Advanced Analysis of Concentrically Braced Frames (CBFs) by Energy Approach

Advanced analysis refers to a method in which the strength and stability of the system and structural members are recognized in an integrated manner and there is no need to separately check the capacity of the structural members. This approach is a suitable method for evaluating the real behavior of structures and makes structural designers better understand the main characteristics affecting the actual behavior of structures. Undoubtedly, one of the most widely used structural systems in the construction industry is Concentrically braced frames (CBFs). Mainly, these kinds of frames collapse because of a soft story formation in one or more stories in which excessive brace buckling occurs. Using second order inelastic analysis, this study provides an intuitive understanding of the collapse mechanism of CBFs with 6 and 18 stories subjected to mainshock-aftershock sequences. Such understanding will support development of design methods that preclude low-capacity collapse modes specially under multi-shock excitations. This paper assesses the collapse mechanism as a stage in which the imposed seismic energy fails to dissipate and eventually leads to uncontrolled kinetic energy in structure. The investigation focuses on the role and distribution of the various energy measures and different dissipating mechanisms throughout the structures. Collapse mechanism is identified for various combinations of the utilized 32 mainshock-aftershock pairs that are gradually scaled following the incremental dynamic analysis (IDA) process. The distribution of input and dissipated energies along various stories reveals the role of upper stories in damping the imposed energy. Furthermore, the similarity between the height profile of the residual drifts and the story imposed energies highlights the characteristics of the structures in adapting their drift response to a mode with the highest energy absorption