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Design of Structural Control Systems Using Viscous Fluid Dampers with Flexible Braces through Optimum Damping and Damping Energy Spectra

Jamshidi, Maziar | 2012

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 43574 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Ahmadizadeh, Mehdi
  7. Abstract:
  8. In this study, an effective method is developed for design of passive control systems using viscous fluid dampers, considering the flexibility of braces used for installation of dampers. Using control design spectra, this approach provides a way to easily determine the optimum properties of the control system without complicated calculations or iterations.In the fist part of this study, it is attempted to find the optimum damping ratio of system by using the input energy spectra of ground motions. First, the relations among the dissipated energy in viscous dampers the strucutre and control system properties and structural response is evaluated.A theorically exact relation is established between the Fourier amplitude spectrum and damping energy spectrum. Next, the dependence of input energy on the damper properties is discussed. It is shown that the for real earthquakes with a given frequency content, input energy is significantly altered by changes in damping of the system. As a result, an increase in damping would not necessarily correspond to higher energy dissipation in the control system and lower energy share of the main structure. Consequetly, no useful relation is found in this study for design of passive viscous fluid control systems that is merely based on the input energy spectra.In the second part of this study, optimum damping spectra are obtained directly trough dynamic analyses of single-degree-of-freedom systems considering brace flexibility. In the first step of this method, suitable damping ratios are determined based on the optimization results of single-degree-of-freedom systems with various natural periods and brace-to-story stiffness ratios. The optimization is carried out using an objective function that is a weighted combination of mean squares of inter-story drift and absolute acceleration responses compared to those in the uncontrolled case. Since the result for optimum damping ratio would differ from one ground motion to another, the search for optimum damping ratio is carried out for a set of scaled ground motion records. The mean values of the resulting optimum damping ratio specta is then adopted as the control design spectrum for the given brace to story stiffness ratio. In the next step, the usage of this method is extended to multi-degree-of-freedom shear building systems by simply adding the above-mentioned optimum damping ratio to the inherent damping of the first natural vibration mode. Finally, based on the assumption of stiffness-proportional classical damping for the structure, the new damping matrix of the system is determined and used to find the required damping coefficients of viscous fluid devices. In order to demonstrate the application of the proposed design method and its effectiveness, control systems are designed for example 5, 7 and 10-story structures and the achieved performances are compared to that of other design methods. It is shown that the control systems designed using the proposed method, provide responses close to those of optimum systems, while simplifying the design process.
  9. Keywords:
  10. Passive Control ; Viscous Damper ; Energy-Based Design Spectra ; Brace Flexibility ; Control Design

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