11.64: Seismic behaviour of steel Chevron bracing systems by non‐linear dynamic analyses

Abstract

The paper focuses on steel Chevron concentric bracing systems (CBF-V) as seismic resistant structures, presenting the evaluation of the structural seismic performance through dynamic analyses, aimed at improving the design methods. A wide numerical study is carried out. Different CBF-V configurations are designed, for a total number of 12 structures, by varying number of storeys (3, 6, 10), design methods (linear and dynamic analysis) and braces sections (H-shaped and circular hollow sections). Nonlinear Incremental Dynamic Analyses (IDA) are performed by the SeismoStruct software, considering 7 earthquakes records, in order to investigate the seismic response. The most relevant evaluation parameters are shown and discussed, IDA curves are caught and the behavior factors calculated. Results show that expected collapse modes, involving the braces only, according to the Capacity Design, are not in agreement with the noted mechanisms, in which damage also involves nondissipative elements, in particular the beams. Hence, it is needed to deepen such key issue, through further experimental and numerical investigations in order to optimize the design procedure.1 INTRODUCTIONThe behaviour of Concentric Braced Frames (CBFs) mainly depends on the capacity of bracing members to resist large inelastic displacement reversals without significant loss of strength and stiffness, more specifically, on compressed members, whose behaviour is governed by buckling. This topic is investigated by several researchers through both experimental and analytical studies on either single braces, or single story single bay, or multi-storey multispan structural systems, by analysing both local and global behaviour of bracing systems. For single braces, the optimization of the slenderness [1] and the influence of the cross-section shapes and other member properties on the structural performance, generally estimated in terms of strength, ductility, energy dissipation and mid-length lateral deformation [2], are evaluated. For structural systems, overall cyclic response and brace failure mechanisms are investigated [3], through static non-linear analyses on CBFs with different stories and span lengths, evidencing the great effect of the geometrical configurations on the response modification factors [4,5]. Specifically for Chevron Braced Frames (CBF-V), the ratio between the beam flexural stiffness and the braces vertical stiffness is recognized as affecting the overall performance and the damage of braces under compression [6]. In this context an overview on the codification aspects according to the Italian technical Code (NTC08) and the Eurocode 8 is presented [7,8], evidencing some critical issues both in the design process and in the seismic capacity of structures evaluated through non-linear static analyses. In particular, it is observed that current design criteria lead to structures that behave better than expected, in terms of ductility and dissipative capability, being the behaviour factors calculated larger than the design value, although...