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. Seismic base isolation, which is aimed at reducing the seismic demand instead of increasing the earthquake-resistant capacity of the structure, is an attractive alternative to conventional earthquake-resistant design methods. Seismic base isolation is one of the most widely implemented and accepted seismic protection systems, in the world. Seismic isolation is a relatively recent and evolving technology. It has been in increased use since 1980s, and has been well evaluated and reviewed internationally. Development of seismic design practices can be seen in its development from a simple assumption of designing buildings for lateral load of 10 % of weight of the building to the recent form of Performance Based Seismic Design (PBSD). Lessons learnt from the past earthquakes have greatly contributed in this development. Base isolation has now been used in numerous buildings in countries like Italy, Japan, New Zealand, and USA. Base isolation is also useful for retrofitting of important buildings (like hospitals and historic buildings). By now, over 1000 buildings across the world have been equipped with seismic base isolation. A four storey hospital building at Bhuj, India has been constructed using base isolation technique where the isolators were brought from New Zealand. 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. Bilinear modelling of iterative method is simple and less time consuming for an isolator but for an isolation system, iterative method is complex and time consuming. 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 iii presented. The proposed bilinear model for laminated lead rubber bearing and experimental results are compared. In general, the force based design method for buildings and bridges is common around the world. Researchers have reviewed that the force based design method and found following limitations (Priestley, 2000, Priestley et al., 2007, and Cardone et al., 2009): (i) assumption of constant and strength independent stiffness; (ii) the constant value of force reduction factor; (iii) displacements are checked at the end of design process, this means a lack of concern about the implied inelastic displacements. 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 direct displacement based design procedure for base isolated RC frame buildings has been proposed. Design of LLRB is important to achieve the level of performance required. The main requirements for the design of a base-isolation system are (i) the ability to sustain gravity loads, (ii) low horizontal stiffness that can lengthen the fundamental time period to a desired value, (iii) large vertical stiffness to minimize amplification in vertical direction and complications arising due to rocking, (iv) energy dissipation capacity to keep displacements at the isolation level within acceptable limits, and (v) sufficient initial stiffness to avoid unwanted vibrations due to wind loads and frequent minor seismic events. In this study, a complete design procedure using the available codes for base isolated buildings has been proposed to fulfill all the criteria. 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 the earthquakes, due to the torsion of stories in asymmetric structures, destructive effects increase on the structures. In this thesis, eccentric base isolated buildings on different soil types has been studied so as to minimize the effect of torsion.