Zhao-Dong Xu

Semi-active control of structures incorporated with magnetorheological dampers using neural networks

Semi-active control of buildings and structures with magnetorheological (MR) dampers for earthquake hazard mitigation represents a relatively new research area. In this paper, the Bingham model of MR damper is introduced, and the formula relating the yielding shear stress and the control current of MR dampers is put forward that matches the experimental data. Then an on-line real-time control method for semi-active control of structures with MR dampers is proposed. This method considers the time-delay problem of semi-active control, which can solve distortion of the responses of structures. Finally, through a numerical example of a three-storey reinforced concrete structure, a comparison is made between controlled structure and uncontrolled structure. The calculated results show that MR dampers can reduce the seismic responses of structures effectively. Moreover, the on-line real-time control method is compared with the traditional elastoplastic time-history analysis method, and the efficacy of the on-line real-time control method is demonstrated. In addition, the Levenberg–Marquardt algorithm is used to train the on-line control neural network, and studies show that the algorithm has a very fast convergence rate.

Earthquake Mitigation Study on Viscoelastic Dampers for Reinforced Concrete Structures

Viscoelastic (VE) dampers are one of the most common earthquake mitigation devices. This paper addresses the mathematical modelling of VE dampers and the dynamic analysis of structures with VE dampers. In this paper, the equivalent standard solid model, a new mathematical model of VE dampers, is used to describe the influence of temperature on the energy absorption features of VE dampers. Elastoplastic time field analysis, frequency field analysis and shaking table tests are used to analyze responses of a 1/5 scale three-story reinforced concrete frame structure with and without VE dampers. Comparisons between the numerical and experimental results show that the VE dampers can be modeled by the equivalent standard solid model and that the VE dampers are effective in reducing the seismic responses of structures.

Simulation of the Effect of Temperature Variation on Damage Detection in a Long-span Cable-stayed Bridge

Damage detection based on structural dynamic characteristics, such as natural frequencies and mode shapes, is an important area of research. Obtaining accurate structural dynamic characteristics is perhaps the most challenging aspect of this research. In particular, changes in environmental temperature, due to seasonal weather or radiation from sunshine, will lead to changes in the dynamic characteristics of structures. In this study, the effects of change in environmental temperature on the frequencies and mode shape curvatures of a cable-stayed bridge are analyzed considering seasonal temperature difference (a uniform temperature difference) and sunshine temperature difference (an asymmetric temperature difference). Changes in dynamic characteristics due to temperature variation are compared to changes in dynamic characteristics due to damage of girders and cables in a cable-stayed bridge. Guidelines are presented in this study to explain how frequencies and mode shape curvatures change due to average and asymmetric temperature variations in the cable-stayed bridge. An important conclusion is that changes in dynamic characteristics of the bridge due to damage in girders or cables may be smaller than changes in dynamic characteristics due to variations in temperature. Suggestions are given to prevent changes in dynamics of the bridge due to damage from being submerged in changes in dynamics due to temperature effects.

Optimal analysis and experimental study on structures with viscoelastic dampers

In this paper, the simplex method, a synthetic optimization analysis method of structures with viscoelastic (VE) dampers, which is used to determine the optimal parameters and location of VE dampers, is presented. When applied to a shaking table test of the reinforced concrete structure with VE dampers, it is seen that the simplex method can act as the synthetic optimization method of structures with VE dampers. It is also found that the shock absorption effect of the VE dampers is best when the location of VE dampers is optimal.

Magnetoviscoelasticity parametric model of an MR elastomer vibration mitigation device

Both experimental and modeling studies of magnetic field induced viscoelastic properties of magnetorheological (MR) elastomers under different loading cases are discussed. Anisotropic MR elastomer (MRE) samples with different concentrations of carbonyl iron powder, natural rubber and additives are fabricated and four MRE vibration mitigation devices are manufactured to investigate the dynamic viscoelastic properties of MREs under varying magnetic fields, displacement amplitudes and frequencies in the shear mode. The characteristics of the dynamic properties of the MRE devices are obtained in terms of the experimentally determined shear storage modulus and loss factor. These results demonstrate that the MREs exhibit variable stiffness and damping properties. Based on the studies of properties of viscoelastic materials and the experimental results of MREs, a parameter model is proposed to describe MRE performances. The four parameters under various working conditions, such as magnetic field, displacement amplitude and frequency, are identified by using the Matlab optimization algorithm. Comparisons between experimental and numerical results are discussed, and the results show that the proposed parameter model can describe the performances of MRE devices very well.

Fuzzy Control Method for Earthquake Mitigation Structures with Magnetorheological Dampers

Magnetorheological (MR) damper has a potential use in the vibration control of large structures due to its lower energy input and fine earthquake mitigation ability. A key problem for controlling structure with MR dampers is choosing the control current quickly and accurately. In order to solve this problem, a new method by using fuzzy controller, namely fuzzy full-state control method, is proposed. Dynamic responses under different inputs of fuzzy controller are compared. At the same time, dynamic responses of the traditional bi-state (BS) controlled structure and the passive full-current (PFC) controlled structure under different earthquake waves are compared with those of fuzzy full-state controlled structure. Through a numerical example about a three-story reinforced concrete structure, it can be concluded that fuzzy control technique can realize choosing of currents of MR dampers quickly and accurately and have good control effect.

Performance tests and mathematical model considering magnetic saturation for magnetorheological damper

As a semiactive control device, magnetorheological dampers have been paid more attention due to their high controllability, fast response, and low power demand. One of the important characteristics for magnetorheological dampers is magnetic saturation, that is, the maximum damping force will reach some value and no longer vary with the increasing input current, especially in the presence of large magnetic flux density. In order to take this problem into account fully, tests on a shear-valve mode magnetorheological damper are carried out to consider the effects of input current, displacement amplitude, and loading frequency on the properties of the magnetorheological damper during magnetic saturation situation first. Then, the magnetic saturation phenomenon of the magnetorheological damper is simulated using the finite element method, and the numerical simulation results are compared with the experimental results. Finally, a magnetic saturation mathematical model is proposed to describe the properties of the magnetorheological damper, and the numerical hysteresis curves of the proposed magnetic saturation mathematical model, the Bingham model, and the Bouc–Wen model are compared with the experimental results. It can be concluded that the magnetic saturation mathematical model can describe the influence of input current, displacement amplitude, and excitation frequency on the properties and the magnetic saturation property of the magnetorheological damper.

Model, tests and application design for viscoelastic dampers:

Many kinds of dampers are invented to reduce the dynamic responses due to earthquake or strong wind. Viscoelastic damper is a kind of commonly used passive control device for vibration mitigation of structures. Its energy dissipation characteristics are affected by environmental temperature and excitation frequency. In order to describe the complex characteristics of viscoelastic dampers changing with temperature and frequency, a new model — the equivalent standard solid model, which is based on the standard linear solid model and temperature-frequency equivalent theory, is proposed. The comparison results between experimental and numerical data of viscoelastic dampers show that the equivalent standard solid model can precisely describe energy dissipation behavior of viscoelastic dampers under different temperatures and frequencies. At the same time, 52 viscoelastic dampers are applied for seismic retrofit design on Xi’an Petrol Hotel, and the finite analysis for structures with and without dampers are carried out under frequent-earthquake and infrequent-earthquake excitations. The dynamic responses of structures with and without dampers are compared. Analytical results show that viscoelastic dampers can reduce the earthquake responses effectively, and they are high-performance energy dissipation devices.

Energy Damage Detection Strategy Based on Strain Responses for Long-Span Bridge Structures

An energy damage detection strategy through disposing strain responses has been developed. First, the strain-based energy dynamic indexes for a system with multiple degrees of freedom were derived from the frequency response function (FRF) of strain responses and energy spectra density. Then, the traditional mode-shape curvature strategy and the proposed strain-based energy damage detection strategy were both used to analyze a long-span cable-stayed bridge, and it was found through comparison that the proposed strain-based energy damage detection strategy solved the shortcomings of the traditional mode-shape curvature strategy. Finally, damage location, damage quantification, and noise pollution resistance analysis for a long-span cable-stayed bridge with different degrees of damage were carried out to verify the effectiveness of the proposed strain-based energy damage detection strategy. The numerical analysis showed that the proposed strain-based energy damage detection strategy can locate damage positions accurately, and it also has good damage quantification and noise pollution resistance abilities.

Experimental and Modeling Study on Magnetorheological Elastomers with Different Matrices

In this paper the physical and dynamic mechanical property tests of magnetorheological elastomers (MREs) are reported. Two kinds of MREs with different matrices, about 12 samples in total, are fabricated by mixing carbonyl iron powder and additives, and cured by using a constant magnetic field. The physical and dynamic viscoelastic properties of these MRE specimens are evaluated with respect to different magnetic fields, displacement amplitudes, and frequencies. The experimental results demonstrate that MREs have variable stiffness and the loss factor of the samples with bromobutyl rubber is high, which shows a good damping property. The proposed magnetoviscoelasticity parameter model is then verified by comparing the experimental and numerical results, which demonstrate that the magnetoviscoelasticity parameter model can describe the MRE performance well.