Component testing and multi-level seismic design of steel braced frames with high post-yielding stiffness and two-phase yielding

Using induction heat (IH) treatment to locally transform one half of a steel brace section to high-strength steel section as well as inducing intentional eccentricity along the brace length has been experimentally proven to increase the limited post-yielding stiffness exhibited by conventional steel braces. This paper develops a multi-level seismic design method for intentionally eccentric IH-treated steel braced frames (IH-FIEB) to support their implementation in building structures. More specifically, experimental results on the cyclic behaviour of IH-treated steel braces with intentionally eccentricity (IH-BIEs) are presented giving an emphasis in quantifying member's resistance to local buckling and fracture initiation. A theoretical model is developed on the force-deformation relationship of the bracing system which is validated with test results and results obtained from a parametric finite element analysis study. On the basis of theoretical model, design expressions that describe the mechanical two-phase yielding behaviour of the bracing system controlled by the alternated flexural-axial behaviour are developed. The high post-yielding stiffness and controllability of the brace response through eccentricity provide the brace the capability of satisfying multiple strength performance objectives simultaneously. Time-history analysis results under three seismic hazard levels demonstrate that IH-FIEBs assure a more uniform overstrength and plastic engagement between each story than the conventional steel braced frames. IH-BIEs smoothly transition into post-buckling response and engage plastically at low drift demands, while seismic forces developed are well controlled and close to the values expected from the design procedure. A significant reduction of the residual deformation is observed at high seismic intensity levels indicating that IH-FIEBs may be able to reduce post-hazard damage compared to conventional braced frames.