Seismic performance of dual-steel moment resisting frames

In seismic design of steel structures, the “dual-steel” concept concerns the combined use of High Strength Steel (HSS) in non-dissipative members and Mild Carbon Steel (MCS) in dissipative zones, in order to control the global frame behaviour into a ductile overall failure mode. In this paper, a comprehensive parametric study devoted to investigate the seismic design and performance of Eurocode 8 compliant dual-steel Moment-Resisting Frames (MRF) is presented and discussed. The overall seismic performance has been analysed through static and dynamic nonlinear analyses against three limit states: damage limitation (DL), severe damage (SL) and near collapse (NC). The investigated parameters cover both geometric and mechanical variables, as the type columns, span length, number of storeys and spectral shape. The analyses showed that the use of HSS in Eurocode 8 compliant MRFs is effective to provide overall ductile mechanism, but it may lead to inefficient and uneconomical structures characterized by limited plastic demand due to the large design overstrength. The comparison between dual steel MRFs with those entirely made of MCS showed that: i) in order to fulfil the codified drift requirements and to limit the stability coefficients, the same shapes for members should be used for both structures in the most of cases; ii) a similar performance can be recognized in both dual steel and single grade steel structures under both damage limitation and significant damage limit state; iii) dual steel frames guarantee a better control of plastic mechanism than single grade steel frames at near collapse limit state.