Mitigation of size effect on shear capacity of steel fiber-reinforced concrete beams without stirrups

As modern buildings become increasingly large-scale, reliable shear design of large reinforced concrete beams has become essential. However, the pronounced size effect often leads to an excessive reduction in their shear strength, necessitating effective mitigation strategies. This study experimentally and statistically evaluated the mitigation of the size effect in steel fiber-reinforced concrete (SFRC) beams without stirrups. Fifteen beam specimens were fabricated and tested under four-point loading, with key variables including steel fiber volume fraction, effective depth, longitudinal reinforcement ratio, and presence or absence of stirrups. The experimental results demonstrated that, unlike plain concrete (PC) specimens, SFRC beams exhibited increased shear strength with increasing effective depth. Additionally, the incorporation of steel fibers influenced the depth of the compression zone, reducing the sensitivity of shear strength to the longitudinal reinforcement ratio. To extend these findings, a database comprising 258 SFRC beam tests was compiled, and a multivariate nonlinear weighted regression analysis was conducted to quantify the reduction in the size effect. As a result, the effective depth at which the size effect becomes significant ( d 0 ) was found to increase by approximately two to five times. Therefore, SFRC not only enhances shear capacity but also significantly reduces the size effect, making it suitable for large-scale structural applications.