SEE6 / 1 / IIEES SOFT STORY DESIGN IN REINFORCED CONCRETE STRUCTURE AND EFFECT OF MASONRY INFILL WALL S. M. Hosseini-Gelekolai 1 , M. R. Tabeshpour 2 1 Department of Civil Engineering, Sharif University, Tehran, Iran, hosseinigelekolai@gmail.com 2 Engineering Faculty, Tarbiat Moallem University, Sabzevar, Iran, tabesh_mreza@yahoo.com ABSTRACT In past few decades researchers and engineers have researched extensively on the effects of masonry infill walls in structural frames and the effects of masonry infill walls on the structural response was a concern for them. One of the sections in Iranian Codes that existence of masonry infill walls in buildings affects it, is the section 2-10 of 2800 Code that accordance to this section, in order to prevent soft story failure when there is a discontinuity in bracing or shear walls in a certain story, columns of that story must be designed for factored loads. Even though in the contest of 2800 Code there is no notification to necessity of increasing the design loads in existence of discontinuity of masonry infill walls, based on experiences of structural response in past earthquakes even in the case of existence of discontinuity of masonry infill walls the columns of the story must be designed for factored loads. The question is that in this case how much the factor is? The main goal of this research is to investigate the appearance of soft story failure in first story in reinforced concrete structures with moment frame system and different arrangements of masonry infill walls in upper stories (in different bays and different thickness) and also suggesting a factor to strengthen the first story columns in order to prevent the soft story failure. We call this factor, the overstrength factor which is the 2.8 factor in section 2-10 of 2800 Code but here the masonry infill walls discontinued in the first story and in that section, braces or shear walls discontinued. The overstrength factor is equal to 2.8 for all cases while it should be calculated exactly so that not results in overdesigning or under designing of structures. If column of soft story be designed basis of this factor the existence capacity will be used optimally and also we will not have terrible events like soft story failures in future earthquakes. Keywords: Design of Soft Story, Masonry Infill Wall, Reinforced Concrete Structure, Equivalent Compressive Brace, Overstrength Factor of Column 1. INTRODUCTION Masonry infill walls exist in several buildings but their role in the rehabilitation and retrofitting of the structures is neglected by most of the structural engineers. Researches on the behavior of frames with masonry infill walls have been started from 1950s. Several lateral loading tests have been done on the full scale and prototype models including masonry infill walls. There are two general methods for considering the effects of masonry infill walls in structural frames in seismic design of structures [1]; i) Modeling the infill separately and neglecting the interaction of masonry infill walls in the structural response Six th International Conference of Seismology and Earthquake Engineering 16-18 May 2011 Tehran, Iran