Jann N. Yang

Semi-active hybrid control systems for nonlinear buildings against near-field earthquakes

Because of the high peak acceleration and high velocity pulse with long period, near-field earthquakes, such as Northridge, Kobe and Chi-Chi earthquakes, are responsible for the destruction and severe damage to civil infrastructures. Under this circumstance, the low-damping passive base isolation system alone cannot ensure the safety of building structures. The purpose of this paper is to present the safety performances of various types of hybrid control systems for nonlinear buildings against near-field earthquakes. Hybrid control systems considered consist of mainly the base isolation system and either the passive control devices or the semi-active dampers or the combination thereof. Passive control systems studied involve viscous dampers and friction dampers, whereas two newly proposed semi-active control systems are investigated including the resetting semi-active stiffness damper and the semi-active electromagnetic friction damper. The elasto-plastic behaviors of both the base-isolation system and the building have been considered in the analysis. Extensive simulation studies have been conducted for control of base-isolated buildings subject to various near-field earthquakes. Simulation results demonstrate that the newly proposed resetting semi-active stiffness damper and semi-active electromagnetic friction dampers are quite effective in protecting the safety of building structures against near-filed earthquakes.

Control of seismic-excited buildings using active variable stiffness systems

Abstract It has been demonstrated that active variable stiffness (AVS) systems may be effective for response control of buildings subjected to earthquake excitations. The applications of active variable stiffness systems involve nonlinear control in which control theories for linear systems are not applicable. Based on the theory of variable structure system (VSS) or sliding mode control (SMC), control methods are presented in this paper for applications of active variable stiffness systems to seismic-excited buildings. In addition to full-state feedback controllers, general static output feedback controllers as well as simple output feedback controllers using only collocated sensors are presented. The principle of active variable stiffness control is interpreted based on the concept of the dissipation of hysteretic energies. Simulation results indicate that the control methods presented are robust and the performance of static output feedback controllers is comparable to that of the fullstate feedback controllers. Simulation results further indicate that the active variable stiffness systems, using locking and unlocking devices, are effective in reducing the interstorey drifts of seismicexcited buildings. However, the floor acceleration of the building may increase significantly, depending on the structure design and earthquake excitation.

Effect of fixed time delay on stability and performance of actively controlled civil engineering structures

In this paper, a state-of-the-art review for the fixed time delay of actively controlled civil engineering structures is presented, including the identification of time delay, the effect of time delay on the stability and performance of the controlled structures, and the evaluation of the critical time delay. In particular, a critical review of the stability analysis methods currently available for the critical time delay of Multiple-Degree-of-Freedom (MDOF) systems is conducted and new simulation results are presented to show its limitation in practical applications. Further, a method of stability analysis for the critical time delay of MDOF systems equipped with single or multiple actuators is presented along with the simulation results to demonstrate its applications to seismic hazard mitigations. Under earthquake excitations, simulation results for the structural response indicate that the degradation of the control performance due to the fixed time delay is not significant until the time delay is close to the critical time delay. It is further demonstrated that the time-delay problem is more serious for structures with closely spaced vibrational modes, such as a building equipped with an active tuned mass damper.

Optimal polynomial control for seismically excited non-linear and hysteretic structures

In this paper, we present an optimal polynomial controller for reducing the peak response quantities of seismically excited non-linear or hysteretic building systems. A performance index, that is quadratic in control and polynomial of any order in non-linear states, is considered. The performance index is minimized based on the Hamilton-Jacobi-Bellman equation using a polynomial function of non-linear states, which satisfies all the properties of a Lyapunov function. The resulting optimal controller is a summation of polynomials in non-linear states, i.e. linear, cubic, quintic, etc. Gain matrices for different parts of the controller are determined from Riccati and Lyapunov matrix equations. Numerical simulation results indicate that the percentage of reduction for the selected peak response quantity increases with the increase of the earthquake intensity. Such load adaptive properties are very desirable, since the intensity of the earthquake ground acceleration is stochastic in nature. The proposed optimal polynomial controller is an effective and viable control method for non-linear or hysteretic civil engineering structures. It is an addition to available control methods in the literature.

Decentralized sliding mode control of a building using MR dampers

This paper presents the structural control results of shaking table tests for a steel frame structure in order to evaluate the performance of a number of proposed semi-active control algorithms using multiple magnetorheological (MR) dampers. The test structure is a six-story steel frame equipped with MR dampers. Four different cases of damper arrangement in the structure are selected for the control study. In experimental tests, the El Centro earthquake and Kobe earthquake ground motion data are used as excitations. Further, several decentralized sliding mode control algorithms are developed in this paper specifically for applications of MR dampers in building structures. Various control algorithms are used for the semi-active control studies, including the proposed decentralized sliding mode control (DSMC), LQR control, and passive-on and passive-off control. Each control algorithm is formulated specifically for the use of MR dampers installed in building structures. Additionally, each algorithm uses measurements of the device velocity and device drift for the determination of the control action to ensure that the algorithm can be implemented in a physical structure. The performance of each algorithm is evaluated based on the results of shaking table tests, and the advantages of each algorithm are compared and discussed. The reduction of story drifts and floor accelerations throughout the structure is examined.

LQG control of lateral–torsional motion of Nanjing TV transmission tower

The 310 m Nanjing TV transmission tower in China will be installed with an active mass driver on the upper observation deck in order to reduce the acceleration responses under strong wind gusts. This paper presents the linear–quadratic–Gaussian (LQG) control strategy using acceleration feedback to reduce the tower responses under coupled lateral–torsional motion. Emphasis is placed on the practical applications, such as the limitations on actuator peak force and stroke, limited number of sensors, etc. The along- and across-wind components of the wind velocity are defined by the cross-power spectra. In the simulation analysis, both deterministic and stochastic approaches have been used, and the power spectral density, rms values and peak values of response quantities have been computed. Comparisons of the responses of the TV tower due to wind loads from different angles of attack have been made. Simulation results demonstrate that (i) the performance of the active mass driver using the LQG control strategy is remarkable in reducing coupled lateral-torsional motions of the tower, and (ii) the LQG strategy is robust with respect to uncertainties in the angle of attack of wind loads. The LQG strategy is suitable for the full-scale implementation of active mass driver on Nanjing Tower. Copyright

Experimental Study of Damage Detection by Data-Driven Subspace Identification and Finite-Element Model Updating

The need to identify the physical properties of a structure given its force-response (input-output) relationship is driven primarily by the need to validate the approximate solution models, such as finite-element models. This paper proposes a method, which combines the structural system identification and model updating techniques, for the damage detection of a steel frame structure and a RC frame. The damage detection procedure consists of two steps: (1) identifying the system dynamic characteristics using the subspace identification (SI) technique from input/output measurements and (2) developing a damage assessment method for structural members (including joints) based on a progressive finite-element model updating and a large-scale optimization using a nonlinear least-square technique. The proposed method was verified through a shaking table experimental study using: (1) a 1/4-scale six-story steel frame structure by loosening the connection bolts for damage simulations and (2) a two-story RC frame subject to different levels of ground excitations back to back. As demonstrated by experimental results, the proposed damage detection method, based on the combination of SI technique and the model updating approach, is very effective for the damage assessment of frame structures. The method not only can detect the damage locations but also can quantify the damage severities.

Control of sliding-isolated buildings using dynamic linearization

Abstract The method of dynamic linearization is presented for the control of seismic-excited buildings isolated by a fractional-type base sliding system. The dynamic behaviour of the building equipped with a base sliding isolation system is highly nonlinear. The method of dynamic linearization is to synthesize the control vector such that the response of the building matches that of a spicified template system, where the dynamic behaviour of the template system is well known. Applications of the method of dynamic linearization to a sliding-isolated building require only a few sensors for the entire control system, making the control method very easy for practical implementation. A shaking table experimental program was conducted to demonstrate the validity of the control method presented. For the shaking table tests, a three-storey 1 4 - scaled building model is mounted on a base mat supported by four frictional bearings. Numerical simulation results under ideal control environments indicate that the performance of the dynamic linearization method is remarkable. However, experimental results show a moderate degradation of the control performance due to noise pollution and system time delays. It is observed that control of sliding-isolated buildings is quite sensitive to a system time delay.

Stochastic hybrid control of hysteretic structures

Abstract A method of stochastic control for seismic-excited hysteretic structures is presented. Emphasis is placed on hybrid control of base-isolated buildings. The earthquake excitation is modeled as a filter shot noise and the equations of motion of the structure are augmented by the earthquake model. Based on the stochastic optimal control algorithm, the resulting control vector consists of a feedforward compensation and a state feedback loop. The nonstationary response statistics, such as the mean square function, are obtained using the methods of random vibration and equivalent linearization. Simulation results indicate that for a range of earthquake model parameters of practical interest, the performance of the control method using both the feedforward compensation and the state feedback loop does not offer a significant improvement over that of the control method using only the state feedback.

Applications of sliding mode control to benchmark problems

In this paper, both the methods of continuous sliding mode control (CSMC) and continuous sliding mode control with compensators (CSMC&C) have been applied to two benchmark structures, namely, a building model equipped with an active mass driver system, and a building model equipped with an active tendon system. The CSMC&C strategy is a modification of CSMC to facilitate the design of static output feedback controllers and to provide a systematic tuning of the control effort. Due to the structural identification scheme used in the benchmark problems, in which the state variables are fictitious, one cannot take the full advantages of static output feedback controllers. As a result, an observer is used in CSMC, whereas a low-pass filter is incorporated for each measurement in CSMC&C. The purpose of using low-pass filters in CSMC&C is to transform the benchmark problems into strictly proper systems. The main advantage of the CSMC&C method is that the on-line computational effort is reduced since the dimension of filters and compensator is much smaller than that of an observer. Simulation results based on the CSMC and CSMC&C methods are presented and compared with that of the LQG method. Robustness of stability and noise rejection for each controller design are also illustrated by examining the loop transfer function. Simulation results for the benchmark problems indicate that the control performances for LQG, CSMC and CSMC&C are quite comparable.