Document Type : Research Article
Civil Engineering School, Engineering Department, University of Zanjan
Civil Engineering Group, Engineering Department, University of Zanjan, Zanjan, Iran.
In this study the seismic performance of special concentrically mega braced frames with different spans ratio is investigated; For this purpose, eight configurations of four and eight-story structures with special concentrically braced frame were designed in three dimensions, with conventional X and mega brace configurations with different spans ratio, then a braced frame of them was modeled in OpenSees in two dimensions, taking into account the second-order effects of the removed gravitational section, through a leaning column. Finally, in order to investigate the seismic performance of structures and perform incremental dynamic analysis, 14 far field earthquakes were selected according to the characteristics of the construction site. Evaluation of analysis results according to NIST GCR 10-917-8 report and Hazus Technical Manual in maximum inter story drift ratio, comparison of fragility curves and comparison of period and weight of structures, indicates that in special concentrically mega braced frames, if the spans are equal, mega braces has a suitable and economical performance, and if the ratio of spans is different, the use of mega braces has a better performance than conventional X braces and is much more economical. For example, in eight-story structures with an span ratio of 1.5, the weight of the structure with mega brace is about 20% less than the similar structure with conventional X brace. Also, the main period of frames with conventional X braces is about 20 to 30% longer than structures with mega braces, which indicates the higher stiffness of mega braces.