Experimental study on triple grades hybrid high-performance steel structure (TGHSS): Members and joints

The triple grades hybrid high-performance steel structure (TGHSS) is an innovative structural system with excellent seismic behaviour proposed by the authors. The TGHSS comprises high strength steel columns, ordinary strength steel beams, and low yield point steel buckling-restrained braces (BRBs). In full-scale cyclic tests on eight single-bay two-story specimens, strains on surfaces of beams, columns, and joints were extensively monitored, and deformations of BRBs at both stories were recorded. The global responses of TGHSSs were introduced and discussed in the first companion paper. This paper focuses on the local responses of members and joints under real boundary conditions in structures, which is deemed to provide more reasonable performance evaluations than independent tests on extracted specimens. The force analyses of beams, columns, and joints were conducted. The hysteresis behaviour, mechanical indexes, energy dissipation, and end rotation of BRBs were studied. Moreover, the plastic development sequences were obtained and discussed following the above contents. Results showed that the low yield point BRBs could provide reliable cyclic behaviour and energy dissipation with the cumulative ductility factor of 312.03 ∼ 1191.74, meeting the requirement in American and Chinese codes. The ordinary strength steel beams produced plastic hinges with considerable rotations of at least 0.02 rad. The high strength steel columns exhibited limited plasticity, especially the panel zones in beam-to-column joints almost kept elastic. Results revealed that the TGHSS featured a sequential plastic development as expected from BRBs to beams and then to columns. Following the test results, some design suggestions were proposed for TGHSSs.