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.
