High-Strength Steel Reinforcement in Critical Regions of Earthquake-Resistant Bridges
Large diameter bars are often used in large civil infrastructure projects such as bridges, power stations, large mat footings, and are occasionally used as reinforcement in buildings where the use of smaller size reinforcement would cause excessive congestion. The use of high-strength Grade 80 reinforcement can reduce the number of bars required in construction, likely reducing congestion, thereby reducing construction time. Current design guidelines only allow the use of A706 Grade 60 reinforcing bars in seismic critical members (SCMs), while allowing the use of straight A706 Grade 80 bars only in capacity protected members. The use of high-strength large-diameter bars in SCMs requires experimental validation since extrapolation of current prescriptive requirements for Grade 60 reinforcement cannot always be deemed satisfactory or appropriate. The research work presented herein comprises of a comprehensive investigation, which addresses, at the bar, bar-to-concrete, and at the component levels, the main areas of research needed to implement the use of ASTM A706 Grade 80 high-strength reinforcement into bridge seismic design practice, and presents findings from proof-of-concept experiments in support of this implementation.
This dissertation presents an experimental and analytical investigation to characterize the response of large-diameter ASTM A706 Grade 80 reinforcement embedded in confined concrete, replicating the boundary conditions of bars developed into extended shafts, bent caps, and footings. The equivalent strain penetration term for this type of reinforcement, which is used to calculate the analytical plastic hinge length of columns, is determined and recommendations are provided to more closely represent experimentally measured results.
The proof-of-concept experiments supporting the implementation of high-strength Grade 80 reinforcement in future design codes consist of a full-scale bridge column extending into an enlarged Type II pile shaft, and a ¾-scale exterior column of a multi-column bent cap connection, both reinforced entirely with ASTM A706 Grade 80 bars. Findings from these experiments are used to calibrate and validate detailed finite element models which can be used to aid future bridge design practice.
At the bar level, to characterize the buckling behavior, post-buckling fracture mechanism, and cyclic fatigue life of large-diameter Grade 80 reinforcing bars, a set of experiments were performed on both commonly available ASTM A706 Grade 80 bars and newly developed Grade 80 bars with a more smoothed-rib-radius. An extensive finite element study is also conducted to develop a simplified equation for design to prevent premature plastic buckling and subsequent fracture of column longitudinal reinforcement.