[justify]It is one in which the lateral stiffness is less than 70 percent of that in the storey above or less than 80 percent of the average lateral stiffness of the three storeys above.
Construction of multi storied buildings with open first storey is a common practice in India. Such a building in which the upper stories having infill wall panel provided for functional and architectural reasons and open ground storey is called soft storey.
Masonry infill is normally considered as non-structural elements and their stiffness contributions are generally not considered while analyzing the beams and columns of the soft storey.
It is noticed from the past earthquakes that strong infill provide most of the lateral resistance and prevent collapse of relatively flexible and weak reinforced concrete frames that are necessarily not designed for the seismic forces.
Possible causes of building failure during earthquake
Damage to RC building structures can be attributed to the combination of the following reasons.
Soft story effects
Poor detailing of structural joints
Inadequate reinforcing steel tie spacing and 90 degree hook
Insufficient reinforcing steel development length in columns with large aspect ratio
Honeycomb concrete at the top of ground floor columns
Lateral force is not considered in design
Inappropriate anchoring of beam
and slab reinforcement
Seismic rehabilitation and retrofit
Earthquake resistance in RC frame structures can be enhanced by either of the following approaches:
Strengthening the components, such as columns and beams, by jacketing with concrete, steel, or fiber wrap overlays
Increasing the overall capacity of the structural system by installing new concrete infill walls or steel bracings.[/justify]