Diagonally-Reinforced Concrete Coupling Beams with High-Strength Steel Bars

The use of high-strength steel in diagonally reinforced coupling beams was investigated with the aims of minimizing reinforcement congestion and increasing the maximum permissible design shear stress without compromising behavior under large displacement reversals. Five large-scale diagonally reinforced concrete coupling beam specimens with clear span-to-depth ratios of 1.9 were tested under fully reversed cyclic loads. The primary variables were yield stress of the diagonal reinforcement (60 and 120 ksi [420 and 830 MPa]), target beam shear stress (10 and 15√(f_c^' ) psi [0.83 and 1.25√(f_c^' ) MPa]), length of the secondary (non-diagonal) longitudinal reinforcement, and axial restraint. All specimens had the same nominal concrete compressive strength and beam dimensions. Chord rotation capacities exhibited by the specimens with Grade 120 (830) reinforcement were between 5.1 and 5.6%, less than that of the control specimen with Grade 60 (420) diagonal reinforcement (7.1%). Neither development of secondary reinforcement nor increases in design shear stress affected specimen chord rotation capacity. The axially-restrained specimen with Grade 120 (830) diagonal reinforcement showed the same chord rotation capacity as a similar specimen without axial restraint, but 14% larger strength. In specimens with secondary longitudinal reinforcement extended into the wall (such that the embedment length exceeded the calculated development length), the localization of damage evident along the beam-wall interface in tests of specimens with bars terminating near the wall face was not observed. Although damage was more distributed throughout the beam span, deformation capacity was not increased. Among the specimens, it was shown that the initial stiffness, area of the shear force-chord rotation hysteresis cycles, and residual chord rotation at zero shear force changed in inverse proportion to the diagonal bar yield stress. A database of results from tests of diagonally reinforced coupling beams was compiled and used to evaluate the sensitivity of coupling beam chord rotation capacity to a range of variables. Variables included aspect ratio, reinforcement grade, transverse confinement reinforcement (type, spacing, and ratio), shear stress, and length of secondary (non-diagonal) reinforcement (whether terminated near the beam-wall interface or developed into the wall). An equation was proposed for calculating coupling beam chord rotation capacity as a function of beam clear span-to-height ratio and the ratio of hoop spacing to diagonal bar diameter. Chord rotation capacity was not correlated with other variables. Modifications are also proposed to the stiffness and deformation capacity modeling parameters recommended in ASCE 41-17 and ACI 369.1-17 for diagonally reinforced coupling beams to account for reinforcement grade

Diagonally Reinforced Concrete Coupling Beams with Grade 120 (830) High-Strength Steel Bars

Four large-scale coupling beams were tested under fully reversed cyclic loads to investigate the effects on behavior of diagonal bar grade (60 or 120 [420 or 830]), beam shear stress (9.5 or 14√fc′, psi [0.79 or 1.17√fc′, MPa]), and longitudinal bar detailing (either terminated near the beam-wall interface or developed into the walls). Coupling beam chord rotation capacity was 7.1% for the beam with Grade 60 (420) bars and between 5.1 and 5.6% for the beams with Grade 120 (830) bars, a difference likely due to having hoops spaced at 3.4db and 4db for Grade 60 and 120 (420 and 830) bars, where db is the diagonal bar diameter. Effective stiffness, energy dissipation, and residual chord rotations were approximately inversely proportional to bar grade. Developing the secondary longitudinal reinforcement reduced rotation demands at beam ends but did not improve deformation capacity. Beam shear stress did not affect beam chord rotation capacity

Diagonally-Reinforced Concrete Coupling Beams with High-Strength Steel Bars

The use of high-strength steel in diagonally reinforced coupling beams was investigated with the aims of minimizing reinforcement congestion and increasing the maximum permissible design shear stress without compromising behavior under large displacement reversals. Five large-scale diagonally reinforced concrete coupling beam specimens with clear span-to-depth ratios of 1.9 were tested under fully reversed cyclic loads. The primary variables were yield stress of the diagonal reinforcement (60 and 120 ksi [420 and 830 MPa]), target beam shear stress (1.0 and 1.5 times the ACI Building Code limit), length of the secondary (non-diagonal) longitudinal reinforcement, and axial restraint. All specimens had the same nominal concrete compressive strength and beam dimensions. Chord rotation capacities exhibited by the specimens with Grade 120 (830) reinforcement were between 5.1 and 5.6%, less than that of the control specimen with Grade 60 (420) diagonal reinforcement (7.1%). Neither development of secondary reinforcement nor increases in design shear stress affected specimen chord rotation capacity. The axially-restrained specimen with Grade 120 (830) diagonal reinforcement showed the same chord rotation capacity as a similar specimen without axial restraint, but 14% larger strength. In specimens with secondary longitudinal reinforcement extended into the wall (such that the embedment length exceeded the calculated development length), the localization of damage evident along the beam-wall interface in tests of specimens with bars terminating near the wall face was not observed. Although damage was more distributed throughout the beam span, deformation capacity was not increased. Among the specimens, it was shown that the initial stiffness, area of the shear force-chord rotation hysteresis cycles, and residual chord rotation at zero shear force changed in inverse proportion to the diagonal bar yield stress. A database of results from tests of diagonally reinforced coupling beams was compiled and used to evaluate the sensitivity of coupling beam chord rotation capacity to a range of variables. Variables included aspect ratio, reinforcement grade, transverse confinement reinforcement (type, spacing, and ratio), shear stress, and length of secondary (non-diagonal) reinforcement (whether terminated near the beam-wall interface or developed into the wall). An equation was proposed for calculating coupling beam chord rotation capacity as a function of beam clear span-to-height ratio and the ratio of hoop spacing to diagonal bar diameter. Chord rotation capacity was not correlated with other variables. Modifications are also proposed to the stiffness and deformation capacity modeling parameters recommended in ASCE 41-17 and ACI 369.1-17 for diagonally reinforced coupling beams to account for reinforcement grade

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√fc′ psi (0.83√fc′ 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

High-Strength Steel Bars in Earthquake-Resistant T-Shaped Concrete Walls

The object of this study was to determine experimentally the influence of selected reinforcing steel mechanical properties on wall deformation capacity. Four large-scale T-shaped reinforced concrete wall specimens with different types of reinforcement were subjected to reversed cyclic displacements. The primary variables were the yield strength () and the tensile-to-yield strength ratio (/) of the reinforcing bars. The study also aimed to identify the minimum uniform elongation () and fracture elongation () required of high-strength reinforcement for use in earthquake-resistant concrete structures. Test data are presented from four walls, T1 with conventional Grade 60 (420) reinforcement and T2, T3, and T4 with high-strength Grade 100 (690) reinforcement. The flexural reinforcement consisted of No. 6 (19) bars inside confined boundary elements and No. 4 (13) bars elsewhere. Confining reinforcement in boundary elements consisted of No. 3 (10) hoops and crossties of the same grade as the flexural reinforcement. Wall T1 had / of 1.34 and 1.39 for the No. 6 (19) and No. 4 (13) bars, respectively. Walls with Grade 100 (690) reinforcement had / of 1.15 and 1.10 for T2, 1.23 and 1.21 for T3, and 1.36 and 1.20 for T4. All walls were loaded with a shear span-to-depth ratio of 3 and had the same nominal dimensions and concrete compressive strength (8 ksi or 55 MPa). Axial load was limited to the self-weight of the wall and testing apparatus. The walls were designed to have nearly the same nominal flexural strength. Flexural yielding controlled the lateral strength of the walls, inducing an average shear stress up to 3.5√′, psi (0.29√′, MPa). To ensure large tensile strain demands in one of the loading directions, the neutral axis depth at nominal flexural strength did not exceed the thickness of the flange. Design of the walls complied with ACI Building Code (ACI 318-14) requirements for special structural walls with additional detailing requirements applied based on ATC 115. Walls designed for a target flexural strength using Grade 60 (420) or Grade 100 (690) reinforcement, with similar / for the primary flexural reinforcement, had similar strength and deformation capacity (defined as the drift cycle completed before a 20% loss of lateral strength). The limited test data indicate that walls with low axial force and reinforcement that satisfies tensile-to-yield strength ratio (/) ≥ 1.2, uniform elongation () ≥ 6%, and fracture elongation () ≥ 10% exhibit a minimum drift ratio capacity of 3%. Walls T1, T2, T3, and T4 exhibited drift ratio capacities of 3.7, 1.8, 3.0, and 3.9%, respectively. Moment-curvature analyses were conducted to evaluate the use of the plastic hinge model for estimating the deformation capacity of the walls and the maximum strain demands. The use of the plastic hinge model was conservative for estimating wall deformation capacity with simple rules for the plastic hinge length depending on whether deformations due to shear and strain penetration are considered. However, the plastic hinge model did not consistently provide conservative estimates of the maximum strain demands.Charles Pankow FoundationConcrete Research Council, American Concrete Institute Foundatio

Chord Rotation Capacity and Strength of Diagonally Reinforced Concrete Coupling Beams

A database of results from 27 tests of diagonally reinforced concrete coupling beams was analyzed to develop improved force-deformation envelopes (backbone curves) for modeling and analysis of coupling beams. The database, which was selected from a larger set of 60 test results, comprises specimens that generally satisfy ACI 318-19 requirements. The analyses show that the chord rotation capacity of diagonally reinforced concrete coupling beams compliant with ACI 318-19 is closely correlated with beam clear span-to-overall depth ratio and, to a lesser extent, the ratio of hoop spacing to diagonal bar diameter. A simple expression is proposed for estimating beam chord rotation capacity. Coupling beam strength was shown to be more accurately estimated from flexural strength calculations at beam ends than other methods. Recommendations are made for obtaining more accurate backbone curves in terms of chord rotation capacity, strength, and stiffness