A Capacity Design Procedure for Columns of Steel Structures with Diagonals Braces

According to modern seismic codes, in concentrically braced frames the seismic input energy should be dissipated by means of the hysteretic behaviour of braces while all the other members (i.e. beams and columns) have to remain elastic. Accordingly, the design internal forces of braces are determined in these codes by elastic analysis of the structure subjected to seismic forces obtained by the design spectrum. The internal forces of the non-dissipative members, instead, are calculated by means of specified rules for the application of capacity design principles. According to some recent numerical analyses, the yielding or buckling of columns may take place before braces achieve their axial deformation capacity. This paper investigates the reasons of this unsatisfactory behaviour and proposes technological suggestions and a design procedure to improve the seismic performance of columns of building structures with diagonal braces.