Seismic behavior of base isolated buildings with soft story for Dhaka region

Abstract

Soft storey buildings are characterized by having a storey which is 70% less stiff than the floor above or below it. Information available in existing literature and experience from past earthquakes suggests that soft storey have high tendency of failure during earthquakes. In Dhaka, many buildings are constructed with a soft ground floor as walls are removed from this particular floor for creating car parking provision, making it vulnerable to earthquake damage. One way of mitigating such vulnerability may be by incorporating base isolation system to the building. In this thesis, a comparative study is presented between base isolated and non-isolated soft storey building in the context of Dhaka, Bangladesh. For analysis, time history of recently occurred Natore earthquake has been normalized for both EW and NS direction considering peak ground acceleration of 0.2g, the recommended value for Dhaka as per Bangladesh National Building Code (BNBC). Corresponding Response Spectrum (RS) has been evaluated from these two earthquake records. A prototype building of six storey height with and without soft storey was considered in this work. Lead rubber Isolator for such soft storey building was also designed using appropriate method. Non-linear time history and response spectrum analysis of this building was performed in the finite element software program SAP2000. Non linearity was considered only for the isolator part of the building. At first, a comparative study was conducted between isolated and non-isolated building. The study revealed that the values of structural parameters like moment and shear was reduced by 39% and 55% respectively in soft storey columns when isolator was used in the building. Use of Isolator was also found to be effective in reducing displacement. Displacement was found to reduce by 45% in fixed base soft storey building and 41% for fixed base without soft storey building when isolator was incorporated. Furthermore, storey drift was also found to be reduced by 43% whereas maximum acceleration was reduced by74% in fixed base soft storey building with isolator. Next, a parametric study was conducted to evaluate the influence of different isolator parameters like initial and post yield stiffness, isolator yield force on overall behavior of soft storey building. It was found that peak shear, moment and displacement of columns decrease with increasing initial isolator stiffness. However, for changes in post peak isolator stiffness, variation in peak shear, moment and displacement of column did not show any particular trend. It was also observed that column peak shear, moment and displacement increased with increasing isolator yield force. Lastly, a comparative cost study was performed between building with and without isolation system. Since use of isolator reduces overall peak shear, moment and displacement in columns and other structural elements of the building, reinforcement requirement is less than conventional non isolated building. Cost of isolator, on the other hand, is added to the isolated building. So, comparing the savings in reinforcement cost with that of isolator cost, it was found that use of base isolator can actually reduce the overall structural cost of the building by 3 to 4%.