ABSTRACT
In the paper some topics relevant to the design procedure and the seismic performance of steel concentrically braced frames (CBFs) are addressed. The design proposal of Eurocode 8 for CBFs is analysed in view of the application to fairly complex structural systems. Application of the “tension only” concept, to be used in strength verifications against ultimate-limit-state seismic effects, is applied employing a new strategy based upon the definition of a set of static equivalent seismic forces, computed from response spectrum analysis. Energy dissipation due to interaction between beams and diagonals in macro-bracing systems is also addressed.
The dynamic response of a complete 3-D structural system, encompassing several frames with different bracing solutions, is studied by means of a step-by-step numerical procedure implemented in an in house developed computer code. Non-linear response of bracing bars due to element buckling and inelastic material behaviour, as well as non-linear response of beams interacting with macro-bracing systems is considered. Results, given either in terms of displacements and interstory drift envelopes or braces and beams hysteretic behavior, show higher modes and torsional effects importance. Effects of connection detailing, which may jeopardize bracing ductility is also considered through the introduction of a “death” option for bracing diagonals exceeding a predeterminate ductility in tension.
