Passive vibration control of framed structures by
base isolation method using lead rubber bearing

Abstract

In recent years considerable attention has been paid to research and development of structural control devices with particular emphasis on mitigation of wind and seismic response of buildings. Many vibration-control measures like passive, active, semi-active and hybrid vibration control methods have been developed. Passive vibration control keeps the building to remain essentially elastic during large earthquakes and has fundamental frequency lower than both its fixed base frequency and the dominant frequencies of ground motion. Base isolation is a passive vibration control system.
Free vibration and forced vibration analysis was carried out on the framed structure by the use of computer program SAP 2000 v12.0.0 and validating the same experimentally. The results of the free vibration analysis like time period, frequency, mode shape and modal mass participating ratios of the framed structure were found out. From modal analysis the first mode time period of fixed base building is found to be 0.56 sec whereas the first mode period of isolated building is found to be 3.11s (approximately 6 times the fixed-base period!). This value is away from the dominant spectral period range of design earthquake. Forced vibration analysis (non-linear time history analysis) was done to determine the response of framed structures and to find out the vibration control efficiency of framed structures using lead rubber bearing. Isolation bearings in this study are modelled by a bilinear model. Under favourable conditions, the isolation system reduces the interstorey drift in the superstructure by a factor of at least two and sometimes by a factor of at least five. Acceleration responses are also reduced in the structure by an amount of 55-75% although the amount of reduction depends upon the force deflection characteristic of the isolators. A better performance of the isolated structure with respect to the fixed base structure is also observed in floor displacements, base shear (75-85% reduction), floor acceleration relative to the ground(less acceleration imparted on each floor and their magnitude is approximately same in each floor), roof displacement. Introduction of horizontal flexibility at the base helps in proper energy dissipation at the base level thus reducing the seismic demand of the super structure to be considered during design.

Keywords: Passive vibration control, Time history analysis, interstorey drift, yielded stiffness,
Design basis earthquake.