Fracture of high-strength bars in concrete frame members under earthquake loads

Fracture of longitudinal bars due to high-strain low-cycle fatigue is a critical failure mode in seismically detailed reinforced concrete frame members because it can lead to rapid strength loss and structural instability. The issue has recently attracted attention due to a national effort aimed at introducing high-strength reinforcing bars (HSRB) with yield strengths of 80 and 100 ksi in concrete construction. The HSRB being produced in the United States possess varying post-yield mechanical properties, such as the tensile-to-yield strength ratio, uniform or fracture elongations, as well as low-cycle fatigue life. The behavior of Special Moment Frame (SMF) members with different types of HSRB subjected to large inelastic demands up to bar fracture is investigated through laboratory testing and analytical examination. Laboratory tests were performed to identify any major issues in the performance of HSRB in concrete members. More specifically, the work aimed to assess the influence of the tensile-to-yield strength (T/Y) ratio, fracture elongation, and shape of stress strain curve of HSRB on the behavior of seismically detailed concrete columns. Four specimens were tested under constant axial load and reverse cyclic lateral loading of increasing amplitudes until fracture of longitudinal bars. Three columns were reinforced with grade 100 bars sourced from different manufacturers and therefore having different post-yield mechanical properties. The fourth column was reinforced with conventional grade 60 ASTM A706 (2016) bars. Concrete columns reinforced with HSRB reached similar lateral drift levels as the specimen reinforced with grade 60 bars before significant loss in lateral strength. A computational framework based on fiber-section elements and mechanics-based behavioral models is proposed to accurately estimate both member-level deformations and strain demands in longitudinal bars and the concrete surrounding them within the plastic hinge regions of frame members. Particularly, the effects of the mechanical properties and steel grade of reinforcing bars on their strain demands are quantified experimentally and estimated by the proposed framework. The strain demands derived through the proposed analytical framework were used to track the damage progress in longitudinal bars that lead to buckling and fracture. A buckling initiation model is proposed that accounts for the mechanical properties of the reinforcing bars, as well as the loading history the bars and the surrounding concrete experience prior to buckling. Material specific bar fatigue relations calibrated through material test results are used to predict the number of half-cycle to bar fracture based on accumulation of strain demands prior and after buckling if it occurs.