Diagonally Reinforced Concrete Coupling Beams: Effects of Axial Restraint

Two pairs of nominally identical large-scale coupling beam specimens were tested under reversed cyclic displacements. Within each pair, one specimen was free to elongate and the other had resistance to elongation during testing. The specimens had clear span-to-overall-depth ratios of 1.9, a nominal concrete compressive strength of 6000 psi (42 MPa), Grade 60 or 120 (420 or 830) diagonal bars, and nominal shear stresses near the ACI Building Code (ACI 318) limit of 10 root f(c)' psi (0.83 root f(c)' MPa). Passive axial restraint resulted in beam axial forces and was correlated with higher coupling beam strength, lower chord rotation capacity, earlier diagonal bar buckling, and greater damage. The importance of these effects increased with the magnitude of the induced axial force. The ACI equation for coupling beam nominal strength (based on the area, yield stress, and inclination of diagonal bars) underestimated beam strength by up to 80%, whereas estimates based on flexural strength were substantially more accurate and allowed consideration of axial force effects.