A multi-performance seismic design procedure to incorporate Crescent Shaped Braces in mid-rise frame structures

This paper introduces a novel procedure for the seismic design of a new class of seismic-resisting systems for mid-rise buildings obtained by incorporating special yielding steel braces known as Crescent Shaped Braces (CSBs) into Not Moment-Resisting Frames (NMRFs). The proposed design procedure is grounded on the Performance- Based Seismic Design (PBSD) framework through the imposition of multiple seismic performance objectives with the aim of obtaining an almost uniform along-the-height seismic behavior. The desired uniform seismic behavior is ensured by imposing: (i) uniform inter-storey drifts under frequent earthquakes; (ii) widespread yielding of the braces along the building height under occasional earthquakes; (iii) minimum ductility capacity at all storeys to be exploited under rare earthquakes; (iv) minimum hardening stiffness for the mitigation of the second order (P-Δ) effects under very rare earthquakes. The desired performances are made possible by the specific features of the CSBs in terms of initial lateral stiffness, yielding strength, ductility capacity and final hardening response. The procedure is articulated in four conceptual phases and several steps to guide the professional engineer through all the main design phases, from the selection of the seismic performance objectives to the preliminary sizing of the CSB devices, up to the final design/verification through non-linear time-history analyses. The effectiveness of the proposed design procedure is finally demonstrated through an applicative example.