ABSTRACT –
Soft story irregularity is one of the main reasons of the building damage during earthquakes.
This irregularity is mentioned in almost all reconnaissance reports, and buildings having total
collapse at some stories, even without unbroken windows at others may be seen after
earthquakes. Soft story may arise not only because of sudden changes in structural system
properties (like height of the stories) but also due to abrupt changes in amount of infill walls
between stories which are usually not considered as a part of load bearing system. In this
study, soft story and effect of infills on this behavior are investigated using nonlinear static
analysis for mid-rise reinforced concrete buildings which are thought to be the most vulnerable
against earthquakes among existing building stock. Four and seven storey buildings are
designed per 1975 Turkish Earthquake Code to reflect existing building stock. Soft story
models of the reference buildings are obtained considering increased floor story height (4m
instead of 2.8), less amount of infill at floor story and both cases. Displacement capacities of
the reference and soft story models are determined at Immediate Occupancy, Life Safety and
Collapse Prevention performance levels according to 2007 Turkish Earthquake Code. Soft
story behavior due to change in story height and/or infill amount is evaluated in view of these
displacement capacities and structural behavior of models. All of the buildings are modeledwith two different transverse steel spacing as 10 and 20 cm to investigate the effect of
transverse steel amount on the behavior.