Seismic Evaluation of Steel Moment Frames Rehabilitated with CBF, EBF and BRBF Braced Systems

Many of the existing buildings are made and designed in according to Standard No.2800 (1st Edition). in recent years Increasing in earthquake engineering knowledge and changing in design criteria regulations and seismic evaluation has caused that the existing structures meet local or general deficiencies in seismic requirements. Therefore, the identification and development of appropriate methods is necessary for seismic rehabilitation of existing structures and providing levels of design performance. In this study, the nonlinear seismic behavior of six steel buildings with steel moment frame systems that have modeled in 4, 8 and 12 stories designed according to the criteria of the First Edition 2800 Standard and Publication 519 and has been investigated using nonlinear static analysis. The evaluation results indicate that the existing steel moment frames that are designed according to the Standard No.2800 (1st Edition) cant provide Stiffness and ductility requirements of the Standard No.2800 (3rd Edition) and FEMA356 and must be rehabilitated. The frames Improved by concentrically steel braces, increases stiffness and reduces lateral displacement however, due to buckling of the compression braces, structures suffered a sudden drop in strength and its behavior is not ductile. The use of eccentrically braces due to concentrated plastic deformation in link beams and elastic braces, structural behavior is more ductile in comparison with concentrically braced frames method. The use of buckling restrained braces, due to compressive brace is not buckling and yields in tensile, has caused increase in stiffness, strength and ductility of structure and has improved structural performance in comparison of concentrically braced frames .