Reinforced Concrete Coupling Beams with Axial Restraint

Seven one-half-scale reinforced concrete coupling beams, designed using ACI 318-19, were tested with constant stiffness axial restraint. The test variables were the span-depth ratio, reinforcement configuration (conventional or diagonal), primary reinforcement ratio and bar diameter, and level of axial restraint. Six beams consisted of three nominally identical pairs, with the two beams in each pair tested at a different level of axial restraint. The two conventionally reinforced beams reached peak strength at 2.0 and 3.0% chord rotation and experienced rapid post-peak strength degradation with the opening of diagonal cracks and the formation of splitting cracks along the longitudinal reinforcement. Strength degradation in diagonally reinforced beams initiated with buckling of diagonal reinforcement, and variation in axial restraint on identical pairs of beams did not lead to a significant difference in deformation capacity. Deformation capacity was larger for beams with a larger diagonal bar diameter, which corresponded to a larger reinforcement ratio and a larger ratio of transverse reinforcement spacing to diagonal bar diameter (s/db). For the diagonally reinforced test beams, the maximum measured shear strength reached as high as 2.4 times the nominal shear strength computed using ____ ACI 318-19 and exceeded the 0.83 root fc ' Acw MPa (10 limit on nominal shear strength by more than a factor of 2.0 in the test with the smallest span-depth ratio. Based on strut-and-tie behavior, modifications to the ACI318-19 equation to include axial load were examined. When the location of the compressive strut and tension tie at the beam ends was consistent with nominal moment calculations, the resulting ratio of the average maximum measured shear strength in the positive and negative loading directions to shear strength calculated using the modified equation ranged from 1.16 to 1.33. For the diagonally reinforced beams, a larger span- depth ratio, bar size, and reinforcement ratio were associated with larger rotation at yielding and larger effective flexural rigidity. ____ root fc ' Acw psi)