Behavior and design of short reinforced ultra-high performance concrete columns

Existing strength design framework for UHPC structural components has focused primarily on the flexural and shear design of UHPC beams. To further facilitate the deployment of UHPC within primary components (i.e., beams and columns), this study presents the component and section-level mechanics necessary for informing the strength design of UHPC columns. The existing strength design framework for UHPC components is explicitly extended to components subjected to axial-flexural and pure concentric axial loading. The extended design framework is evaluated for its accuracy and statistical robustness by comparing predicted strength values to the strength values of 79 experimental UHPC columns. Considerations for service and ultimate limit states are presented, with newly derived service limit states that account for the unique mechanical behavior of UHPC in compression as well as the statistical variations within the design framework. In addition, a simple effective lateral stiffness expression, which accounts for cracking, is developed to enable a more accurate estimation of load demands on axially loaded UHPC components. The results of this study highlight the high predictive performance of the extended design framework with mean predicted-to-experimental strength ratios of 0.96 and 1.00 for columns subject to axial-flexural and pure axial loading, respectively. Moreover, the influence of conventional confining reinforcement was found to be negligible on the strength prediction of columns under axial-flexural loading between axial load ratios of 4%–36%. However, conventional and fiber reinforcement ratios were found to influence the nominal axial strength capacity of columns under pure axial loading. Lastly, shortcomings and concerns within the extended design framework are discussed and highlighted.