Showing papers in "Structural Control & Health Monitoring in 2008"

A review of loading conditions for railway track structures due to train and track vertical interaction

Abstract: Train and track interactions during services normally generate substantial forces on railway tracks. Such forces are transient by nature and of relatively large magnitude and are referred to as impact loading. There has been no comprehensive review of the typical characteristics of the loading conditions for railway track structures, in particular, impact loads due to the wheel/rail interaction, published in the literature. This paper presents a review of basic design concepts for railway tracks, abnormalities on tracks, and a variety of typical dynamic impact loadings imparted by wheel/rail interaction and irregularities. The characteristics of typical impact loads due to wheel and rail irregularities, e.g. rail corrugation, wheel flats and shells, worn wheel and rail profiles, bad welds or joints, and track imperfections, are presented with particular emphasis on the typical shapes of the impact load waveforms generally found on railway tracks. Copyright © 2007 John Wiley & Sons, Ltd.

A review of nonlinear dynamics applications to structural health monitoring

Abstract: The process of implementing a damage detection strategy for aerospace, civil and mechanical engineering infrastructure is referred to as structural health monitoring (SHM). In many cases damage causes a structure that initially behaves in a predominantly linear manner to exhibit nonlinear response when subject to its operating environment. The formation of cracks that subsequently open and close under operating loads is an example of such damage. The damage detection process can be significantly enhanced if one takes advantage of these nonlinear effects when extracting damage-sensitive features from measured data. This paper will provide a review of examples from nonlinear dynamical systems theory and from nonlinear system identification techniques that are used for the feature-extraction portion of the damage detection process. This paper is not intended as a comprehensive review of all damage detection methods rooted in nonlinear dynamics, but rather to provide a number of illustrations of complimentary approaches where damage-sensitive data features are based on nonlinear system response. These features, in turn, can either be used as a direct diagnosis of damage or as input to statistical damage classifier. Copyright © 2007 John Wiley & Sons, Ltd.

Testing and modeling of square carbon fiber‐reinforced elastomeric seismic isolators

Abstract: Steel-reinforced elastomeric isolators (SREIs), as an effective seismic isolation device, are the most common isolators in use. However, SREIs are typically heavy and expensive and, as a result, application of these devices is often limited to large and expensive structures. A reduction in the cost and weight of elastomeric isolators would permit a significant increase in their application to many ordinary residential and commercial buildings. Fiber-reinforced elastomeric isolators (FREIs) are a new type of elastomeric bearing that employs fiber as the reinforcement material rather than steel. FREIs have several advantages over traditional SREIs including superior damping properties, lower manufacturing cost, light weight, and the possibility of being produced in long rectangular strips with individual isolators cut to the required size. This paper presents a brief literature review on experiments with FREIs, and reports on an experimental study conducted on carbon FREIs from which the mechanical properties of the bearings, including displacement characteristics and damping values are evaluated. A brief description of an analytical approach to model the cyclic response of the bearings is another component of this paper. As a special application, the bearings considered in this study were not bonded to the test platens. For bearings having suitable aspect ratio values, this particular type of application resulted in a stable rollover deformation, which reduced the horizontal stiffness and increased the efficiency of the bearing as a seismic isolator device. Test results suggest that for many high seismic risk regions worldwide, the application considered in this study can be viable for the base isolation of ordinary low-rise buildings. Copyright © 2007 John Wiley & Sons, Ltd.

Impulse response analysis of the Van Nuys 7-storey hotel during 11 earthquakes and earthquake damage detection

Abstract: Wave travel times of vertically propagating waves are measured in the Van Nuys 7-storey hotel, located in the Los Angeles metropolitan area, from impulse response functions computed by deconvolution of the recorded earthquake response. East-West response data from 11 earthquakes over a period of 24 years are analysed. Changes in wave travel times are used to infer about local (between sensors) and global changes of structural stiffness, from one event to another, and with time during the earthquakes that damaged the building (San Fernando and Northridge). Also, wave travel times are used to estimate the fundamental fixed-base frequency of the building, f1, which is compared with independent estimates of the soil–structure system frequency fsys during the same earthquakes and during five ambient vibration tests. The analysis shows that monitoring only the changes of fsys can be misleading for structural health monitoring and can lead to erroneous alarms, while monitoring changes of f1 over suitably chosen time windows (before, during, and after excitation by strong earthquake motions) can be a powerful and robust tool for structural health monitoring. It is concluded that, under favourable conditions, this method can be used as a tool for global and local structural health monitoring. Copyright © 2007 John Wiley & Sons, Ltd.

Negative stiffness characteristics of active and semi-active control systems for stay cables

Abstract: This paper examines the characteristics and effects of negative stiffness with active and semi-active control of stay cables for vibration reduction. The characteristics of negative stiffness of stay cable active and semi-active control are presented through numerical simulations. Three indices are defined to quantify the degree of negative stiffness. A pseudo-viscoelastic (P-VE) damper is proposed to replace an active or semi-active device to carry out numerical and theoretical analysis. An asymptotic solution of damping ratio of the combined cable/P-VE damper is obtained and the approximate optimal damping ratio of the combined cable/P-VE damper is further derived. The relationship between optimal damping ratio and the damping coefficient of the P-VE damper with various stiffness (both of negative and positive stiffness) is calculated by using the asymptotic solution and compared with that evaluated by using numerical analysis. The effect of negative stiffness on response reduction of cables is demonstrated by the improved energy dissipation ability of the damper and the increased approximate damping ratio because of the enhanced displacement of the cable at the location where it is attached to the damper. Copyright © 2007 John Wiley & Sons, Ltd.

Stiffness identification and damage localization via differential evolution algorithms

Abstract: The goal of structural health monitoring is to identify which discrepancies between the actual behaviour of a structure and its reference undamaged state are indicative of damage. For this purpose, an objective function, which minimizes the difference between the measured and theoretical modal characteristics of the structure, is formulated. By selecting the stiffness parameters as optimization variables, a differential evolution algorithm is applied to create successive generations that better reflect the measured response, until a certain tolerance is met. At each step of the algorithm, the current modal parameters are re-calculated from the new generation of stiffness matrices to estimate the value of the objective function. This procedure represents a favourable path to solve the so-called ‘inverse problem’. Furthermore, the comparison of the identified stiffness matrix with the initial one allows for damage detection and localization. A numerical example, where a generic structure is discretized into finite elements, is provided. Copyright © 2008 John Wiley & Sons, Ltd.

Cable-deck dynamic interactions at the International Guadiana Bridge: On-site measurements and finite element modelling

Abstract: SUMMARY The International Guadiana Bridge is a cable-stayed bridge crossing the Guadiana River, which marks the southern border between Portugal and Spain. The bridge has a central span of 324 m and a total length of 666 m and was open to traffic in 1991. Despite the globally satisfying behaviour under the common environmental loads, which largely ensures the bridge serviceability, frequent episodes of cable vibration have been observed since completion of the construction. In this paper, the bridge modal properties and dynamic behaviour, identified from repeated campaigns of vibration data acquisitions, are compared with the response of a three-dimensional finite element model, including the description of the cable transversal motion. The model, after minor updating, furnishes a realistic reproduction of the current bridge dynamic behaviour. Then different possible justifications of the local vibrations are evaluated, briefly scanning the known sources of large amplitude cable oscillations both in the linear and the nonlinear field. In particular, the occurrence of different internal resonance conditions is deeply discussed, in order to verify whether the experimental observations could be really justified by a cable–deck dynamic interaction mechanism. Among different possibilities, a beating phenomenon between two resonant modes, amplified by the lower damping and inertial characteristics of the local mode with respect to the global one, is selected as the most critical cable excitation source. Since the cable vibrations are proved to persist for different wind conditions, the heavy traffic load on the bridge deck is investigated as one possible source of the global mode direct excitation. On this respect, the model response to random load and moving forces acting on the bridge deck is numerically evaluated evidencing how some particular features of the real bridge behaviour can be qualitatively reproduced. Copyright # 2008 John Wiley & Sons, Ltd.

Implementation of a closed-loop structural control system using wireless sensor networks

Abstract: Wireless sensor networks have rapidly matured in recent years to offer data acquisition capabilities on par with those of traditional tethered data acquisition systems. Entire structural monitoring systems assembled from wireless sensors have proven to be low cost, easy to install, and accurate. However, the functionality of wireless sensors can be further extended to include actuation capabilities. Wireless sensors capable of actuating a structure could serve as building blocks of future generations of structural control systems. In this study, a wireless sensor prototype capable of data acquisition, computational analysis and actuation is proposed for use in a real-time structural control system. The performance of a wireless control system is illustrated using a full-scale structure controlled by a semi-active magnetorheological (MR) damper and a network of wireless sensors. One wireless sensor designated as a controller automates the task of collecting state data, calculating control forces, and issuing commands to the MR damper, all in real time. Additional wireless sensors are installed to measure the acceleration and velocity response of each system degree of freedom. Base motion is applied to the structure to simulate seismic excitations while the wireless control system mitigates inter-storey drift response of the structure. An optimal linear quadratic regulation solution is formulated for embedment within the computational cores of the wireless sensors. Copyright © 2007 John Wiley & Sons, Ltd.

Semi-active control of structures using neuro-predictive algorithm for MR dampers

Abstract: A semi-active control method for a seismically excited nonlinear benchmark building equipped with a magnetorheological (MR) damper is presented and evaluated. A linear quadratic Gaussian (LQG) controller is designed to estimate the optimal control force. The required voltage for the MR damper to produce the control force estimated by LQG controller is calculated by a neural network predictive control algorithm (NNPC). The LQG controller and the NNPC are linked to control the structure. The coupled LQG and NNPC system are then used to train a semi-active neuro-controller designated as SANC, which produces the necessary control voltage that actuates the MR damper. The effectiveness of the NNPC and SANC is illustrated and verified using simulated response of a 3-story full-scale, nonlinear, seismically excited, benchmark building excited by several historical earthquake records. The semi-active system using the NNPC algorithm is compared with the performances of passive as well as active and clipped optimal control (COC) systems, which are based on the same nominal controller as is used in the NNPC algorithm. The results demonstrate that the SANC algorithm is quite effective in seismic response reduction for wide range of motions from moderate to severe seismic events, compared with the passive systems and performs better than active and COC systems. Copyright © 2008 John Wiley & Sons, Ltd.

Loading models and response control of footbridges excited by running pedestrians

Abstract: New trends in the architectural design of footbridges feature an unprecedented slenderness, especially when these are located in the urban environment. For this reason, static analyses and a design towards the ultimate limit state have proven inadequate in many circumstances, and the main objective in the structural design is becoming that of assessing the serviceability limit state through dynamic analyses. On the other hand, the key issue of dynamic analyses is the availability of reliable models for the structure and for loads, and in the particular case of pedestrian action the lack of commonly accepted models for walking, running and jumping has become the weak link in the whole structural design process. In a first stage of the present work, vibration measurements were taken on a recently built cable-stayed footbridge, whose second vibration mode was excited by runners. As a second step, a dynamic loading model for the vertical component of the running-induced force was developed, which was used for the finite element analyses of the footbridge. Finally, tuned mass dampers (TMDs) represent a quite mature technology for reducing the resonant response of flexible structures, but their effectiveness is heavily dependent on the tuning ratio. In the case of footbridges, pedestrians can act as a significant part of the vibrating mass; thus, varying the vibration frequency, which makes it difficult to properly tune the damper frequency. Semi-active TMDs can be looked at as passive devices able to adjust their dynamic parameters according to a given control logic. A physical description of a control algorithm is given in the paper, and its performance is discussed. Copyright © 2008 John Wiley & Sons, Ltd.

Superelastic semi‐active damping of a base‐isolated structure

Abstract: A hybrid base isolation system that is composed of linear elastomeric bearings (EB), friction-pendulum bearings (FPB), shape memory alloy (SMA) wires, and magnetorheological (MR) dampers is proposed for the mitigation of seismic motions. Each subcomponent of the isolation system is employed for a unique task in managing superstructure response when ground motions are experienced. EB are provided to couple the superstructure and substructure in the vertical direction while partially decoupling the superstructure from lateral ground motion. FPB provide support for gravity loads and a restoring force when base drifts become large. SMA wires supply recoverable hysteretic behavior and serve as an additional restoring force. Finally, MR dampers provide variable viscous damping that can be altered in real time for intelligent amelioration of superstructure response. Neuro-fuzzy techniques are incorporated to model SMA and MR elements. A fuzzy logic controller is generated using a multi-objective genetic algorithm for optimal modulation of MR damper resistance levels. To evaluate performance of the proposed isolation system the Phase II, Part IV Base Isolation Benchmark Problem is adopted and standard performance metrics are considered. The nominal isolation system of the problem statement is augmented with SMA and MR devices. Hysteretic behavior of each device is analyzed and their complimentary behavior is identified. Results of several control cases are provided that include semi-active and passive operation of the MR dampers and several configurations of SMA wires. Results show that the proposed superelastic semi-active base isolation system can reduce base drifts by 18% and maintain favorable superstructure response. Copyright © 2008 John Wiley & Sons, Ltd.

Variable friction pendulum system for near‐fault ground motions

Abstract: The seismic response of a flexible multi-storey building isolated with variable friction pendulum system (VFPS) is investigated under normal component of six near-fault ground motions. The VFPS, an advanced friction base isolator, is proposed in this study. The variation of the friction coefficient in VFPS is such that up to a certain value of displacement the frictional force increases and then it decreases with further increase in displacement. Such variation is selected with the criterion that the isolator displacement and building base shear decrease significantly without much alteration to superstructure acceleration under near-fault ground motions. The governing equations of motion of building isolated with VFPS are derived and solved in the incremental form by using Newmark's step-by-step method assuming linear variation of acceleration over small time interval. In order to verify the effectiveness of VFPS, the seismic responses are compared with that of the same building isolated by conventional friction pendulum system (FPS). A parametric study is also carried out to critically examine the behaviour of building isolated with VFPS. The important parameters considered are the number of stories of superstructure, initial time period and isolation period of the VFPS. In addition, the seismic response of building isolated with VFPS under simple cycloidal pulses is also investigated. From the above investigations, it was concluded that the VFPS is quite effective for controlling the seismic response of buildings under near-fault ground motions. Copyright © 2007 John Wiley & Sons, Ltd.

Instantaneous damage detection of bridge structures and experimental verification

Abstract: An extended Kalman filtering (EKF) method was developed and applied to instantaneously identify elemental stiffness values of a structure during damaging seismic events based on vibration measurement. This method is capable of dealing with nonlinear as well as linear structural responses. Identification of the structural elemental stiffness enables location as well as quantification of structural damage. The instantaneous stiffness values during an event can provide highly useful information for post-event capacity estimation. In this study, a large-scale shaking table test of a 3-bent concrete bridge model was performed in order to verify the proposed damage detection method. The bridge model was shaken to different damage levels by a sequence of earthquake motions with increasing intensities. The elemental stiffness values of the structure were instantaneously identified in real time during the damaging earthquake excitations using the EKF method. The identified stiffness degradations and their locations agreed well with the structural damage observed by visual inspection and strain measurements. More importantly, the seismic response accelerations analytically simulated using the instantaneous stiffness values thus identified agreed well with the measured accelerations, demonstrating the accuracy of the identified stiffness. This study presents an experimental verification of a structural damage detection method using a realistic bridge model subjected to realistic seismic damage.

Statistical model‐based damage localization: A combined subspace‐based and substructuring approach

Abstract: This paper addresses the damage localization problem with a statistical model-based approach applied to vibration-based measurements. Damages are viewed as changes in modal parameters. Damage detection is achieved with a subspace-based residual and a global test, which performs a sensitivity analysis of the residual to the modal parameters, relative to uncertainties in those parameters and noises on the data. Damage localization is achieved by plugging the sensitivities of the modal parameters with respect to structural (finite element model) parameters in this decision framework. For large structures that have thousands of elements, a statistical substructuring method, in which the columns of the latter sensitivity matrix are clustered into different classes, is employed. This paper investigates further the clustering step. Numerical results obtained on the finite element model of a bridge deck with a large number of elements are reported. Copyright © 2007 John Wiley & Sons, Ltd.

Acoustic emission source location methods using mode and frequency analysis

Abstract: Traditional acoustic source location techniques are greatly affected by the threshold value of the acoustic signals. Based on the theory of modal acoustic emission, acoustic signals have characteristics indicative of multi-mode, broad band and dispersion. One-dimensional acoustic source location theory is developed to incorporate these characteristics. The arrival times and wave velocities needed for source location are influenced by mode and frequency. Based on the number of sensors used, two universal source location techniques have been investigated. To obtain the arrival time of one mode at certain frequency, the Gabor wavelet transform is applied to the analysis of acoustic signals. One-dimensional acoustic source location experiments in a thin plate have been performed using a lead break as acoustic source. The acoustic source location techniques have been greatly improved and the accuracy of these acoustic source location methods has been verified. Copyright © 2007 John Wiley & Sons, Ltd.

Damage Detection Based On Damping Analysis Of Ambient Vibration Data

Abstract: Enabling an automated, remote and rapid detection of structural damage, sensor-based structural health monitoring is becoming a powerful tool for maintenance of civil engineering structures. In this study, a baseline-free, time-domain damage detection method was developed for concrete structures, which is based on analysis of nonlinear damping from measured structural vibration responses. The efficacy of the proposed method was demonstrated through a large-scale concrete bridge model subjected to different levels of seismic damage caused by shaking table tests. By applying the random decrement signature technique, the proposed method successfully identified, from its ambient vibration responses, nonlinear damping of the bridge associated with the seismic damage. The amount of the nonlinear damping increases as the seismic damage becomes more severe. This paper also compares the damage detection results with those obtained by stiffness-based methods, demonstrating a strong correlation between the increase in nonlinear damping and the decrease in structural stiffness associated with the increase in damage severity. Copyright © 2008 John Wiley & Sons, Ltd.

Damage indicator defined as the distance between ARMA models for structural health monitoring

Abstract: SUMMARY A new damage indicator denoted by the distance between ARMA models is proposed in this paper to identify structural damage including its location and severity. Two definitions are introduced as the distance, either the cepstral metric or subspace angles of ARMA models. However, the accuracy is deteriorated when the multiple inputs have strong correlations. To overcome this difficulty, a pre-whitening filter is applied. Thus, the proposed damage indicator is applicable for varieties of excitation types in civil engineering, such as wind, traffic loading and earthquake excitations. A five-storey building model is used for performance verification when subjected to different excitations. Ambient force and earthquake input have been used as excitations acting on the structure. Two calculating methods of the proposed damage indicators are both evaluated. When the excitations are mutually correlated, by using the pre-whitening filter, the damage identification ability of the proposed damage indicator improves significantly, especially for damage localization. The damage indicator increases monotonically with damage severity, which provides the potential for damage quantification. Copyright # 2007 John Wiley & Sons, Ltd.

A method for large‐scale vulnerability assessment of historic towers

Abstract: The methods commonly pursued for vulnerability or risk analysis, when carried out at large scale (territorial, regional), are mainly based on qualitative parameters, due to the necessity of processing a huge number of structures. As a matter of fact, the final target of these methods is the correlation of the most representative parameters of the structural behaviour, so as to finally rank the same structures according to their level of vulnerability or associated risk. These methods are undoubtedly effective for large-scale analyses and statistical elaborations, although affected by a certain level of subjectivity. Moreover, the vulnerability is commonly defined by hybrid indices (more often obtained as score assignment or expert opinions) that do not represent physical entities. A validation of these methods can be pursued by data on earthquake damage, which are very helpful for calibrating the analysis and formulating fragility curves. Experimental data and in situ investigations can also help to enhance the effectiveness of the method and to finally calibrate the same results. These are particularly needed for very slender structures, as towers and similar structures, the behaviour of which is strongly influenced by the dynamic performance. This paper presents a new method, named Vulnerability of Towers (VULNeT), for assessing the seismic vulnerability of slender structures (particularly towers) according to two different levels of accuracy. The method is based on qualitative parameters, collected through a new survey form, on purpose developed with an online data storage, capable of running speedy analyses. Once the method has been introduced, some applications on different samples are shown and the result compared with those of other recent works obtained from the literature on the topic. The final part of this paper provides a general framework on possible enhancements of VULNeT. Data on the dynamic behaviour of the structure obtained through experimental campaigns and structural modelling are introduced, and these are conceived for validating the results achieved as well as to check the feasibility of the possible failure mechanisms included in the form. Finally, the first results obtained on a pilot application on the Febonio tower of Trasacco (Abruzzo, Italy, where in situ investigations and F.E. structural modelling were carried out), are presented, and some conclusions are drawn on the future development of the work. Copyright © 2008 John Wiley & Sons, Ltd.

The use of macro‐fibre composites for pipeline structural health assessment

Abstract: Pipeline structures are susceptible to cracks, corrosion, and other ageing defects. If left undetected, these forms of damage can lead to the failure of the pipeline system, which may have catastrophic consequences. Most current forms of health monitoring for pipeline systems involve non-destructive evaluation (NDE) techniques. These techniques require sophisticated instruments and direct access to the structure, which is not always possible for civil pipeline applications. This research proposes the use of macro-fibre composite (MFC) transducers for real-time structural health monitoring in pipeline systems. In particular, we propose the coupled implementation of impedance-based and Lamb wave-based methods that are simultaneously used to accurately determine the health of a pipeline network. The self-sensing impedance methods are used to detect structural damage occurring at pipeline connection joints, while the Lamb wave propagation measurements identify cracks and corrosion along the surface and through the wall thickness of the pipe structure. Both methods utilize the same MFC active sensors, which are flexible, durable, relatively inexpensive, and can be permanently bonded to the surface of a pipe during installation. Therefore, measurements for damage identification can be performed at any time, even while the system is in operation. Based on the success of this study, guidelines are outlined for the full-scale development of a low-cost, active-sensing-based SHM system suitable for pipeline applications. Published in 2007 by John Wiley & Sons, Ltd.

Vibration analysis and dynamic characterization of the Colosseum

Abstract: Extensive measurements of the dynamic response of the Colosseum induced by road traffic were performed The vibration level was evaluated in accordance with international standards to investigate the potential for structural damage Using only the response, the natural frequencies and mode shapes of the monument were determined and compared with numerical ones, which were obtained by a finite element model The mechanical properties of the model were updated in order to adapt the analytical response to the experimental one Details of the test methods, measurements and analysis procedures and the major findings of the investigation are presented Copyright © 2008 John Wiley & Sons, Ltd

Probabilistically robust nonlinear design of control systems for base‐isolated structures

Abstract: A stochastic-simulation-based nonlinear controller design for base-isolation systems is discussed in this study. The performance objective is the maximization of structural reliability, quantified as the probability, based on the available information, that the structural response trajectory will not exceed acceptable thresholds. A simulation-based approach is implemented for evaluation of the performance of the controlled system. This approach explicitly takes into account nonlinear characteristics of the structural response and the control law in the design process. A realistic probabilistic model for representation of near-fault ground motions is adopted in the design stage. The variability of future earthquake events is addressed by incorporating a probabilistic description for the ground-motion model parameters, leading to a design approach that is robust to probabilistic uncertainty. The methodology is illustrated through application to the base-isolated benchmark building with elastomeric and friction pendulum isolators and an array of regenerative force actuators. Skyhook control implementation is considered and an efficient scheme is presented for the clipping of the control forces in order to satisfy the actuator force constraints. The performance of the controlled system is evaluated under seven earthquake records using a number of metrics. Comparison with the performance of a similar network of viscous dampers is also discussed.

Bayesian wavelet methodology for structural damage detection

Abstract: Accurate structural damage detection is still a challenging problem due to the complicated nonlinear behavior of structural system, incomplete sensed data, presence of noise in the data, and uncertainties in both experimental measurement and analytic model. This paper presents a Bayesian wavelet probabilistic methodology for nonparametric damage detection to address the above-mentioned issues. A Bayesian discrete wavelet packet transform-based denoising approach is employed to perform data cleansing prior to damage detection. A nonparametric system identification method, based on fuzzy wavelet neural networks, is applied to predict dynamic responses of the structure subjected to external excitation. Bayesian hypothesis testing is developed to assess the difference between the sensed data and model prediction. The Bayesian assessment metric is treated as a random variable and its probability density function is constructed using Monte Carlo simulation technique to incorporate possible uncertainties. The evaluation method provides quantitative information about the condition of a structural system. The methodology is validated using the sensed data collected from a 5-story test steel frame and a 38-story concrete building model. Both the original and denoised data are used in the damage detection to investigate the effects of noise on detection accuracy. Numerical results demonstrate that the proposed methodology provides an effective metric to quantify the confidence in the damage detection. Copyright © 2007 John Wiley & Sons, Ltd.

Robust active control of hysteretic base-isolated structures: Application to the benchmark smart base-isolated building

Abstract: The main objective of applying robust active control to base-isolated structures is to protect them in the event of an earthquake. Taking advantage of the discontinuous control theory, a static discontinuous active bang-bang type control is developed using only the measure of the velocity at the base as a feedback. Moreover, due to that in many engineering applications, accelerometers are the only devices that provide information available for feedback; our velocity feedback controller could be easily extended by using just acceleration information through a filter. The main contributions of this paper are the development and application of (a) a static velocity feedback controller design and (b) a dynamic acceleration feedback controller design to a benchmark problem, which is recognized as a state-of-the-art model for numerical experiments of seismic control attenuation. The performance indices show that the proposed controller behaves satisfactorily and with a reasonable control effort. Copyright © 2008 John Wiley & Sons, Ltd.

Condition assessment and dynamic system identification of a historic suspension footbridge

Abstract: The Morca footbridge is a historic suspension bridge, originally constructed in 1928. The bridge is characterized by the timber deck supported by two pairs of cables that were recently replaced. After the strengthening, an extensive program of non-destructive global tests was carried out; the field tests included complete geometric survey of the deformed configuration due to dead loads, static load tests and ambient vibration measurements. The experimental investigation was complemented by the development of a 3D finite element model and the correlation between theoretical and experimental results suggested a satisfactory state of preservation of the deck timber elements but also the potential weakness of some nodes. Hence, the dynamic tests were repeated in order to evaluate any changes in the modal parameters and the correlation with structural modifications or damage. Copyright © 2008 John Wiley & Sons, Ltd.

Structural control benchmark problem: Phase II—Nonlinear smart base-isolated building subjected to near-fault earthquakes

Abstract: SUMMARY Many branches of engineering, mathematics, and sciences, have relied on benchmark problems as a standard means to compare different solution techniques. Since 1996, the ASCE Structural Control and Monitoring Committee and Task Group on Benchmark Problems, the U.S. Panel on structural control, and IASCM have developed a series of benchmark control problems that offer a set of carefully modeled real-world structures in which different control strategies can be implemented, evaluated, and compared using a common set of performance indices. First-, second- and third-generation benchmark problems focusing on the response control of seismic and wind-excited buildings, and seismically excited long-span cable-stayed bridges have been developed and evaluated. The U.S. Panel on structural control and monitoring (currently chaired by Professor Satish Nagarajaiah, Rice University, Houston, TX), IASCM, and the ASCE structural control and monitoring committee have developed a new benchmark study to compare control strategies designed for a base-isolated building subjected to strong near-fault pulse-like ground motions. The special issue on phase I smart base-isolated building benchmark problem with a linear isolation system was successfully completed and published. This special issue focuses on the phase II smart base-isolated building benchmark problem with nonlinear isolation systems—friction or elastomeric system. Copyright r 2008 John Wiley & Sons, Ltd.

A novel health assessment technique with minimum information

Abstract: SUMMARY A novel structural health assessment method is proposed for detecting defects at the element level using only minimum measured response information considering imperfect mathematical model representing the structure, various sources of uncertainty in the mathematical model, and uncertainty in the measured response information. It is a linear time domain finite element-based system identification procedure capable of detecting defects at the local element level using only limited response measurements and without using any information on the exciting dynamic forces. The technique is a combination of the modified iterative least-squares technique with unknown input excitation (MILS-UI) proposed earlier by the research team at the University of Arizona and the extended Kalman filter with a weighted global iteration (EKF-WGI) procedures to address different sources of uncertainty in the problem. The authors denote the new method as GILS-EKF-UI. To implement the concept, a two-stage substructure approach is used. In the first stage, a substructure is selected that satisfies all the requirements for the MILS-UI procedure. This provides information on the initial state vector and the input excitation required to implement any EKF-based procedure. In the second stage, the EKF-WGI is applied to identify the whole structure. This way the whole structure, defect free or defective, can be identified with limited response measurements in the presence of uncertainty and without using excitation information. The theoretical concept behind this novel method is presented in this paper. Copyright # 2007 John Wiley & Sons, Ltd.

Acceleration-based damage indicators for building structures using neural network emulators

Abstract: SUMMARY In this paper we propose the use of artificial neural network (ANN) emulators for an acceleration-based approach to evaluating building structures under earthquake excitation. The input layer of the ANN is a forced vibration, described as ground acceleration and the acceleration data of several floors. The approach is improved by using the acceleration at later time steps as the output of the neural network (NN). This time delay is optimized as a tunable band to provide the most sensitive signals. Minimally, this approach requires only one sensor, making it highly practicable and flexible. It is applicable to structures under diverse excitations including even very small impacts. Based on numerical simulation of a 5-story shear structure, we determined appropriate parameters for use of an NN and studied the generality and efficacy of the approach. The damage index, the relative root mean square error, was calculated for the case of a single structural damage as well as for cases of double damages at different damage locations, and appropriate parameters for the NN emulator were proposed according to the damage patterns. Variant ground motions were used to certify the generality of the approach. The numerical simulations of the proposed approach were verified experimentally. Copyright # 2007 John Wiley & Sons, Ltd.

Mitigation of seismic responses on building structures using MR dampers with Lyapunov‐based control

Abstract: As losses of human lives and damages to buildings frequently occur during earthquake periods, it is crucial to mitigate structural vibrations. This paper describes the development of a Lyapunov-based control approach for magnetorheological (MR) dampers integrated in building structures to suppress quake-induced vibrations. In this work, MR dampers are used as semi-active devices, taking the advantages of the fail-safe operation and low power consumption. The control of MR dampers is, however, hindered by their hysteretic force–velocity responses and usually leads to indirect strategies compromising controllability and performance. To enhance the system performance, a Lyapunov-based controller is proposed here for direct control of the supply currents to the dampers for a multi-storey building. The dampers are configured in a differential mode to counteract the force-offset problem from the use of a single damper. The effectiveness of the proposed technique is verified, in simulations, by using a building model subject to quake-like excitations. Copyright © 2007 John Wiley & Sons, Ltd.

Application of angular-mass dampers to base-isolated benchmark building

Abstract: The angular-mass damper referred herein is a wheel that has concentrated mass on its perimeter. The rotational inertia of the wheel is employed to dampen the relative acceleration between two joints. The application of this kind of damper to civil engineering structures would be practical since the damper works passively. Since the angular mass is much more significant than the translational mass, the rotational mass damper does not significantly increase the translational earthquake-induced force. To study the effectiveness of the device in the seismically excited structures, the device is applied to the base-isolated benchmark building. The results show that the device can further reduce the responses of the already dampened benchmark building. Copyright © 2008 John Wiley & Sons, Ltd.

Adaptive control of nonlinear smart base‐isolated buildings using Gaussian kernel functions

Abstract: In this paper, a direct adaptive control scheme using Gaussian kernel functions is presented for the active control of nonlinear base-isolated buildings. The control scheme is based on direct adaptive control where the system response is made to follow a desired trajectory. The number of kernel functions is adaptively estimated using a growing and pruning strategy which results in the reduction of the computational overhead. Stable adaptive parameter update laws for Gaussian kernels are derived using Lyapunov approach. Performance of the proposed control scheme is evaluated on the recently developed nonlinear three-dimensional base-isolated benchmark structure. The analytical model of the benchmark structure is highly complex due its three-dimensional nature incorporating lateral and torsional responses, the biaxial interaction of the nonlinear bearings at the isolation layer, and strong coupling between the isolation level forces and the superstructure responses. Control action is provided by eight actuators distributed at the isolation level in each principal direction of the structure, and utilizing the state information corresponding to the base of the structure only. Results are presented using a comprehensive set of the performance indices to realistically quantify the trade-offs associated with the control of nonlinear base-isolated buildings. The main advantages of the adaptive controller presented in this paper are: (i) the control algorithm does not require estimating the system parameters, specifically, mass, stiffness and damping, (ii) the exact nature of the nonlinear dynamics need not be known, and (iii) the control synthesis is noniterative, and on-line. Copyright © 2007 John Wiley & Sons, Ltd.