Case Study on Seismic Performance of Soft Stories in Short Steel Structures and Replacement of Braces with Equivalent Moment Resisting Frame

Many of changes in ground floor of buildings may result in elimination of braces.This phenomenan, which called “soft story”, decreases the stiffness of stories and leads to collapse of building under severe earthquakes. Study of last earthquakes such as Bam on December 26 ,2003 and chichi-Thaiwan on September 20 ,1999 shows that many peaple missed their life because of soft stories created due to elimination of structural infills by changing residential buildings to commercial or architectural reasons.In this research, the soft story studied in short steel buildings with four stories by removing X-braces at first and second stories using nonlinear static and dynamic analyses. After removing braces, the stiffness of soft story increased gradually by providing rigid connections and using moment resisting frame and the replacement of braces with equivalent moment frame (with different stiffnessess) studied. The seismic design parameters such as transient and inherent dissplacements, ductility and energy dissipation capacity calculated and compared in various diagrames. In addition, the sttiffness of X-braced and moment resisting frames calculated using classical relations of solid mechanics and discussed. According toobtained results, the replacement of X-braces by an equivalent moment frame in a building leads to excessive drifts and less energy dissipation capacity. In addition, the findings of this paper is coin with specification requirements to avoid soft stories in buildings by avoiding stories with stiffness of 70 percent relative to above story.