366 publications from this institution
This paper presents the benchmark problem definition for seismically excited base-isolated buildings. The objective of this benchmark study is to provide a well-defined base-isolated building with a broad set of carefully chosen parameter sets, performance measures and guidelines to the participants, so that they can evaluate their control algorithms. The control algorithms may be passive, active or semi-active. The benchmark structure considered is an eight-storey base-isolated building similar to existing buildings in Los Angeles, California. The base isolation system includes both linear and nonlinear bearings and control devices. The superstructure is considered to be a linear elastic system with lateral–torsional behavior. A new nonlinear dynamic analysis program has been developed and made available to facilitate direct comparison of results of different control algorithms. Copyright © 2005 John Wiley & Sons, Ltd.
Elastomeric isolation bearings are required to be stable at high shear strains, which occur during strong earthquakes. Hence, rigorous determination of the critical axial load during design is important. Currently, the critical load is determined using the small displacement Haringx theory and modified to account for large shear strains by an approximate correction factor. The objective of this study is to experimentally determine the effect of horizontal displacement or shear strain on critical load and to study the validity of the approximate correction factor. Experiments were conducted on a series of elastomeric bearings with low shape factors. Test procedure and test results are presented in detail. It is shown that the critical load decreases with increasing horizontal displacement or shear strain. It is also shown that substantial critical load capacity exists at a horizontal displacement equal to the width of the bearing and is not zero, as predicted by the correction factor. It is further shown that the approximate formula is not conservative at smaller displacements and overly conservative at larger displacements. The critical loads obtained from experiments are compared with results from finite element analyses and nonlinear analytical solutions; the comparisons indicate that the effect of large horizontal displacements on the critical load can be reliably predicted.
This paper presents sparse and low-rank methods for explicit modeling and harnessing the data structure to address the inverse problems in structural dynamics, identification, and data-driven health monitoring. In particular, it is shown that the structural dynamic features and damage information, intrinsic within the structural vibration response measurement data, possesses sparse and low-rank structure, which can be effectively modeled and processed by emerging mathematical tools such as sparse representation (SR), and low-rank matrix decomposition. It is also discussed that explicitly modeling and harnessing the sparse and low-rank data structure could benefit future work in developing data-driven approaches towards rapid, unsupervised, and effective system identification, damage detection, as well as massive SHM data sensing and management.
This study validates an adaptive control algorithm capable of compensating for online sensor failure. Online failure is a relevant problem when considering actively damped, multi-story smart buildings experiencing a disturbance event. In recent years, Artificial Neural Networks (ANNs) have proven very efficient in pattern classification and control applications. In this study, the unique application of ANNs involving Radial Basis Functions (RBFs) combined with H∞ optimal control has demonstrated three significant characteristic advantages: (1) real time adaptability, (2) optimal convergence and computation time, and (3) most importantly, no offline training. The novelty of the proposed controller is elucidated by performing disturbance rejection tests involving a scaled two degree of freedom shear frame subjected to a combined H∞ and ANN control. A bench scale structural model is instrumented with piezoelectric sensors and actuators. After the onset of a first mode disturbance, the structural frame is subjected to a complete sensor failure. The proposed controller is shown to enhance the performance of a baseline H∞ controller in the presence of sensor failure.
A semi-active algorithm for edgewise vibration control of the spar-type floating offshore wind turbine (SFOWT) blades, nacelle and spar platform is developed in this paper. A tuned mass damper (TMD) is placed in each blade, in the nacelle and on the spar to control the vibrations for these components. A Short Time Fourier Transform algorithm is used for semi-active control of the TMDs. The mathematical formulation of the integrated SFOWT-TMDs system is derived by using Euler-Lagrangian equations. The theoretical model derived is a time-varying system considering the aerodynamic properties of the blade, variable mass and stiffness per unit length, gravity, the interactions among the blades, nacelle, spar, mooring system and the TMDs, the hydrodynamic effects, the restoring moment and the buoyancy force. The aerodynamic loads on the nacelle and the spar due to their coupling with the blades are also considered. The effectiveness of the semi-active TMDs is investigated in the numerical examples where the mooring cable tension, rotor speed and the blade stiffness are varying over time. Except for excessively large strokes of the nacelle TMD, the semi-active algorithm is considerably more effective than the passive one in all cases and its effectiveness is restricted by the low-frequency nature of the nacelle and the spar responses.
In recent years, it has been recognized that Negative Stiffness Devices (NSD) may be suitable for seismic protection of structures. Although the concept of negative stiffness may appear to be a reversal on the desired relationship between the force and displacement in structures (i.e., the product of restoring force and displacement is nonnegative), when implemented in parallel with a structure having positive stiffness, the combined system appears to have substantially reduced stiffness while remaining stable. Thus, there is an "apparent weakening and softening" of the structure that results in reduced forces and increased displacements (where the weakening and softening is of a non-damaging nature in that it occurs in the NSD's rather than within the structural framing system). Any excessive displacement response can then be limited by incorporating a damping device in parallel with the adaptive stiffness device. The combination of adaptive negative stiffness and passive damping provides a large degree of control over the expected performance of the structure. In this paper, a numerical study is presented on the performance of a seismically-isolated bridge model that is subjected to various strong earthquake ground motions. The results demonstrate that the addition of negative stiffness devices reduces the base shear substantially, while the deck displacement is limited to acceptable values. The seismic performance of the bridge model is being investigated as part of the NEES-ADAPT Project which includes shaking table tests of the bridge to validate the results from the numerical simulations.
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 recentering mechanism that avoids permanent deformation in the composite structure-device assembly unless the main structure itself yields. Essentially, a yielding-structure is mimicked with no, or with minimal, permanent deformation or yielding in the main structure. As a result, the main structural system suffers less acceleration, less displacement, and less base shear, while the ANSS absorbs these effects. 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 with a structural system with supplemental passive dampers is presented. A companion paper presents the NSD and its mechanics in detail.
Recent studies integrating vibration control and structural health monitoring (SHM) use control devices and control algorithms to enable system identification and damage detection. In this study real-time SHM is used to enhance structural vibration control and reduce damage. A newly proposed control algorithm, including integrated real-time SHM and semi-active control strategy, is presented to mitigate both damage and seismic response of the main structure under strong seismic ground motion. The semi-active independently variable stiffness (SAIVS) device is used as semi-active control device in this investigation. The proper stiffness of SAIVS device is obtained using a new developed semi-active control algorithm based on real-time damage tracking of structure by damage detection algorithm based on identified system Markov parameters (DDA/ISMP) method. A three bay five story steel braced frame structure, which is equipped with one SAIVS device at each story, is employed to illustrate the efficiency of the proposed algorithm. The obtained results show that the proposed control algorithm could significantly decrease damage in most parts of the structure. Also, the dynamic response of the structure is effectively reduced by using the proposed control algorithm during four strong earthquakes. In comparison to passive on and off cases, the results demonstrate that the performance of the proposed control algorithm in decreasing both damage and dynamic responses of structure is significantly enhanced than the passive cases. Furthermore, from the energy consumption point of view the maximum and the cumulative control force in the proposed control algorithm is less than the passive-on case, considerably.
A carbon nanotube film (see Figure) can be used as a strain sensor at the macroscale, due to the dependence of the electrical properties of single‐walled carbon nanotubes on mechanical deformation at the nanoscale.
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 adding a passive damper. In this paper an analytical study is carried on an inelastic multistoried shear building to demonstrate the effectiveness of placing NSDs and dampers at multiple locations along the height of the building. It has been shown that by placing a NSD in a particular story the superstructure above that story can be isolated. It has also been shown through simulation studies that the NSD will limit the amount of energy transmitted to the superstructure from the ground excitation. Essentially, NSD acts as a vibration isolator. Large base deformations is one major limitation in base-isolating the structural systems but using NSDs this can be overcome as the isolation is achieved over the height of the building and not confined to the base. It has been shown through the simulation studies that by placing NSDs in all the lower story's the acceleration of the superstructure and base shear can be reduced significantly without affecting the drifts. Simulation results of a nine-storied 1:3 scale inelastic shear building subjected to periodic ground motion and Kobe fault normal ground motion demonstrate the response of the multi-storied frame can be attenuated by incorporating NSDs.
The use of a controller based on fuzzy set theory, to control a hybrid isolated structure is considered in this work. The objective of the hybrid system is to obtain an ideal sliding system with "complete" base isolation for structures subjected to earthquakes. As the hybrid system approaches a state of "complete" isolation, the effects of imperfections, measurement noise, model limitations, and compensation errors start to play a dominant role in the system performance. Fuzzy logic provides a simple framework to capture the effects of nonlinearities and uncertainties in a real problem, without an explicit model of the plant or controller. The applicability of this approach is investigated on a benchmark experimental model consisting of a 1:4 scale sliding base isolated system complemented at its base by a servo controlled actuator, with a digital computer to provide real-time control. The results of this experimental and analytical study show the feasibility of the implementation of fuzzy logic based control to highly nonlinear civil engineering structures.
Origami structures have been receiving a lot of attention from engineering and scientific researchers owing to their unique properties such as deployability, multi-stability, negative stiffness, etc. However, dynamic properties of origami structures have not been explored much due to a lack of validated analytical dynamic modeling approaches. Given the range of interesting properties and applications of origami structures, it is important to study the dynamic behavior of origami structures. In this study, a dynamic modeling approach for origami structures is presented considering distributed mass modeling, which has the potential to be a generalizable approach. In the proposed approach, stiffness is modeled using the bar and hinge modeling approach while the mass is modeled using the mass distribution approach. Various candidate mass distribution approaches were investigated by comparing their responses to the finite element method responses for various geometric conditions, loading and boundary conditions, and deformation modes. It was observed that a dynamic modeling approach with triangle circumcenter mass distribution was able to capture most of the dynamics satisfactorily consistently. Subsequently, a Miura-ori specimen was manufactured and its free vibration response was determined experimentally and then compared to the prediction of the analytical model. The comparison demonstrated that the analytical model was able to capture most of the dynamics in the longitudinal direction.
