Simplified seismic design methods for low-ductile steel multi-tiered concentrically braced frames

• Author / Creator

  Eshagh Derakhshan Houreh

• Multi-Tiered Concentrically Braced Frames (MT-CBFs) represent a bracing configuration where two or more concentric braced panels are stacked between the storey levels in multi-storey buildings or between the ground and roof levels in single-storey buildings such as sports facilities, airplane hangers, or industrial buildings. This configuration is commonly used in tall storeys when it is impractical or uneconomical to employ one braced panel along the frame height. The multi-tiered configuration is preferable to avoid the use of large brace sections and impractical brace connections in design. A large proportion of MT-CBFs in Canada is located in low-to-moderate seismicity regions where low-ductile frames, such as Limited Ductility (Type LD) or Conventional Construction (Type CC) category, are often preferred to avoid relatively complicated seismic design and detailing requirements. Limited research studies have focused on the seismic stability and design of such low-ductile MT-CBFs. The current special seismic design provisions for Type LD MT-CBFs in the current Canadian steel design standard (CSA S16), which often requires performing multiple tedious analyses, particularly for tall frames, lack sufficient background research. For Type CC MT-CBFs, the Canadian steel design standard does not provide specific seismic design requirements. This research aims to investigate the seismic behaviour of Type LD and Type CC steel MT-CBFs with the focus on the stability of their columns, assess the current seismic design provisions, and propose enhanced and yet simplified design guidelines for the design of such frames in low-to-moderate seismicity regions.
  An industrial building located in Montreal, QC, Canada representing a moderate seismicity region, was selected. A parametric study matrix consisting of 64 low-ductile steel MT-CBFs was developed by varying various parameters, including the frame ductility level, frame height, tier height ratio, brace and column cross-sections. The selected frames were then designed to CSA S16-14 provisions, excluding the key design requirements for MT-CBF columns of Type LD and Type CC. The nonlinear numerical models of the frames were then developed using fibre-based elements in OpenSees, including the connection effects. The models were then used to perform nonlinear response history analyses under 30 ground motion records.
  Results of the NLRH analyses showed multiple column buckling cases in both Type LD and Type CC frames. It was confirmed that the inelastic response in the selected frames was limited. No tension yielding has occurred in the tension-acting braces of the majority of the selected frames. Although the majority of compression-acting braces knuckled in compression, none of them entered the post-buckling zone. Storey drifts in all frames were significantly lower than the 2015 NBCC limit. Similarly, tier drifts were found to be noticeably lesser than the drift that can cause low-cycle fatigue fracture in braces. For Type LD frames, the axial compression forces did not exceed the design forces obtained using the capacity design principle. Additionally, in-plane and out-of-plane moments induced in the columns of Type LD frames were found to be lower than 6% and 8% of the respective column plastic moments in the plane and out of the plane of the frame. The columns of Type CC MT-CBFs experienced larger compression forces than their design forces. Column in-plane and out-of-plane moments observed in the columns of Type CC frames were lower than 8% and 28% of the respective column plastic moments in the plane and out of the plane of the frame. Improved and simplified seismic analysis and design methods were proposed for the design of Type LD and Type CC MT-CBFs with the emphasis on their columns. The adequacy of the proposed methods was examined for six case study frames, which confirmed that the proposed methods lead to a satisfactory seismic performance without column instability. The column size remained unchanged in Type LD frames and reduced in Type CC frames when compared to the columns designed to CSA S16-14.

• Subjects / Keywords

  • double
  • analysis
  • ductility
  • seismic
  • construction
  • limited
  • braced
  • multi-tiered
  • column
  • structure
  • response
  • angle
  • concentrically
  • MT-CBF
  • OpenSees
  • model
  • low-ductile
  • steel
  • Nonlinear
  • design
  • Fiber-based
  • numerical
  • frame
  • HSS
  • earthquake
  • conventional

• Graduation date

  Fall 2020

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-zgby-k502

• License

  Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.