on the desired maximum drift. Also, the location of the sensors significantly impacts the efficiency of the active controller in reducing interstory drifts. In simulation trials, the proposed gene manipulation method obtained layouts that distributed devices and sensors more evenly over the building height than layouts obtained using standard multi-objective methods, resulting in greater control efficiency. The primary benefit of implementing the proposed gene manipulation was in reducing the number of multi-objective genetic algorithm generations required by up to 40% without negatively impacting the quality of Pareto-optimal device/sensor layout solutions obtained. Copyright © 2011 John Wiley & Sons, Ltd.

System identification of buildings by wave travel time analysis and layered shear beam models—Spatial resolution and accuracy

Abstract: A previously explored method for one-dimensional structural system identification and earthquake damage detection in buildings, based on measuring wave travel time through the structure and its changes, is formalized, and its spatial resolution and accuracy are analyzed. The method identifies the velocity of propagation of shear waves in the structure as function of height. The wave travel time is measured from impulse responses obtained from recorded response at different locations in the structure. The main advantages of this SHM method over other methods are its robustness in application to real buildings and large amplitude response, insensitivity to the effects of soil–structure interaction, and local in nature achieved with relatively small number of sensors. The identification is based on a layered shear beam model of the building. In this paper, analytical impulse response functions are presented for such model, which provide theoretical basis to define identification algorithms. The derivation of one such algorithm, which involves approximations, from the exact wave propagation solution of the model is presented, and its spatial resolution and accuracy are critically examined. Termed here ‘direct algorithm’, it involves measuring the pulse time shifts and amplitudes and identifies shear wave velocities and quality factor Q in the layers. Various issues identified, e.g. the trade-off between accuracy and detail of the identification, are illustrated on a full-scale densely instrumented nine-story RC building (Millikan Library in Pasadena, CA, USA, excited by 2002 Yorba Linda earthquake). Copyright © 2012 John Wiley & Sons, Ltd.

Finite element model updating using frequency response functions and numerical sensitivities

Abstract: SUMMARY A new method is presented for finite element model (FEM) updating using frequency response functions and numerical sensitivities. The proposed method differs from methods currently in the literature by using numerical sensitivities to solve the inverse problem instead of analytical sensitivities. This method combines the usefulness of commercially available finite element modeling programs with advanced optimization algorithms to solve the inverse problem while requiring neither model reduction nor data expansion. The method is applied to several simulated test cases in which damage is detected and the usefulness of the method is shown. The method proved to be robust, stable, error tolerant, and successful in estimating unknown structural parameters using frequency response functions for FEM updating. Copyright © 2013 John Wiley & Sons, Ltd.

Experimental and numerical studies on model updating method of damage severity identification utilizing four cost functions

Abstract: SUMMARY As the final stage of damage identification, damage severity identification has great significance to structural safety assessment and decision-making in maintenance. Take the damage detection of truss structures for instance; the stochastic damage locating vector method has great advantages. However, the method is a localization technique designed to provide information in damage location only. Many present damage severity identification methods suffer from great error due to high noise. Therefore, it is imperative to develop a new identification method for truss structural health monitoring. To solve this problem, this paper presents the model updating method of damage severity identification based on four cost functions: (i) correlation coefficient of free vibration accelerations; (ii) correlation coefficient of local mode shapes; (iii) free vibration accelerations assurance criterion; and (iv) local modal assurance criterion. In these functions, correlation coefficient and correlation degree of free vibration accelerations of measured nodes are first proposed to identify damage severity. Moreover, a simple supported bailey steel-truss bridge Benchmark Model has been designed and constructed. The span is 8 m with the scaled ratio 1:25. Based on the model, both experimental and numerical simulation results using these procedures under pulse excitation indicate that they are feasible and effective. In addition, the proposed techniques exhibit high-noise insusceptibility. Copyright © 2011 John Wiley & Sons, Ltd.

Modelling and numerical simulations of vibrothermography for impact damage detection in composites structures

Abstract: The paper investigates modelling aspects related to application of vibrothermography for detection of barely visible impact damage in composite structures. Low-velocity impact tests were performed to introduce multiple delaminations into carbon/epoxy composite plate. Damage severity was revealed using well-established non-destructive evaluation techniques. Vibrothermography was used subsequently to show good agreement with classical damage detection techniques. Following these experimental investigations, numerical simulations were performed to assess feasibility and sensitivity of vibrothermography for impact damage detection. Numerical results were validated using experimental data showing very good qualitative and encouraging quantitative agreement. The study demonstrates that virtual impact damage detection using vibrothermography can be performed as part of structural design to assess sensitivity of the method in real engineering applications. Copyright © 2012 John Wiley & Sons, Ltd.

Energy‐efficient deployment strategies in structural health monitoring using wireless sensor networks

Abstract: SUMMARY Structural health monitoring using wireless sensor networks has drawn considerable attention in recent years. The ease of deployment of tiny wireless devices that are coupled with sensors and actuators enhances the data collection process and makes prognostic and preventive maintenance of an infrastructure much easier. In this paper, the deployment problem is considered for finding node locations to reliably diagnose the health of a structure while consuming minimum energy during data collection. A simple shear structure is considered and modal analysis is performed. The example verifies the expectation that placing nodes further apart from each other reduces the mode shape errors but increases the energy consumption during data collection. A min–max, energy-balanced routing tree and an optimal grid separation formulation are proposed that minimize the energy consumption as well as provide fine grain measurements. Copyright © 2012 John Wiley & Sons, Ltd.

Optimal design formulas for viscous tuned mass dampers in wind‐excited structures

Abstract: SUMMARY Optimal design for tuned mass dampers (TMDs) with linear or nonlinear viscous damping is formulated in order for design practitioners to directly compute the optimal parameters of a TMD in a damped structure subjected to wind excitations. The optimal TMD tuning frequency ratio and damping coefficient for a viscous TMD system installed in a damped structure under 10 white noise excitations are determined by using the time-domain optimization procedure, which minimizes the structural response. By applying a sequence of curve-fitting schemes to the obtained optimal values, design formulas for optimal TMDs are then derived. These are expressed as a function of the mass ratio and damping power-law exponent of the TMD as well as the damping ratio of the structure. The feasibility of the proposed optimal design formulas is verified in terms of formulary accuracy and of comparisons with existing formulas from previous research works. In addition, one numerical example of the Taipei 101 building with a nonlinear TMD, which is redesigned according to the proposed optimal formulas, is illustrated in effort to describe the use of the formulas in the TMD design procedure and to investigate the effectiveness of the optimal TMD. The results indicate that the proposed optimal design formulas provide a convenient and effective approach for designing a viscous TMD installed in a wind-excited damped structure. Copyright © 2011 John Wiley & Sons, Ltd.

Optimum parameters of tuned liquid column–gas damper for mitigation of seismic-induced vibrations of offshore jacket platforms

Abstract: SUMMARY The effectiveness of tuned liquid column–gas damper, TLCGD, on the suppression of seismic-induced vibrations of steel jacket platforms is evaluated in this study. TLCGD is an interesting choice in the case of jacket platforms because it is possible to use the structural elements as the horizontal column of the TLCGD. In this study, optimum parameters of the TLCGD are obtained, considering nonlinear damping of the TLCGD and water–structure interaction between jacket platform and sea water. Equation of motions and other related formulas are derived, and using a SimuLink model, the frequency and the head loss coefficient of the TLCGD are optimized. Results are in general agreement with those obtained in earlier studies for typical building structures. However, effects of period of the structure and ground motion characteristics on the optimum parameters are also evaluated in this study. Results show that until a particular threshold for the mass ratio, the higher the mass ratio, the higher the efficiency of the damper. After that, by increasing the mass ratio, there is no improvement on the damper efficiency. It is also found that PGA and frequency content of a ground motion have no important effect on the optimum frequency ratio, but they have a noticeable effect on the optimum head loss coefficient. Besides, frequency content has some effect on the TLCGD efficiency. It is shown that the optimum frequency of a TLCGD is uncoupled with the area ratio and the head loss coefficient, and they have no effect on the optimum frequency ratio. Copyright © 2011 John Wiley & Sons, Ltd.

Substructure identification for shear structures I: Substructure identification method

Abstract: A substructure identification method for shear structures is proposed herein. A shear structure is partitioned into many simple substructures; an inductive identification procedure is derived to estimate the structural parameters from top to bottom. In each identification step, the dynamic equilibrium of a one-floor substructure is utilized to construct a substructure identification problem, estimating the story stiffness and damping coefficient. An identification error analysis for least-square error identification problems, based on the linearization of the least-square error problem, is proposed and applied to the substructure identification method. The results show that the identification errors are closely related to two important structural responses: the frequency response of the interstory acceleration of the identified story and the frequency response ratio between adjacent interstory accelerations. Further, these responses are critical to the substructure identification only near a certain frequency, the substructure natural frequency of the identified story substructure. The larger the first response is and/or the smaller the second response is, the more accurate the estimation results will be. A numerical example of a five-story shear structure is given to demonstrate the effectiveness of the proposed substructure identification method and to verify the identification error analysis results. From the results of the identification error analysis, a companion paper proposes a controlled substructure identification method, utilizing specially designed algorithms for structural control devices to change the two key structural responses that affect the identification accuracy, to improve the accuracy of the substructure identification. Copyright © 2012 John Wiley & Sons, Ltd.

Semi-active direct control method for seismic alleviation of structures using MR dampers

Abstract: In the present study, a direct semi-active control method is introduced to mitigate the seismic responses of structures equipped with magnetorheological (MR) dampers. Bouc–Wen model is utilized to investigate the nonlinear behavior of the MR dampers, and an efficient controller is proposed based on this model features. In this method, for conducting the MR damper control force close to the estimated optimal control force at any moment, the optimal magnitude of current voltage that is applied to produce a magnetic field in the MR damper is calculated through the use of linear quadratic regulator optimal control algorithm. This algorithm is applied to control seismic vibrations of a three-story and an 11-story sample shear building that have been equipped with the MR damper control system, and the performance of the controller is evaluated. Copyright © 2012 John Wiley & Sons, Ltd.

Optimal design of friction pendulum tuned mass damper with varying friction coefficient

Abstract: SUMMARY Tuned mass dampers with viscous damping and isolation systems with friction pendulums have been proposed and widely applied over the past several decades. By combining these two ideas, a friction pendulum tuned mass damper (FPTMD) is proposed in this study. Because the restoring and friction forces are provided by the spherical surface of the FPTMD, springs and dampers are not needed. Moreover, suspension is not necessary and the installation space is greatly reduced. The optimal design of the FPTMD with varying friction coefficients for wind-excited high-rise structures is investigated. The optimization procedures are demonstrated by the FPTMD implemented on Taipei 101 under white-noise wind force, and the optimization results are validated by three-dimensional graphs. From the results of the sensitivity study, the effectiveness of the FPTMD with two different patterns of friction coefficients is sensitive to the tuning frequency ratio but not very sensitive to the friction parameters. Moreover, an FPTMD with a friction coefficient that linearly varies with displacement is even less sensitive to the friction parameters and the amplitude of excitation. The feasibility of the FPTMD with two different patterns of friction coefficient is illustrated by Taipei 101 subjected to the design wind force with a return period of 50 years. Following design optimization and numerical verification, the effect of vibration reduction for Taipei 101 is demonstrated. Copyright © 2012 John Wiley & Sons, Ltd.

Optimal design of an array of active tuned mass dampers for wind-exposed high-rise buildings

Abstract: In this paper, a comprehensive procedure is developed for the optimization of a hybrid control system for tall buildings subjected to wind-induced vibrations. The control system is made of active tuned mass dampers (ATMDs) and is conceived to mitigate the flexural and torsional response in serviceability limit state conditions. The feedback information necessary to compute the control forces is provided by a limited number of accelerometers arranged over the building's height. To reduce the computational effort, subsequent optimization subprocedures are employed that take advantage of the genetic algorithm to find the solution of the nonlinear, constrained optimization problems. At first, the optimization of the ATMDs' number and positions over the top floor of the building is carried out. Then, the optimal location of the accelerometers over the building's height is obtained. The reduction of the flexural and torsional accelerations is chosen as target of the optimization problem. The technical limitations of the ATMDs, such as the actuators saturation and the limited stroke extensions, are the constraints to the problem. As an illustrative example, a control system is optimized for the response mitigation of a tall building subjected to wind load. Copyright © 2012 John Wiley & Sons, Ltd.

Reference-free delamination detection using Lamb waves

Abstract: SUMMARY This paper presents a new Lamb wave-based delamination detection technology that allows detection of delamination in a single wave propagation path without using prior baseline data or a predetermined decision boundary. This study shows that, if delamination exists along a wave propagation path, the first arrival antisymmetric (A0 )m ode is followed by other A0 modes reflected from the inside of the delamination. Unlike other conventional Lamb wave techniques, the proposed technique takes advantage of the first A0 mode reflected from the inside delamination to instantly identify the existence of delamination. First, the proposed study employs a dual piezoelectric transducer (PZT), which is composed of a concentric ring and disk PZT segments, for Lamb wave excitation and a circular PZT for sensing. By activating either the circular or ring PZT segment separately, two pitch-catch Lamb wave signals are obtained from the single wave propagation path. Then, a normalized A0 mode signal is decomposed from the measured Lamb wave signals using a previously developed mode decomposition technique, and the first A0 mode reflected from the delamination is further extracted by using a matching pursuit algorithm. Finally, a reference-free damage classifier is built on the extracted A0 mode reflection from the delamination. Because the proposed technique does not require baseline signals during the entire delamination detection process, robust delamination detection has been achieved even under varying temperature conditions. Copyright © 2013 John Wiley & Sons, Ltd.

Damage detection in slab‐on‐girder bridges using vibration characteristics

Abstract: This paper develops and applies a multi-criteria procedure, incorporating changes in natural frequencies, modal flexibility and the modal strain energy, for damage detection in slab-on-girder bridges. The proposed procedure is first validated through experimental testing of a model bridge. Numerically simulated modal data obtained through finite element analyses are then used to evaluate the vibration parameters before and after damage and used as the indices for assessment of the state of structural health. The procedure is illustrated by its application to full scale slab-on-girder bridges under different damage scenarios involving single and multiple damages on the deck and girders.