Seismic evaluation of concentrically braced frames – Influence of the design approach on the V-bracing

The Concentrically Braced Frames (CBF) are commonly used in steel construction especially for tall structures and areas with high seismicity thanks to both higher lateral stiffness and smaller material consumption than those for Moment-resisting Frames. The seismic response of this structural system is strongly affected by the performance of bracings, which are conceived as the dissipative elements by means of the yielding in tension. The other members are designed with adequate overstrength in order to ensure that the yielding occurs in bracing members before it occurs yielding or buckling of both columns and beams. In order to obtain this kind of performance and to avoid undesirable deterioration or collapse, the EN1998-1 [1] provides specific requirements aiming also to have a homogeneous damage distribution over the height of the building. This study aims to investigate the influence of design criteria on seismic behaviour of concentric frames with V-bracing using the "dual-steel" concept, where the High Strength Steel (HSS) is used in non-dissipative members while the Carbon Mild Steel (MCS) is applied to the dissipative elements. The combined use of HSS with MCS may allow an easier application of capacity design criteria. The expected design improvement would be obtained in terms of smaller member sizes than those obtained when using MCS only. Indeed, the non-dissipative members should be designed to withstand the full plastic strength of the dissipative ones, and consequently, large overstrength demands to these elements or zones lead to high material consumption. Therefore, static and dynamic nonlinear analyses were carried out. Furthermore, the seismic performance was based on three limit states defined by EN1998-3 [3]: Damage Limitation (DL), Significant Damage (SD) and Near Collapse (NC); evaluating the influence of design criteria on the behaviour factor.