Improvement of Hysteretic Energy Dissipation of Concrete Members by Multi-yielding Longitudinal Reinforcement

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A new reinforcing method to improve the inelastic behavior of reinforced concrete was proposed. The method combines use of high-strength and ordinary-strength longitudinal bars. When the column is subjected to bending moment, the ordinary strength longitudinal bars yield first and then the high-strength bars yield later in large post-yield range. Up to the stage of yielding of the high-strength bars, the column does not show significant degradation of load-carrying capacity. To confirm the advantages of the new reinforcing method, flexural analyzes on reinforced concrete columns with different grades of longitudinal bars were conducted. The results of the analysis indicate that the combined use of different grades of longitudinal bar improves the inelastic behavior of reinforced concrete columns. An experimental research work on the proposal was also conducted to verify the effectiveness. The parameters used were the ratio of the amount of high-strength reinforcement to that of ordinar … More y-strength reinforcement, the axial load level on the column and bi-directional loading.A new type of prestressing strand, graded composite strands, has been developed. The seven-wire strand consists of three high-strength wires (a yield stress of 1400 MPa) and four ordinary-strength wires (400 MPa). The high-strength wires provide prestressing force along the member axis even under large deformations. The ordinary-strength wires, which yield during the prestressing operation, are expected to contribute to hysteretic energy dissipation. Therefore, these graded composite strands make precast connections have many of the desirable characteristics of monolithic connections. The effectiveness of the strands is verified by loading tests on beam-to-column connections. Eight external beam-to-column connections were constructed and tested. Four specimens were connected by ordinary prestressing stands and the others were connected by the graded composite strands. The parameters used were the amount of prestressing force introduced and the distance of the location of prestressing strands from the centroidal axis of the beam section. The test results indicate that specimens with graded composite strands had much larger hysteretic energy dissipatiion than did the specimens with the ordinary strands.Another series of loading tests were carried out on column-to-foundation connections post-tensioned by 'graded composite prestressing steel bars'. The steel bar cross section consists of two parts : normal strength and high strength. Thus, it has a monotonic load-deformation relationship which is better modeled by a trilinear than a bilinear approximation. This nonlinear behavior results in energy dissipation. Nine column-to-foundation connections were constructed using the graded composite steel bars or ordinary prestressing steel bars. However, the test results revealed that the graded composite steel bars were not so effective to increase hysteretic energy dissipation capacity. Less