LOW-CYCLE FATIGUE BEHAVIOR OF REINFORCING STEEL

ASTM A615 grade 40 ordinary deformed-steel reinforcing bars with a specified minimum yield strength of 276 MPa (40 ksi) and ASTM A722 high-strength prestressing thread bars with a specified ultimate strength of 1,083 MPa (157 ksi) were experimentally evaluated for their low-cycle fatigue behavior under axial-strain-controlled reversed cyclic tests with strain amplitudes ranging from yield to 6%. All tests were performed on virgin (unmachined) specimens to closely simulate seismic behavior in structural concrete members. A methodology is suggested to identify incipient (first-cracking) failure of test specimens. The experimental data were evaluated with existing fatigue models, which related stress-strain quantities to the failure life. Additional energy-based fatigue models are proposed that relate various stress and/or strain quantities to the dissipated energy. The study demonstrates that the modulus of toughness and low-cycle fatigue life for both the low- and high-strength materials are similar. Based on fatigue considerations, it is concluded that existing design codes are overly restrictive in not permitting the use of high-strength thread bars in seismic-resisting elements.