The purpose of the paper is to introduce a system that reduces the occurrence of weak-story in the event of earthquake. Weak-story concentrates deformation on the story and causes all member to collapse before the capacity of all member is reached. This paper introduces Strong-Back system (SB) to protect weak story. SB is a hybrid of zipper frame, tied eccentrically braced frame, and elastic truss system and it is divided into elastic and inelastic areas. Elastic areas prevent the generation of weak story by distributing energy, and inelastic areas dissipate energy through buckling or yielding. In this paper, the seismic performance is evaluated by comparing the four type braced frame with SB through push-over analysis. The four criteria are compared from the base shear, the ductility capacity, the column failure order, and the quantity of brace. As a result, SB proved to have sufficient performance to protect the weak-story.

In this study, the non-linear seismic response analysis of the 2F piloti type building model with varying stiffness and strength distribution between soft-weak story(1F) and upper story(2F). The standard model, whose stiffness and strength ratio of the first story is 10% and 100% comparing the upper story respectively, suffered from shear failure of the core wall on the first story. Otherwise, the models with decreased stiffness or strength ratio of the first story damaged more than the standard model and the lack of the strength was more impactful. The model with improved strength contributed to reduce damage on the first story, while the improved stiffness only was not effective on the reduction of the damage level. Overall, in case that the strength and stiffness ratio of the first story increased, damage distributed in balance along the story.