Cyclic response of concrete beams reinforced with ultrahigh strength steel

Seismic applications of concrete members reinforced with ultrahigh strength steel bars (yield strength greater than 80 ksi) have been limited by U.S. building codes since the introduction of Strength Design in 1963. The limitation has been primarily due to the paucity of experimental data. This investigation aimed to provide benchmark data for studying the cyclic response of concrete beams reinforced with steel bars having yield strengths approaching 100 ksi.;Seven specimens were subjected to large transverse displacement reversals: three specimens were reinforced longitudinally with conventional steel bars (Grade 60) and four with ultrahigh strength steel bars (Grade 97). All transverse reinforcement was Grade 60. Other experimental variables were: volume fraction of steel-hooked fibers (0 or 1.5%), spacing of transverse reinforcement ( d/4 or d/2), and ratio of compression-to-tension longitudinal reinforcement (rho'/rho = 0.5 or 1). The nominal concrete compressive strength was 6000 psi.;Test beams reinforced with Grade 97 bars had drift ratio capacities in excess of 10%, comparable to the deformation capacities of similar beams reinforced with conventional bars. Numerical models were developed for the calculation of moment-curvature and force-displacement relations. The calculated values showed good correlation with the measured data.;For beams with nearly identical strength, the measured yield displacement of beams reinforced with Grade 97 bars was about 25% greater than in beams reinforced with Grade 60 bars. However, nonlinear seismic analyses of single-degree-of-freedom systems indicate that the displacement calculated for systems representing the beams with Grade 97 bars are, on average, less than 10% larger than the displacement calculated for systems representing the beams with Grade 60 bars.;The evidence presented shows that the use of ultrahigh strength steel (Grade 97) as longitudinal reinforcement is a viable option for earthquake-resistant construction.