Showing papers in "Structural Control & Health Monitoring in 2014"

Crack detection and characterization techniques - An overview

Abstract: SUMMARY Crack occurrence and propagation are among critical factors that affect the performance and lifespan of civil infrastructures such as bridges, pipelines, and so on. As a consequence, numerous crack detection and characterization techniques have been researched and developed in the past decades in the areas of SHM and non-destructive evaluation (NDE). The significant amount of performed studies and the large number of publications give rise to the need to systematize, condensate, and summarize this enormous effort. The aims of this paper are to summarize the knowledge about cracking and its sources, review both existing and emerging methods for crack detection and characterization, and identify the advantages and challenges for these methods. In general, this paper identifies two sensing approaches (direct and indirect) and two data analysis approaches (model-based and model-free or data-driven) along with a range of associated technologies. The advantages and challenges of each approach and technology are discussed and summarized, and the future research needs are identified. This paper is intended to serve as a reference for researchers who are interested in crack detection and characterization as well as for those who are generally interested in SHM and NDE. Copyright © 2014 John Wiley & Sons, Ltd.

Constructing statistical models for arch dam deformation

Abstract: In its lifetime, a dam can be exposed to significant water level variations and seasonal environmental temperature changes. The structural safety control of a concrete dam is supported by monitoring activities and is based on models. In practice, the interpretation of recorded concrete dam displacements is usually based on HST (hydrostatic, seasonal, time) statistical models. These models are widely used and consider that the thermal effect can be represented by a seasonal function. The main purpose of this paper is to present an HTT (hydrostatic, thermal, time) statistical model to interpret recorded concrete dam displacements. The idea is to replace the seasonal function with the use of recorded temperatures that better represent the thermal effect on dam behavior. Two new methodologies are presented for constructing HTT statistical models, both based on principal component analysis applied to recorded temperatures in the concrete dam body. In the first method, principal component analysis is used to choose the thermometers for the construction of the HTT model. In the second method, the thermal effect is represented by the principal components of temperature of selected thermometers. The advantage of these methods is that the thermal effect is represented by real temperature measured in the concrete dam body. The HTT statistical models proposed are applied to the 110 m high Alto Lindoso arch dam, and the results are compared with the HST displacement model. Copyright © 2013 John Wiley & Sons, Ltd.

Value of information: impact of monitoring on decision-making

Abstract: Structural health monitoring (SHM) is a process aimed at providing accurate and in-time information concerning structural health condition and performance, which can serve as an objective basis for decision-making regarding operation, maintenance, and repair. However, at the current state of practice, SHM is less used on real structures, and one reason for this is the lack of understanding of the Value of Information obtained from SHM. Consequently, even when SHM is implemented, bridge managers often make decisions based on experience or common sense, frequently considering with reserve and sometimes disregarding the suggestions arising from SHM. Managers weigh the SHM results based on their prior perception of the state of the structure and the confidence that they have in the specific applied SHM system and then make decisions considering the perceived effects of the actions they can undertake. In order to address and overcome the aforementioned identified limitations in the use of the SHM, a rational framework for assessment of the impact of the SHM on decision-making is researched and proposed in this paper. The framework is based on the concept of Value of Information and demonstrated on the case study of the Streicker Bridge, a new pedestrian bridge on Princeton University campus.

SMC structural health monitoring benchmark problem using monitored data from an actual cable-stayed bridge

Abstract: A structural health monitoring (SHM) system provides an efficient way to diagnose the condition of critical and large-scale structures such as long-span bridges. With the development of SHM techniques, numerous condition assessment and damage diagnosis methods have been developed to monitor the evolution of deterioration and long-term structural performance of such structures, as well as to conduct rapid damage and post-disaster assessments. However, the condition assessment and the damage detection methods described in the literature are usually validated by numerical simulation and/or laboratory testing of small-scale structures with assumed deterioration models and artificial damage, which makes the comparison of different methods invalid and unconvincing to a certain extent. This paper presents a full-scale bridge benchmark problem organized by the Center of Structural Monitoring and Control at the Harbin Institute of Technology. The benchmark bridge structure, the SHM system, the finite element model of the bridge, and the monitored data are presented in detail. Focusing on two critical and vulnerable components of cable-stayed bridges, two benchmark problems are proposed on the basis of the field monitoring data from the full-scale bridge, that is, condition assessment of stay cables (Benchmark Problem 1) and damage detection of bridge girders (Benchmark Problem 2). For Benchmark Problem 1, the monitored cable stresses and the fatigue properties of the deteriorated steel wires and cables are presented. The fatigue life prediction model and the residual fatigue life assessment of the cables are the foci of this problem. For Benchmark Problem 2, several damage patterns were observed for the cable-stayed bridge. The acceleration time histories, together with the environmental conditions during the damage development process of the bridge, are provided. Researchers are encouraged to detect and to localize the damage and the damage development process. All the datasets and detailed descriptions, including the cable stresses, the acceleration datasets, and the finite element model, are available on the Structural Monitoring and Control website (http://smc.hit.edu.cn). Copyright © 2013 John Wiley & Sons, Ltd.

Study on semi‐active tuned mass damper with variable damping and stiffness under seismic excitations

Abstract: SUMMARY In the present study, a semi-active tuned mass damper (STMD) with variable damping coefficient and stiffness is evaluated under seismic excitations. Variation of the damping ratio of the STMD is implemented through tracking the displacement of the STMD.If the tracked amplitude of the STMD is increasing, damping ratio of the STMD is set to zero, or else it is set to an appropriate nonzero value. Stiffness of the STMD is tuned through tracking the displacement of the primary structure, which is analyzed using a short-time Fourier transform-based control algorithm. Both far-field and near-fault ground motions are used to examine the effectiveness of the STMD and the control algorithm. Displacement time history and response (displacement and acceleration) spectra are obtained for the cases of an optimal passive TMD and an STMD. It is found that the STMD with variable damping ratio and frequency can effectively attenuate the seismic responses and outperform the optimal passive TMD. In addition, results are obtained for the case that damage occurs to the primary structure during an earthquake. The study indicates that the STMD controlled by the proposed algorithm can rapidly capture the variation of the structure and remains tuned with the primary structure, whereas the optimal TMD becomes off-tuned when damage occurs. Copyright © 2013 John Wiley & Sons, Ltd.

A European Association for the Control of Structures joint perspective. Recent studies in civil structural control across Europe

Abstract: SUMMARY Structural control has been comprehensively studied over the world as a multidisciplinary research field. The present work is motivated by an attempt to give a common frame to the recent research and applications of structural control technology in civil engineering across Europe. They include novel passive dampers, functional materials and semi-active dampers, active control systems, and their performance investigations. Design methods for the vibrations reduction of buildings, bridges, and wind turbines are discussed with reference to case studies. Control algorithms and dimension reduction techniques are also studied. Adaptation strategies and techniques based on the potential offered by piezoelectricity are reviewed. Copyright © 2014 John Wiley & Sons, Ltd. Copyright © 2014 John Wiley & Sons, Ltd.

Optimum design for more effective tuned mass damper system and its application to base‐isolated buildings

Abstract: SUMMARY Base-isolated structures are vulnerable to long-period ground motions due to resonances. The hybrid control strategy combining traditional tuned mass dampers (TMDs) with base-isolation systems has been proved by some researchers to be effective in preventing the resonant behaviors. However, large space for TMDs is required because of large stroke lengths of TMDs, which may be difficult to realize in practical applications. In this paper, a non-traditional TMD that is directly connected to the ground by a dashpot is adopted to mitigate the resonant behavior of a structure. It is found that the conventional design method of traditional TMDs based on the quasi-fixed points theory cannot provide the global minimum value of the objective function for non-traditional TMD systems. An optimum design method for obtaining a wide suppression bandwidth is proposed. Seismic-induced vibration control for a three degree-of-freedom base-isolated structural system with a non-traditional TMD is studied. The control effect of the optimally designed non-traditional TMD is significantly improved, and the stroke length of the non-traditional TMD is greatly reduced, compared with the traditional TMD during near-field long-period earthquakes. In these regards, non-traditional TMDs may provide a better solution for retrofitting or constructing base-isolated structures. Copyright © 2013 John Wiley & Sons, Ltd.

Simultaneous identification of structural parameters and dynamic input with incomplete output‐only measurements

Abstract: SUMMARY A hybrid heuristic optimization strategy is presented to simultaneously identify structural parameters and, when possible, dynamic input time histories from incomplete sets of output measurements. The proposed strategy combines a novel swarm intelligence algorithm, the artificial bee colony algorithm, with a local search operator, Nelder–Mead simplex method, integrated in a search space reduction approach, so as to improve the convergence efficiency of the overall identification process. Because of the independent nature of the algorithm, a parallel scheme is implemented so as to improve the computational efficiency. If the time histories of the structural response and information about the mass of the structural system are available, then the algorithm can also be used for the identification of the time histories of the dynamic input force through a modified Newmark integration scheme, using the current estimates of the structural parameters. To investigate the applicability of the proposed technique, three numerical examples, two shear-type building models and a coupled building system model under different conditions of data availabilities and noise corruption levels are presented. The results show that the proposed technique is powerful, robust and efficient in the simultaneous identification of the structural parameters and input force even from an incomplete set of noise-contaminated structural response measurements. Copyright © 2013 John Wiley & Sons, Ltd.

Three-dimensional dynamic deflections and natural frequencies of a stiff footbridge based on measurements of collocated sensors

Abstract: SUMMARY A redundant system of collocated geodetic sensors and an accelerometer were used to measure the displacements of a stiff, 40-m-long footbridge excited by essentially vertical synchronized jumps. A main output of the measurements was that excitations produced vertical deflections (of the order of several mm), lateral deflections with an amplitude about half that of the vertical, an effect known for various bridges, and also smaller-scale longitudinal deflections, probably induced by the vertical ones. This combination of deflections in three axes is not an artifact of measuring errors, or of limitations of specific instruments, and do not reflect random effects, for they have been observed in different experiments by different sensors (Global Navigation Satellite System-GNSS, i.e., Global Positioning System-GPS plus the Russian Global Navigation Satellite System-GLONASS), robotic total stations-RTS (or robotic theodolite), and an accelerometer, and were tested through control of the measurement errors from time series reflecting no bridge oscillations. In all cases, sensors provided similar estimates of deflections and of deflection-derived dominant frequencies, which were found equal to those derived from the accelerometer. The proposed instrumentation and methodology seem suitable to control deflections of bridges in cases that visibility of satellites is poor or is disrupted by vehicle circulation. Measurement-based modeling of bridges described might be useful for understanding the structural behavior of bridges under several types of dynamic loads and can contribute to the displacement-based design of structures. Copyright © 2013 John Wiley & Sons, Ltd.

Performance assessment of buildings isolated by shape‐memory‐alloy rubber bearing: Comparison with elastomeric bearing under near‐fault earthquakes

Abstract: The shape-memory-alloy supplemented rubber bearing (SMARB) has been recently proposed as a superior alternative to traditional elastomeric bearing, such as lead-rubber bearing (LRB), because of the dubious performance of conventional bearings under near-fault earthquakes. The present study establishes the significant improvement of performances in SMARB over the LRB in isolating multi-storeyed building frame against earthquakes. The response of the isolated building is evaluated through nonlinear dynamic time-history analysis under a set of recorded, near-fault, fault-normal component of ground motions. The optimal characteristic strengths for both the bearings are obtained through parametric study. The robustness of the improved performances are studied under varying characteristics of the superstructure, isolation bearing, as well as scenarios of seismic loading. It is demonstrated that the improvement of isolation efficiency, accompanied by considerable reduction of peak and residual bearing displacements can be attained by the SMARB over the LRB. The SMARB is also found to be more effective in suppressing transference of high-frequency components of ground motions to the floor acceleration, which is expected to be beneficial for frequency-sensitive equipment, mounted on the floors. Copyright © 2013 John Wiley & Sons, Ltd.

Applications of laser scanning to structures in laboratory tests and field surveys

Abstract: SUMMARY The capabilities and advantages of laser scanners used for structural engineering applications to assess structural damage in laboratory tests and post-earthquake field survey are discussed. High definition laser scans have many advantages over conventional instrumentation in laboratory setting: (i) capture three dimensional global deformations of large objects with accuracy; (ii) enable graphical post-processing based on a location of the acquired points in space and utilizing the light intensity red–green–blue (RGB) values of scanned images; (iii) access to objects without spatial constraint and remote monitoring by scanning from a distance; and (iv) reduce workload for installing instruments. Therefore, high definition laser scans have been successfully used in laboratory tests, and several case studies are presented to demonstrate the accuracy of laser scanner measurements. In addition, this paper makes the first attempt to demonstrate the use of high-definition laser scanning technology in field surveys of structures damaged during January 2010 Haiti earthquake. Damage assessments were performed for several structures using the scans, and the results were compared with qualitative results of visual ground survey. The techniques of gathering and disseminating critical information employed herein is expected to have major impact on future structural laboratory tests and field investigations. Copyright © 2013 John Wiley & Sons, Ltd.

Real-time identification of time-varying tension in stay cables by monitoring cable transversal acceleration

Abstract: SUMMARY Stay cables are critical components in bridges. However, stay cables suffer from severe fatigue damage. Therefore, a monitoring technique to obtain the time history of the tension in stay cables is important. Because the acceleration of stay cables is readily measurable, approaches to identify cable tension based on frequency analysis and monitored cable acceleration have been widely investigated and used in practice. However, this type of approach can only identify a time-invariant tension of a stay cable over a specified duration, not the time-varying tension. This paper proposes an approach to identify the time-varying tension of stay cables by monitoring cable accelerations. The tension variation in stay cables is caused by vehicles passing over the bridge. The real-time identification algorithm that determines the time-varying tension of stay cables is proposed using an extended Kalman filter based on both the transversal monitored acceleration at a single location on the cable and the monitored wind speed on the bridge, where the time-varying tension is a state variable that is identified. A stay cable from the Nanjing Yangtze River No. 3 Bridge was used for the numerical study. The time-varying tension of the stay cable can be identified when either a single vehicle or multiple vehicles pass over the bridge. The robustness of the proposed approach is also investigated through deviations in the initial tension, initial displacement, and velocity of the stay cable. An experiment was conducted on a scaled stay cable with time-varying tension excited by wind. The time-varying cable tension of the cable was identified by the proposed approach and compared with the real time-varying cable tension. The identification accuracy and robustness of the proposed approach is verified through the experiment and numerical study. Copyright © 2013 John Wiley & Sons, Ltd.

An optimised tuned mass damper/harvester device

Abstract: SUMMARY Much work has been conducted on vibration absorbers, such as tuned mass dampers (TMD), where significant energy is extracted from a structure Traditionally, this energy is dissipated through the devices as heat In this paper, the concept of recovering some of this energy electrically and reuse it for structural control or health monitoring is investigated The energy-dissipating damper of a TMD is replaced with an electromagnetic device in order to transform mechanical vibration into electrical energy That gives the possibility of controlled damping force whilst generating useful electrical energy Both analytical and experimental results from an adaptive and a semi-active tuned mass damper/harvester are presented The obtained results suggest that sufficient energy might be harvested for the device to tune itself to optimise vibration suppression Copyright © 2014 John Wiley & Sons, Ltd

Monitoring of structural prestress loss in RC beams by inner distributed Brillouin and fiber Bragg grating sensors on a single optical fiber

Abstract: SUMMARY For the safety of a prestressed structure, prestress loss is a critical issue. Unfortunately, up to date, with consideration of the harsh environment and the durability, no qualified technique is available. This paper proposes a novel technique to monitor prestress loss in RC beams by using combined Brillouin and fiber Bragg grating sensors on a single optical fiber. A smart steel strand based on the newly developed sensor was introduced. Laboratory tests of a series of prestressed RC beams were used to verify the concept of monitoring of prestress loss by using the smart steel strands. The prestress loss obtained from the Brillouin and the fiber Bragg grating sensors are compared with those from the load cell and the structural analysis, which validated the feasibility of the proposed sensing system. The monitoring results from the proposed optical sensors can reveal both the spatial distribution and the time history of prestress loss during the construction and the in-service phase. Copyright © 2013 John Wiley & Sons, Ltd.

Optimal multiaxial sensor placement for modal identification of large structures

Abstract: SUMMARY Research on optimal sensor placement has become a very important topic because of the need to obtain effective testing results with limited testing resources in modal identification and structural health monitoring. An integer-encoding multi-swarm particle swarm optimisation (IMPSO) algorithm is proposed to place multiaxial sensors optimally on large structures for modal identification. The concepts of grade evaluation and migration strategy and the mutation operators of genetic algorithm are introduced into the integer-encoding particle swarm optimisation algorithm. Three different fitness functions for optimal multiaxial sensor placement (OMSP) are investigated. The second fitness function considers spatial correlation based on Moran's I to solve the information redundancy of multiaxial sensor placement, whereas the other two functions evolve from the existing methods for comparison with the second fitness function. The novel algorithm and three fitness functions are further applied to the Laxiwa arch dam for verifications. The results show that the proposed IMPSO outperforms two existing algorithms in its global optimisation capability. The results also prove that the second fitness function has advantages in sensor distribution and ensuring the well-conditioned information matrix and observability of multidimensional modal shapes. The multiaxial sensor placement scheme determined by the proposed method is applied to the modal test of the Laxiwa arch dam under simulative ambient excitation. The results show that the scheme determined by the second fitness function can identify the frequencies and multidimensional mode shapes accurately, indicating that this method may be used to provide guidance for OMSP in various types of large structures. Copyright © 2013 John Wiley & Sons, Ltd.

Monitoring of typhoon effects on a super-tall building in Hong Kong

Abstract: SUMMARY A super-tall building with height of 420 m and 88 floors is located in central Hong Kong. Field monitoring of wind effects on the high-rise structure was conducted during the passage of several typhoons on the basis of a wind and movement monitoring system installed in the building. Field data such as wind speed, wind direction, pressures on cladding, acceleration and displacement responses were simultaneously recorded during the typhoons and then analyzed. Typhoon wind parameters including turbulence intensity, gust factor, peak factor, turbulence integral length scale and power spectral density were presented and discussed. The dynamic properties of the high-rise structure were determined from the field measurements and compared with those calculated at the design stage. The damping ratios of the super-tall building were evaluated by a random decrement technique, which demonstrates amplitude-dependent characteristics. The relationships between the structural dynamic responses and the approaching wind speed were analyzed. Wind tunnel tests were conducted to investigate the wind effects on the super-tall building. The field-measured acceleration responses were found to be consistent with the model test results. Finally, the serviceability performance of the super-tall building during the typhoons was assessed on the basis of the field measurement results. The findings of the paper are expected to be of considerable interest and practical use to professionals and researchers involved in wind-resistant designs of super-tall buildings. Copyright © 2013 John Wiley & Sons, Ltd.

Real‐time hybrid simulation approach for performance validation of structural active control systems: a linear motor actuator based active mass driver case study

Abstract: SUMMARY Small-scale models have been commonly utilized in testing of performance of active mass driver (AMD) control systems. The utmost reason is that physical testing of AMD system at full scale is usually too expensive to afford, as well as hard to implement on site. With reference to the real-time hybrid simulation technology, a real-time AMD subsystem testing method is proposed in this paper. In this method, the entire system is composed of AMD as physical subsystem and target structure as numerical subsystem. The physical test is conducted on AMD subsystem, whereas the numerical simulation is carried out on the structure subsystem. Meanwhile, the real-time data are being communicated between these two subsystems. This method is then applied to the performance validation of a novel AMD control system, which is driven by a linear motor. In the test, a benchmark three-storey frame structure is employed as the numerical subsystem, and earthquake excitations are used as the external input. On the basis of a series of tests, both the time history and the statistical criteria show that the results of AMD subsystem and structure subsystem obtained in the real-time AMD subsystem test agree well with the simulation results. Furthermore, all the test results show good repeatability. Therefore, the feasibility and reliability of the proposed real-time AMD subsystem testing approach for performance validation of AMD subsystem has been demonstrated. Such kind of experimental method is efficient in terms of reducing cost associated with performance validation of large-scale active control systems prior to the implementation. Copyright © 2013 John Wiley & Sons, Ltd.

Blind denoising of structural vibration responses with outliers via principal component pursuit: BLIND DENOISING VIA PRINCIPAL COMPONENT PURSUIT

Abstract: SUMMARY Structural vibration responses themselves contain rich dynamic information, exploiting which can lead to tackling the challenging problem: simultaneous denoising of both gross errors (outliers) and dense noise that are not uncommon in the data acquisition of SHM systems. This paper explicitly takes advantage of the fact that typically only few modes are active in the vibration responses; as such, it is proposed to re-stack the response data matrix to guarantee a low-rank representation, through which even heavy gross and dense noises can be efficiently removed via a new technique termed principal component pursuit (PCP), without the assumption that sensor numbers exceed mode numbers that used to be made in traditional methods. It is found that PCP works extremely well under broad conditions with the simple but effective strategy no more than reshaping the data matrix for a low-rank representation. The proposed PCP denoising algorithm overcomes the traditional PCA (or SVD) and low-pass filter denoising algorithms, which can only handle dense (Gaussian) noise. The application of PCP on the health monitoring data of the New Guangzhou TV Tower (Canton Tower) shows its potential for practical usage. Copyright © 2013 John Wiley & Sons, Ltd.

Durability evaluation of reinforced masonry by fatigue tests and acoustic emission technique

Abstract: SUMMARY An experimental analysis has been carried out on the long-term behaviour of composite specimens and walls made with historical brickwork. The test pieces were subjected both to static and to cyclic loading tests (accelerate static creep) and to freezing-thawing thermo-hygrometric tests in order to study the durability of different strengthening mortars. The acoustic emission (AE) monitoring technique was also used to assess the damage localization and to predict the time to failure of the strengthened walls during fatigue tests. The AE is a very effective non-destructive technique that permits to estimate the amount of energy released during fracture propagation. Damage evolution during fatigue compressive loading, in terms of measured strains and the AE counting number, can be divided into three stages; fatigue life can be predicted from the slope of the AE counting number rate diagram. Starting from an AE monitoring, a special methodology has been developed to predict the service life of creep damaged masonry structures. The novelty of this work consists in the experimental confirmation that the AE technique is able to analyse the creep curves of masonry strengthening mortars. Therefore, it demonstrated that by the AE technique, it is also possible to evaluate the durability of strengthening materials subjected to load histories in the time. Copyright © 2013 John Wiley & Sons, Ltd.

Application of sensor technologies for local and distributed structural health monitoring

Abstract: SUMMARY The work presented in this paper deals with the application of different sensor technologies for fatigue crack damage monitoring of metallic helicopter fuselages A test programme has been conducted, consisting of seven fatigue crack propagation tests on aluminium panels (skin with riveted stringers) representative of the rear fuselage of a helicopter Electrical crack gauges and comparative vacuum monitoring sensors have been used locally to monitor propagating cracks A network of optical fibre Bragg gratings is presented as a valid possibility for distributed monitoring (based on strain field dependence on damage), alternative to consolidated electrical resistance-based strain gauges A Smart Layer based on piezoelectric transducers that emit and receive Lamb wave signals has also been analysed in this paper for distributed monitoring Two damages have been considered: a skin crack artificially initiated on a panel bay and a skin crack propagating from a rivet hole after stringer failure Damage sensitivity has been evaluated and compared among the considered technologies, thus providing useful recommendations for each considered system, together with advantages and drawbacks concerning their suitability for on-board installation and monitoring The damage index sensitivity to operative condition, load in particular, is verified and compared with damage effect for distributed networks A finite element model able to describe any selected feature sensitivity to the monitored damage is also presented as a useful tool for the optimization of the structural health monitoring system design process Copyright © 2013 John Wiley & Sons, Ltd

Free vibration of taut cable with a damper and a spring

Abstract: SUMMARY The damping and frequency of taut cable with a damper and a spring are investigated in this paper, which is motivated by cross-ties and damper that are both utilized in mitigation of oscillation of the stays in cable-stayed bridges or damper located near cable anchorage with rubber bushing. The dynamic characteristics of the cable-damper-spring system are analyzed on the basis of the taut string theory and considering the compatibility requirements on each constraint point. By using a transfer matrix method, the complex frequency equation of the cable-damper-spring system is derived. The complex frequency equation is further re-written in terms of real and imaginary parts. The special limiting solutions are presented. Asymptotic approximate solutions for damper and spring close to cable ends are developed with small frequency shifts between free cable and damped system mode. The effects of spring stiffness and location to maximum cable vibration damping, optimum damper constant, and frequency are also addressed when spring is not located near cable anchorage. The mode behaviors when damper and spring is parallel connected are given. The general solutions for arbitrary location of damper and spring along the cable are further discussed. The results of this study are helpful to understanding the damper parameter optimization of cable-damper-rubber-bushing system and the basic dynamics of the complex cable-cross-ties-damper system. Copyright © 2013 John Wiley & Sons, Ltd.

Robust design optimization of friction dampers for structural response control

Abstract: SUMMARY It is known that the use of passive energy dissipation devices, as friction dampers, reduces considerably the dynamic response of a structure subjected to earthquake ground motions. However, the parameters of each damper as well as the best placement of these devices remain difficult to determine. Thus, in this paper, robust design optimization of friction dampers to control the structural response against earthquakes is proposed. In order to take into account uncertainties present in the system, some of its parameters are modeled as random variables, and consequently, the structural response becomes stochastic. To perform the robust optimization of such system, two objective functions are simultaneously considered: the mean and variance of the maximum displacement. This approach allows finding a set of Pareto-optimal solutions. A genetic algorithm, the NSGA-II (Nondominated Sorting Genetic Algorithm), is applied to solve the resulting multi-objective optimization problem. For illustration purposes, a six-story shear building is analyzed. The results showed that the proposed method was able to reduce the mean maximum displacement in approximately 70% and the variance of the maximum displacement in almost 99% with only three dampers. Copyright © 2014 John Wiley & Sons, Ltd.

Feasibility of tension braces using Cu–Al–Mn superelastic alloy bars

Abstract: SUMMARY This paper investigates the feasibility of tension braces using Cu–Al–Mn superelastic alloy bars as energy dissipating and self-centering elements for steel frames by performing 1/3 scale shaking table tests. The difficulty with conventional steel tension braces lies in pinching or significant deterioration of stiffness and strength under cyclic loading. When a steel frame with conventional tension braces is subjected to intense earthquakes, pinching may lead to a large residual drift and/or instability. To overcome the difficulty, this paper examines the effectiveness of Cu–Al–Mn superelastic alloy bars, facilitated by their large recovery strain, low material cost, and high machinability, as a partial replacement of steel bars in tension braces. In the shaking table tests, a 1/3 scaled 1-bay, 1-story steel frame with the present tension braces is subjected to quasi-static cyclic loading and dynamic harmonic ground motions of 6 Hz. Both the static and dynamic test results demonstrate the effectiveness of the present braces in avoiding pinching under the ductility ratio up to 3. The dynamic test results also demonstrate the capability of the present tension braces in reducing the peak response acceleration within the base shear capacity. To study the rate dependence of the frame response, further, time-history analyses are performed by using a SDOF model based on a uniaxial rate-independent model, calibrated with the quasi-static tests. A comparison of the analytical results with the dynamic test results demonstrates that the rate dependence of the frame response is negligible up to the loading frequency of 6 Hz. Copyright © 2014 John Wiley & Sons, Ltd.

Dynamic response of identical adjacent structures connected by viscous damper

Abstract: In this paper, the dynamic behavior of two identical adjacent structures connected with viscous dampers is investigated under base acceleration. The base acceleration is modeled as harmonic excitation as well as stationary white-noise random process. Each adjacent structure is modeled as a two-degree-of-freedom system. The governing differential equations of motion of the coupled system are derived and solved for relative displacement and absolute acceleration responses. A parametric study is conducted to study the influence of important system parameters on the response behavior of damper connected structures. The important parameters considered are excitation frequency, mass ratio, and stiffness ratio of the structure. It is observed that the viscous damper is quite effective in controlling the dynamic response of identical connected structures. For a given coupled structure and excitation, it is found that there exists an optimum value of damping coefficient of damper for which the peak responses under harmonic excitation and the mean square response quantities under stationary white-noise excitation attain the minimum value. The close-form expressions for optimum parameter and corresponding response are derived for an undamped system. Finally, it is observed that the damping ratio of the connected structures does not have noticeable effects on the optimum damper damping. This implies that the proposed close-form expressions of undamped structures can be used for the damped connected structures as well. Copyright © 2013 John Wiley & Sons, Ltd.

Uncertainty analysis of system identification results obtained for a seven‐story building slice tested on the UCSD‐NEES shake table

Abstract: A full-scale seven-story reinforced concrete building section/slice was tested on the Network for Earthquake Engineering Simulation (NEES) shake table at the University of California San Diego during the period of October 2005 to January 2006. Three output-only system identification methods were used to extract the modal parameters (natural frequencies, damping ratios, and mode shapes) of the test structure at different damage states. In this study, the performance of these system identification methods is investigated in two cases: (Case I) when these methods are applied to the measured dynamic response of the structure and (Case II) when these methods are applied to the dynamic response of the structure simulated using a three-dimensional nonlinear finite element model thereof. In both cases, the uncertainty/variability of the identified modal parameters due to the variability of several input factors is quantified through analysis of variance (ANOVA). In addition to ANOVA, meta-models are used for effect screening in Case II (based on the simulated data), which also capture the effects of linear interactions of the input factors. The four input factors considered in Case I are amplitude of input excitation, spatial density of measurements, length of response data used for system identification, and model order used in the parametric system identification methods. In the second case of uncertainty analysis, in addition to these four input factors, measurement noise is also considered. The results show that for all three methods considered, the amplitude of excitation is the most significant factor explaining the variability of the identified modal parameters, especially the natural frequencies. Copyright © 2013 John Wiley & Sons, Ltd.

The technique of digital image correlation to identify defects in glass structures

Abstract: SUMMARY The article describes an experiment using the technique of digital image correlation (DIC) to identify and analyse defects in glass structures. The investigation involved three sets of glass structures, that is, float glass sheets, with or without any defects and subjected to bearing tests. The DIC technique perfectly identified not only the position and size of the defect but also the magnitude of deformations around the defect itself. Low stress values also indicate that strains concentrate around the defects. This enables the DIC method to be used to investigate the quality of glass elements, to detect the presence of defects and to monitor the most critical structural elements. The results obtained with the DIC technique were confirmed by comparing them with data gathered from the analysis of the fracture surface. Copyright © 2014 John Wiley & Sons, Ltd.

A methodology of design for seismic performance enhancement of buildings using viscous fluid dampers

Abstract: SUMMARY A methodology of design for seismic performance enhancement of buildings by using linear viscous fluid dampers (VFDs) is proposed. It also gives the procedure for arriving at an efficient distribution of VFDs in the building. The peak base shear and inter-storey drifts determined from a time history analysis of the building subjected to design basis earthquake (DBE) are used for satisfying the Uniform Building Code 1997 specified target performance criteria for base shear and inter-storey drifts. The methodology proposed is used for designing the linear VFDs to increase the effective damping with chevron, upper toggle, and scissor jack mechanisms in a 20-storey benchmark building subjected to DBE to meet the performance criteria. The time histories of the N–S component of El Centro, N–S component of Kobe, N–S component of Northridge, and S–E component of Taft scaled to a PGA of 0.2 g are considered to be representatives of DBE for the place where the 20-storey benchmark building is located. It is observed that the optimum location of the dampers with different mechanisms in the building is the ground floor or the first few storeys from the ground floor. Copyright © 2013 John Wiley & Sons, Ltd.

Corrosion monitoring of post-tensioned concrete structures using fractal analysis of guided ultrasonic waves

Abstract: SUMMARY This work presents a new approach based on fractal analysis of guided ultrasonic waves (GUWs) for monitoring the corrosion evolutionary path in post-tensioned systems. Fractal analysis is a new scientific paradigm that has been used successfully in many fields including biological and physical sciences. However, its application in the SHM community has been modest. The proposed approach utilizes piezoelectric transducers, permanently attached to the steel tendon, to transmit and receive GUWs. The corrosion monitoring is performed through the examination of the fractal dimension of GUW measurements over time. Accelerated corrosion tests were carried out on two 7 wire steel strands embedded in two concrete blocks to validate the proposed system. Finally, an outlier detection algorithm is proposed to enhance the sensitivity of the technique to the corrosion-induced damage. Copyright © 2013 John Wiley & Sons, Ltd.

Local positioning systems versus structural monitoring: a review

Abstract: SUMMARY Structural monitoring and structural health monitoring could take advantage from different devices to record the static or dynamic response of a structure. A positioning system provides displacement information on the location of moving objects, which is assumed to be the basic support to calibrate any structural mechanics model. The global positioning system could provide satisfactory accuracy in absolute displacement measurements. But the requirements of an open area position for the antennas and a roofed room for its data storage and power supply limit its flexibility and its applications. Several efforts are done to extend its field of application. The alternative is local positioning system. Non-contact sensors can be easily installed on existing infrastructure in different locations without changing their properties: several technological approaches have been exploited: laser-based, radar-based, vision-based, etc. In this paper, a number of existing options, together with their performances, are reviewed. Copyright © 2014 John Wiley & Sons, Ltd.

An isolation device for near‐fault ground motions

Abstract: Seismic isolation is an appreciable control strategy that reduces the vibrations of structural and nonstructural systems induced by strong ground motions. However, under near-fault (NF) ground motion, the seismic isolation devices might perform poorly because of large isolator displacements caused by long-period large velocity and displacement pulses associated with such strong motion. The objective of this paper is to assess the effectiveness of a new seismic isolation device, referred to as roll-in-cage (RNC) isolator, in protecting against NF ground motions. The device is intended to achieve a balance in controlling isolator displacement demands and structural accelerations. The RNC isolator provides in a single unit all the necessary functions of rigid support, horizontal flexibility with enhanced stability, and energy dissipation characteristics. Moreover, it is distinguished from other isolation devices by two unique features: (i) it has a built-in energy-absorbing buffer to limit the design displacement under strong excitation, and (ii) it has a built-in linear recentering mechanism that prevents residual displacement after earthquakes. The seismic response of multistory buildings isolated by the RNC isolator is investigated under three recorded NF earthquakes and three synthetic ground motions. The results show that the RNC isolator is a convenient isolation system in protecting against NF earthquakes.