Enhanced Modified Compression Field Theory-Based Shear Capacity Prediction of Ultra-High-Performance Concrete (UHPC) Beams with Steel Fibers and Stirrups

Most design provisions for the shear capacity of ultra-high-performance concrete (UHPC) beams require a measured tensile input, such as the localization tensile stress or a residual flexural strength, that is unavailable before the mix is finalized. This study proposes an Enhanced Modified Compression Field Theory (EMCFT) that predicts the shear capacity from inputs available at the design stage, with the fiber volume fraction (ρf), the fiber aspect ratio (lf/df), and the compressive strength (fc′) in place of a measured tensile property. The principal tensile stress is decomposed into a composite tension stiffening baseline and a separable fiber bridging term that depends on the fiber reinforcing index ψf=ρf·lf/df. The fiber coefficients are determined from independent pullout mechanics as αf=0.19 and αp=135, with αp matched to the crack width at 50% fiber stress reduction. Consequently, the model contains only two fitted parameters, namely the composite baseline coefficient αm=1.085 (95% confidence interval [0.988, 1.181]) and an I-section flange participation factor k=0.307 (95% confidence interval [0.184, 0.470]). For 45 beams, the leave-one-out cross-validated mean experimental-to-predicted ratio is 1.007, with a coefficient of variation of 9.3%.