Seismic Behaviour and Design of Two-Bay Steel Multi-Tiered Braced Frames and Other Special Steel Concentrically Braced Frames in Single-Storey Buildings

In tall single-storey steel structures such as sports facilities, airplane hangars, industrial buildings or warehouses, multi-tiered concentrically braced frames (MT-CBFs) are commonly used.MT-CBFs consist of a bracing system where multiple braced panels are stacked over each other along the storey height.Each panel is referred to as a tier.This bracing configuration results in shorter braces, which typically leads to minimized brace sections and easier compliance with slenderness and width-to-thickness requirements.Intermediate struts are provided at tier levels, to resist unbalanced brace axial loads that develop after the compression braces have buckled.This redistribution of unbalanced axial loads through struts prevents undesirable K-braced frame behaviour, where the columns must resist large in-plane flexural demands.The columns typically consist of I-shaped members oriented such that strong-axis bending is utilized to resist out-of-plane bending moments over the frame height.Struts allow the columns to be laterally braced at tier levels for weak-axis buckling.Results from previous studies showed that under seismic loading inelastic deformations in MT-CBFs tend to concentrate in one critical tier.Two major concerns arise from this phenomenon.On one hand, significant inelastic deformations can occur in the bracing members of the critical tier, which may result in brace fracture due to low cycle fatigue.On the other hand, concentration of inelastic drifts in one tier leads to in-plane bending demands in the columns, which may cause instability.Both Canadian and American seismic design provisions require MT-CBFs to be designed for seismic loading assuming a concentration of inelastic deformations in one tier.This research project focuses on the seismic response of bracing systems that can presumably exhibit behaviour similar to the one described above.The first studied bracing system is two-bay MT-CBFs, where two multi-tiered braced frames are placed side by side in adjacent bays.The second studied bracing system consists of two-bay X-CBFs, where a standard braced frame spans over two column bays, intersecting with a middle column.The third and last studied bracing system is a split-X braced frame, where struts are placed at mid height of a standard CBF to laterally brace the columns against in-plane buckling.The objective of this research project is to study the design and the seismic behaviour of the aforementioned bracing systems, for prototype buildings located in Vancouver, British Columbia.This objective was achieved by developing and refining an OpenSees numerical model, capable of x simulating the inelastic behaviour of bracing members and columns.The design of a two-tiered MT-CBF was carried out, following both Canadian and American seismic provisions.Finally, nonlinear response history analyses were realized for all three studied bracing systems.The design of a two-tiered CBF allowed to point out key differences in the Canadian and American design approaches.Variation in anticipation of the ductility demand led to different building seismic weight and design base shears for the same brace selection.Additionally, column design differs in the two codes.The Canadian standard requires progressive analysis of the frame to be realized, whereas the American standard requires the columns to be sufficiently resistant in bending to accommodate the unbalanced probable shear resistance occurring between tiers.Nonlinear response history analyses (NLRHA) of six two-bay MT-CBFs were conducted.The results show that progressive yielding of tiers occurs in this system, in a similar manner to what is observed in single-bay MT-CBFs.Progressive yielding can occur in various tiers, as well as in various bays.The implemented design procedure was able to predict the yielding sequence of the tiers, as well as the in-plane flexural demand on columns.Discrepancies in this prediction were noted because of the assumed brace forces in design.It was found that design inelastic drifts were consistently underestimated.On the contrary, the out-of-plane bending moments on columns were largely overestimated by the design procedure, based on CSA S16-19.NLRHA on the two studied two-bay X-CBFs showed that frame behaviour was satisfactory, regardless whether an intermediate strut was included or not.Brace behaviour resembled what is typically observed in traditional CBFs.When an intermediate strut was used, the exterior columns developed significant in-plane bending moments, which were anticipated and considered in design.Furthermore, when accounting for the inherent variability of brace yield stress, it was determined that the interior column can also be subject to in-plane flexure.Finally, NLRHA was conducted for a split-X CBF.The intermediate strut allowed the column to be laterally braced in the in-plane direction.However, after tension brace yielding, the brace intersection point exhibited in-plane displacements resulting in strut axial force and in-plane bending moments on the columns.These were larger than anticipated in design, and the frame response showed inelastic demand concentrated in the upper half of the column members.

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