Effect of superstructure stiffening in base isolated tall buildings

Seismic isolation enables the reduction in earthquake forces by lengthening the period of vibration of the structure. It can result in significant reduction of structural respones for buildings less than 10 storeys. Very tall buildings of more than 20 storeys already have higher fundamental periods and, therefore, do not need to be base isolated. In order to investigate the feasibility of base isolation for this range of buildings, analytical study is carried out for buildings of 10 to 20 storeys height. Three reinforced concrete framed buildings of 10 storeys, 14 storeys and 20 storeys, respectively are considered. The idea of superstructure stiffening is explored to enhance the effectiveness of base isolation for this range of buildings. The isolation system considered in this study is low damping rubber bearings with external dampers at isolation level. It is observed that base isolation is feasible for stiffened tall buildings though stiffening of superstructure of base isolated buildings does not decrease the response significantly.