OPTIMIZATION OF SEISMIC ISOLATION SYSTEMS WITH VISCOUS FLUID DAMPERS USING GENETIC ALGORITHMS

Abstract eng:
This paper presents evolutionary-based optimization procedure for designing natural rubber seismic isolation systems with viscous fluid dampers. The proposed technique is applied to the design of seismic isolation systems with viscous fluid dampers. A lumped-mass stick model representing a realistic five-story MDOF system with natural period of vibration ranging between 2.5 to 4.5 seconds. A suite of 24 Near-Field (NF) earthquake records representing different seismic hazard levels are utilized in the analysis and design. The damping coefficient (C), damping exponent (α) and fundamental natural period (T) are used as design variables for the seismic isolation system. The minimization of the key response parameter, the top story acceleration ratio (TSAR) of the isolated structure compared to an identical fixed base structure, is selected as the objective function in this optimization problem. The total maximum displacements (DTM), the peak damper force (PDF), as well as the maximum drift ratio (δmax) are presented as constraints in the optimization problem. In order to achieve global optimum performance [TSAR] while handling competing constraints [DTM, PDF, δmax], the presented GA-based technique is utilized. The commercial structural analysis software SAP2000 is utilized to perform the dynamic analysis for the MDOF system through direct Time-History Analysis (THA). The optimization algorithm is programmed in MATLAB and linked to SAP2000 through its OAPI feature. In agreement with conventional wisdom and sound engineering experience, it is found that combining low values of (C), highly nonlinear (α) [low values] with highly flexible rubber isolators [long T] produces the optimum performance.

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