Gnazzo, Massimino (2026) Enhancing the seismic design of steel concentrically braced frames in the framework of the second generation of EC8. [Tesi di dottorato]

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Abstract

Steel concentrically braced frames (CBFs) are widely used in seismic regions as the lateral load-resisting system of buildings due to their high lateral stiffness, strength, and cost-effectiveness. Within the European design framework, the second generation of Eurocode 8 (EC8) has introduced significant advances in the seismic provisions for steel CBFs, including improved capacity design concepts and broader harmonisation of rules across different lateral brace configurations. Nevertheless, recent numerical and experimental studies indicate that the seismic performance of CBFs remains affected by unresolved critical issues that depend strongly on the adopted ductility class, brace configuration, and member and connection detailing. This thesis presents a comprehensive numerical investigation on the seismic design and response of steel CBFs designed according to both the first and second generation of Eurocode 8. The study aims to assess the effectiveness of current code provisions and to develop improved seismic design criteria for both low-dissipative and dissipative CBF systems. Nonlinear numerical models were developed in OpenSees and validated against experimental results from the literature to accurately capture the cyclic behaviour of braces, the local response of gusset plates, brace end connections and beam-to-column joints, and the global seismic response of CBFs. Comprehensive parametric numerical studies were conducted to evaluate the influence of ductility class, brace configuration, and design assumptions on the structural response at different performance levels by means of time history analyses. Moreover, incremental dynamic analyses were carried out to (i) investigate the structural response from the elastic range to structural collapse, (ii) estimate behaviour factors, and (iii) derive fragility curves. The results indicate that, despite the improvements introduced in the second generation of EC8, relevant shortcomings remain. Low-dissipative (DC1) systems designed without capacity design requirements are susceptible to brittle failure mechanisms at brace end connections, thereby adversely affecting their seismic response. In medium- and high-dissipative CBFs, the seismic demand on columns is generally underestimated by current code provisions, ultimately leading to potential buckling phenomena for columns. Based on the numerical outcomes, revised seismic design recommendations are proposed for both low-dissipative and dissipative CBFs, focusing on connection overstrength requirements and on a more reliable estimation of column seismic demand. The effectiveness of the proposed criteria was numerically assessed, demonstrating an improvement in the overall seismic performance of the structural system. Overall, the research advances seismic design practice for steel concentrically braced frames and provides a consistent technical basis for refining the design provisions of Eurocode 8 to better align with the actual nonlinear response and failure mechanisms of CBF systems under strong seismic actions.

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