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Performance-based seismic design and evaluation of steel eccentrically braced frames with tubular links as bridge bents Shafiq, Ahmad

Abstract

The latest release of the Canadian Highway Bridge Design Code (CHBDC), S6-14, incorporates performance-based design (PBD) provisions for bridges in Canada for the first time. CHBDC S6-14 establishes two different design approaches, with the PBD being the standard method of design and force-based design (FBD) being permitted for special cases. The focus of this study is on ductile eccentrically braced frames (EBFs) as bridge substructure. For member proportioning, the CHBDC S6-14 refers to the Canadian steel design standard for buildings, CSA S16-14, stating a force reduction factor, R=4. This is a FBD, and there is need to evaluate the design in terms of the performance descriptions and damage states by carrying out the analyses recommended by CHBDC S6-14. For this case study, an existing bridge is considered as a Major Route bridge, and an EBF with built-up tubular shear link has been chosen as an earthquake-resisting system (ERS). Four different cases have been designed including two using FBD and two for PBD approach for comparison purposes. Due to the lack of strain/rotation criteria in CHBDC S6-14 at multiple service states for EBFs as bridge bents, different acceptance criteria for rotations and corresponding damage states have been proposed by using fragility curves from the literature. The link total rotation has been considered as a demand parameter and different methods of repairs consistent with each damage state have also been provided. The response spectrum analysis coupled with inelastic static pushover analysis is used for global displacement demands and for demonstrating local component performance compliance of shear links. Nonlinear time-history analysis is also used to check and provide a comparison of the first approach. The code requires no-yielding for the 475-year return period event. This criterion governs the design and makes the sizes large and inefficient, while the link plastic rotations corresponding to higher return period events are very low compared to the allowable limits provided in the literature for links mainly used in buildings. Through different cases, it is demonstrated that if the links are made replaceable and allowed to have limited yielding at 475-year earthquake, it makes the design more practical.

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