DIRECT DISPLACEMENT BASED DESIGN OF BASE ISOLATED RC FRAME BUILDINGS

Abstract

Past earthquakes, especially in developing countries, have indicated that major loss of life often occurs due to the collapse of poorly constructed buildings. If the level of seismic demand on these buildings is reduced through a simple but reliable engineering solution, this would result in much safer design. The isolation system reduces the effects of an earthquake by essentially isolating the superstructure and its contents from potentially damaging ground motion. Accurate evaluation of the structural properties and precise modeling of isolation devices are of utmost importance in predicting the response of the structure during the earthquakes. The most common isolation system used is Laminated Lead Rubber Bearings (LLRB). They combine the function of isolation and energy dissipation in a single compact unit, giving structural support, horizontal flexibility, damping, and a re-centering force in a single unit. The force deformation behavior of LLRB is modeled as bilinear system with viscous damping. Iterative method is complex and time consuming for bilinear modeling of an isolation system. In this thesis, non-iterative bilinear modeling of LLRB bearing has been proposed. Experimental results of laminated rubber bearing with and without lead core have been presented. The proposed bilinear model for laminated lead rubber bearing and experimental results are compared. Several codes such as FEMA, NEHRP, Euro code, ASCE7-10, ASCE41-06 and AASHTO have been referred for analysis of base isolated structure. From the study, it is observed that FEMA 45 I provides a detailed explanation on analysis procedures. In general, the force based design method for buildings and bridges is common around the world. Recognizing the limitations of force based design, it seems rational to adopt a seismic design method directly addressing the displacements and deformations in the structure right from the beginning of the design process. The displacement based design procedures are developed to overcome the limitations of force based design methods. In this thesis, a detailed Direct Displacement Based Design (DDBD) procedure has been proposed. The DDBD procedure is applied to the symmetric base isolated infilled frame buildings. The procedure is applicable to the open ground storey buildings also. The effect of infill panels is included by appropriately considering its strength, stifthess and energy dissipation. From the analysis and results, it is observed that, the required performance is achieved. Design of LLRB is important to achieve the level of performance required. In this study, a complete design procedure using the available codes for base isolated buildings has been proposed to fulfill all the criteria. Software has been developed for the design of LLRB which requires minimum inputs and provides detailed information required for the analysis, design and fabrication of isolator. Extensive studies on the behavior of base-isolated symmetrical structures have been done. Nevertheless, studies on the behavior of base-isolated asymmetric structures are limited. During an earthquake, due to the torsion of stories in asymmetric structures, destructive effects increase on the structures. In this thesis. eccentric base isolated huildins on different soil types has been