Mechanical model for the shear strength of steel fiber reinforced concrete (SFRC) beams without stirrups

Numerous studies have shown that adding steel fibers to the concrete mixture improves both the shear strength and ductility of reinforced concrete beams. Most current shear design formulations for steel fiber reinforced concrete beams are empirically based and their predictions provide acceptable results when compared with tests results, but only in limited ranges of the parameters involved. On the other hand, many shear theoretical models suggested in the literature are derived by extending previous formulations for conventional reinforced concrete beams, just introducing the stresses transferred across the critical shear crack. Since the effects of steel fibers on the others shear resisting mechanisms are not accounted for, “adjusting” empirical factors must be used to fit the experimental results. There is, therefore, the need for developing mechanical models capable to rationally account for the effects of steel fibers on the global shear strength and on each shear resisting mechanisms. In this paper, the previously derived and validated Multi Action Shear Model for RC elements is extended to steel fiber reinforced concrete beams without stirrups. The effects of steel fibers on each resisting mechanism have been identified and incorporated in the formulation of each shear component and in the Multi-Action Shear Model equilibrium equations. The residual tensile stresses of fibrous concrete, obtained through a simple formulation, has been used to: capture the enhancement of the compression chord contribution; the shear transferred by bridging effect of the fibres along the critical shear crack and by dowel action. The proposed model is shown to properly estimate the shear resistance of a large set of available test data, being able to account for most influencing parameters, like fibers types and amounts, concrete strength, longitudinal reinforcement ratios and beam geometry.