New Design Procedure for Inverted-Y-Braced Systems For Seismic Retrofit Of Existing R.C. Frames

This paper illustrates a new structural design procedure for the seismic retrofit interventions on reinforced concrete frame buildings with eccentric steel bracings. This work analyzes inverted-Y-braced systems, whose link is represented by an ad-hoc stiffened double-T-shaped profile. The design method here proposed works entirely in the linear range, and aims at the simultaneous plasticization of all the storey links, so as to induce a global damaging mechanism that maximizes the hysteretic damping effect. The procedure complies with the “resistance hierarchy” criterion, whereby the bracing system enters the plastic range before the existing reinforced concrete frame. It thus provides the retrofitted structure with an overall ductile breaking mechanism. Consequently, the reinforced concrete structure is lightened and less subjected to seismic action, and it is preserved from significantly entering the nonlinear field. Moreover, the introduction of eccentric steel bracings permits to regularize in plane the existing structure in order to minimize, at every level, the distance between the centroid of the masses and the center of stiffness This method has been validated by means of static and dynamic nonlinear numerical analyses both on regular and irregular frames. A practical application to an irregular frame is here described.