Performance Characterization of Beams with High-Strength Reinforcement

A laboratory test and analytical research program was undertaken to characterize the performance of reinforced concrete beams with high-strength reinforcement subjected to reversed cyclic lateral loading simulating earthquake effects. The beams are representative of beams used in special moment frames. Four beams were tested in the laboratory test investigation, one with A706 Grade 60 reinforcement, one with Grade 100 reinforcement having tensile-to-yield strength ratio (T/Y) of 1.17, one with Grade 100 reinforcement with T/Y = 1.26, and one with A1035 Grade 100 reinforcement. In each beam, the noted reinforcement grade was used for both longitudinal and transverse reinforcement, except for beam with Grade 100 T/Y = 1.17 that had transverse reinforcement of Grade 100 with T/Y = 1.26. Overall, all beams achieved rotation capacity of at least 0.045 radians. The beams with A706 Grade 60 and Grade 100 (T/Y = 1.26) reinforcement failed by buckling of longitudinal bars over several hoop spacings. The other two beams with Grade 100 reinforcement failed by fracture of longitudinal bars at the maximum moment section. Strain gauges installed on longitudinal bars indicated that beams with higher T/Y achieved greater spread of plasticity compared to beams with lower T/Y. In the analytical study, the seismic performance of tall reinforced concrete special moment resisting frames with high-strength reinforcement was investigated through nonlinear dynamic analyses. Four 20-story reinforced concrete moment frames, three reinforced with Grade 100 steel and one with Grade 60 steel were designed in accordance with ASCE 7-16 and ACI 318-14 at a hypothetical site in San Francisco, California. All four frames had the same dimensions and concrete properties, resulting in identical design drifts. Frames with Grade 100 reinforcement were designed to have reduced amount of longitudinal reinforcement to provide equivalent nominal strength as was provided in the Grade 60 reinforcement model. Tests had demonstrated that frames with higher-grade reinforcement had greater strain penetration into beam-column joints, resulting in greater slip of reinforcement from connections. This effect combined with reduced reinforcement ratios caused the frames with Grade 100 reinforcement to be more flexible than the frame with Grade 60 reinforcement. In addition, some currently available types of Grade 100 reinforcement have lower tensile-to-yield strength ratio and lower uniform elongation compared with Grade 60 reinforcement. The reduced T/Y results in reduced strain-hardening, increased strain localization, and increased P-Delta effects. The effects of these local behaviors on overall frame performance are studied through the nonlinear dynamic analyses. The various types of reinforcement were found to result in minor differences in overall frame seismic performance