Mahdi Azizi - PhD Candidate in Structural Engineering, Department of Civil Engineering, Islamic Azad University,Tabriz, Iran.

Steel braced frame is one of the structural systems used to resist earthquake loads in buildings. The unexpected failure of steel structures during past strong seismic excitation led to full fill adequate strength for modern structures in seismic areas. Many existing steel structures with this system need retrofitting to overcome deficiencies and to resist seismic loads. In the present study, the seismic behavior of concentrically bracedframe (CBF) structures was investigated and the zipper column idea for retrofitting was proposed. Two types of structures, including CBF and CBF with zipper columns for 5, 12 and 20 stories were designed per the Iranian Seismic Code, the Iranian Steel Structures Design Code and the Seismic Provisions. Dynamic analysis using 4 earthquake records were carried out to obtain dynamic responses. According to the analysis results, the maximum displacement, base Shear and the inter-story drift ratio in CBF systems are decreased as zipper columns are utilized. Also the frames showed good dissipating-energy capacity and large deformation ductility without significant strength losses. Moreover the seismic behavior of CBF system is improved.

concentrically braced frames, chevronbraced frames, zipper columns, near-fault region and seismic design