Title:
Nonlinear Modeling Parameters and Acceptance Criteria for Reinforced Concrete Coupling Beams
Author(s):
Christopher Motter, Baha’a Al-Khateeb, Dakota Saathoff
Publication:
CRC
Volume:
Issue:
Appears on pages(s):
126
Keywords:
DOI:
Date:
5/1/2022
Abstract:
A database of diagonally reinforced concrete coupling beam tests was formulated and used to assess strength, stiffness, and deformation capacity. The shear strength equation provided in ACI 318-19 considers only the transverse strength of the diagonal bars and was found to be overly conservative. A new equation that includes shear strength of concrete and transverse reinforcement was found to provide a better fit to test data. Existing recommendations were found to underestimate deformation capacity. A plastic hinge model that includes bond slip was formulated to estimate deformation capacity based on strain at crushing of confined concrete and strain at onset of diagonal reinforcement buckling. Favorable agreement was found between the model and test data. An empirical equation based on ratio of diagonal bar diameter to section depth and ratio of spacing of transverse reinforcement to diagonal bar diameter was fit to data. The empirical equation led to reduced scatter relative to the plastic hinge model. A parametric study was conducted using the plastic hinge model and the empirical equation, and reasonable agreement was found between the two models over this practical range of parameters. New recommendations for determining the deformation capacity of diagonally reinforced concrete coupling beams are provided. Damage patterns observed after the 2010-2011 Canterbury earthquake sequence in New Zealand showed instances in which coupled walls did not behave as intended in design, as plastic hinges formed at the base of the wall piers but not at the beam ends. A potential cause was coupling beam axial restraint from walls and floors increasing the strength of the coupling beams. The deformation capacity model was not intended to predict axial elongation and capture the resulting influence of axial restraint on coupling beam deformation capacity.To better understand the effect of axial restraint on coupling beam strength and deformation capacity, seven one-half-scale reinforced concrete coupling beams, designed using ACI 318-19, were constructed and tested under constant axial compressive stiffness. Test variables were reinforcement configuration (conventional or diagonal), span-to-depth ratio, primary reinforcement ratio and bar diameter, and level of axial restraint. Six beams consisted of three identical pairs, with the two beams in each pair tested at a different level of constant stiffness axial restraint. The conventionally reinforced beams were observed to yield in shear. The onset of significant strength degradation in the diagonally reinforced beams was associated with buckling of diagonal reinforcement rather than crushing of confined concrete. As a result, deformation capacity was more sensitive to variation in the ratio of transverse reinforcement spacing to diagonal bar diameter, s/db, than variation in axial compression. The diagonally reinforced beams with #4 and #6 reinforcement had deformation capacity of at least 6% and 10%, respectively. The deformation capacity was at least 15% larger than that predicted using the empirical model, suggesting that axial restraint did not result in a reduction of deformation capacity. It is recommended to design coupled walls for expected coupling beam demands. Results from this study provide experimentally derived values for the level of overstrength in diagonally reinforced coupling beams with axial restraint. Values of constant axial compressive stiffness used in the tests ranged from 0.69Agf’c to 1.38Agf’c per inch and led to development of peak compressive stresses of 0.27-0.51Agf’c. This resulted in an increase in beam strength as high as 120% above nominal moment strength when computed without consideration of axial restraint and as high as 53% when computed at peak measured axial force. The difference between Mn computed at peak measured axial force and computed without axial force suggested an increase in beam shear strength due to axial restraint as high as 64%, with larger values associated with lower longitudinal reinforcement ratio.