Abstract:
In urban areas of Bangladesh it is a very common practice to keep the ground floor
of multi storied buildings open for parking facility. These buildings-are generally
designed as RC framed buildings without considering the masonry infill
contribution. Conventional practice is to consider the masonry infill as non structural
element. But in reality masonry infills make the structure stiffcr; resulting stiffness
irregularity, in buildings with open ground floor. In this thesis, an extensive
computational study has been conducted to find out the nature of buildings with
open ground floor and to determine the seismic vulnerability of such a building.
A numerical investigation has been performed to evaluate the naturc of RC framed
building with open ground floor, i.e. soft story building. Several soft story buildings
with variation of number of floors, number of span, number of bays, floor panel size,
percentage of infills etc. are considered during the investigation. The infills are
modeled as equivalent diagonal strut. Beams and columns are modeled using threedimensional
frame element and floor slab using shell element. Loads considered
during analysis are dead load, live load, wind and earthquake load. Bangladesh
National Building Code (BNBC, 1993) is followed in the analysis. Earthquake load
is studied with both equivalent static force method and response spectrum method.
The time period, force, bending moment, sway pattern as well as stecl percentage of
the same buildings is also evaluated and compared using equivalent static force
method and response spectrum method.
It has been found from analysis that natural period of the building decreases with
increasing amount of in fill as found from dynamic analysis while conventional
Equivalent Static Force Method (ESFM) does not show any variation in period with
varying amount of infill. Presence of infill in upper stories makes those floors much
stiffer compared to ground floor. As a result the upper floors behave togethcr in a
rigid manner causing excessive sway at ground floor level. This soft story behavior
is clearly identified in dynamic analysis in prescnce of infill while conventional
static analysis cannot predict such behavior. It has been found from analysis that the
design base shear is significantly increased due to presence of structurally active
infill as compared to static analysis. This clearly indicates that the ground floor
columns in buildings with soft story are, in general significantly under-designed for
earthquake loading. As such the possibility of collapse of such buildings by soft
story mechanism is very high during earthquakes.
Based on the findings of the study suggestions are made to safcguard buildings with
open ground floor form soft story collapse. Two alternative suggestions arc made,
namely, a) base shear amplification for design of ground floor columns at design
stage and b) Isolation of infill form the frame elements at construction phase to
make them structurally inactive. It is expected that these suggestions would be
useful for safeguarding buildings with open ground floor and human life.
