The structural configuration with a soft first
story proved to be very vulnerable and performed poorly during the past
earthquakes. Like other many countries, brick infill masonry is used in
Bangladesh as a nonstructural element. Its usages in upper stories and keeping
building’s ground floor open result in lateral stiffness difference and cause
soft first story state. The scarcity of land in Bangladesh has compelled to
construct multi storied RC buildings with an open ground to be used as vehicle
parking, stores or other facilities. This research committed to assess the
seismic vulnerabilities of RC buildings with a soft first story, causes behind
the collapse of soft first story during earthquakes, seismic performance
difference with bare frames and sustainable approach to retrofit them. Seismic
performance and vulnerabilities of the soft first story were assessed by the
JBDPA guidelines of seismic evaluation, FEMA-356, BNBC-2015 and nonlinear
static pushover analysis. Flexural moment magnification at the soft first story
columns during earthquakes was determined. Sustainable retrofitting approaches
to upgrade seismic performance and prevent catastrophe during earthquakes were
proposed with cost analysis. This research found that, seismic behavior,
ductility demand, inter story drift pattern and damage distribution of RC
buildings with a soft first story were totally different than the RC buildings
designed by only bare frame analysis. The soft first story suffered huge
ductility demand, extreme inter story drift change and concentrated in severe
damage. Magnification of flexural moment at soft story columns was detected as
a variable entity. Retrofitting of soft first story was found different from
conventional RC buildings. A combination of RC column jacketing and adding
steel bracing proved to be effective to eliminate stiffness difference and
control the excessive inelastic lateral drift.
Keywords: Soft story, Seismic performance, Nonlinear analysis, Retrofit.