Seismic Capacity of Top Corner Joint in RC Building Reinforced with Headed Longitudinal Bars S. Y. Seo Chungju National University, Chungju, Korea K. B. Choi Kyungwon University, Seongnam, Korea K. S. Kim The University of Seoul, Seoul, Korea

ABSTRACT Recently several researches have been presented on headed reinforcement as an alternative to the standard hooks that are used for the joint area of RC members. The previous experimental works show that the head improves anchorage capacity of the reinforcements and reduces the congestion of them in joint. In case of top corner joints in building, as column and beam are connected together, severe reinforcement congestion is likely to occur; thus a detailing can be very useful by using mechanical anchoring. Headed reinforcement is expected to improve the performance in corner joints, but if the concrete is not adequately restrained, it could easily lead to brittle failure. This study offers a various details for corner joint with reinforcements anchored by head, examines their structural capacity through experimental work, and endeavors to provide a database to develop efficient details for the fabrication and construction. The research result contains that the exterior joint is exposed to the risk of brittle failure when a repetitive stress is applied in the beam-column corner joint where headed reinforcement is anchored. To prevent this, additional transverse reinforcement is needed within the joint to restrain concrete adequately. Keywords: RC top corner joint, reinforcement congestion, headed reinforcements, experimental work 1. INTRODUCTION When a building structure is under repetitive stress (e.g. seismic loads), the stress is converged into the joint where vertical and horizontal members cross, and causes inelastic deformation. In constructing a building against lateral loads, the joint parts are especially important because their non-linear behavior affects the entire structure. Generally the first inelastic deformation takes place in the bottom joint, which is why the research has been concentrated on the subject so far; however, further research on the top corner joint is needed because issues frequently arise such as reinforcement congestion or lack of concrete filling in the area. The demand for research is increasing as the diminishing section area and the use of high-strength materials makes it hard to secure sufficient length for placing reinforcements. Recently researches have been presented on headed reinforcement, a combination of anchoring hardware over the reinforcing steel bars, as an alternative to the standard hooks that are used for the joint area. The experiments show that the head improves anchorage capacity of the reinforcing bars, thus requiring shorter length for embedment. In case of top corner joints, as columns and beams are connected together, reinforcement congestion is likely to occur; thus a detailing can be very useful by using mechanical anchoring. Headed reinforcement is expected to improve the performance in corner joints, but if the concrete is not adequately restrained, it could easily lead to brittle failure. This study offers a various details for corner joint anchored with headed reinforcement, examines their structural properties, and endeavors to provide a database to develop efficient details for the fabrication and construction. 

2. BEHAVIOR OF CORNER JOINT IN RC BUILDING In the top corner joint RC members, the principle tensile stress occurs at the line between center of joint and inner corner when the positive moment acts. And the compressive stress at the outside of diagonal crack pushes triangle so that only internal tensile force resists the diagonal force. On the contrary, when the negative moment acts, the tensile stress in the joint governs. The force causing the diagonal cracks in joint is assumed to be induced as a type of equal shear stress by bond force. The summed equal shear stress makes the splitting cracks. These diagonal cracks appear when concrete reaches the tensile strength.

     Stress distribution Crack Stress distribution Cracks

(a) Positive (Closing) moment (b) Negative (Opening) moment

Figure 1. Stress and cracks in corner joint

Generally, from previous studies (Mayfield et al., 1971; Y. H. Luo et al, J. W. Wallace et al, 1998; M. S. Hashim et al, 1989; M. Joahnsson, 2001), it can be found that the opening moment is generally resisted by reinforcement. Accordingly, most experiments have focused on the opening moment) and details that influence the opening moment. However, as was explained earlier, the joint can be easily burdened with reinforcement congestion, which complicates the construction and also discourages effective resistance against lateral force. Moreover, the corner joint has low restraint strength due to the absence of an upper column, which further deteriorates shearing strength. To improve the construction workability in the joint, headed bar that can be replaced by bar with standard hook was suggested and the evaluation of its performance under various details was conducted by previous researchers such as Wallace et al (1998). Headed reinforcement was reported to enforce the internal force of the joint, but under closing moment, the strength plummeted. As a result, the additional transverse reinforcement was recommended within the joint. Based on this research work, ASTM A970M specifies the specification of headed bar and its’ development length for design is presented in ACI 318 code (2008). Also Eurocode 2 (2004) allows the use of mechanical devices. As an application, Wallace (1998) studied the structural behavior of corner joint reinforced with headed bar and suggested that it is practically effective in knee joint. However a confinement of head anchorage area is required to prevent push out failure of concrete. This study evaluates the structural performance of RC top corner joint in building with various details of headed anchorage system through an experimental work. 3. EXPERIMENTAL PROGRAM 3.1. Specimen design To evaluate the structural performance of RC corner joint that is subject to cyclic load, six specimens were designed as shown in Table 1. In CJ-1 specimen, head was used to anchor beam top bar after 90-deg-hook; the reinforcement in beam bottom and column were also anchored with headed

reinforcement. In CJ-2, only headed reinforcement was used without a hook. In CJ-3, to oppress push-out around beam in CJ-2, vertical stirrups were added. In CJ-4, to distribute the stress of out corner on the joint, outer bars of column and top bars of beam were anchored with headed reinforcement in 90 deg hook. CJ-5 is non-seismic standard detail which has standard hook in all longitudinal ends except bottom bar of beam. In CJ-6, the amount of reinforcing within the joint was diminished compared to CJ-4, and the head was removed from the lower beam reinforcements. This is to observe the minimum reinforcement requirement and the effect of the head in the lower beam. The dimension and reinforcement details are illustrated in Figure 2. Table 1. List of test specimen

Specimen name Anchorage detail of column bars Anchorage detail of beam bars Horizontal shear

reinforcement in joint Outer bar Center bar Inner bar Upper bar Bottom bar

CJ-1

Head Head

Head

90°Hook+Head

Head 3-D10 CJ-2 Head

CJ-3* Head

CJ-4 90°Hook+Head 90°Hook+HeadCJ-5 Standard hook U cap

None Standard hook

None 1-D10 CJ-6 90°Hook+Head Head 90°Hook+Head

* Three additional vertical stirrups are added in joint.

CJ-1 CJ-2 CJ-3

CJ-4 CJ-5 CJ-6

Figure 2. Dimension and detail of test specimen (Unit: mm)

3.2. Material properties

The head used in the experiment is composed of connecting part and anchoring plate as illustrated in Figure 3. In the head, as for the connecting part, the diameter is 28mm, thickness 23mm; for anchoring plate, diameter 38mm, and thickness 10mm. Right one in Figure 3 shows the pull-out test of headed reinforcement by S.C. Chun (2001), its internal strength by different embedment depth. It compares the hypothetical outcome of formula ACI 349 (1997) and CCD (Concrete Capacity Design; Werner et al, 1995) and the actual result from the experiment. When the anchoring depth was 9, 12, 15cm, “cone” failure occurred. When the depth exceeded 18cm, the deformed bar yielded first then the “cone” failure followed. And in case of 21, 24cm, the bars were broken. In the figure, Ft indicates maximum tensile strength; Fy for yield strength.  

Ready-mixed concrete was used. Its’ compressive strength is 24MPa and slump is 15cm. SD400 deformed bar, D19 for main bars and D10 for stirrup and hoop, is used. Tensile strength test for the bars showed that fy and fu were 408 and 549 MPa for D10, 511 and 697 MPa for D19, respectively.

  

 

 

 

 

 

 

 

 

Figure 3. Headed bar detail and pull out strength (S. Chun, 2001)

3.3. Loading and measuring of instruments

In order to simulate the hysteretic behavior of the corner joint under earthquake loads, an actuator is installed on the end of beam. Also, the end of column is supported with pin like the contra-flexure point in real building. The opening or closing moment can be applied to the joint by changing the actuator stroke. Figure 4 shows the test layout. Blank line 11 pt Blank line 11 pt

Figure 4. Test set up

The displacement is controlled by the prepared story drift following the procedure proposed by ACI (1999) that is codified for the test to simulate the hysteretic behavior of structure in high seismic region. Three cycle loadings are applied at every story drifts such as 1/1000, 1/700, 1/350, 1/250, 1/180, and 1/130.     4. TEST RESULTS 4.1 Crack pattern In CJ-1 and CJ-4, the initial crack took place at inner joint in vertical and horizontal pattern. Then, as the diagonal crack appeared due to opening and closing moment, the cracks became prominent in the center part of the joint. Finally the crushing of concrete followed, and the concrete split. In CJ-2 and CJ-3 with the head instead of ACI standard hook, the initial crack pattern was same as above, and deterioration occurred by the failure of anchorage and bonding, and “push-out” occurred as the cover was stripped by the head. In the final stage, the failure of anchorage and bonding was observed as the beam top and the joint area fell apart. In CJ-3 where additional shear reinforcements were added, “push-out” as well as the failure of anchorage and bonding were observed. CJ-5 specimen with

non-seismic standard hook detail failed showing compressive crushing in the concrete core confined shear reinforcement. In CJ-6 where the amount of reinforcements within the joint was diminished and the beam top and outer column bars were anchored with 90 deg hook, a diagonal crack started from the inner part of the joint and progressed to the outer side as the stress increases; finally, X-shaped crack occurred at the inner joint leading to splitting. Typical failure patterns of specimens are shown in Figure 6.

(b) CJ-2 (c) CJ-5 (c) CJ-6 Figure 6. Typical failure patterns

4.2. Load-displacement curve 

 Figure 7 plots curves that show the correlation between the applied load and the displacement. In CJ-1, the maximum strength showed 110.6kN for positive stress and 119.6kN for negative stress: a hysteresis loop that shows symmetrical result for two loads. In CJ-2, in applying positive loading (closing moment), the strength decreased after the diagonal crack took place, then further declined by the “push-out”. In CJ-3, the maximum strength increased 80% compared to CJ-2; yet the “push-out” still kept the strength low. In all cases except CJ-5 and CJ-6 without anchor in bottom bar of beam, for the opening moment, the hysteresis loop showed similar patterns and sufficient strength and ductility capacity can be acquired. Most stable curve was found in CJ-5 with standard hooks even if it has less shear reinforcements in the joint and doesn’t have anchor in bottom bar of beam. In the manner of strength, CJ-4 showed the highest maximum strength, 37% more than CJ-3 under positive stress. However, the degree of declining was still unavoidable as the stress increases. CJ-6 showed performance that reached 89% and 73% of CJ-4’s maximum strength under positive and negative stress, respectively. Its weak performance resulted from the lack of transverse reinforcement within the joint under positive stress, and missing head in the beam bottom under negative stress. On the other hand, CJ-5with standard hook, which is designed to compare with CJ-6, shows more stable hysteresis loop and higher strength than CJ-6 for both closing and opening moment even if the anchorage detail of beam bottom bar is same. Moreover, nevertheless CJ-5 is for non-seismic detail, its’ behavior looks better in dissipating the energy than CJ-4 and CJ-3 designed for seismic detail. From this, there is a possibility that the energy may be lost a lot if head bars are used in the top corner joint and unless the anchorage part is suitably confined by shear reinforcements. In the load-displacement curve, it was noted that the ultimate strength in each cycle declined due to the crack and the yield of reinforcement. Figure 8 shows that CJ-4 reaches the maximum strength under the positive stress; CJ-2, reinforced with only the head, showed lowest resistance. In contrast, CJ-3 reinforced with a stirrup in joint, showed relatively satisfactory performance. CJ-6, with less reinforcement compared to other specimens, showed relatively higher strength under negative moment as the hook held the column and the beam; yet under positive moment, the lack of the head in beam bottom critically reduced the strength. It can be induced that the reinforcement inside the joint is critical in preparation for the repetitive stress.

Figure 5. Push-out behavior in compression

(a) CJ-1 (b) CJ-2

(c) CJ-3 (d) CJ-4

(e) CJ-5 (f) CJ-6

Figure 7. Load-displacement curves

Figure 8. Envelop of all specimens 

Table 2 shows the accumulative energy from the maximum strength to the point where the maximum strength dropped by 75% (starting point of deterioration). CJ-1 and CJ-4 with hook-shaped head showed better ductility capacity and dissipation of energy compared to CJ-2 and CJ-3 with only a head after the maximum strength was reached. CJ-2 that simply anchored the head for beam and column exterior bars showed lowest dissipation of energy; this indicates that to apply this detail to the cornet joint to resist a seismic force, a proper reinforcement is required to prevent brittle failure. Also, it can be found that highest dissipated energy is acquired in CJ-5 with conventional hooks.

  

Table 2. Test result

Specimen Name

Py(kN)1) Δy(㎜)2) Pu(kN)3) Δu(㎜)4) Ductility(μu) 5)

Dissipated energy (kN․㎜)6)

CJ-1 (+) 81.18 14.95 110.60 25.69 1.72

18710 (-) 89.43 19.39 119.60 33.98 1.75

CJ-2 (+) 42.18 4.09 65.77 6.96 1.70

826 (-) 94.73 15.39 126.85 35.16 2.28

CJ-3 (+) 84.66 10.80 118.44 19.79 1.83

12385 (-) 92.99 18.23 124.21 49.66 2.72

CJ-4 (+) 120.80 21.30 161.68 33.31 1.56 19383

(-) 80.14 16.96 108.41 35.12 2.07

CJ-5 (+) 129.11 16.92 172.58 33.97 2.01

35299 (-) 71.75 8.00 97.63 19.14 2.39

CJ-6 (+) 107.44 15.61 144.47 25.20 1.61

11643 (-) 58.38 10.36 79.04 25.18 2.43

1) Py: Yield load, 2) Δy: Yield displacement, 3) Pu: Ultimate load, 4) Δu: Displacement at Pu, 5) μu: Δu/Δy, 6) Accumulated dissipation energy (kN․㎜) at failure point (at 75% load beyond the peak load)

4.2. Strain variation of beam reinforcements in joint Figure 9 shows the strain variation of beam top bars that are anchored in the inner joint of CJ-2 and CJ-3. In CJ-2, there was little strain of beam top bars for both directions loading; this means it slipped at head anchorage since the concrete around head anchorage was not suitably confined. On the contrary, the top bar in beam in CJ-3 strained a little bit under tensile stress during closing moment. This is because the concrete of head anchorage part was somewhat confined by the additional vertical shear reinforcements. From this, it is required that shear reinforcements shall be positioned around head anchorage to prevent the brittle failure.  

 

 

 

 

 

 

 

(a) CJ-2 (with only head) (b) CJ-3 (with head + vertical shear reinforcements)

Figure 9. Load-strain curve of beam top bar in joint 5. CONCLUSIONS 1) When cyclic stress is applied in the RC beam-column corner joint where headed longitudinal bars

were in both beam and column, the outer area of joint was exposed to the risk of brittle failure. 2) Especially when headed bars are only placed in both outer of column and upper of beam, the

structure was subject to low inner strength and rapid brittle failure under opening moment. To

prevent this, additional transverse reinforcement is needed within the joint to restrain concrete adequately.

3) If the exterior column bars and upper beam bars were 90 deg hooked then anchored with the head, the placement was most effective; even better than using additional reinforcement without 90 deg hooks. However, it is hard to prevent weakening under closing moment, which calls for a further study of details.

4) If the bottom reinforcement of beam was not anchored with 90 deg hook or the head, the compressive force under the opening moment could not be transmitted sufficiently to the anchoring parts, and the strength declined substantially.

5) On the comparison with the joint that has standard hooks, the strength which is compatible to that of standard hook can be acquired by using headed bars. However, additional methods confining the joint concrete needs to be developed in order to keep stable hysteretic behavior for earthquake

ACKNOWLEDGEMENT This research was supported by Ministry of Construction & Transportation of Korean government under a grant (06 Construction Core B04) from Construction Core Technology Program. B. C. Kim is acknowledged for assistance in doing the experimental work and data analysis and Boowon BMS Corporation for providing information and technology regarding the headed bars used in the experiment.

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