Enhancement of Seismic Performance of Reinforced Concrete Columns with Buckling-

Restrained Reinforcement

P. Lukkunaprasita,*, T. Tangbunchooa, K. Rodsinb

aDepartment of Civil Engineering, Chulalongkorn University Bangkok, Thailand

bDepartment of Civil and Environmental Engineering Technology, King Mongkut’s Univesity of Technology North Bangkok Bangkok, Thailand

Abstract

Reinforced concrete (RC) columns with light confinement prevalent in developing countries exhibit low ductility with brittle shear failure, especially when buckling of longitudinal rebars takes place. This study applies the buckling restraining concept widely used in seismic resistant steel structures to reinforcing bars. Two RC columns 270mm x 300mm in cross section with a height of 1200 mm and minimum (non-seismic) transverse reinforcement were tested under cyclic lateral loading. Buckling-restrained reinforcement was provided over the critical zone. The buckling-restraining casing effectively prevented buckling of slender vertical bars under a substantially high axial load level, resulting in a more ductile mode of failure with evident formation of plastic hinge at the base of the column. Prior to gravity load collapse, the drift capacities and the degraded concrete shear capacities of the specimens were significantly increased compared to their counterparts without casings. Thus, it is important to prevent longitudinal bar buckling in investigation of degraded concrete shear capacity of RC columns.

Keywords: Seismic performance; Gravity load collapse; Reinforced column tests; Light transverse reinforcement; Longitudinal bar buckling; Buckling-restrained reinforcement; Degraded concrete shear capacity

1. Introduction

Reinforced concrete columns with light longitudinal and transverse reinforcement are prevalent in existing low rise buildings in regions of low or even moderate seismicity, espe-cially in developing countries. These structures are vulnerable to damage or even collapse in the event of a strong earthquake. Unfortunately, research work on lightly reinforced concrete columns is quite limited. RC columns with light transverse steel subjected to cyclic lateral load exhibit rapid loss of lateral load resistance soon after attaining the peak capacity. Shear mode of failure often prevails with small drift capacity [1],[2]. Under moderate to high axial load ratios, longitudinal bars tend to buckle, with the consequence of abrupt shear failure as reported by Wibowo et al. [3]. Sezen and Moehle [4] earlier reported that for columns with light axial load, shear failure would be triggered due to apparent strength degradation after development of the flexural strength whereas columns with high axial load would suffer abrupt shear compression failure.

*Corresponding author. Tel.: +66 2 2186571; fax: +662 2186571

E-mail address: lpanitan@chula.ac.th (P. Lukkunaprasit).

It was speculated that preventing longitudinal bar buckling would greatly enhance the seis-mic performance of RC columns since it eliminates the transfer of gravity load from the steel to concrete which would otherwise be caused by bar buckling, thereby reducing the shear de-mand on concrete on the diagonal crack plane. The buckling restraining concept successfully applied in seismic resistant steel structures was adopted to provide buckling – restrained rein-forcement (BRR). RC column specimens with BRR were cyclically loaded in the horizontal direction under constant axial load, and their performances compared with their counterparts without BRR.

2. Performance of control columns without buckling-restraining casing

The specimens S2 and S3 tested by Wibowo et al. [3] serve as the control specimens. The columns, 270mm×300mm in cross section, were reinforced with four ø16 mm Grade 400 MPa longitudinal steel bars. Hoop ties, 6 mm in diameter, were provided at 300mm spacing corresponding to a transverse reinforcement ratio rH of 0.0007. The nominal concrete com-pressive strength fc' was 20 MPa. The column was loaded in single curvature under cyclic loading with the lateral load applied at a height of 1200 mm from the base. A constant axial load of 20% the axial load capacity based on fc'Ag was applied to specimen S2, while that for S3 was 40%.

The specimens exhibited flexure dominated inelastic behavior with well distributed flexu-ral cracks up to the peak strength at about 1.5% and 1.0% drifts for specimens S2 (20% axial load ratio) and S3 (40% axial load ratio), respectively.1.0 %. Soon after the peak load, previ-ously developed vertical cracks widened, indicating impending vertical bar buckling. The drift capacity was (1.75 strain data)…2.0% with sustainable lateral load capacity of (98??)about…97 % of the peak value for S2. The corresponding values for S3 are 1.25% drift and 97 % of the peak capacity, respectively. Note that these drifts are very close to those at peak loads. At impending failure upon visible longitudinal bar buckling (2.5% and 1.5% drifts for S2 and S3, respectively), vertical bars buckled in the potential buckling zone near the base followed by an abrupt transfer of the force carried by steel to the concrete core with a consequence of significant increase in shear along the cracked shear plane. This triggered an abrupt shear failure (due to deterioration of cyclic shear resistance) and loss of gravity load capacity, which resulted in a sharp drop in the descending branch of the envelope curve. Furthemore, the failure shear plane cut through the concrete core at roughly 45º in between the ties. Thus, practically no shear resistance was provided by the transverse reinforcement at the failed section.

( Please read the first 7 line of this paragraph again)

3. Buckling-restrained reinforcement

The concept of buckling restrained brace successfully used in steel structures for enhancing seismic performance of steel buildings was adopted. In our pioneer study, ø 28 mm steel tubes with 4mm-thickness were used to encase the ø16 mm longitudinal bars. The deformed bars were coated with silicone sealant compound to a practically smooth surface and subse-quently wrapped with plumber’s tape. Non-shrink grout was then injected into the void be-tween the wrapping and the steel casing. The stress-strain relationships of the steel bar with and without casing under compressive loading are shown in Fig. 1.

Fig. 1: Compressive stress-strain relationships of 16 mm diameter bar with and without cas-ing.

The length of the bars between the grips was 300 mm. For BRR, the length of the casing was 250mm. As expected, right after peak load, the capacity of the bare steel bar sharply drops due to bar buckling. In contrast, the buckling-restrained reinforcing bar could sustain almost constant load after yielding since the lateral restraint provided by the casing prevents the bar from buckling. Interestingly, even at a large axial strain of 4%, BRR could sustain a stress as high as 90% of the peak value.

4. Test specimens

The specimens, designated by SC1and SC2, were practically identical to S3 and S2, in di-mensions and reinforcement details, respectively, except for the concrete compressive strengths and provision of BRR. Due to some prior mistake in quality control, the contractor

Table 1

Details of Column Specimen

Speci-men

rV Main

Reinforc-ment

rH Ties

(@mm)

Axial load

ratio

fc'

(MPa)

Remark

S2 1.0 % 4 ø 16 0.07 % Ø 6@300 0.2 21.0

S3 1.0 % 4 ø 16 0.07 % Ø 6@300 0.4 18.4

SC1 1.0 % 4 ø 16 0.07 % ø 6@300 0.4 27.7 With BRR

SC2 1.0 % 4 ø 16 0.07 % ø 6@300 0.2 29.8 With

BRR

Notation : rV is the longitudinal reinforcement ratio; axial load ratio is the ratio of the ap-plied axial load to axial load-carrying capacity based on fc' and gross concrete area.

Fig. 2. (a) Geometry and reinforcement details of column specimens; (b) Buckling-restrained reinforcement (BRR).

somehow modified their mix design which resulted in about 40% increase in concrete strength over the specified value of 20 MPa. As for the reinforcement, buckling restraining casings were provided over the critical failure zone as previously experienced by specimens S2 and S3, i.e. over the lowest tie spacing of 300mm. The yield strengths of the rebars were 530 MPa and 240 MPa for the longitudinal and transverse reinforcement, respectively. Rele-vant details of the specimens are presented in Table 1 and Figure 2.

5. Test setup

Figure 3 shows the test setup. The specimens were tested in the testing frame developed by Warnitchai and Rodsin [personal communication]. The axial load was applied by means of a vertical hydraulic jack constrained to move horizontally on guided rollers, thereby ensuring verticality of the axial load. The lateral load was applied using an actuator with 1000 kN ca-pacity. During testing, special care was taken to ensure that the axial load was maintained constant to within 10% of the initial load. The drifts as well as the axial displacement of the columns were measured using linear variable displacement transducers (LVDTs). Electrical strain gauges were installed above and below the casings at a distance as close as practicable to the casings to measure the strains in the longitudinal rebars.

The displacement controlled loading sequence consisted of drift-controlled mode with two increments of 0.125% drift followed by increments of 0.25% up to 2% drift, after which the drift increments were 0.5%. Two cycles of loading were repeated in each drift ratio to ensure that stable response could be maintained. The test was performed until the vertical load ca-pacity was practically lost.

Fig. 3. Test setup.

6. Experimental results

6.1. Effectiveness of BRR

Figure 4 shows the hysteretic loops of specimens SC1 and SC2 together with S3 and S2 for comparison. Specimens SC1 and SC2 with BRR experienced relatively less cracking than their counterparts at the same drift ratio as illustrated in Fig. 5 for SC1 and S3 . As expected, the buckling-restraining casing effectively prevented buckling of rebars in the zone rein-

forced with BRR. Specimens SC1 and SC2 could sustain the gravity load with stable hys-teretic loops past the peak load with relatively more gradual drop in lateral load capacity as evident in the envelope curves depicted in Fig. 6. While specimens S3 and S2 practically lost their load carrying capacities at 1.5% and 2.5% drift ratio, respectively, their counterparts with BRR could still sustain more than 90% of the peak capacities at these levels. Prior to gravity load collapse, the drift capacity of specimen SC1 was 1.75%, a 40% increase com-pared to the specimen without casing. The improvement was much greater for specimen SC2 (with half the axial load ratio of 0.2) which sustained the gravity load up to a drift of 4.5% whereas the specimen without BRR could do so up to 2% drift only.

An investigation of the strains in the longitudinal rebars reveals that, soon after the peak loads, they reached compression yield strain at about 1.25% and 2.5% drift ratios for speci-mens SC1 and SC2, respectively (see Fig. 7). Interestingly, the rebars in their counterparts S3 and S2 buckled soon after reaching the compressive yield strain. It should be noted that these results are indicative only because some of the strain gauges got damaged soon after the peak load. Furthermore, the strains in specimens SC1 and SC2 were obtained from interpolations of the readings from strain gauges above and below the casings, whereas those in specimens S2 and S3 (reported by Wibowo et al.) were values from strain gauges close to the lowest tie. Nevertheless, the results clearly indicate that the rebars in the specimens with buckling-re-strained casings, being highly stressed to the compressive yield strain level, would have buckled if they were bare bars, with subsequent trigger of abrupt failure not far from the drift levels when the compression yield was reached in a similar manner as in specimens S2 and S3.

Fig. 4. Hysteretic curve of specimen (a) S3; (b) SC1; (c) S2; (d) SC2.

Fig. 5: Comparison of damage at same drift ratio of 1.5%(cycle 1) (a) specimen S3(note sign of impending collapse) ; (b) specimen SC1.

Fig. 6. Normalized envelope curves of specimens

S2,S3(without BRR) and SC1,SC2 (with BRR).

Fig. 7. Compressive strains in the longitudinal rebars versus lateral drift.

Figures 8 and 9 show the modes of failure of all specimens for comparison. Clearly, the spec-imens without BRR failed in an undesirable shear mode of failure, whereas the ones with BRRs could develop plastic hinges at the bases of the columns. However, splitting as well as shear distress in SC1 and SC2 also prevailed at impending collapse, but failure was more gradual than the case without BRRs. It should be noted that the tie spacing/diameter ratio of the vertical rebars was well over 18, and yet BRRs worked effectively and remained practi-

cally straight without buckling even under a significantly high axial load ratio of 40% (see Fig. 8 (d)).

6.2. Degraded shear capacity

The ductility of Seismic performance It is highly desirable to have Under earthquake load-ing, structural components need to possess sufficient post-peak shear capacity in order to per-form satisfactorily. As depicted in the lateral load - deflection envelopes in Fig. 10, due to bar buckling, the shear capacity of specimen S3 at 1.5% drift dropped to almost zero, and it would naturally be reported as total loss of shear capacity at this drift level. In contrast, at this drift, specimen SC1, the counterpart of S3 with (almost) the same column configurations ex-cept for the provision of buckling-restrained casings and a somewhat difference in concrete compressive strength, could still sustain a lateral load of more than 90% the peak shear lateral load capacity. It should be noted that, due to the large tie spacing much in excess of half the section depth, the contribution of transverse reinforcement is negligible. Therefore, the lateral load capacity derives solely from contribution of concrete.

It is important to note that concrete shear degradation models in the literature do not take into account the influence of longitudinal bar buckling, and hence they could much underestimate the actual shear capacity of concrete near impending collapse. This is because previous re-searchers did not have an effective way to separate the effect of bar buckling, nor were they able to prevent buckling from taking place for a given reinforcement configuration in slender longitudinal bars. Thus, BRR makes it possible evaluation of actual concrete shear capacity with practically no interference from longitudinal bar buckling.

Fig. 8. (a) Specimen S3 at gravity load collapse at 1.5% drift; (b); (c) Specimen SC1 at 1.75% drift with sustainable gravity load and 2% drift with loss of gravity load capacity, resp. Note no buckling of BRR as evident in (d).

Fig. 9. (a) Specimen S2 at 2% drift; (b); (c) Specimen SC2 at 4.5% drift with sustainable gravity load and 5% drift with loss of gravity load capacity, resp.

7. Conclusions

From the limited number of tests conducted, the buckling-restrained reinforcement developed has demonstrated its potential in preventing buckling of slender vertical bars under a significantly high axial load level. Shear failure, witnessed in specimens when bar buckling takes place, can be deferred or even eliminated when BRR is provided in the critical zone to prevent bar buckling. This results in a more ductile mode of failure with evident formation of plastic hinge at the base of the column. Prior to gravity load collapse, the drift capacities and the degraded concrete shear capacities of the speci-mens are significantly increased compared to their counterparts without casings. Thus, the deterio-rated concrete shear capacity reported in the literature without distinction of longitudinal bar buckling can be significantly in error, especially for lightly reinforced columns. at displacement ductility close to gravity load collapse. Furthermore, BRR would be a useful device in experimental in-

vestigation of actual degraded concrete shear capacity without influence from longitudinal bar buckling.

BRR makes it possible evaluation of degraded shear capacity contributed by concrete with practically no interference from longitudinal bar buckling. This could be a significant factor to consider in investigation of actual degraded shear capacity of concrete in the future.

Clearly more extensive experiments are needed to fully investigate the effectiveness of BRR over a wide range of applications.

• Buckling of longitudinal bars with large tie spacing (L/db >18 !) leads to an abrupt trans-fer of axial load from the steel bars to the concrete, thereby triggering shear failure due to deterioration of concrete shear strength during cyclic loading.

• Past concrete shear strength degradation models without accounting for longitudinal bar buckling can be in error, especially for lightly reinforced columns.

Obviously, the practicality and the effectiveness of BRR over a wide range of applications are subject to further extensive investigations.

Acknowlegements

The authors are grateful for the funding from the Commission on Higher Education, Min-istry of Education and Chulalongkorn University.

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