جستجوی مقالات مرتبط با کلیدواژه "shear frame" در نشریات گروه "عمران"

Recently, nonlinear viscous dampers have been widely used to improve the seismic performance of structures. These dampers dissipate the kinetic energy caused by the earthquake by producing a damping force. These dampers are designed in such a way that the force is proportional to the velocity. The Maxwell model is the most common model for modeling the behavior of nonlinear viscous dampers. In this model, the damping coefficient, the velocity exponent and the axial stiffness of the dampers are the key parameters. In most previous studies, the uncertainty of the underlying parameters in behaviour of viscous dampers has been ignored while it can has a significant effect on the seismic response of structures. In this study, first, the reliability analysis of a shear frame equipped with a nonlinear viscous damper was performed using the Monte Carlo sampling method. The results show that increasing the maximum drift from 0.015 to 0.02 reduces the probability of failure by 72%. Then, reliability-based sensitivity analysis of the studied frame was performed in order to determine the most effective random variable on the reliability of the frame. The results show that the velocity exponent is the most effective random variable on the reliability of the frame. Also, results indicate that importance of random variables depends on the used limit state function in the reliability analysis. For example, the imporatance value of the damping coefficient is 59% and 9.5% less than the velocity exponent with respect to the maximum drift of 0.015 and 0.025, respectively.

A designer needs to design a structure with the aim of obtaining the maximum possible load expected for the structure during its lifetime. In this paper, with regards to the information obtained from the earthquakes, the critical earthquakes were computed for a shear frame building, equipped with a belt truss system and subjected to two constraint scenarios. For this purpose, a nonlinear optimization problem has been solved, in which the objective function was the maximization of the roof displacement. In the first constraint scenario, the computed critical earthquake was known as the first state critical earthquake. In addition, for the second constraint scenario, the earthquake was named as the second state critical earthquake. In the first scenario, the energy and peak ground acceleration was considered as the constraints, while in the second scenario, the upper bound Fourier spectrum was added to these constraints. Finally, properties of the initial and critical earthquakes were investigated using the Fourier analysis method and continuous wavelet transform. The numerical results showed that the Fourier spectrum of the first critical earthquake was 6.86 times higher than the maximum values for the same parameter for other earthquakes at a frequency near the first natural frequency of the structure. Also, using time-frequency curve, it was shown that duration of the strong ground motion of all earthquake places within the dominant duration of the frequencies of the same earthquake was more than 10 sec.

Due to high capacity and low energy consumption of Magneto-Rheological (MR) dampers، they are vastly being utilized to control seismic responses of structures. Presenting more precise methods for control algorithm، and including more realistic physical chara­­cteristics of MR dampers (e. g. nonlinearities، uncertainties and …) will help engineers to employ this kind of damper more efficiently. In order to achieve a controller that quickly and accurately determines the input voltages to the MR dampers، in this paper، a new strategy is proposed. The proposed strategy utilizes Adaptive Network based Fuzzy Inference System، (ANFIS) for optimal control of structures that are equipped with MR dampers. To obtain optimal time histories of demanded voltages، a new objective functional (J) that is a combination of some control criteria including reduction of relative drifts، absolute accelerations and absorbed energy is suggested. The optimization problem is such formulated that the set of equations of motions and equations representing the nonlinear model of MR dampers (here Bouc-Wen) are solved simultaneously. The optimization problem is solved by the enhanced method of steepest descend algorithm by Moharrami and Fayezi [3]. In this way، for a 15-storey building frame subjected to two deterministic earthquakes، the time histories of optimal input voltages of dampers are numerically computed. Next، the optimal voltages associated with the data on drifts، velocities and accelerations of stories are used as desired input- output data pairs to train the ANFIS as a quick and accurate controller. Three ANFISs were trained by different weights for drift (q1) and absolute acceleration (q2) data versus voltages. The weights of q1 and q2 controlling data were assumed to be (1،0)، (0،1) and (1، 0. 42) for ANFIS1، ANFIS2 and ANFIS3، respectively. Finally، to establish a context for assessment of the effectiveness of the proposed strategy in comparison with other conventional methods and to analyze the effects of weights in the objective functional، two numerical cases are presented. In the first case، the aforementioned 15-storey building frame is controlled against some earthquakes which were not applied for training process of ANFIS. Results show that ANFIS1 has decreased maximum and time-averaged relative drifts more than other control methods. In addition، this controller has somehow attenuated base shear similar to passive-on but has not been successful in reduction of absolute acceleration values. The ANFIS2 has controlled absolute accelerations better than other controllers whereas drifts have been reduced fairly well. By the ANFIS3، one can achieve reasonable decrease in all controlling criteria. Their values are between ANFIS1 and ANFIS2. It can be concluded that depending on the relative importance of control on drifts or accelerations of stories، one can chose proper weights for q1 and q2. In the second example، a benchmark six-storey building that is equipped with 2 dampers in the first، and 2 dampers in the second storey، has been controlled by the three proposed controllers. The results are compared with several conventional methods. The proposed strategy show more flexibility in reduction of the structural control criteria in comparison with some other conventional methods.