SOFT STOREY EFFECT
According to IS 1893:2002, " A soft storey is one in which the lateral stiffness is less than 70% of that in the story above or less than 80% of average lateral stiffness of three storeys above."
In shaking a building, an earthquake ground motion will search for every structural weakness. These weaknesses are usually created by sharp changes in stiffness, strength or ductility and the effects of these weaknesses are accentuated by poor distribution of reactive masses. Severe structural damage to modern buildings during recent earthquakes illustrates the importance of avoiding sudden changes in lateral stiffness and strength. A typical example of harmful effects these discontinuities can bring is seen in case of a building with soft storey.
A soft storey is defined as a storey in a building that has comparatively less resistance or stiffness than the stories above or below it.
A soft storey has inadequate shear resistance or inadequate ductility ( energy absorption capacity) to resist earthquake induced building stresses. Usually the location of soft storey is at ground floor of building. Soft storey buildings are characterized by having a storey which has a lot of open space.
BEHAVIOUR OF SOFT STOREY DURING EARTHQUAKE :-
The presence of walls in upper storeys makes them much stiffer than open ground storey. Thus, upper storeys move almost together as a single block, and most of the horizontal displacement occurs in soft ground storey itself. Thus, such buildings swing back and forth like inverted pendulums during earthquake shaking and the columns in open ground storey are severely stressed. If columns are weak or if they do not have adequate ductility, they may be severely damaged or the building may collapse.
