The soft first story RC buildings proved to be vulnerable in Nepal during the 2015 Gorkha earthquake. The RC buildings in Nepal are constructed with a weak frame, lack of ductile detailing, and brick masonry infill. In the city area, due to urbanization and lack of land area, the upper story is used as brick masonry, and the first story is used for shopping and parking purpose. This research intended to assess and compare the seismic vulnerabilities of RC buildings with soft story in Nepal namely, (i) building constructed by following NBC 205:1994 i.e. Mandatory Rule of Thumb (MRT) with poor detailing and modified later by adding story (NBC) and (ii) building constructed based on modified MRT of 2010 recommended by DUDBC, with poor detailing and modified later by adding story (NBC+). Seismic performance is evaluated by JBDPA guidelines of seismic evaluation, FEMA-356, NBC 1994, nonlinear static pushover analysis and dynamic analysis by Gorkha Earthquake motion. The results are discussed in terms of story shear, capacity curve, maximum roof displacement, inter story drift and damage pattern. Retrofit by wing wall and shear wall is proposed. The research found that seismic behavior, ductility demand, and inter-story drift pattern of RC building with the soft first story are different from those with analysis of bare frame. The soft first story suffered extreme inter story drift change causing severe damage. Seismic vulnerability of both buildings before and after retrofit showed NBC+ building performed better than NBC building. Both model buildings sustain Gorkha earthquake motion which reveals the building collapsed during earthquake are of low concrete strength, poor detailing, and lack of reinforcement constructed before the application of NBC. A combination of RC shear wall and wing wall proved to be effective in eliminating stiffness differences and controls excessive inelastic lateral drift to keep the usability of open space in the first story.
Keywords: Soft-first story, Seismic Performance, Retrofit, Wing wall, Shear wall.