A simplified and computationally efficient numerical model for the analysis of knee-braced steel frames under lateral loads

Abstract

This study focuses on the capacity design of earthquake-resistant steel structures incorporating knee bracing systems, which serve as dissipative elements. It also presents the development and validation of a numerical model for simulating the behavior of these systems. The proposed model accurately captures the response of the fuse element by accounting for the interaction among axial force, shear, and bending moment, including second-order effects, while maintaining low computational demands. Model validation was performed through comparison with two independent experimental campaigns: (1) three-point bending tests on isolated fuses to characterize their local behavior, and (2) cyclic push-over tests on a full-scale, single-story frame to evaluate the global response of the system under lateral loading. Following the validation, an extensive parametric study was carried out via nonlinear static analyses on a multi-story, multi-bay steel frame with semi-rigid joints equipped with knee bracing systems. The investigation examined the influence of the axial forces in the fuses, the fuse cross-sectional typology, the joint rotational stiffness, and the lateral load distribution. The proposed model provides a practical and computationally efficient tool to support performance-based design and optimization of energy-dissipative devices in steel frame structures. The paper also provides a series of design recommendations for practitioners involved in designing earthquake-resistant structures with knee bracing systems.