The capacity design criterion in the seismic design of X concentric braced frames. Critical analysis and new approach

This paper begins with the observations of several authors on the design methodologies proposed by the current codes for the seismic design of X concentric braced frames (CBFs). It has in fact been observed that the introduction of the capacity design criterion for CBFs, which assumes that the diagonals are the “weak” elements, together with detailed design indications, leads to excessive overstrength requirements for the “strong” elements and, consequently, to oversized structural solutions. In this paper the authors propose a design approach whose aim is to reduce the overstrength requirements by preserving good CBF ductility levels. For this purpose, after a critical discussion of the specific indications contained in the European and Italian seismic codes, the proposed approach is presented in detail. Several numerical examples are then presented with a view to assessing the reliability of the proposed approach in terms of the nonlinear response of designed braces.