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The use of ductile concrete materials such as High Performance Fiber Reinforced Cementitious Composites (HPFRCCs) within plastic hinge regions of structural components has garnered research interest in order to improve the seismic resistance of reinforced concrete structures. While experimental and numerical results appear promising in reducing component damage and probability of system level collapse, accurate nonlinear analysis tools capable of capturing the influence of axial load well into a component’s inelastic regime is needed. In this study, a series of 180 high fidelity numerical simulations of HPFRCC beam–column elements are simulated and used to calibrate new plastic hinge length expressions for concentrated and distributed plasticity models for use in system level structural analysis. The numerical models cover a range of HPFRCC material properties, reinforcement ratios, shear span lengths, and axial load levels. The ability of the newly developed expressions to predict component inelastic rotations are subsequently compared to hinge length expressions in the literature and the inelastic rotations of 47 experimental components. The results of this study provide new insights into the effects of axial load on the plastic hinge behavior of HPFRCC components, significantly improves on the accuracy of past plastic hinge length expressions allowing for more accurate modeling of HPFRCC component responses and system level behavior. 

While widely adopted prescriptive-based design practices work to limit the probability of complete collapse, relatively little attention and emphasis is placed on the damage levels and functionality of structures after seismic events. High-performance fiber reinforced cementitious composites reinforced with steel (R/HPFRCCs) have been of growing interest for such seismic applications to improve structural level damage and performance. In order to progress the implementation of these materials at the structural level, a systematic approach toward understanding the mechanics of R/HPFRCC columns is warranted. Therefore, in this study, an existing numerical framework for R/HPFRCC beams was extended to the analysis of columns across a range of materials, reinforcement ratios, and axial load levels to evaluate the change in component level response. It was observed that axial load can considerably increase the nominal bending moment capacity of R/HPFRCC columns as well as affect the drift capacity. A shift from failure on the tension side of the element (e.g., reinforcement fracture) to the compression side (e.g., crushing of the HPFRCC) of the numerically tested column occurred between an axial load ratio of 10 and 20%. Lastly, changes in bond stress due to the material level tensile strength were shown to considerably impact the ultimate component drift capacity. 

The unique mechanical properties of ultra-high performance concrete (UHPC) causes changes in failure modes and ductility in reinforced components. Numerous experiments have shown these materials, and others with similar ductile characteristics in tension, can improve the damage tolerance, strength, and ductility of members subjected to large deformations from seismic loading and similar extreme conditions. The use of these materials, however, has not been systematically studied to understand their application at a system-level performance and design procedures have been complicated due to their unconventional failure mechanism. This project aims to fill this gap by testing a targeted set of components subjected to combined effects of axial loads and bending with variations in axial load ratio and longitudinal reinforcement ratio. Additional experiments are planned to compare performance across other ductile concrete materials with variations in mechanical properties. The experimental results including load-deformation, reinforcement strain, concrete surface strain will be used to understand the parameters that have the highest influence on plastic hinge length and moment-rotation response which can ultimately help to validate analytical models against experiments based on these key parameters. 

Researchers have explored the high energy absorption capacity and strength of UHPC materials to improve the seismic performance of structural components. Experimental results in the literature of reinforced UHPC members have indicated superior damage tolerance, higher strength and deformation capacities, and lower potential for collapse across a range of structural components. Investigations into the underlying failure mechanisms have highlighted the significance of the synergy between material tensile strength and reinforcement properties on member flexure response. Although research into the seismic application of reinforced UHPC continues to expand, relatively little is known about the effects of varying axial load on the plastic hinge response of beam-column elements across a range of UHPC tensile properties and reinforcement levels. Therefore, in this study, the effects of varying tensile properties on beam-column elements through numerical simulations across a range of axial load ratios were investigated. Two dimensional numerical models accounting for material nonlinearities (e.g., bond-slip, UHPC tensile strength and strain capacity) were used to capture component responses. Trends in the moment-drift responses and plastic hinge lengths have highlighted the diminishing returns of using higher fiber volume percentages (2%) as higher axial loads tend to relieve tensile demands. Additionally, existing plastic hinge length expressions for RC components were found to over-predict hinge length consistently while those developed for HPFRCC components accurately predict plastic hinge lengths at low axial load levels.