Satish Nagarajaiah | Rice University (original) (raw)

Papers by Satish Nagarajaiah

Research paper thumbnail of Seismic Protection of Highway Bridges with Negative Stiffness Devices 13-0013.pdf

Research paper thumbnail of Seismic Response Control of Structures Using a Novel Adaptive Passive Negative Stiffness Device Appendices 13-0004_Appendices.pdf

Research paper thumbnail of Negative stiffness device for seismic protection of structures: An analytical and experimental study

Research paper thumbnail of Output-only modal identification and structural damage detection using time–frequency and wavelet techniques for assessing and monitoring civil infrastructures

Research paper thumbnail of Adaptive length SMA pendulum smart tuned mass damper performance in the presence of real time primary system stiffness change

Smart Structures and Systems, 2014

ABSTRACT In a companion paper, Pasala and Nagarajaiah analytically and experimentally validate th... more ABSTRACT In a companion paper, Pasala and Nagarajaiah analytically and experimentally validate the Adaptive Length Pendulum Smart Tuned Mass Damper (ALP-STMD) on a primary structure (2 story steel structure) whose frequencies are time invariant (Pasala and Nagarajaiah 2012). In this paper, the ALP-STMD effectiveness on a primary structure whose frequencies are time varying is studied experimentally. This study experimentally validates the ability of an ALP-STMD to adequately control a structural system in the presence of real time changes in primary stiffness that are detected by a real time observer based system identification. The experiments implement the newly developed Adaptive Length Pendulum Smart Tuned Mass Damper (ALP-STMD) which was first introduced and developed by Nagarajaiah (2009), Nagarajaiah and Pasala (2010) and Nagarajaiah et al. (2010). The ALP-STMD employs a mass pendulum of variable length which can be tuned in real time to the parameters of the system using sensor feedback. The tuning action is made possible by applying a current to a shape memory alloy wire changing the effective length that supports the damper mass assembly in real time. Once a stiffness change in the structural system is detected by an open loop observer, the ALP-STMD is re-tuned to the modified system parameters which successfully reduce the response of the primary system. Significant performance improvement is illustrated for the stiffness modified system, which undergoes the re-tuning adaptation, when compared to the stiffness modified system without adaptive re-tuning.

Research paper thumbnail of Semi-active frequency tracking algorithm for control of flapwise vibrations in wind turbine blades

IET Irish Signals and Systems Conference (ISSC 2009), 2009

The increased size and flexibility of modern multi-Megawatt wind turbines has lead to the dynamic... more The increased size and flexibility of modern multi-Megawatt wind turbines has lead to the dynamic behaviour of these structures becoming an important design consideration. The aim of this paper is to study the potential use of Semi-Active Tuned Mass Dampers (STMDs) in reducing vibrations in the flapwise direction. The model developed in this study considers only the structural dynamics of the turbine and includes the coupling between the blades and tower. The semi-active a lgorithm employs a frequency tracking technique based on the Short Time Fourier Transform (STFT). This allows real-time tuning of the dampers to the dominant frequencies in the system. Numerical simulations have been carried out to study the effectiveness of the STMDs for steady and turbulent wind loading.

Research paper thumbnail of A new structural modification approach for seismic protection based on adaptive negative stiffness device: Conceptual analysis

Structures Congress 2011 - Proceedings of the 2011 Structures Congress, 2011

Yielding can be emulated in a structural system by adding an adaptive "negative stiffness device"... more Yielding can be emulated in a structural system by adding an adaptive "negative stiffness device" (NSD) and shifting the "yielding" away from the main structural system-leading to the new idea of "apparent weakening" that occurs ensuring structural stability at all displacement amplitudes. This is achieved through an adaptive negative stiffness system (ANSS), a combination of NSD and a viscous damper. By engaging the NSD at an appropriate displacement (apparent yield displacement that is well below the actual yield displacement of the structural system) the composite structure-device assembly behaves like a yielding structure. The combined NSD-structure system presented in this study has a re-centering mechanism thereby avoids permanent deformation in the composite structure-device assembly unless, the main structure itself yields. Essentially, a yielding-structure is "mimicked" without any, or with minimal permanent deformation or yielding in the main structure. As a result, the main structural system suffers less accelerations, less displacements and less base shear, while the ANSS "absorbs" them. This paper presents comprehensive details on development and study of the ANSS/NSD. Through numerical simulations, the effectiveness and the superior performance of the ANSS/NSD as compared to a structural system with supplemental passive dampers is presented.

Research paper thumbnail of Control of inelastic structures by weakening and damping

20th Analysis and Computation Specialty Conference - Proceedings of the Conference, 2012

Control of structures can be achieved by adding suitable control devices such as actively control... more Control of structures can be achieved by adding suitable control devices such as actively controlled actuators, strengthening and stiffening elements, and/or adding passive damping devices. However, the control demands often require reducing the induced forces in structures, and eliminating essential structural elements and masses that contribute to generating inertial forces during earthquakes. Recently, Reinhorn and his colleagues developed design and implementation concepts that weaken the structural system and reduce the induced forces at the expense of increased deformations, while correcting and controlling such increases with supplemental damping. This results in an improved behavior, in particular when applied to existing structures, as well as when introduced into new construction. The concept and implementation was studied by the author's team theoretically using control methods and experimentally using structural models with weakening, or softening, devices and simulated earthquakes. The implementation of such a concept requires particular attention to and balance of safety and stability. The presentation will introduce the concept, the development of weakening components (such as rocking columns), the innovation of true negative stiffness devices, the theoretical and experimental verification of the concept using simulated earthquakes in the laboratory, and the development of design procedures using active control theories.

Research paper thumbnail of Negative stiffness device for seismic response control of multistory buildings

20th Analysis and Computation Specialty Conference - Proceedings of the Conference, 2012

ABSTRACT Weakening and damping of structures has proven to be an effective method for mitigating ... more ABSTRACT Weakening and damping of structures has proven to be an effective method for mitigating the structure's response. This approach has drawn further attention after the invention of negative stiffness device (NSD), developed by the authors. Preliminary analytical and experimental studies reported on the NSD have revealed that by adding the NSD to a single story structure the base shear demands and peak acceleration of the main structure are reduced significantly and the inter-story deformations are contained by ...

Research paper thumbnail of Blind denoising of structural vibration responses with outliers via principal component pursuit

Structural Control and Health Monitoring, 2013

Structural vibration responses themselves contain rich dynamic information, exploiting which can ... more 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.

Research paper thumbnail of Blind identification of damage in time-varying systems using independent component analysis with wavelet transform

Mechanical Systems and Signal Processing, 2014

This paper proposes a novel output-only damage identification method based on the unsupervised bl... more This paper proposes a novel output-only damage identification method based on the unsupervised blind source separation (BSS) technique termed independent component analysis (ICA). It is discovered that ICA biases to extract sparse component, which typically indicates damage, from the observed mixture signals. The measured structural responses are first preprocessed by wavelet transform (WT). The wavelet-domain signals are then fed as mixtures into the BSS model, which is solved by ICA. The obtained ''interesting'' source with sharp spike and its associated spatial signature in the recovered mixing matrix reveal damage instant and location respectively. Following which, identification of the time-varying modes is carried out by ICA using the structural responses before and after the identified damage instant. For illustration, numerical simulations are conducted, where damage is modeled by abrupt stiffness variation in the time-varying system. Experimental and real-world seismic-excited structure examples with time-varying stiffness are also presented to illustrate the capability of the developed WT-ICA method. Results show that the WT-ICA algorithm realizes accurate and robust blind identification of damage instant and location in single or multiple damage events.

Research paper thumbnail of Structural damage identification via a combination of blind feature extraction and sparse representation classification

Mechanical Systems and Signal Processing, 2014

This paper addresses two problems in structural damage identification: locating damage and assess... more This paper addresses two problems in structural damage identification: locating damage and assessing damage severity, which are incorporated into the classification framework based on the theory of sparse representation (SR) and compressed sensing (CS). The sparsity nature implied in the classification problem itself is exploited, establishing a sparse representation framework for damage identification. Specifically, the proposed method consists of two steps: feature extraction and classification. In the feature extraction step, the modal features of both the test structure and the reference structure model are first blindly extracted by the unsupervised complexity pursuit (CP) algorithm. Then in the classification step, expressing the test modal feature as a linear combination of the bases of the over-complete reference feature dictionary-constructed by concatenating all modal features of all candidate damage classes-builds a highly underdetermined linear system of equations with an underlying sparse representation, which can be correctly recovered by ℓ 1-minimization; the non-zero entry in the recovered sparse representation directly assigns the damage class which the test structure (feature) belongs to. The two-step CP-SR damage identification method alleviates the training process required by traditional pattern recognition based methods. In addition, the reference feature dictionary can be of small size by formulating the issues of locating damage and assessing damage extent as a two-stage procedure and by taking advantage of the robustness of the SR framework. Numerical simulations and experimental study are conducted to verify the developed CP-SR method. The problems of identifying multiple damage, using limited sensors and partial features, and the performance under heavy noise and random excitation are investigated, and promising results are obtained.

Research paper thumbnail of Time-Frequency Blind Source Separation Using Independent Component Analysis for Output-Only Modal Identification of Highly Damped Structures

Journal of Structural Engineering, 2013

Output-only algorithms are needed for modal identification when only structural responses are ava... more Output-only algorithms are needed for modal identification when only structural responses are available. The recent years have witnessed fast development of blind source separation (BSS) as a promising signal processing technique, pursuing to recover the sources using only the measured mixtures. As the most popular tool solving the BSS problem, independent component analysis (ICA) is able to directly extract the time-domain modal responses, which are viewed as virtual sources, from the observed system responses; however, it has been ...

Research paper thumbnail of Data Compression of Structural Seismic Responses via Principled Independent Component Analysis

Journal of Structural Engineering, 2014

AbstractThis paper proposes a novel lossy data compression scheme for structural seismic response... more AbstractThis paper proposes a novel lossy data compression scheme for structural seismic responses based on principled (truncated) independent component analysis (PICA). It is first shown that independent component analysis (ICA) is able to transform a multivariate data set into a sparse representation space where is optimal for coding and compression, such that both the intradependencies and interdependencies (i.e., redundant information) between the multichannel data are removed for efficient data compression. Two examples are presented to demonstrate the compression performance of PICA, using the real-measured structural seismic responses from the 1994 Northridge earthquake, of the Fire Command Control (FCC) building and the USC hospital building, respectively. It is compared with the popular wavelet transform coding technique, which is only able to handle single-channel data separately. Results show that PICA achieves dramatically higher compression ratio (CR) than the wavelet method while retaining e...

Research paper thumbnail of Output-only modal identification with limited sensors using sparse component analysis

Journal of Sound and Vibration, 2013

Blind source separation (BSS) based methods have been shown to be efficient and powerful to perfo... more Blind source separation (BSS) based methods have been shown to be efficient and powerful to perform output-only modal identification. Existing BSS modal identification methods, however, require the number of sensors at least equal to that of sources (active modes). This paper proposes a new modal identification algorithm based on a novel BSS technique termed sparse component analysis (SCA) to handle even the underdetermined problem where sensors may be highly limited compared to the number of active modes. The developed SCA method reveals the essence of modal expansion that the monotone modal responses with disjoint sparsest representations in frequency domain naturally cluster in the directions of the mode matrix's columns (modeshapes), which are readily extracted from the measured system responses using a simple clustering algorithm. Then, in determined case where sensor number equals that of modes, the estimated square mode matrix directly decouples the system responses to obtain the modal responses, whereby computing their frequencies and damping ratios; whereas with limited sensors, the modal responses are efficiently recovered via the ℓ 1-minimization sparse recovery technique from the incomplete knowledge of the partial mode matrix and the system responses of inadequate sensors. Numerical simulations and experimental example show that whether in determined or underdetermined situations, the SCA method performs accurate and robust identification of a wide range of structures including those with closely-spaced and highly-damped modes. The SCA method is simple and efficient to conduct reliable output-only modal identification even with limited sensors.

Research paper thumbnail of Control of Sliding‐Isolated Bridge with Absolute Acceleration Feedback

Journal of Engineering Mechanics, 1993

This paper presents an experimental and analytical study of hybrid control of bridges using slidi... more This paper presents an experimental and analytical study of hybrid control of bridges using sliding bearings, with recentering springs, in parallel with servohydraulic actuators. A new control algorithm with absolute acceleration feedback, based on instantaneous optimal control laws, is developed. The developed control algorithm is implemented in a shake-table study of an actively controlled sliding-isolated bridge. The objective of implementing the hybrid system is to evaluate its advantages in addition to those due to the passive sliding system. The experimental system used in the shake-table test is described and the results of the experiments are presented. It is shown that substantial reduction of response acceleration is possible, using hybrid control, while confining the sliding displacement within an acceptable range, and eliminating almost completely postearthquake permanent offsets. Comparisons of the results of hybrid system with results of passive system are presented. The advantages of hybrid control, in addition to those due to passive control, are discussed.

Research paper thumbnail of Multiscale Wavelet-LQR Controller for Linear Time Varying Systems

Journal of Engineering Mechanics, 2010

This paper proposes a multiresolution based wavelet controller for the control of linear time var... more This paper proposes a multiresolution based wavelet controller for the control of linear time varying systems consisting of a time invariant component and a component with zero mean slowly time varying parameters. The real time discrete wavelet transform controller is based on a time interval from the initial until the current time and is updated at regular time steps. By casting a modified optimal control problem in a linear quadratic regulator ͑LQR͒ form constrained to a band of frequency in the wavelet domain, frequency band dependent control gain matrices are obtained. The weighting matrices are varied for different bands of frequencies depending on the emphasis to be placed on the response energy or the control effort in minimizing the cost functional, for the particular band of frequency leading to frequency dependent gains. The frequency dependent control gain matrices of the developed controller are applied to multiresolution analysis ͑MRA͒ based filtered time signals obtained until the current time. The use of MRA ensures perfect decomposition to obtain filtered time signals over the finite interval considered, with a fast numerical implementation for control application. The proposed controller developed using the Daubechies wavelet is shown to work effectively for the control of free and forced vibration ͑both under harmonic and random excitations͒ responses of linear time varying single-degree-of-freedom and multidegree-of-freedom systems. Even for the cases where the conventional LQR or addition of viscous damping fails to control the vibration response, the proposed controller effectively suppresses the instabilities in the linear time varying systems.

Research paper thumbnail of Torsion in base-isolated structures with elastomeric isolation systems

International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, 1994

Research paper thumbnail of A wavelet-based time-varying adaptive LQR algorithm for structural control

Engineering Structures, 2008

A modified form of the conventional linear quadratic regulator (LQR) control algorithm has been p... more A modified form of the conventional linear quadratic regulator (LQR) control algorithm has been proposed in this paper. The formulation of the modified LQR algorithm uses the information derived from the wavelet analysis of the response in real time, to obtain the local energy distribution over frequency bands. This information reflecting the effect of excitation on the structural system are used to adaptively design the controller by updating the weighting matrices to be applied to the response energy and the control effort. The optimal LQR control problem is solved for each time interval with updated weighting matrices, through the Ricatti equation, leading to time-varying gain matrices. The advantage of the proposed control algorithm is that, it does not require an a priori (offline) choice of the weights as in the classical case and adaptively calculates the gains using the weights decided on the response characteristics in real time (online). The proposed wavelet-based adaptive timevarying LQR (TVLQR) controller is applied to both single-degree-of-freedom (SDOF) and multi-degree-of-freedom (MDOF) systems and the results are compared with the case of conventional LQR controller. Simulations based on single and multiple controllers indicate that the proposed TVLQR controller achieves significant reduction in the displacement response of the structures as compared to the reduction obtained from the use of LQR controller in some cases with acceptable increment in peak control force and marginal increment in control energy demand.

Research paper thumbnail of Nonlinear elastic and inelastic spectra with inherent and supplemental damping

Earthquake Engineering & Structural Dynamics, 2013

SUMMARYDuring severe seismic events, structures designed according to current standards yield and... more SUMMARYDuring severe seismic events, structures designed according to current standards yield and develop inelastic deformations. While the acceleration responses are limited by the yielding strength, these structures develop permanent deformations (and possible damage) due to such yielding. Spectra developed for inelastic structures can help in determining the desired yield levels and the associated inelastic deformations. Some structures made of special materials or equipped with innovative structural systems may yield, but can recover the deformation upon unloading and, thus, may avoid permanent deformations. These structures are known as nonlinear elastic. Often the post yielding excursions are very large and may exceed their toughness (or deformability). By introducing damping in form of supplemental devices, it is possible to control such deformations and keep them within acceptable limits. Spectra for such nonlinear elastic structures and inelastic structures are developed he...

Research paper thumbnail of Seismic Protection of Highway Bridges with Negative Stiffness Devices 13-0013.pdf

Research paper thumbnail of Seismic Response Control of Structures Using a Novel Adaptive Passive Negative Stiffness Device Appendices 13-0004_Appendices.pdf

Research paper thumbnail of Negative stiffness device for seismic protection of structures: An analytical and experimental study

Research paper thumbnail of Output-only modal identification and structural damage detection using time–frequency and wavelet techniques for assessing and monitoring civil infrastructures

Research paper thumbnail of Adaptive length SMA pendulum smart tuned mass damper performance in the presence of real time primary system stiffness change

Smart Structures and Systems, 2014

ABSTRACT In a companion paper, Pasala and Nagarajaiah analytically and experimentally validate th... more ABSTRACT In a companion paper, Pasala and Nagarajaiah analytically and experimentally validate the Adaptive Length Pendulum Smart Tuned Mass Damper (ALP-STMD) on a primary structure (2 story steel structure) whose frequencies are time invariant (Pasala and Nagarajaiah 2012). In this paper, the ALP-STMD effectiveness on a primary structure whose frequencies are time varying is studied experimentally. This study experimentally validates the ability of an ALP-STMD to adequately control a structural system in the presence of real time changes in primary stiffness that are detected by a real time observer based system identification. The experiments implement the newly developed Adaptive Length Pendulum Smart Tuned Mass Damper (ALP-STMD) which was first introduced and developed by Nagarajaiah (2009), Nagarajaiah and Pasala (2010) and Nagarajaiah et al. (2010). The ALP-STMD employs a mass pendulum of variable length which can be tuned in real time to the parameters of the system using sensor feedback. The tuning action is made possible by applying a current to a shape memory alloy wire changing the effective length that supports the damper mass assembly in real time. Once a stiffness change in the structural system is detected by an open loop observer, the ALP-STMD is re-tuned to the modified system parameters which successfully reduce the response of the primary system. Significant performance improvement is illustrated for the stiffness modified system, which undergoes the re-tuning adaptation, when compared to the stiffness modified system without adaptive re-tuning.

Research paper thumbnail of Semi-active frequency tracking algorithm for control of flapwise vibrations in wind turbine blades

IET Irish Signals and Systems Conference (ISSC 2009), 2009

The increased size and flexibility of modern multi-Megawatt wind turbines has lead to the dynamic... more The increased size and flexibility of modern multi-Megawatt wind turbines has lead to the dynamic behaviour of these structures becoming an important design consideration. The aim of this paper is to study the potential use of Semi-Active Tuned Mass Dampers (STMDs) in reducing vibrations in the flapwise direction. The model developed in this study considers only the structural dynamics of the turbine and includes the coupling between the blades and tower. The semi-active a lgorithm employs a frequency tracking technique based on the Short Time Fourier Transform (STFT). This allows real-time tuning of the dampers to the dominant frequencies in the system. Numerical simulations have been carried out to study the effectiveness of the STMDs for steady and turbulent wind loading.

Research paper thumbnail of A new structural modification approach for seismic protection based on adaptive negative stiffness device: Conceptual analysis

Structures Congress 2011 - Proceedings of the 2011 Structures Congress, 2011

Yielding can be emulated in a structural system by adding an adaptive "negative stiffness device"... more Yielding can be emulated in a structural system by adding an adaptive "negative stiffness device" (NSD) and shifting the "yielding" away from the main structural system-leading to the new idea of "apparent weakening" that occurs ensuring structural stability at all displacement amplitudes. This is achieved through an adaptive negative stiffness system (ANSS), a combination of NSD and a viscous damper. By engaging the NSD at an appropriate displacement (apparent yield displacement that is well below the actual yield displacement of the structural system) the composite structure-device assembly behaves like a yielding structure. The combined NSD-structure system presented in this study has a re-centering mechanism thereby avoids permanent deformation in the composite structure-device assembly unless, the main structure itself yields. Essentially, a yielding-structure is "mimicked" without any, or with minimal permanent deformation or yielding in the main structure. As a result, the main structural system suffers less accelerations, less displacements and less base shear, while the ANSS "absorbs" them. This paper presents comprehensive details on development and study of the ANSS/NSD. Through numerical simulations, the effectiveness and the superior performance of the ANSS/NSD as compared to a structural system with supplemental passive dampers is presented.

Research paper thumbnail of Control of inelastic structures by weakening and damping

20th Analysis and Computation Specialty Conference - Proceedings of the Conference, 2012

Control of structures can be achieved by adding suitable control devices such as actively control... more Control of structures can be achieved by adding suitable control devices such as actively controlled actuators, strengthening and stiffening elements, and/or adding passive damping devices. However, the control demands often require reducing the induced forces in structures, and eliminating essential structural elements and masses that contribute to generating inertial forces during earthquakes. Recently, Reinhorn and his colleagues developed design and implementation concepts that weaken the structural system and reduce the induced forces at the expense of increased deformations, while correcting and controlling such increases with supplemental damping. This results in an improved behavior, in particular when applied to existing structures, as well as when introduced into new construction. The concept and implementation was studied by the author's team theoretically using control methods and experimentally using structural models with weakening, or softening, devices and simulated earthquakes. The implementation of such a concept requires particular attention to and balance of safety and stability. The presentation will introduce the concept, the development of weakening components (such as rocking columns), the innovation of true negative stiffness devices, the theoretical and experimental verification of the concept using simulated earthquakes in the laboratory, and the development of design procedures using active control theories.

Research paper thumbnail of Negative stiffness device for seismic response control of multistory buildings

20th Analysis and Computation Specialty Conference - Proceedings of the Conference, 2012

ABSTRACT Weakening and damping of structures has proven to be an effective method for mitigating ... more ABSTRACT Weakening and damping of structures has proven to be an effective method for mitigating the structure's response. This approach has drawn further attention after the invention of negative stiffness device (NSD), developed by the authors. Preliminary analytical and experimental studies reported on the NSD have revealed that by adding the NSD to a single story structure the base shear demands and peak acceleration of the main structure are reduced significantly and the inter-story deformations are contained by ...

Research paper thumbnail of Blind denoising of structural vibration responses with outliers via principal component pursuit

Structural Control and Health Monitoring, 2013

Structural vibration responses themselves contain rich dynamic information, exploiting which can ... more 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.

Research paper thumbnail of Blind identification of damage in time-varying systems using independent component analysis with wavelet transform

Mechanical Systems and Signal Processing, 2014

This paper proposes a novel output-only damage identification method based on the unsupervised bl... more This paper proposes a novel output-only damage identification method based on the unsupervised blind source separation (BSS) technique termed independent component analysis (ICA). It is discovered that ICA biases to extract sparse component, which typically indicates damage, from the observed mixture signals. The measured structural responses are first preprocessed by wavelet transform (WT). The wavelet-domain signals are then fed as mixtures into the BSS model, which is solved by ICA. The obtained ''interesting'' source with sharp spike and its associated spatial signature in the recovered mixing matrix reveal damage instant and location respectively. Following which, identification of the time-varying modes is carried out by ICA using the structural responses before and after the identified damage instant. For illustration, numerical simulations are conducted, where damage is modeled by abrupt stiffness variation in the time-varying system. Experimental and real-world seismic-excited structure examples with time-varying stiffness are also presented to illustrate the capability of the developed WT-ICA method. Results show that the WT-ICA algorithm realizes accurate and robust blind identification of damage instant and location in single or multiple damage events.

Research paper thumbnail of Structural damage identification via a combination of blind feature extraction and sparse representation classification

Mechanical Systems and Signal Processing, 2014

This paper addresses two problems in structural damage identification: locating damage and assess... more This paper addresses two problems in structural damage identification: locating damage and assessing damage severity, which are incorporated into the classification framework based on the theory of sparse representation (SR) and compressed sensing (CS). The sparsity nature implied in the classification problem itself is exploited, establishing a sparse representation framework for damage identification. Specifically, the proposed method consists of two steps: feature extraction and classification. In the feature extraction step, the modal features of both the test structure and the reference structure model are first blindly extracted by the unsupervised complexity pursuit (CP) algorithm. Then in the classification step, expressing the test modal feature as a linear combination of the bases of the over-complete reference feature dictionary-constructed by concatenating all modal features of all candidate damage classes-builds a highly underdetermined linear system of equations with an underlying sparse representation, which can be correctly recovered by ℓ 1-minimization; the non-zero entry in the recovered sparse representation directly assigns the damage class which the test structure (feature) belongs to. The two-step CP-SR damage identification method alleviates the training process required by traditional pattern recognition based methods. In addition, the reference feature dictionary can be of small size by formulating the issues of locating damage and assessing damage extent as a two-stage procedure and by taking advantage of the robustness of the SR framework. Numerical simulations and experimental study are conducted to verify the developed CP-SR method. The problems of identifying multiple damage, using limited sensors and partial features, and the performance under heavy noise and random excitation are investigated, and promising results are obtained.

Research paper thumbnail of Time-Frequency Blind Source Separation Using Independent Component Analysis for Output-Only Modal Identification of Highly Damped Structures

Journal of Structural Engineering, 2013

Output-only algorithms are needed for modal identification when only structural responses are ava... more Output-only algorithms are needed for modal identification when only structural responses are available. The recent years have witnessed fast development of blind source separation (BSS) as a promising signal processing technique, pursuing to recover the sources using only the measured mixtures. As the most popular tool solving the BSS problem, independent component analysis (ICA) is able to directly extract the time-domain modal responses, which are viewed as virtual sources, from the observed system responses; however, it has been ...

Research paper thumbnail of Data Compression of Structural Seismic Responses via Principled Independent Component Analysis

Journal of Structural Engineering, 2014

AbstractThis paper proposes a novel lossy data compression scheme for structural seismic response... more AbstractThis paper proposes a novel lossy data compression scheme for structural seismic responses based on principled (truncated) independent component analysis (PICA). It is first shown that independent component analysis (ICA) is able to transform a multivariate data set into a sparse representation space where is optimal for coding and compression, such that both the intradependencies and interdependencies (i.e., redundant information) between the multichannel data are removed for efficient data compression. Two examples are presented to demonstrate the compression performance of PICA, using the real-measured structural seismic responses from the 1994 Northridge earthquake, of the Fire Command Control (FCC) building and the USC hospital building, respectively. It is compared with the popular wavelet transform coding technique, which is only able to handle single-channel data separately. Results show that PICA achieves dramatically higher compression ratio (CR) than the wavelet method while retaining e...

Research paper thumbnail of Output-only modal identification with limited sensors using sparse component analysis

Journal of Sound and Vibration, 2013

Blind source separation (BSS) based methods have been shown to be efficient and powerful to perfo... more Blind source separation (BSS) based methods have been shown to be efficient and powerful to perform output-only modal identification. Existing BSS modal identification methods, however, require the number of sensors at least equal to that of sources (active modes). This paper proposes a new modal identification algorithm based on a novel BSS technique termed sparse component analysis (SCA) to handle even the underdetermined problem where sensors may be highly limited compared to the number of active modes. The developed SCA method reveals the essence of modal expansion that the monotone modal responses with disjoint sparsest representations in frequency domain naturally cluster in the directions of the mode matrix's columns (modeshapes), which are readily extracted from the measured system responses using a simple clustering algorithm. Then, in determined case where sensor number equals that of modes, the estimated square mode matrix directly decouples the system responses to obtain the modal responses, whereby computing their frequencies and damping ratios; whereas with limited sensors, the modal responses are efficiently recovered via the ℓ 1-minimization sparse recovery technique from the incomplete knowledge of the partial mode matrix and the system responses of inadequate sensors. Numerical simulations and experimental example show that whether in determined or underdetermined situations, the SCA method performs accurate and robust identification of a wide range of structures including those with closely-spaced and highly-damped modes. The SCA method is simple and efficient to conduct reliable output-only modal identification even with limited sensors.

Research paper thumbnail of Control of Sliding‐Isolated Bridge with Absolute Acceleration Feedback

Journal of Engineering Mechanics, 1993

This paper presents an experimental and analytical study of hybrid control of bridges using slidi... more This paper presents an experimental and analytical study of hybrid control of bridges using sliding bearings, with recentering springs, in parallel with servohydraulic actuators. A new control algorithm with absolute acceleration feedback, based on instantaneous optimal control laws, is developed. The developed control algorithm is implemented in a shake-table study of an actively controlled sliding-isolated bridge. The objective of implementing the hybrid system is to evaluate its advantages in addition to those due to the passive sliding system. The experimental system used in the shake-table test is described and the results of the experiments are presented. It is shown that substantial reduction of response acceleration is possible, using hybrid control, while confining the sliding displacement within an acceptable range, and eliminating almost completely postearthquake permanent offsets. Comparisons of the results of hybrid system with results of passive system are presented. The advantages of hybrid control, in addition to those due to passive control, are discussed.

Research paper thumbnail of Multiscale Wavelet-LQR Controller for Linear Time Varying Systems

Journal of Engineering Mechanics, 2010

This paper proposes a multiresolution based wavelet controller for the control of linear time var... more This paper proposes a multiresolution based wavelet controller for the control of linear time varying systems consisting of a time invariant component and a component with zero mean slowly time varying parameters. The real time discrete wavelet transform controller is based on a time interval from the initial until the current time and is updated at regular time steps. By casting a modified optimal control problem in a linear quadratic regulator ͑LQR͒ form constrained to a band of frequency in the wavelet domain, frequency band dependent control gain matrices are obtained. The weighting matrices are varied for different bands of frequencies depending on the emphasis to be placed on the response energy or the control effort in minimizing the cost functional, for the particular band of frequency leading to frequency dependent gains. The frequency dependent control gain matrices of the developed controller are applied to multiresolution analysis ͑MRA͒ based filtered time signals obtained until the current time. The use of MRA ensures perfect decomposition to obtain filtered time signals over the finite interval considered, with a fast numerical implementation for control application. The proposed controller developed using the Daubechies wavelet is shown to work effectively for the control of free and forced vibration ͑both under harmonic and random excitations͒ responses of linear time varying single-degree-of-freedom and multidegree-of-freedom systems. Even for the cases where the conventional LQR or addition of viscous damping fails to control the vibration response, the proposed controller effectively suppresses the instabilities in the linear time varying systems.

Research paper thumbnail of Torsion in base-isolated structures with elastomeric isolation systems

International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, 1994

Research paper thumbnail of A wavelet-based time-varying adaptive LQR algorithm for structural control

Engineering Structures, 2008

A modified form of the conventional linear quadratic regulator (LQR) control algorithm has been p... more A modified form of the conventional linear quadratic regulator (LQR) control algorithm has been proposed in this paper. The formulation of the modified LQR algorithm uses the information derived from the wavelet analysis of the response in real time, to obtain the local energy distribution over frequency bands. This information reflecting the effect of excitation on the structural system are used to adaptively design the controller by updating the weighting matrices to be applied to the response energy and the control effort. The optimal LQR control problem is solved for each time interval with updated weighting matrices, through the Ricatti equation, leading to time-varying gain matrices. The advantage of the proposed control algorithm is that, it does not require an a priori (offline) choice of the weights as in the classical case and adaptively calculates the gains using the weights decided on the response characteristics in real time (online). The proposed wavelet-based adaptive timevarying LQR (TVLQR) controller is applied to both single-degree-of-freedom (SDOF) and multi-degree-of-freedom (MDOF) systems and the results are compared with the case of conventional LQR controller. Simulations based on single and multiple controllers indicate that the proposed TVLQR controller achieves significant reduction in the displacement response of the structures as compared to the reduction obtained from the use of LQR controller in some cases with acceptable increment in peak control force and marginal increment in control energy demand.

Research paper thumbnail of Nonlinear elastic and inelastic spectra with inherent and supplemental damping

Earthquake Engineering & Structural Dynamics, 2013

SUMMARYDuring severe seismic events, structures designed according to current standards yield and... more SUMMARYDuring severe seismic events, structures designed according to current standards yield and develop inelastic deformations. While the acceleration responses are limited by the yielding strength, these structures develop permanent deformations (and possible damage) due to such yielding. Spectra developed for inelastic structures can help in determining the desired yield levels and the associated inelastic deformations. Some structures made of special materials or equipped with innovative structural systems may yield, but can recover the deformation upon unloading and, thus, may avoid permanent deformations. These structures are known as nonlinear elastic. Often the post yielding excursions are very large and may exceed their toughness (or deformability). By introducing damping in form of supplemental devices, it is possible to control such deformations and keep them within acceptable limits. Spectra for such nonlinear elastic structures and inelastic structures are developed he...