In this paper, a procedure has been presented to develop fragility curves of structures equipped with optimal variable damping or stiffness semi-active tuned mass dampers (SATMDs). To determine proper variable damping or stiffness of semi-active devices in each time step, instantaneous optimal control algorithm with clipped control concept has been used. Optimal SATMDs have been designed based on minimization of maximum interstory drift of nonlinear structure which genetic algorithm(GA) has been used to solve the optimization problem. For numerical analysis, a nonlinear eight-story shear building with bilinear hysteresis material behavior has been used. Fragility curves for the structure equipped with optimal variable damping and stiffness SATMDs have been developed for different performance levels and compared with that of uncontrolled structure as well as structure controlled using passive TMD. Numerical analysis has shown that for most range of intensity measure optimal SATMDs have been effective in enhancement of the seismic fragility of the nonlinear structures which the improvement has been more than passive TMDs. Also, it has been found that, although variable stiffness SATMD shows to be more reliable in lower mass ratios, however in higher mass ratios variable stiffness and damping SATMDs performs similarly to improve reliability of the structure.

In this paper, designing variable stiffness semi-active tuned mass damper (SATMD) for mitigating the responses of nonlinear structures under earthquake excitation has been studied. Two semi-active control algorithms based on instantaneous optimal control and clipping control concept as well as modified balance control have been developed to determine the optimal stiffness of SATMD for nonlinear structures in each time step. For determining optimal parameters of semi-active control system including the weighting matrices in performance index of control algorithm as well as the maximum and minimum values of SATMD stiffness, an optimization problem for minimization of structure maximum response has been defined where genetic algorithm (GA) has been used for optimization. For numerical simulations, an eight-story nonlinear shear building with bilinear hysteresis behavior has been subjected to a white noise excitation and optimal SATMDs have been designed. The results showed that optimal variable stiffness SATMD using both control algorithms has been effective in suppressing the seismic responses of nonlinear structure. Also, variable stiffness SATMD shows better performance than TMD and variable damping SATMD in structural response controlling. Comparing the performance of the variable stiffness SATMD under testing earthquakes which were different from design record, showed that the efficiency of SATMD depends on the characteristics of excitation, hence design record needs to be chosen properly.

One of the new challenges in structural engineering is the mitigation of seismic hazards from structures using flexibility and energy dissipation approaches. This is in contrast with the typical seismic design methodologies in which strength and ductility resources of structural members are tapped to tackle earthquake demands. In this approach, adding to the flexibility of the structure should be in accordance with the energy dissipation potential in the system. In this case, earthquake demands for lateral strength in structures reduces, but energy dissipation devices are needed to subside the lateral deformation of such flexible structural systems.To meet the huge demand for energy dissipation potential in these structural systems, large scale damping devices are required. Such equipment, to mention a few, comes in the form of metallic, frictional, viscoelastic, memory shaped alloys and viscous dashpots. Among the others, viscous dashpots are considered the most favorite ones to use in large structural systems due to their sizable capacity and impartiality to ambient vibration and temperature loads. Moreover, since these devices are velocity dependent energy dissipaters, they are capable of reducing both deformation and acceleration responses of the structural systems more effectively.Viscous dashpots are typically made from a metallic cylinder, a piston, a shaft, cylinder caps and elastomeric seals (to provide confinement on the liquid inside of the cylinder). The existence of elastomeric seals in configuration assembly of these devices is considered a weak point in mechanical design of such dashpots considering maintenance issues. To address this problem, a contractible viscous dashpot was introduced earlier in which there was no need for elastomeric seals. In this work, a new version of this dashpot with variable damping constant have been tested for determination of its functionality and characteristics.Contractible viscous dashpots are made from two flexible chambers that axially contract or expand to accommodate liquid movement between the two. In this mechanism all parts of the device are made of steel and there is no relative movement between cylinder caps and the main shaft. Therefore, there is no need for elastomeric seals to confine the liquid inside of cylinders at cylinder caps.The Dashpot was designed for load capacity of and the maximum stroke of. In the test procedure, however, due to some limitations in the test setup, the attainable load was around 310 KN. The test results show stable hysteretic loops under sinusoidal excitations with the amplitude of in the frequency range of 0.1-0.25 Hz. The hysteresis loops resemble a viscous device with viscoelastic behavior that can be roughly represented by Kelvin model. As expected, damping constant of the dashpot reduces by an increase in excitation frequency. The capability of change in damping characteristics of the dashpot was embedded in the device. This ability was shown in the experiments where damping constant of the device became almost tripled during the test process by adjusting the embedded mechanism in the device.The contractible dashpot used in this study has shown an initial frictional behavior due to imperfection in its manufacturing process. Increase in the internal liquid pressure in the device expands this frictional behavior to about 10% of total capacity of the dashpot. The initial frictional force in the device can be easily improved to help the functionality of the dashpot.This dashpot has shown acceptable performances in all the experimental investigations carried out in the course of this study. Considering its simplicity and practicality (low maintenance costs), there would be a good chance for such devices to be used in large structures in near future.

The dynamic stability is studied for thin-walled structural elements with variable stiffness subjected to periodically alternating axial force in this paper. Here, the variation stiffness means that it changes with periodically alternating axial force as for nonlinear geometry stiffness matrix of thin-walled member. damping is considered and the governing equations are expressed in terms of a system of two second-order differential equations of the Mathieu type, with periodic coefficients. MATLAB package is used to determine the stability boundary. Numerical example is presented for the dynamic stability boundary of a simply supported beam with I-shaped cross section. Comparison is made with finite element analysis. Considered damping, some conclusions are drawn out: Excited zone of thin-walled member is continuous, the dynamic instability is highly dominant in the first region while the second and third instability regions are of much less practical importance, The larger the ratio of damp, the less the dynamic instability region, The larger the ratio of damp, the more time dependent components of the load wanted, absorption of damping is commonly of no effect to prevent parametrically excited vibration from dynamic instability, Parametrically excited vibration considering damping is much more different from damped forced vibration in nature.

Bending plate structures are important in mechanical and civil engineering. Therefore, a great deal of research has been done to describe their behavior. Moreover, closed-form and numerical solutions are proposed for these structures. …

The dynamic stability is studied for thin-walled structural elements with variable stiffness subjected to periodically alternating axial force in this paper. Here, the variation stiffness means that it changes with periodically alternating axial force as for nonlinear geometry stiffness matrix of thin-walled member. damping is considered and the governing equations are expressed in terms of a system of two second-order differential equations of the Mathieu type, with periodic coefficients.MATLAB package is used to determine the stability boundary. Numerical example is presented for the dynamic stability boundary of a simply supported beam with I-shaped cross section. Comparison is made with finite element analysis. Considered damping, some conclusions are drawn out: Excited zone of thin-walled member is continuous, the dynamic instability is highly dominant in the first region while the second and third instability regions are of much less practical importance, The larger the ratio of damp, the less the dynamic instability region, The larger the ratio of damp, the more time dependent components of the load wanted, absorption of damping is commonly of no effect to prevent parametrically excited vibration from dynamic instability, Parametrically excited vibration considering damping is much more different from damped forced vibration in nature.

The dynamic stability is studied for thin-walled structural elements with variable stiffness subjected to periodically alternating axial force in this paper. Here, the variation stiffuess means that it changes with periodically alternating axial force as for nonlinear geometry stiffuess matrix of thin-walled member. damping is considered and the governing equations are expressed in terms of a system of two second-order differential equations of the Mathieu type, with periodic coefficients. MATLAB package is used to determine the stability boundary. Numerical example is presented for the dynamic stability boundary of a simply supported beam with I-shaped cross section. Comparison is made with finite element analysis. Considered damping, some conclusions are drawn out: Excited zone of thin-walled member is continuous, the dynamic instability is highly dominant in the first region while the second and third instability regions are of much less practical importance; The larger the ratio of damp, the less the dynamic instability region; The larger the ratio of damp, the more time dependent components of the load wanted, absorption of damping is commonly of no effect to prevent parametrically excited vibration from dynamic instability; Parametrically excited vibration considering damping is much more different from damped forced vibration in nature.

Semi active devices can be used to control the responses of a continuous bridge during earthquake excitation. They are capable of offering the adaptability of active devices and stability and reliability of passive devices. This study proposes two semi-active control method protection of bridge using variable stiffness and damping systems and MR dampers. The first method is variable stiffness and damping with eight different (on-off) control schemes which is optimized with genetic algorithm. Genetic algorithm is used to define the parameters of this method. In the second method, an intelligent controller using fuzzy control of MR damper is developed. In particular, a fuzzy logic controller is designed to determine the command voltage of MR dampers. In order to evaluate the effectiveness of the proposed method, the performances of the proposed controllers are compared in numerical study. Results reveal that the developed controllers can effectively control both displacement and acceleration responses of the continuous bridge.

Power system stabilizer (PSS) does not have a significant impact on inter-area modes and FACTS devices are used to damping these modes and to enhance power system stability. In this article, an objective function based on different and variable weight coefficients according to eigenvalues condition is proposed and optimization parameters of power system stabilizer and variable impedance parameters include static VAR compensator (SVC) and thyristor controlled series capacitor (TCSC), (Including amplifying gain rate and time constant of phase-compensating blocks) is done using genetic algorithm in harmony. Also, in the process of optimization, the location of the FACTS devices and the control signal are considered as optimization parameters. Simulation results on IEEE 68-bus system show improvement damping of inter-area modes using the proposed method.