Deformation analysis of UHPC beams under shear loading: A flexure-shear coupled method

To investigate the shear behavior and establish a deformation prediction method for UHPC beams, seven UHPC beams with varying steel fiber contents, stirrup ratios, and longitudinal reinforcement ratios were tested under shear loading. The failure modes, load-deflection responses, and stirrup strain evolution were systematically analyzed. Test observations revealed a dominant shear-compression failure mode for all specimens. The experimental results indicate that the shear capacity and deformation response are governed by the combined effects of steel fibers, compressive strength, and reinforcement configuration. For specimens with identical geometric and reinforcement parameters, increasing the steel fiber content from 1.0% to 3.0% was accompanied by increases of approximately 52.8% and 95.9% in the ultimate shear capacity and peak deflection, respectively. However, because the compressive strength also varied among the mixtures, these observed differences should not be attributed solely to the increase in fiber content. Steel fibers effectively restrained diagonal crack propagation, reduced crack widths, and delayed post-cracking stiffness degradation, thereby contributing to improved shear resistance and deformation capacity. Higher stirrup ratios were also associated with improved crack control and shear resistance. Based on the experimental observations, a flexure-shear coupling method was developed by considering flexural deformation, shear deformation, stiffness degradation, and the synergistic interaction between steel fibers and stirrups. Furthermore, an analytical method for estimating shear strength was developed by considering the contributions of both truss and arch resisting mechanisms. Evaluation against experimental data showed that the average predicted-to-measured ratios for cracking shear force, total deflection at shear cracking, ultimate shear capacity, and peak deflection were 1.08, 0.99, 1.02, and 1.04, respectively, with corresponding coefficients of variation of 0.10, 0.13, 0.05, and 0.10. The proposed method provides a useful analytical framework for estimating the deformation and assessing the shear response of UHPC beams within the investigated parameter ranges.