Abstract The second-order blind identification (SOBI) and its variants have been extensively explored for output-only modal identification of civil structures under varied excitations. At the core of these methods is the matrix joint approximate diagonalization (JAD) technique, while their efficiency and accuracy are largely determined by how the target-matrices for JAD are constructed from multi-channel structural responses. This study first formulates the JAD framework for structural identification, where different techniques in formulating the target-matrices are summarized and mathematical tools to conduct JAD are also presented. Then two novel ways stemming from conventional identification methods are presented as alternatives to construct the target-matrices for ambient identification, to maintain a low-order formulation and even avoiding the formation of covariance matrix. Subsequently, in view of the large number of candidate target-matrices which are analytically usable, a guiding principle is proposed for selecting reliable target-matrices, where the closeness of the eigenvectors of the target-matrices are compared beforehand, therefore eliminating of distorted target-matrices and also improving the efficiency of the subsequent JAD. The proposed techniques are applied to modal identification of the Donghai Bridge from monitoring data and the proposed JAD-based methods are compared in this context. The results suggest the effectiveness of the proposed techniques and also provide a performance evaluation of these methods.
This paper presents the application of a probabilistic method to estimate the complete dynamic response and state of damage of a large-scale structural system subjected to strong base excitations. The structure considered consists of a sevenstory slice of a shear wall building tested at the George E. Brown Jr. Network for Earthquake Engineering Simulation site at the University of California at San Diego. The acceleration response measured at limited locations is combined with a nonlinear model to estimate the complete dynamic response at unmeasured degrees of freedom. To improve the predicting capability of the mathematical model a subset of the parameters is jointly estimated with the response, a strategy known as joint state-parameter estimation or augmented state estimation. The estimated response is used to compute a damage index as a quantitative measure of damage.
A new control algorithm is developed for reducing the response of smart base isolated buildings with variable friction semiactive control systems in near-fault earthquakes. The central idea of the control algorithm is to design a H∞ controller for the structural system and use this controller to determine the optimum control force in the semiactive device. The H∞ controller is designed using appropriate input and output weighting filters that have been developed for optimal performance in reducing near-fault earthquake responses. A novel semiactive variable friction device is also developed and with the H∞ controller shown to be effective in achieving response reductions in smart base isolated buildings in near-fault earthquakes. The new variable friction device developed consists of four friction elements and four restoring spring elements arranged in a rhombus configuration with each arm consisting of a friction–stiffness pair. The level of friction force can be adjusted by varying the angle of the arms of the device leading to smooth variation of friction force in the device. Experimental results are presented to verify the proposed analytical model of the device. The H∞ algorithm is implemented analytically on a five storey smart base isolated building with linear elastomeric isolation bearings and variable friction system located at the isolation level. The H∞ controller along with the weighting filters leads to the smooth variation of friction force, thus eliminating the disadvantages associated with rapid switching. Several recent near-fault earthquakes are considered in this study. The robustness of the H∞ controller is shown by considering a stiffness uncertainty of ±10%. Copyright © 2006 John Wiley & Sons, Ltd.
As a useful class of techniques for structural health monitoring, vibration-based structural damage detection has been intensively studied in civil engineering community for decades. Structural damage is commonly assumed to be linear stiffness reduction, which cannot represent nonlinear damage from the real world, such as plastic deformation, cracks, and joint looseness. Novelty detection-based and supervised classification-based methods have been used for nonlinear damage detection, but direct physical interpretation of the damage cannot be accessed. To address this limitation, the major scope of this work is to detect and physically characterize linear/nonlinear-type structural damage in a semi-supervised way. The proposed method first uses previously proposed sparse identification to establish a baseline (undamaged) model. Then, damage is considered as a variation of restoring forces in the structural system. It can be further treated as a pseudoforce along with the external disturbance force applied to the structural system. Thus, the damaged system is transformed into an equivalent linear system and nonlinear restoring force. By comparing the corresponding outputs from the prediction of baseline model and newly measured data, a variation of mass-normalized restoring (pseudo) force is identified, which can be used for further damage characterization. An illustrative example and three experimental tests are introduced in this work to verify the effectiveness of the proposed method.
A novel sensor failure detection method is developed in this paper. Sensor failure considered in this paper can be any type of measurement error that is different from the true structural response. The sensors are divided into two groups; sensors that correctly measure the structural responses are termed "reference sensors" and sensors that may fail to correctly measure the structural responses are termed "uncertain sensors" henceforth. A sensor error function is formulated to detect the instants of failure of the corresponding uncertain sensor, using the measurements from reference sensors and the uncertain sensor examined. The sensor error function is derived using indirect and direct approaches. In the indirect approach, the error function is obtained from the state space model in combination with the inverse model and interaction matrix formulation. The input term is eliminated from the error function by applying the inverse model and the interaction matrix is applied to eliminate the state and all unexamined uncertain sensors except for the one examined from the error function. The direct approach uses the singular value decomposition method to establish the coefficients of the error function from the healthy measured data. The sensor failure detection formulation is investigated numerically using a four degree-of-freedom spring-mass-damper system and experimentally using a 4-m-long NASA eight-bay truss structure. It is shown by means of numerical and experimental results that the sensor failure formulation developed correctly detects and isolates the instants of sensor failure and can be implemented in real structural systems for sensor failure detection.
The installation of dampers close to cable anchorages is a common approach for stay-cable vibration mitigation. Inspired by the force-deformation relationship produced by actively controlled dampers, passive negative-stiffness dampers (NSDs) were proposed for stay cables in the past to achieve excellent vibration-control performance. However, a systematic comparison between passive NSDs and active control in cable vibration mitigation has rarely been reported in literature. This paper systematically compares vibration mitigation performances of a passive NSD to the performance of two active control methods, linear quadratic regulator (LQR) and output feedback control. The comparison indicates that a passive NSD can offer a stay cable with a high damping level comparable to that of LQR control. However, passive NSD will also decrease the modal frequencies of a stay cable, whereas LQR will increase the frequencies slightly. The dynamic response results also indicate that the active LQR control offers slightly better control performance than the passive NSD in various loading cases. The superiority of the LQR control over the passive NSD can be explained through an output feedback control approach. It is noteworthy that the NSD is regarded as more practical and reliable because it offers protection by completely passive means without the need for the feedback and actuation that are required in active control.
Under high velocity, pulse type near source earthquakes semi-active control systems are very effective in reducing seismic response base isolated structures. Semi-active control systems can be classified as: 1) independently variable stiffness, 2) independently variable damping, and 3) combined variable stiffness and damping systems. Several researchers have studied the effectiveness of independently varying damping systems for seismic response reduction of base isolated structures. In this study effectiveness of a combined system consisting of a semi-active independently variable stiffness (SAIVS) device and a magnetorheological (MR) damper in reducing seismic response of base isolated structures is analytically investigated. The SAIVS device can vary the stiffness, and hence the period, of the isolation system; whereas, the MR damper enhances the energy dissipation characteristics of the isolation system. Two separate control algorithms, i.e., a nonlinear tangential stiffness moving average control algorithm for smooth switching of the SAIVS device and a Lyapunov based control algorithm for damping variation of MR damper, are developed. Single and multi degree of freedom systems consisting of sliding base isolation system and both the SAIVS device and MR damper are considered. Results are presented in the form of nonlinear response spectra, and effectiveness of combined variable stiffness and variable damping system in reducing seismic response of sliding base isolated structures is evaluated. It is shown that the combined variable stiffness and variable damping system leads to significant response reduction over cases with variable stiffness or variable damping systems acting independently, over a broad period range.
This study presents a new crosswind mitigation strategy by using negative stiffness damped outrigger (NSDO) system for tall buildings. Using the "assisting motion" feature of negative stiffness, NSDO amplifies the motion of viscous damper resulting in and significant improvement of energy dissipation for tall buildings. Systematical evaluation and comparison on crosswind performance are carried out for tall buildings using (a) conventional outrigger (CO), (b) conventional damped outrigger (CDO), and (c) NSDO. It is shown that the proposed NSDO is able to achieve a damping amplification factor larger than a unit, which would never be achieved by CDO in practical due to the actual deformation of perimeter columns. In this way, NSDO has the most satisfactory crosswind reduction effect, especially when perimeter column stiffness is insufficient. For example, replacing CDO with NSDO not only attenuates crosswind-induced harmful drift and structural acceleration by 22% and 36%, respectively, but also further saves about 75% of the size of viscous dampers. In other words, NSDO adopts less outrigger damping coefficient but reduces more crosswind-induced vibration.
