SEISMIC RESPONSE OF BUILDING BASE ISOLATED WITH FILLED RUBBER BEARINGS UNDER EARTHQUAKES OF  DIFFERENT CHARACTERISTICS

EDI MARDIONO

1404101010027

Student of Civil Engineering University of Syiah Kuala, Aceh, Indonesia.

Seismic base isolation is now an accepted practice, in many parts of the world, for protecting important buildings from earthquake forces. For low to moderate height buildings, natural rubber

bearing has often been considered suitable because of its durability and easier fabrication. Many recent studies showed that low stiffness of base isolation systems could cause unacceptably large displacements of ground floor of the building necessitating substantial amount of damping in the isolation system.

INTRODUCTION

The base isolation system consists in introducing low stiffness bearings and damper between foundation and superstructure. Bearings provide the needed flexibility to the combined system so as to shift its first mode natural frequency away from predominant frequency of the design earthquake motion. This results in the reduction of inertial forces and accelerations several times. The idea behind base isolation has been proposed again and again for at least a century. Earlier the elastomeric bearings were made up of natural rubber, which possess very low energy dissipating capacity resulting in unacceptably high displacements at isolation level. Recent advances in rubber technology have enabled the manufacturers of these bearings to have better control over the structural properties of the rubber. This is achieved by mixing suitable amount of certain chemicals in the natural rubber. These filled rubbers can be used efficiently in the manufacture of isolation bearings. The laminated rubber bearings made by these filled rubbers have high initial stiffness, which reduces, at high level of shear strains. Therefore, behavior of these bearings can be simulated by bilinear curve. The area of hysteretic loop i.e. damping for an earthquake event is controlled by yield force and post-to-pre yield stiffness ratio of the bearing which can be controlled by fillers. The purpose of this paper is to investigate the effectiveness of base isolation by filled rubber bearing and influence of bearing parameters viz. post-to-pre yield stiffness ratio and yield displacement on the isolation damping of a six storey reinforced concrete building excited undirectionally with ground motions having different characteristics.

FILLED RUBBER BEARINGS

The isolation system considered in this study is elastomeric bearing. The most common elastomers used in elastomeric bearings are natural rubber, neoprene rubber, butyl rubber and nitrile rubber. The mechanical properties of natural rubber are superior to those of most synthetic elastomers used for seismic isolation bearings. Therefore natural rubber is most frequently recommended material for use in elastomeric bearings followed by neoprene. Butyl rubbers are suitable for low temperature applications and nitrile rubber have limited application in offshore oil structure.

METHOD OF ANALYSIS OF BASE ISOLATED BUILDING

The time history analysis of the base isolated building is carried out using the computer program 3D-BASISTABS. A full three-dimensional representation of the structure is used in time history analysis of the base isolated buildings. The isolation system is

modelled as a bilinear hysteretic element. The details of method of analysis are described by Reinhorn et al. (1994). Following assumptions are made for the analysis of base isolated building:

·         The superstructure is elastic at all time and the non-linear behaviour is restricted in isolators only.

·         All frame substructures are connected at each floor level by a diaphragm, which is infinitely rigid in its own plane.

·         Each floor has three degrees of freedom (two translations and one rotation) attached to the centre of mass of each floor. The isolation devices are rigid in the vertical direction and each has negligible torsion resistance.

·         Effect of infilled walls on stiffness of the structure is negligible. These are considered in computation of mass only..

CONCLUSIONS

Filled rubber bearings are suitable for ground motions having high dominating frequencies but is not effective for ground motions whose dominant frequencies are close to natural frequency of base isolated structure. The fundamental period for a bearing is unique for given bearing parameters but the damping is found to be dependent upon the characteristics of input ground motion also. Yield displacement can be used as an important parameter for controlling damping in order to achieve desired base displacement.

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