Evaluation of the Seismic Design Methods for Steel Multi-Tiered Concentrically Braced Frames

• Author / Creator

  Cano, Pablo A

• Multi-tiered concentrically braced frames (MT-CBFs) are widely used in North America as the lateral load-resisting system of tall single-storey buildings such as airplane hangars, recreational facilities, shopping centres, and industrial buildings. MT-CBFs consist of multiple concentrically braced panels along the height of the frame separated by horizontal struts. Multi-tier arrangements are typically used when it is not practical nor economical to use a single bracing panel along the height of the frame between the ground and roof levels. In multi-tiered braced frames, the length of braces is reduced, which allows the selection of smaller brace sizes and easily satisfying code-specified brace slenderness limits. The column buckling length in the in-plane direction is also reduced due to the application of intermediate horizontal struts, which permits selection of a smaller column section. When using shorter braces, result in smaller design forces on the adjacent forced-controlled members including struts, beams, columns, and connections.Past studies have shown that inelastic frame deformations tend to concentrate in one of the tiers over the frame height, which induces large in-plane bending moments in braced frame columns and high deformation demands in braces. This behaviour may lead to column buckling and/or brace fracture. Design requirements have been included in the Canadian steel design standard (CSA S16) and the U.S. Seismic Provisions to prevent such limit states. In the U.S., the Seismic Provisions have included the design of multi-tiered ordinary and special concentrically braced frames (MT-OCBFs and MT-SCBFs). However, there are no detailed numerical models or experimental research to validate the design requirements.This M.Sc. thesis research focuses on the evaluation of the seismic behaviour and design methods for MT-CBFs. A two-tiered CBF prototype frame was first designed as a special concentrically braced frame using the 2010 and 2016 AISC Seismic Provisions. Then, a detailed numerical model was developed and was analyzed using the cyclic pushover (static) analysis and the nonlinear response history (dynamic) analysis. The global and local response of the prototype frames together with the force demands in the columns were examined using the results obtained from the numerical analyses. Special attention was paid to the stability condition of the column as well as in-plane and out-of-plane moments induced in this member.Results obtained for the prototype frame designed excluding the special seismic design provisions confirmed column buckling and nonuniform distribution of the frame inelastic lateral deformations in the tier where brace tensile yielding takes place first. A total of 13 column buckling cases were observed using the dynamic analysis method among an ensemble of 40 ground motion records. Moreover, excessive deformations, which is an indication of brace low-cycle fracture, were observed in the yielding tier of this prototype frame. In contrast, the prototype braced frame that was designed in accordance with the recent special seismic design provisions performed satisfactorily. No column buckling occurred and the frame lateral response was stable. Braces in both tiers yielded under most ground motion records and frame inelastic lateral deformations were shared between both tiers. It was found that the column moment demands prescribed by the current design provisions over estimates the moment demands obtained under a major earthquake event. Additionally, expected storey drift was found to be higher than the code-specified design storey drift, which resulted in large ductility demands in braced tiers, which poses concerns regarding the adequacy of the current drift requirements. New brace force adjustment factors are proposed to achieve more realistic brace nonlinear forces when computing column force demands and tier drifts.

• Subjects / Keywords

  • Multi-tiered column buckling seismic design

• Graduation date

  Spring 2019

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-635c-7v78

• License

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