Lap-Loi Chung

SEISMIC RESPONSE ANALYSIS OF BRIDGES ISOLATED WITH FRICTION PENDULUM BEARINGS

SUMMARY A systematic method is developed for the dynamic analysis of the structures with sliding isolation which is a highly non-linear dynamic problem. According to the proposed method, a unified motion equation can be adapted for both stick and slip modes of the system. Unlike the traditional methods by which the integration interval has to be chopped into infinitesimal pieces during the transition of sliding and non-sliding modes, the integration interval remains constant throughout the whole process of the dynamic analysis by the proposed method so that accuracy and eƒciency in the analysis of the non-linear system can be enhanced to a large extent. Moreover, the proposed method is general enough to be adapted for the analysis of the structures with multiple sliding isolators undergoing independent motion conditions simultaneously. The superiority of the proposed method for the analysis of sliding supported structures is verified by a three-span continuous bridge subjected to harmonic motions and real earthquakes. In addition, the side e⁄ect of excessive displacement of the superstructure induced by the sliding isolation is eliminated by replacing one of the roller supports on the abutments with hinge support. Therefore, both reductions in the forces of the substructure and the displacements of the superstructure can be achieved simultaneously. ( 1998 John Wiley & Sons, Ltd.

Seismic performance of steel beams to concrete-filled steel tubular column connections

Abstract A nonlinear force-deformation model to simulate shear transfer behavior in the panel zone of CFT (Concrete-Filled Steel Tube) beam-column connections is proposed. In this model, influence of axial load on the shear transfer behavior is accounted for. To validate the proposed theory, five circular CFT beam-column connections were constructed and tested. Test results showed that all specimens failed by the welding fracture while entering nonlinear stage. It is found that the higher the axial load was applied, the better the ductility of connections was obtained. Comparison of analytical and experimental results shows that the proposed prediction for panel shear falls in a reasonable range for higher axial load tests, but tends to be conservative for lower axial load tests.

Acceleration feedback control of seismic structures

The essential feedback signals for direct output feedback control are limited to velocities and direct acceleration feedback control is found to be unfeasible even though accelerometers are favorable sensors from the viewpoint of measurement. In this paper, acceleration feedback control becomes feasible when the feedback data is extended to cover accelerations of the previous time-steps. A multi-step acceleration feedback control algorithm is derived through an optimization process such that a certain prescribed quadratic performance index is minimized. Control forces are simply generated from acceleration information of the current and previous time-steps. The feasibility of the proposed control algorithm is verified numerically by the tendon control of structures subjected to real earthquake excitation. The control effectiveness of the proposed control algorithm is close to that of state feedback control algorithm. Simple on-line calculation and availability of accelerometers make the proposed control algorithm favorable to realtime control implementation.

A General Method for Semi-active Feedback Control of Variable Friction Dampers

Although there are many well-established control methods for vibration mitigation of seismic structures with active devices, their direct application for structures with semi-active control devices are limited. This limitation is primarily contributed by the fact that a semi-active device can only provide a resistant (passive) force to the controlled structure. In this paper, a general method for semi-active feedback control of seismic structures with variable friction dampers (VFD) is proposed. In order to overcome the force limitation of friction dampers, the method forms a semi-active feedback gain by multiplying an active gain with Heaviside functions. Based on this method, two newly developed control laws, i.e., semi-active modal control and semi-active optimal control were numerically investigated. A multiple DOF structural system with various sensor deployments, for either full-state or direct-output feedbacks was considered in the numerical study. The performances of both semi-active control laws for seismic vibration mitigation were compared with those of passive and active controls. The numerical results showed that both semi-active controls resulted in better acceleration reductions than the passive case and were able to closely imitate the performance of their active control counterparts.

Modal control of seismic structures using augmented state matrix

In conventional methods of modal control, the number of controllable structural modes is usually restrained by the number of sensors that feedback the structural signals. In this paper a modal control scheme where the feedback gain is formulated in an augmented state space is proposed. The advantage of the proposed method is that it increases the number of the controllable modes without adding extra sensors. The method is verified experimentally by an earthquake simulation test with a full-scale building model. The proposed modal control was also compared with the conventional ones in the test. For the building model tested, the performance of the proposed control with only one feedback signal can be as efficient as that of modal control with full state feedback. Copyright

Optimal discrete-time structural control using direct output feedback

An optimal direct discrete-time output feedback control algorithm is developed. According to the proposed algorithm, optimal discrete-time output feedback gain is derived through a variational process such that a certain prescribed quadratic performance index is minimized. It is verified that the classical optimal discrete-time state feedback control algorithm is just a particular case of the proposed control algorithm. With the introduction of special matrix operations, optimal output feedback gain is obtained systematically by solving simultaneously linear algebraic equations iteratively. Control forces are then generated directly from output measurements multiplied by the pre-calculated shift-invariant output feedback gain. A small number of sensors and controllers, and simple on-line calculation make the proposed algorithm favorable to real-time control implementation. Numerical verification is illustrated by the control of a single degree-of-freedom (DOF) and a three degree-of-freedom structure subjected to real earthquake excitations. Differences between discrete-time and continuous-time output feedback control are discussed.

Instantaneous control of structures with time-delay consideration

A modified instantaneous control algorithm with time-delay consideration for the active control of structures is developed in discrete-time formulation. The shift-invariant feedback gain matrix is obtained through an optimization process such that a prescribed instantaneous quadratic performance index is minimized. The effect of time-delay is taken into consideration by introducing a compensation scheme where the feedback gain matrix for the ideal control system is modified by the effective system matrix. It is proved that the real control system with time-delay compensation conserves the eigen-properties of the ideal control system with no time-delay. In the presence of time-delay, the equation of motion of the discrete-time control system remains a set of difference equations which makes the evaluation of the control system simple and straightforward. The feasibility of the proposed control algorithm is verified numerically through eigenvalue, frequency-domain and time-domain analyses by the tendon control of a structure. With the proposed control algorithm, the control system is still effective in spite of the presence of time-delay.

Modified predictive control of structures

A modified predictive control algorithm for the active control of structures is developed in the discrete-time formulation. Before the control algorithm is modified, the predictive control forces are kept constant throughout the predictive time-steps in the predictive control algorithm so that dynamic instability is induced to the control systems for certain numbers of predictive time-steps. After the control algorithm is modified, the predictive control forces are assigned to be linearly related with the predictive structural states so that dynamic stability of the control systems is guaranteed. In addition to the dynamic stability of the control systems, the control performance of the modified control algorithm is also superior to that of the original one. The feasibility of the modified control algorithm is verified through eigenvalue analysis, frequency-domain analysis and time-domain analysis. The tendon control systems of a single-degree-of freedom structure and a three-degree-of-freedom structure are illustrated to demonstrate the control effectiveness of the modified predictive control algorithm.

Effectiveness of an eccentric rolling isolation system with friction damping

Recently, the benefit of nonlinear isolation systems under resonance or near-fault earthquake has been investigated. In this paper, an eccentric rolling isolation system (ERIS) with additional friction damping is proposed. The isolation object is eccentrically pinned on a set of circular isolators so that the restoring force is nonlinear. To investigate the advantage of the ERIS, a corresponding isolation system with linear restoring force is also considered for comparison. The friction parameters of the two systems with linear and nonlinear restoring force are designed under the far-field El Centro earthquake. The performances of the two isolation systems are inspected under excitations other than the design one. In free vibration, the response of the ERIS decays faster than the corresponding linear system. In resonance sinusoidal excitation, the responses are divergent for the linear system but convergent for the ERIS. The linear system is ineffective but the ERIS is effective due to the nonlinearity under the near-fault Chi-Chi earthquake with various peak ground accelerations.

Use of the active member concept in vibration mitigation of seismic structures

Abstract This paper aims to investigate the possible use of a control system that combines the concept of active members and a direct output feedback method to mitigate the seismic vibration of civil engineering structural systems. An active member, according to the definition used in this paper, is an added structural member in which an actuator, a sensor and a simple controller are highly integrated. By integrating these fundamental control devices together, the active members will be less vulnerable to environmental influence; moreover, each active member can be used as an independent control mechanism. In this study, an output feedback control algorithm called the Modal Truncated Output Feedback (MTOF), suitable for the active member control, is adopted. With this algorithm, the failure or malfunction of one active member will not impair the entire control system. A numerical example of the shear building type is used to demonstrate the proposed control system. It is shown that the concept of active members and the MTOF control algorithm together form a very stable, effective, and reliable control system for vibration control of seismic structures.