Presented By: Jessi Thangjitham, North Carolina State University

Description: In quantifying the seismic performance of high strength reinforcing steel, parameters such as rebar geometry have been found to influence the seismic behavior of Grade 80 reinforcing steel. Rebar geometry was assessed with 3D laser scanning and the resulting mesh was used to develop computational finite element models. These models showed that stress concentrations at the base of ribs are significant and may impact uniform elongation capacity. While the stress concentrations also impact Grade 60 steel, the effect on Grade 80 steel is more significant due to intrinsically lower strain capacity of high-strength steel. This led to the idea of using high-strength smooth round bars as reinforcing steel in plastic hinge regions. Material tests that simulate seismic loading of reinforcing steel showed that smooth reinforcing bars do not exhibit the stress concentrations that are localized around the rib radius of conventional bars. Therefore, bars without ribs have higher bending strain capacity than traditional reinforcing steel. Results of large-scale quasi-static and dynamic columns tests with two different smooth bar designs will be presented. The first design limits the smooth bar to the plastic hinge region thus allowing for friction bond from the ribs to develop in the remainder of the member. The second design is composed of smooth bars along the entire length of the member while deploying heads at each end to ensure proper anchorage. In general, these unconventional designs with smooth bars could improve the seismic performance of RC members that form plastic hinges compared to traditional reinforcing steel.

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