Improved Systems for Seismic Design of Buildings with Soft Ground Storey

Abstract

Due to the demand of various facilities like open utility or parking space, open ground storey has become a typical feature in the modern multi-storey construction in urban areas. This causes vertical stiffness irregularity leading to the development of soft storey meehanism. Soft storey mechanism has been proved to be the most potential failure mode of reinforccd concrete (R.C.) structures as experienced during the past earthquakes. Although some of the national eodes do have certain dcsign guide to avoid soft storey problem, the Bangladesh National Building Code (BNBC, 1993) does not have any such provision. This thesis has been conductcd to find out improved struetural systems to mitigate the vulnerability of soft storey structures. Thc objective is to achieve safety of open ground storey structures without too much changing the constructional practice. For this purpose, a total of ten numbcrs of four to ten storied buildings havc bcen studied to evaluate the performance of soft storey building under seismic lateral load. Nine alternate simplified remedial design procedures were considered and the structures have been studied with the possible remedial measures to achieve the objectives. The performances of these structures have been evaluated through nonlinear static pushover analysis as per the procedure of ATC-40, 1996. It has been observed that performance of the structure having soft ground storey is highly unsatisfactory. Inter-storey drift as well as displacement and ductility demand is very high at soft ground storey. Stiffness of open ground storey is significantly less than that of the storey above. So, it is a vital concern for the design engineer to provide adequate measures to achieve acceptable performance in open ground storey structure under seismic action. The perfonnanees of the structures with possible remedial measures have been studied. Two remedial provisions have been found which could completely eliminate the soft storey problem. It has been shown that provision of central service core or concrete shear wall having twice thickness at the ground storey than in upper xix storeys, would significantly improve the performance of the open ground storey structure. The second remedy involves careful placement of partial concrete walls only at ground floor to provide compensatory stiffness at ground storey to mitigate soft storey problem. These provisions may be incorporated for open ground storey constructions in Bangladesh National Building Code (BNBC) as well as in other building codes.