Hysteretic Behavior of Conventionally Reinforced Concrete Coupling Beams in Reinforced Concrete Coupled Shear Wall
© The Author(s) 2017
Received: 2 August 2017
Accepted: 23 October 2017
Published: 7 December 2017
This paper presents the experimental results of four full-scale coupling beams in which only horizontal reinforcements are placed, without diagonal reinforcements, with the aim to develop reinforcement details for coupling beams used in connecting side walls in a wall-slab structural system. Each coupling beam specimen was designed according to the deep-beam design procedure that does not use diagonal reinforcements and that is found in current standards. Two cases for basic deep-beam design specimens were investigated wherein (1) U-type reinforcement was added to prevent sliding shear failure of the joints and (2) horizontal intermediate reinforcements were placed. The coupling beam specimens were fabricated with a shear span-to-depth ratio (aspect ratio) of 1.68 and were connected to walls only by horizontal reinforcements, i.e., without diagonal reinforcement. The experimental results indicate that the strength of the beams was about 1.5 times the designed strength of a strut-and-tie model, which suggests that the model is available for predicting the strength of coupling beams with conventional reinforcement layouts such as horizontal and transverse reinforcement bars. The deformation capacity of these conventionally reinforced concrete coupling beams ranged from 1.48 to 3.47% in accordance with the reinforcement layouts of the beams. Therefore, this study found that the performance of conventionally reinforced concrete coupling beams with an aspect ratio of 1.68 can be controlled through the implementation of reinforcement details that include U-type reinforcement and the anchorage of intermediate horizontal bars.
The basic concept behind reinforcements for coupling beams for a special structural wall is to confine the beam using closely spaced transverse reinforcements to prevent compressive buckling of the diagonal reinforcements. Accordingly, the currently used codes (ACI 2014; Eurocode 8 2004; Korea Concrete Institute (KCI) 2012) for different areas of the world stipulate that at least four diagonal reinforcements should be placed and laterally confined. Such construction details, however, make actual construction difficult at field sites. Consequently, the current ACI code (2014) requires that lateral confinement of coupling beams should be used instead of diagonal reinforcements, as shown in Fig. 3b, which were recommended in the ACI Committee 318 2011 edition of the ACI code (ACI 2011). These construction details, however, still pose difficulties in construction due to the excessive use of lateral confinement reinforcements and diagonal reinforcements.
In the current standards (ACI 2014; Eurocode 8 2004; KCI 2012), all the shear forces and moments that act on coupling beams are assumed to be borne only by the diagonal reinforcements. Therefore, transverse and longitudinal reinforcements that confine the diagonal reinforcements are not reflected in the design at all. Consequently, all the reinforcements that are placed in the longitudinal direction of the beam are cut off at the interface, without being affixed to the walls. In other words, plastic behavior of the diagonal reinforcements is encouraged in order to dominate the behavior of the coupling beams at the ends of such beams where the moment and shear forces are greatest. Such construction details may induce sufficient plastic behavior of the coupling beam but may pose difficulties in construction, as stated earlier, when the shear and longitudinal reinforcements that are placed for the lateral confinement of the diagonal reinforcements are excessive in quantity. In particular, in the case of a wall-slab type apartment building with no columns, the walls may be as thin as 200 mm or 300 mm, making rebar placement especially difficult. Furthermore, the mandatory placement of diagonal reinforcements, which pose difficulty in construction, makes it very difficult to perform rebar work in the actual field, which in turn may lead to faulty construction.
A performance-based design method (TBI Guidelines Working Group 2010) was developed recently for the design of reinforced concrete (RC) members. Thus, the development of construction details for coupling beams that can be selected according to the design requirements also is needed. Specifically, depending on the shear span-to-depth ratio and shear stress of the coupling beam, diagonal reinforcements should be used if high deformation capacity is required. If not, alternative details must be developed. Because such walls have high stiffness values, the shearing rate of the lateral force is also high, but the actual deformation is small. For example, a wall-slab type of structure with no columns has numerous long walls that experience little deformation. Therefore, the amount of deformation that is required for the coupling beams that connect these walls likewise becomes relatively small. In other words, high deformation capacity is not required for the coupling beams; thus, suitable construction details for coupling beams should be investigated.
In order to develop such reinforcement details for coupling beams that connect walls with less deformation in a wall-slab structural system, this study sought to design coupling beams in which only conventional reinforcements are placed, without diagonal reinforcements, and thus to examine the hysteretic behavior of the coupling beams based on the results of cyclic loading tests for proposed reinforcement details.
2 Review of Previous Studies
The significant damage of conventional RC coupling beams during the Alaska earthquake that occurred in 1964 showed that coupling beams with orthogonal reinforcements that consist of traditional longitudinal and transverse reinforcements are susceptible to severe damage under large shear reversals. Paulay and Binney (1974) proposed the idea of placing two intersecting diagonal reinforcement groups, confined by closely-spaced transverse reinforcements, for shear-dominant RC coupling beams. Many researchers (Barney 1976; Shiu et al. 1978; Tegos and Penelis 1988; Tassios et al. 1996; Galano and Vignoli 2000; Kwan and Zhao 2002; Fortney 2005; Naish 2010) have shown that diagonal reinforcement groups are effective in improving the strength, ductility, and energy dissipation capacity of RC short coupling beams. Diagonal reinforcement groups are generally recognized as the most effective type of reinforcement detail for providing ductile behavior of RC short coupling beams that have a span-to-depth ratio of less than or equal to 2.0. Placing two groups of diagonal bars in RC coupling beams that have an aspect ratio of less than 4.0 has been specified since 1995 in the ACI Building Code (ACI 318-95 1995), which specifies that each group of diagonal bars shall have the same quantity as the transverse reinforcements for the columns in the special moment frame to suppress the buckling of each diagonal-bar group. However, the placement of transverse bars around the diagonal reinforcement groups, as specified in ACI 318-05, leads to significant construction difficulties. In order to overcome such difficulties, ACI 318-08 includes an alternative detail option where transverse reinforcement is placed around the beam’s full section, without directly placing transverse reinforcement around the diagonal bar groups. However, it is almost impossible to place diagonal reinforcement groups at a right angle to all the required transverse bars required for the diagonal confinement and full-section confinement found in ACI 318-05 (2005) and ACI 318-08 (2008), respectively (Hajyalikhan 2015).
To resolve these construction difficulties, several alternative construction details for RC short coupling beams have been considered, including rhombic reinforcement, diagonal reinforcement without transverse ties, bent-up reinforcement, double beams, and long and short dowel reinforcement layouts (Tegos and Penelis 1988; Tassios et al. 1996; Galano and Vignoli 2000; Hajyalikhan 2015). However, none of these construction details allows for performance that is equivalent to that of coupling beams strengthened with bundled diagonal reinforcements according to the existing details specified in the standards and that significantly improve constructability. Recently, as the building design concept has changed to performance-based design, improving constructability and reducing economic costs have been accomplished by utilizing coupling beams with proper reinforcement details that are based on the required deformation capacity under the design loads rather than based on the aspect ratio, as in the current standard.
More recently, studies have focused on developing construction details for seismic performance evaluation and improvement of coupling beams that are strengthened with horizontal and vertical reinforcements. Breña and Ihtiyar (2010) investigated the effects of different amounts of longitudinal and transverse reinforcement on the seismic behavior of four RC coupling beams and discussed the strength, deformation components, and response parameters that are needed to construct backbone curves for conducting nonlinear analyses of a coupled shear wall system. Hajyalikhan (2015) proposed a simplistic reinforcement scheme that consists of two separate cages that are similar to those used for typical beams in RC special moment frames to minimize the construction problems that are associated with diagonal reinforcement groups. Hajyalikhan (2015) reported that proposed details for RC short coupling beams can transform shear-dominated, brittle behavior into flexure-dominated, ductile behavior. Cai et al. (2016) conducted experimental tests using steel fiber-reinforced concrete (SFRC) coupling beams with conventional reinforcements and proposed a simplified model that applies the Mohr–Coulomb failure criterion to predict the seismic shear strength of SFRC coupling beams. Their test results indicate that the inclusion of steel fibers can enhance the seismic performance of SFRC coupling beams and that their proposed model provides accuracy and reliability. Lim et al. (2016) investigated the seismic performance of intermediate aspect ratio coupling beams using proposed alternatives to mitigate the construction difficulties associated with diagonal reinforcement by combining conventional and diagonal reinforcement construction details. Nabilah and Koh (2017) tested four conventional RC coupling beams with aspect ratios of 2.5 and 3.1 and reported that the shear stiffness of an intermediate length coupling beam was reduced by 0.1% of the initial stiffness value upon the yielding of the reinforcement.
In Korea since the 2000s, numerous researchers have conducted studies to evaluate performance and simplify construction details of coupling beams. For example, Park and Yun (2011) investigated the seismic performance of strain-hardening cement-based composite (SHCC) coupling beams that contained different types of reinforcement. They found that ductile cement-based composites such as SHCC are effective in improving the ductility and strength of shear-dominant coupling beams. Shin et al. (2014) tested three high-performance fiber-reinforced cement composite (HPFRCC) coupling beams with an aspect ratio of 3.5. Their test results showed that HPFRCC greatly contributes to the reduction in crack damage and shear distortion in slender coupling beams. Jang et al. (2015) examined the feasibility of replacing additional transverse reinforcement that is required for short coupling beams that contain 1.5% hooked-end steel fiber. Also, the Korea Land and Housing Institute (2012, 2014) proposed simplified reinforcement details for transversely confined diagonal reinforcement in short coupling beams in coupled shear walls.
3 Deformation Capacity Required for Coupling Beams
The behavioral characteristics of coupling beams are related to the deformation of the lateral forces of the walls to which the beam is connected, as shown in Fig. 1. The beam exhibits the deformation of a double curvature due to the deformation of the left and right walls. In the figure, the drift required for the beam is the same as the story/floor drift of the walls. If the stiffness of the left wall is different from that of the right wall, the drift of the wall with less stiffness will be greater than the one that is more stiff. In this case, it is desirable to consider the drift of the wall that has the large deformation also as the drift of the beam. The drift required for the walls can be determined by the stiffness values of all the walls on the floor if the story drift of the floor is considered the same due to the diaphragm behavior of the floor. However, determining the drift required for each wall separately is preferable in order to evaluate whether the coupled walls provide sufficient ductility with respect to the largest drift. That is, the drift required for the coupling beams cannot be less than that required for the walls.
The ACI code (2007) provides a recommendation for the application of precast concrete walls, which is not covered in the design criteria, to an area that experiences strong earthquakes if the performance can be proven through appropriate performance tests and analyses. In addition, the ACI presents guidelines for related tests to evaluate whether the structural members provide sufficient strength, stiffness, ductility, and energy dissipation capacity via performance testing. These guidelines also are quoted in the National Earthquake Hazards Reduction Program (NEHRP) provisions for seismic regulations for new buildings (NEHRP 450-1 2003).
4 Experiment Plan
4.1 Design of the Specimens
Reinforcement detail at joint
Flexural bars in beam
M nb (kN · m)
Flexural bars at joint
M nj (kN · m)
Shear bars in beam
V n (kN)
All anchored in wall
Stirrup and headed cross tie
6-HD22 + 8-HD10
6-HD22 + 8-HD10
(2-HD13 + 1-HD10)@100
Top and bottom bars are only anchored in wall and other is cut off at joint
4-HD22 + 4-HD16 + 8-HD10
4-HD22 + 4-HD16
2-(HD13 + HD10)@100
All anchored in wall
U-type middle bars
Stirrup and hooked cross tie
6-HD22 + 8-HD10
6-HD22 + 8-HD10 + (U-bar)
(2-HD13 + 1-HD10)@100
All anchored in wall
Stirrup and headed cross tie
For the B-1-H specimen, 3-HD22 (1161.3 mm2) was placed as flexural reinforcement on the upper and lower parts in accordance with the previously described design process. The horizontal reinforcement 8-HD10 and the stirrup HD13@100 were decided using Eqs. (5), (6), and (7) for the minimum reinforcements in the vertical/horizontal direction. The horizontal reinforcement 8-HD10 at the center of the section is the reinforcement for the lateral confinement of the beam rather than for the flexural strength, but this reinforcement was affixed sufficiently to the walls so that it could contribute to the flexure and shear at the joints. In addition, headed reinforcement was used instead of a 90-degree hook to improve the lateral-confinement performance.
The B-1-HA specimen was almost the same as the B-1-H specimen, but its reinforcement for the lateral confinement of the beam was cut off at the beam-wall interface, without being affixed to the walls. To enhance the confinement effect by increasing the number of reinforcements, 2-(HD22 + HD16) (1171.4 mm2) was placed as the upper and lower reinforcements of the beam; these reinforcements were affixed to the walls. Unlike the B-1-H specimen, horizontal reinforcement 8-HD10 was not affixed to the wall, so these reinforcements did not contribute to the flexure at the joints. For the beam stirrup, 2-(HD13 + HD10) was placed at 100-mm intervals.
4.2 Material Properties
Material properties of reinforcement bars.
Design yield strength (MPa)
Test results (MPa)
4.3 Test Method and Measurements
5 Experimental Results
5.1 Cracking and Failure Shape
The failure of the B-1-H specimen revealed that initial cracks occurred along the interface at the corner of the joint between the coupling beam and the upper and lower parts of the wall at the drift of 0.2% and that cracks at the upper right part of the coupling beam increased at 1.0%. At the drift of 1.4%, many horizontal reinforcements yielded, and the width of the cracks increased rapidly as the load increased. Finally, delamination of the concrete cover at the upper part of the coupling beam occurred at the drift of 2.2%. Failure did not occur evenly at the joints of the two walls, but was concentrated only at the part of the joint that was connected to the upper wall.
For the B-1-HA specimen, initial horizontal cracks occurred at the center and at the lower left and upper right corners of the coupling beam at the positive loading of 0.2% drift. At the drift of 0.4%, the inclined cracks progressed at the upper and lower parts and at the central part of the beam. At the drift of 1.0%, the width of the inclined cracks increased, and delamination of the concrete cover occurred at the upper and lower parts of the coupling beam. Finally, at the drift of 3.0%, delamination of the concrete surface occurred at the upper and lower parts of the coupling beam. Then, plastic hinges formed at both ends of the coupling beams.
In the case of the B-2 specimen, initial horizontal cracks occurred at the left interface between the coupling beam and the lower parts of the wall at the positive loading of 85.23 kN, and diagonal cracks began to appear as the load increased. In addition, as the diagonal cracks rapidly increased at the drift of 0.4%, they were found to have been distributed throughout the beam. At the drift of 1.0%, delamination in the concrete surface occurred at the upper left part of the beam, and the width of the diagonal cracks increased by 1.4%. The test was terminated at the drift of 1.8%. As diagonal tension failure occurred at the center of the span, not at the interface with the walls, delamination in the concrete surface near the cracks occurred, leading to brittle fracture.
For the B-2-H specimen, initial cracks occurred at the left interface of the lower part of the wall and the right interface of the upper part of the wall in the first cycle of positive loading at the drift of 0.2%, and diagonal cracks occurred near the center of the coupling beam at 0.4% drift. Finally, the specimen underwent brittle failure as partial delamination of the concrete cover occurred at the center of the coupling beam at the drift of 2.2%. The U-type reinforcement that was placed to prevent sliding shear failure of the joints is thought to have contributed to the flexure of the joint between the coupling beams and the walls so that shear failure occurred in the beam.
5.2 Load–Drift Curves
P y (kN)
δ y (mm)
P u (kN)
δ u (mm)
P f (kN)
δ f (mm)
δ u /δ y
δ f /δ y
P n (kN)
P u /P n
The strength of a coupling beam that is connected to walls with only horizontal reinforcements, without the use of diagonal reinforcements, was calculated based on the strut-and-tie model. As shown in Table 3, this strength value was found to be 1.21–1.64 times higher than the design strength. Except for the B-2 specimen with a significantly lower strength value in the negative direction, the ratios were 1.46–1.64 times higher than the design strength, indicating that the average is about 1.5 times the design strength. The drift of the specimens at the maximum load ranged from 1.13 to 2.11%. Compared to the B-1 series specimens whose horizontal reinforcements of the beams were anchored in the walls, the B2 series specimens whose main bars were anchored in the walls showed low drift percentages. The wall drift of Eq. (2) becomes 1.63% when it is considered as the required drift until failure. When this value is compared with the test results, all the specimens, except for the B-2 specimens, can be said to have exceeded the required drift of 1.63%. Of course, more than 1.5 times the design response displacement of the building is required for an actual building, which needs to be taken into consideration.
5.3 Energy Dissipation Capacity
5.4 Reinforcement Strains
From the comparison of the B-2 and B-2-H specimens tested to investigate the confinement effect of headed cross ties, the cross ties with the standard hook shows a slightly greater strain than the headed cross ties. Therefore, no further increase in the confinement effect is likely when headed bars are used as the horizontal and vertical cross ties.
Coupling beams whose shear span-to-depth ratio was 1.68 and which were connected to walls only by horizontal reinforcements, without diagonal reinforcement, showed strength that is about 1.5 times the design strength for a strut-and-tie model, thus indicating that proper design strength is possible using these construction details. Overall, the deformation capacity was about 2%, which indicates a certain amount of deformation capacity. A pinching phenomenon, however, that occurred after the load reversals indicated a low level of energy dissipation.
Horizontal reinforcements that were anchored in the walls for the lateral confinement of the beam led to an increase in beam strength. Even in cases where part of the horizontal reinforcement was not anchored into the wall, the design strength and a certain degree of ductility capacity were provided, and plastic hinges could be induced completely in the wall-beam joints. However, the strength gradually decreased after reaching the maximum force, which suggests that if the wall and beams are connected only with upper and lower horizontal reinforcements, the overall behavior is dominated by the flexural behavior of the joints, and the stress from the wall may not be transferred properly to the coupling beam when the connection is weak.
A comparison of the B-2 and B-2-H specimens indicates that the connection reinforcement used in standard hook construction details can lead to a slightly high strain distribution. Therefore, no further increase in the confinement effect would be expected in cases where headed reinforcements are used as the connection reinforcement.
When U-type reinforcements were placed at the joints to control slippage due to the plasticization of the joints, excessive shear deformation occurred as plastic hinges were induced into the center of the beam, without forming at both ends. Consequently, brittle failure occurred when only horizontal reinforcements were placed.
This research was supported by a grant from the Korea Land & Housing Institute. And this work was also supported by the Brain Korea 21 Plus Project of Dept, of Architectural Engineering, Chungnam National University in 2017.
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