To resist strong intensity earthquakes, the design of reinforced concrete members often relies on well-detailed plastic hinge regions for a ductile response. Within these plastic hinges, large strain deformations are expected in both the concrete and steel reinforcement. In the case of ordinary bridge columns, experimental testing of full-scale specimens has shown that fracture of longitudinal reinforcement, following visible plastic buckling is a common failure mode in the plastic hinge of columns designed according Caltrans’ Seismic Design Criteria (SDC). This type of failure is defined herein as “Plastic Buckling-Straightening Fatigue” (PBSF), as the name “Low-cycle fatigue” commonly used for it is inconsistent with limitations set forth by ASTM standards for this phenomenon. The mechanism under which the PBSF type of failure occurs starts with micro-cracks developing at the root of bar deformations, in the concave side of a buckled bar, which start to propagate for an abrupt fracture when the bar stretches. Knowing the process under which the fracture occurs, the buckling behavior of the longitudinal reinforcement, considering the effect of material properties and the configuration of transverse hoops is studied, evaluating the fatigue life of the bars. Based on the regression of multiple Finite Element Model results, a simple design and verification procedure is proposed to control the PBSF limit state in ordinary bridge columns, recommended for inclusion in Caltrans SDC specifications.

The use of large diameter reinforcement has proven an effective method to expedite the construction process of reinforced concrete bridges (Marsh, et al., 2011). The fatigue life of large bars however, has not been well documented yet, with the use of large diameter reinforcement in seismic regions limited to #11 bars vertical members in most ordinary bridges. For this research work, a series of Grade 60 #18 bars were tested under large amplitude cyclic strain histories, evaluating the fatigue life of the reinforcement for different length to diameter ratios and amplitude of deformations. The tests performed herein correspond to the first successful examination of the response under large amplitude strain reversals, including the fracture under PBSF, for reinforcement of this size.