Use of Opensees for the validation of a simplified procedure for the seismic assessment and retrofit of steel concentric braced frames

Concentric braced frames (CBFs) is one of the most used structural typology for steel buildings. Indeed, the truss mechanism, which activates in presence of lateral forces due to wind or earthquake loads, leads to levels of lateral stiffness significant higher in comparison to other structural typologies such as moment resisting frames (MRFs). On the other hand, the possible occurrence of buckling phenomena in the members of CBFs, which are prevalently subjected to axial forces, can lead to either fragile global failure mechanisms, when buckling occurs in beams, columns and local failures affects the connections, or to a significant reduction of the global ductility and energy dissipation of CBFs, when buckling only involves diagonal members. For these reasons, for a long time CBFs were not included among structural typologies of steel buildings to be used in seismic prone area. Design codes followed elastic concepts, principally assuring that the strength of each CBFs’ member was adequate to resist the external loads neglecting the contribution of diagonal in compression. Only in recent years, CBFs have been considered as a seismic resistant structural typology. In particular the capacity design approach inspiring the modern codes suggests provisions aimed to assure to CBFs a ductile ultimate mechanism, characterized by the yielding (reasonably distributed along the building height) of diagonals, whilst the buckling of beams and columns is prevented.