Title: Reinforced Concrete Coupling Beams with Axial Restraint

Author(s): Baha’a Al-Khateeb and Christopher J. Motter

Publication: Structural Journal

Volume:

Issue:

Appears on pages(s):

Keywords: axial restraint; coupling beam; earthquake; link beam; reinforced concrete; seismic; shear wall; structural wall

DOI: 10.14359/51742135

Date: 7/30/2024

Abstract:
Seven one-half-scale reinforced concrete coupling beams, designed using ACI 318-19, were tested with constant stiffness axial restraint. Test variables were span-to-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 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 larger diagonal bar diameters, which corresponded to a larger reinforcement ratio and a larger ratio of transverse reinforcement spacing to diagonal bar diameter. For the diagonally reinforced test beams, the maximum measured shear strength reached as high as 2.4 times the nominal shear strength computing using ACI 318-19 and exceeded the limit on nominal shear strength by more than a factor of 2.0 in the test with the smallest span-to-depth ratio. Based on strut-and-tie behavior, modifications to the ACI 318-19 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 direction to shear strength calculated using the modified equation ranged from 1.16 to 1.33. For the diagonally reinforced beams, a larger span-to-depth ratio, bar size, and reinforcement ratio were associated with larger rotation at yielding and larger effective stiffness.