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.
Recently, a benchmark problem has been developed to study seismically excited highway bridges. In the second phase of the aforementioned study, the bridge is isolated both at the abutments and at the central pier location. The isolation, though effective in reducing the superstructure responses such as mid-span accelerations, results in increased mid-span and isolator displacements. The performance of a newly developed Lyapunov semiactive controller in reducing the isolator and mid-span displacements is investigated analytically on this newly developed phase II full-scale three-dimensional seismically excited highway bridge. The bridge is isolated using nonlinear hysteretic bearings with a lead core on the inside and an elastomer surrounding the lead core. Magneto-rheological (MR) dampers are used to control the seismic responses of the bridge semiactively. The semiactive control devices are installed at the isolation level between the deck and the isolators on bridge piers and center column at ten locations, each location consisting of a single orthogonal pair to control the responses in both directions. The outputs allowed in the benchmark problem definition are used to design the controller and where velocity measurements are required, the accelerations are integrated using a filter that simulates integration. The performance of the controller is analyzed in terms of the performance indices defined in the benchmark problem definition. The results of the Lyapunov controller are compared with the results of the sample controller presented in the benchmark problem.
Two phases of the benchmark problem on base-isolated buildings concluded recently, culminating in two separate special issues in the Journal of Structural Control and Health Monitoring. The base-isolated building considered in the benchmark problem is based on the USC hospital building in Southern California. The goal of this benchmark is to provide a common computational test-bed to analyze competing control strategies on base-isolated buildings, including devices, algorithms and sensors. To achieve this goal, a 3-D finite-element model was developed in MATLAB to represent the complex behavior of the full-scale base-isolated building with lateral-torsional behavior. The model allows users to model both linear and nonlinear isolation systems. A nonlinear structural analysis tool was developed in MatlabTM and distributed to the participants for nonlinear dynamic analysis. Over twenty papers in two special issues in the Journal of Structural Control and Health Monitoring were published as a result of this effort. This paper presents an overview of this benchmark effort.
Vibration attenuation devices are used to reduce the vibrations of various mechanical systems and structures. In this work, an analytical method is proposed to provide the means to investigate the influence of system parameters on the dynamic response of a system. The method of multiple scales is used to calculate an approximate broadband solution for a two degree-of-freedom system consisting of a linear primary structure and a nonlinear tuned mass damper. The model is decoupled, approximate analytical solutions are calculated, and then they are combined to produce the desired frequency-response information. The approach is initially applied to a linear two degree-of-freedom system in order to verify its performance. The approach is then applied to the nonlinear system in order to study how varying the values of parameters associated with the nonlinear absorber affect its ability to attenuate the response of the primary structure.
In this paper a new adjustable passive fluid spring and damper (APSD) is studied experimentally. The APSD is capable of varying the stiffness and damping independently; thus, the device can be used as an adjustable damper or as an adjustable fluid spring or as a combined stiffness and damping device. This allows a range of possibilities to modify structural stiffness and damping properties by placing the APSD at appropriate locations in the bracing system of the structure. In this paper the experimental results of the APSD under cyclic and ramp loading are presented. The versatile properties of the APSD are shown by means of experimental for displacement loops.
The increase in bearing displacements of sliding isolated buildings due to near-fault earthquakes, with long-period pulse type of ground motion, is an important problem. Often, supplemental nonlinear passive dampers are incorporated into the isolation system to reduce the base displacements; however, this may increase the interstory drifts and accelerations in the superstructure. Hence, there is a need to examine whether controllable nonlinear dampers can reduce the base displacements without a further increase in superstructure response. In this study, the effectiveness of variable damping, provided by magnetorheological (MR) dampers, in reducing the response of sliding isolated buildings during near-fault earthquakes is investigated using a 1:5 scale steel two-story model. A nonlinear analytical model is developed with due consideration given to the nonlinearities of the friction bearings and the MR damper. A Lyapunov-based control algorithm is developed for control of the MR damper and the building model, and tested on a shake table. Results of passive low/high damping cases and semiactive cases are compared. It is shown that the variable damping reduces base displacements and superstructure responses further than passive low/high damping cases.
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.
By Satish Nagarajaiah and Ian Buckle One of the most common seismic isolators in use today is the elastomeric bearing. The combination of rubber layers and reinforcing steel shims gives a device that is axially very stiff but soft laterally. Flexibility may be increased, and large period shifts achieved, simply by increasing the number and/or thickness of the rubber layers. But increasing the shear flexibility of these short columns can lead to relatively low buckling loads, which may be further reduced when high shear strains are simultaneously imposed. As a consequence, many design procedures require the axial load rating of a bearing to be reduced as the shear displacement increases (e.g., AASHTO 1999). These reductions are based on engineering judgment and very little science. For example, for a rectangular bearing of width B, the critical load P
By introducing negative stiffness devices, this study further improves the maximum achievable damping ratio of conventional damped outrigger (CDO) structures with flexible perimeter columns. Dynamic characteristics of tall buildings with this novel negative stiffness damped outrigger (NSDO) are parametrically studied by solving the transcendental characteristic equations. An NSDO is able to improve the maximum achievable damping ratio to about 30% with less consumption of an outrigger damping coefficient (or a less amount of a viscous damper) as compared with a CDO. Numerical results showed that the NSDO is effective for both winds and earthquakes. For instance, an NSDO further decreases the maximum seismic interstory drift by 18.9% and reduces the total-wind-excited acceleration by 34.9%, with only a 20% outrigger damping consumption, as compared to a CDO. Because neither an NSDO nor CDO provides extra stiffness at low amplitudes of motion, an extra conventional outrigger (CO) is suggested to be placed at the top of a tall building when applying an NSDO in practical applications, and the effectiveness of an NSDO is also not compromised when an extra CO is placed.
