Seismic Response and Design of Steel Multi-Tiered Eccentrically Braced Frames
Creators: Ashrafi, Abolfazl
Publisher: name: University of Alberta Library
Year: 2023

<p>Multi-tiered braced frames are commonly used as the lateral load-resisting system of tall single-storey buildings or tall storeys of multi-storey buildings. This framing system divides the frame or storey height into multiple bracing panels, resulting in more economical member sizes and practical connections. Although multi-tiered concentrically braced frames are preferred in practice, multi-tiered eccentrically braced frames (MT-EBFs) offer an alternative solution in high seismic regions due to their high ductility, stable and reliable yielding mechanism, and architectural versatility. Despite extensive research studies conducted to examine the seismic response of and develop design requirements for multi-tiered concentrically braced frames, there is limited research on the seismic performance of steel MT-EBFs. Furthermore, there exist no special design requirements for MT-EBFs given that there are several concerns associated with the stability response of their unbraced intermediate links and columns. Due to the lack of background research, the current edition of the Canadian steel design standard, CSA S16-19, does not recognize MT-EBFs as ductile EBF. This Ph.D. research project aims to improve understanding of the behaviour of MT-EBFs under seismic loads and develop new analysis and design requirements to improve their seismic stability of MT-EBFs with emphasis on their link beams and columns. <br>
A set of prototype MT-EBFs part of an industrial building is selected and used to perform a numerical parametric study by varying the frame height, tier height ratio, number of tiers, link lateral bracing condition, and flexural rigidity of the brace-to-beam connections. The frames are designed as per 2019 CSA S16 assuming shear yielding mechanism in their links. Nonlinear response history analyses are then performed to evaluate their global and local response and quantify seismic-induced demands in the intermediate beams, braces, and columns as well as inelastic link rotation. The continuum-based numerical model of a two-tiered EBF with continuous I-section links and three- and four-tiered EBFs with continuous built-up tubular links are developed and used to further evaluate the seismic response of MT-EBFs taking into consideration out-of-plane and torsional response of the links and columns. The results of the dynamic analyses performed on the MT-EBFs not specifically designed for the multi-tier response confirm that intermediate links had a tendency to buckle out-of-plane because of the loss of stiffness after shear yielding and the absence of out-of-plane bracing, which results in appreciable out-of-plane flexural bending on the columns. Moreover, progressive yielding of the links until full plastic mechanism develops causes non-uniform distribution of inelastic lateral deformation along the frame height, where the largest inelastic deformation tends to occur in the tier with the largest design shear to shear strength ratio. This response imposes in-plane flexural bending in the columns. MT-EBF columns subjected to in-plane and out-of-plane bending together with a large axial compression force caused by gravity and seismic loads are prone to buckling. Seismic analysis and design requirements are developed for two-tiered EBFs with continuous wide-flange links, and three- and four-tiered EBFs with continuous built-up tubular link beams. The requirements for two-tiered frames make use of intermediate beams to limit out-of-plane deformation of diagonal braces, torsionally brace the link beam in the intermediate level using diagonal braces, take advantage of column flexural stiffness and strength to brace the intermediate beam out-of-plane, estimate and account for column in-plane bending demands, and limit the inelastic link rotation. The proposed guidelines for three- and four-tiered EBFs with continuous built-up tubular links involve requirements to laterally brace diagonal braces and intermediate beams, requirements to verify inelastic link rotation at each tier, and requirements to verify the strength and stability of columns under the combined axial force plus in-plane and out-of-plane flexural demands. Nonlinear dynamic analyses are conducted to validate the proposed requirements, which confirm that the improved MT-EBFs can better distribute the inelastic lateral deformation between tiers, experience lower inelastic link rotations in their links, and undergo limited out-of-plane response in their intermediate beams. Furthermore, column in-plane and out-of-plane flexural demands are properly predicted by the proposed methods.</p>