Experimental and Theoretical Investigation of Shear Transfer Mechanisms in GFRP-RC Deep Beams without Stirrups

This study focuses on the shear transfer mechanisms of glass fiber–reinforced polymer (GFRP)-RC deep beams without stirrups and proposes a unified shear model applicable to both steel- and FRP-RC deep beams. Seven concrete deep beams reinforced with GFRP and steel bars were tested under four-point bending. The influences of reinforcement material, shear span-to-depth ratio, reinforcement ratio, and effective depth on the shear performance of the beams were studied. The test results indicated that, after the formation of the critical shear crack, the shear force was directly transferred from the loading point to the support, creating a strut-and-tie action. GFRP-RC deep beams without stirrups exhibited a significant size effect on shear strength. The contributions of different shear transfer mechanisms were quantified based on the kinematics of the critical shear crack measured using two-dimensional digital image correlation. At the peak load, the total contributions of aggregate interlock, dowel action, and residual tensile stresses in the fracture process zone to the shear strength of both GFRP- and steel-RC deep beams were <4%. A large amount of the shear force was transferred through the concrete strut. Because of the influence of the critical shear crack, concrete crushing in the pure bending region of the GFRP-RC deep beams occurred at loads lower than the flexural capacity. A unified shear model was established to predict the shear strength of both steel- and FRP-RC deep beams. The model was validated by comparing its predictions with the test results of 265 beams. The unified shear model accurately predicted the shear strength, with a mean value of the tested-to-predicted shear strength ratio of 1.04 and a coefficient of variation of 0.23. Furthermore, the proposed model effectively reflected the influence of the shear span-to-depth ratio and the effective depth on the shear strength.