Reinforced Concrete (RC) frame buildings with masonry infill panels are becoming increasingly common in urban and semi-urban areas all over India. Many such buildings constructed in recent times have a special feature of the ground storey being left open for the social purpose of parking, i.e., columns in the ground storey do not have any partition walls (of either masonry or RC) between them. These buildings are immanently vulnerable to collapse or damage due to earthquake. In this study, an attempt has been made to conduct a nonlinear static analysis (pushover analysis) of a 6-storey RC soft-storey building followed by retrofitting with different combinations of masonry infill and steel bracings. It is concluded that the performance of the soft-storey building can be increased by providing a few infills at the ground without affecting the parking.

Earthquake is a major threat for soft-storey buildings due to their structural behavior against ground motion. Reinforced Concrete (RC) frame buildings with masonry infill walls have been widely constructed for commercial, industrial and multifamily residential uses in seismic-prone regions worldwide. These buildings are generally designed as RC-framed structures without any regard to the structural action of the masonry infill walls present in the upper floors. However, in reality, masonry infill walls in the upper floors make those floors much stiffer and stronger against lateral load (e.g., earthquake) compared to ground floor, rendering these buildings into soft-storey buildings. Infill can also reduce structural drift and structural damage. The plinth beam is suggested when infill panels are considered in the modeling. Some national codes, like the Indian seismic code (IS: 1893, Part-1, 2002), require members of the soft storey (storey stiffness less than 70% of that in the storey above, or less than 80% of the average lateral stiffness of the three storeys above) to be designed for 2.5 times the seismic storey shears and moments, obtained without considering the effects of masonry infill in any storey. Various national codes can be broadly grouped into two categories—those that consider or do not consider the role of masonry infill walls while designing the RC frames. A few codes specifically recommend isolating the masonry infill from the RC frames such that the stiffness of masonry infill does not play any role in the overall stiffness of the frame. However, construction of such a building with isolated masonry infill wall requires high construction skill and may not be appropriate for the developing nations. These codal provisions are specified for all the buildings with soft storeys irrespective of the extent of irregularities; and the methods are quite empirical and may be too conservative, and thus have further scope for improvement. Experience of different nations with the poor and devastating performance of such buildings during earthquakes always seriously discouraged the construction of such buildings, i.e., with soft ground floor.

Structural Engineering Journal, Pushover analysis, Soft storey, Masonry infill walls, Steel bracing.