A footing-to-reinforced concrete (RC) pier connection resists the lateral load induced by earthquakes as well as the gravity load. The footing-to-RC pier connection is the vulnerable part to strong earthquake loading. Several studies have been conducted on improving the seismic performance of the connection by using high-strength reinforcing bars and by adding special structural components, such as steel tube and fiber-reinforced polymer sheet. In this study, reinforcing bars made of high-manganese steel (HMSBs) with high strength and ductility were installed in the connection instead of conventional reinforcing bars to improve the seismic performance. Test specimens were fabricated with HMSBs, and the strength, ductility, and dissipated energy of the connection were evaluated through a cyclic loading test. Three-dimensional finite-element analysis was also performed to investigate the effects of various axial forces on the behavior of the connection with HMSBs. The results show that the connection with HMSBs exhibits better seismic performance, represented by flexural strength, ductility, and energy dissipation, than that with ordinary reinforcing bars.
The substructure of a bridge generally consists of a coping, pier, and footing. In a reinforced-concrete pier (RC pier), reinforcement steel inside the pier extends into the coping or footing to connect each component. The substructure of the bridge resists the gravity load as well as lateral loads induced by events such as earthquakes. The connection between the footing and pier is the most vulnerable part to earthquake loading. Thus, to improve the seismic performance, it is necessary to reinforce the footing-to-RC pier connection.
Several studies have been conducted for this purpose by using high-strength reinforcing bars [
The effect of the deformation capacity (ductility) of the reinforcing bar is an important parameter that affects the seismic performance of RC pier connections. Recently, a new high-manganese steel (HMS) was developed by POSCO, a steel company from the Republic of Korea. This steel has excellent deformation capacity, such that its energy dissipation ability is much greater than that of ordinary conventional steel. The percentage of manganese in HMSs is approximately 18–22%, whereas that of ordinary steel is under 0.6–1.6%. According to Sasaki et al. [
The stress-strain curve of reinforcing steels.
In this study, a high-manganese steel bar (HMSB) was fabricated and installed in the connection between the footing and circular RC pier to investigate the seismic performance of the footing-to-circular RC pier connection. A series of tests were conducted, and the strength and ductility were evaluated for the connection with HMSBs. Furthermore, three-dimensional (3D) finite-element analysis (FEA) was performed for in-depth analysis of the behavior of the proposed connection and to investigate the effect of the axial load on the connection behavior. The results showed that the connection reinforced by HMSBs had excellent seismic performance compared to that of ordinary RC connections.
In this study, two different test specimens were constructed to investigate the effect of HMSB on the seismic performance of a circular RC connection to the footing. The reference specimen, named “T-SD400,” contains longitudinal reinforcing bars of cross-sectional area 132.73 mm2 (diameter 13 mm), made of SD400-D13, which has a nominal yield strength of 400 MPa. The T-HMSB specimen has the same dimensions as T-SD400. However, the longitudinal SD400-D13 reinforcing bars were replaced by HMSBs, which have almost the same cross-sectional area (132 mm2), as shown in Figure
Cross section of reinforcing bars: (a) SD400 bar and (b) HMSB.
Because commercial reinforcing bars made of HMS have not been produced yet, the HMSBs were specially fabricated by cutting an HMS plate, as shown in Figures
The process of fabricating HMSBs and rebar assembly for an RC pier connection: (a) HMS plate; (b) water-jet cutting; (c) rebar assembly (T-HMSB); and (d) rebar assembly (T-SD400).
The layout of the test specimens and the arrangement of the reinforcing bars are shown in Figures
Test specimen layout: (a) dimensions and (b) reinforcement arrangement.
Dimensions of test specimens.
Specimen |
|
|
|
|
|
|
|
|
|
---|---|---|---|---|---|---|---|---|---|
T-SD400 | 512 | 2,200 | 800 | 2,470 | 1,000 | 205,887 | 2123.72 | 1.031 | 0.23 |
T-HMSB | 512 | 2,200 | 800 | 2,470 | 1,000 | 205,887 | 2112 | 1.026 | 0.23 |
Sixteen SD400-D13 bars and HMSBs were installed for the T-SD400 and T-HMSB specimens. The reinforcement ratios in the longitudinal direction (
The SD400-D10 bars, which were 10 mm in diameter and had nominal yield strength of 400 MPa, were used as transverse spiral reinforcing bars for both test specimens and placed at 70 mm intervals near the connection and at 140 mm intervals elsewhere. The AASHTO LRFD design code [
According to the AASHTO LRFD design code [
Material tests were conducted for the reinforcing bars and concrete. From the results, the yield stress of an SD400 bar and HMSB was 440 MPa and 540 MPa, respectively. The ultimate tensile stress of an SD400 bar and HMSB was 550 MPa and 945 MPa, respectively. The compressive strength of the concrete was 35 MPa.
It should be noted that the circular pier and the HMSB with rectangular section and plain surface are used in this study. Thus, the results in this study should be limited for these conditions.
Given the dimensions shown in Table
The strain and stress distributions of an RC section based on the strain compatibility method.
As shown in Figure
From the calculation results,
Theoretical
Figure
Test setup: (a) layout of loading and (b) the specimen on the frame.
Lateral displacement was applied at the top of the RC pier by an actuator. The vertical axial load to the RC pier was not applied, such that pure flexural behavior could be obtained. Because of the limited test resources, axial load testing was not conducted in this study. Instead, the effect of the axial force of the RC pier was examined by a series of verified FEAs, as discussed in Section
Linear variable differential transformers were installed to measure the lateral displacement at the center of the loading point (2,200 mm from the bottom of the pier). To measure the extreme strain in the reinforcing bar, strain gauges were attached at the outermost longitudinal reinforcing bars at the heights of 100, 300, and 500 mm from top of the footing, as shown in Figure
The location of the strain gauges.
Cyclic lateral displacement control protocol.
Cyclic loading tests for the footing-to-circular RC pier connections were conducted. Figure
Connection damages: (a) T-SD400 at 0.67% drift; (b) T-HMSB at 0.67% drift; (c) T-SD400 at 5.48% drift (failure); and (d) T-HMSB at 10.09% drift (failure).
At 0.68% drift, similar flexural cracks were observed for both test specimens. Upon increasing the drift ratio, more flexural cracks developed. Then, the concrete at the connection part was crushed by excessive compression. Additionally, at the final stage, the reinforcing bars were buckled and ruptured in both directions in both specimens. The concrete damage level of T-SD400 was higher than that of the T-HMSB, even if the drift ratio of the last stage of T-HMSB (10.09%) was much larger than that of T-SD400 (5.48%), because HMSB has a higher yield and ultimate stress than ordinary reinforcing bars, and less compression developed in the concrete of T-HMSB. Moreover, the width of the flexural cracks at the failure of T-HMSB was longer than that of T-SD400, as shown in Figures
The base moment-drift ratio curves obtained from the test are shown in Figure
Base moment-drift ratio curves.
Test results are summarized in Table
Test results and comparison with the theoretical value.
Specimen |
|
|
|
Drift (%) at 0.8 |
||
---|---|---|---|---|---|---|
Positive | Negative | Positive | Negative | |||
T-SD400 | 213.03 (3.05) | 180.82 (2.15) | 214.2 | 0.99 | 0.84 | 5.48 |
T-HMSB | 260.81 (7.15) | 261.60 (7.12) | 271.7 | 0.96 | 0.96 | 10.09 |
Figure
Strain distributions in reinforcing bars with the
For the T-HMSB specimen, the strain data were almost linear up to a 2% drift ratio, as shown in Figure
The yield strains (
Displacement ductility and energy dissipation are very important factors for evaluating seismic performance. They are evaluated from the test in this section. The displacement ductility (
Displacement ductility calculation: (a) T-SD400 and (b) T-HMSB.
Summary of displacement ductility calculation.
Specimen | Direction | Idealized yield displacement, |
Failure displacement, |
Displacement ductility, |
Rate of increase (%) |
---|---|---|---|---|---|
T-SD400 | Positive | 0.68 | 5.48 | 8.56 | — |
Negative | 0.62 | 4.56 | 7.35 | — | |
T-HMSB | Positive | 1.05 | 10.09 | 9.61 | 12.35 |
Negative | 1.08 | 9.01 | 8.34 | 13.53 |
The idealized yield displacement (
Energy dissipation represents the energy absorbed by the structure during the cyclic loading excitation. According to Paulay et al. [
The cumulative dissipated energy of each model was calculated, and the results are shown in Figure
Cumulative dissipated energy for test specimens.
Summary of cumulative dissipated energy calculation results.
Specimen | Cumulative drift ratio (%) (up to 0.8 |
Cumulative dissipated energy, |
Rate of increase (%) |
---|---|---|---|
T-SD400 | 138.97 | 10,112 | — |
T-HMSB | 300.03 | 23,327 | 131 |
FEA was conducted by using ABAQUS [
Typical finite-element model: (a) concrete elements, boundary condition, and loading; (b) reinforcing bars in the pier; and (c) reinforcing bars in the footing.
The concrete of the FEA model was formed by using eight-node solid elements, as shown in Figure
For the efficiency of the analysis, a half model was used. The 1-2 plane cut in half of the model was symmetrically bounded, as shown in Figure
The uniaxial compressive stress-strain relationship proposed by Saenz [
The average stress-strain relationship of an embedded reinforcing bar differs from that of a bare reinforcing bar, because of cracking and the associated stress concentration near the cracking zone. To include these effects in the analysis model, the average stress-strain relationship of embedded reinforcing bars proposed by Hus and Mo [
The stress-strain curve used in FEA: (a) concrete and (b) embedded reinforcing bar.
GTN (Gurson–Tvergaard–Needleman) model is suitable to simulate the ductile failure of the steel member such as reinforcing bar failure [
The FEA was conducted for the test specimen for verification. Figures
Verification results: (a) F-SD400 vs. T-SD400 and (b) F-HMSB vs. T-HMSB.
The stress distributions and crack patterns of the analysis model at final stage are shown in Figures
F-SD400 at the final stage (4.46%): (a) reinforcement stress distributions; (b) concrete stress distributions; and (c) concrete cracks.
F-HMSB at the final stage (7.91%): (a) reinforcement stress distributions; (b) concrete stress distributions; and (c) concrete cracks.
A parameter analysis was performed to evaluate the effect of the axial load. The pier is generally subjected to an axial load, and the moment capacity and failure mechanism change depending on the magnitude of the axial load.
Axial load ratios and corresponding theoretical moment capacity.
Parameter | F-SD400 | F-HMSB | ||
---|---|---|---|---|
Axial load ratio, |
Axial load, |
Ultimate moment, |
Axial load, |
Ultimate moment, |
0 | 0 | 214.21 | 0 | 271.7 |
0.1 | 706 | 300.00 | 727 | 341.24 |
0.15 | 1,059 | 336.77 | 1,091 | 373.58 |
0.30 | 2,118 | 389.54 | 2,182 | 393.52 |
For the four axial loads shown in Table
The results of parameter study: (a) F-SD400 and (b) F-HMSB.
Based on the analysis results shown in Figure
Displacement ductility of FEM analysis models.
Summary of the parameter analysis results.
Model | Axial load ratio, |
|
Idealized yield displacement, |
Failure displacement, |
Displacement ductility, |
---|---|---|---|---|---|
F-SD400 | 0 | 1.06 | 0.87 | 4.48 | 5.15 |
0.1 | 1.11 | 0.67 | 7.7 | 11.49 | |
0.15 | 1.13 | 0.65 | 7.23 | 11.12 | |
0.3 | 1.24 | 0.6 | 2.39 | 3.98 | |
|
|||||
F-HMSB | 0 | 1.02 | 0.99 | 7.99 | 8.07 |
0.1 | 1.10 | 0.76 | 10.63 | 13.99 | |
0.15 | 1.10 | 0.69 | 9.77 | 14.16 | |
0.3 | 1.27 | 0.63 | 3.05 | 4.84 |
In this study, HMSBs were used as reinforcing bars to improve the seismic performance of the footing-to-circular RC pier connection. Circular RC pier connection specimens with HMSBs and ordinary reinforcing bars were constructed and tested to investigate the pure flexural behavior of the connection. Furthermore, FEA was conducted to examine the effect of the axial load on the proposed connection. From the test and analysis results, the following conclusions are drawn: From the flexural test with zero axial load, it was found that the test specimen with HMSBs (T-HMSB) has 22%, 13%, and 131% higher ultimate strength, displacement ductility, and dissipated energy, respectively. The use of HMSB could provide better seismic performance than can the conventional reinforcing bar. From the test, it was found that the flexural crack width of T-HMSB is larger than that of the specimen with ordinary reinforcing bars (T-SD400), because the HMSB used in this study had a plain surface and rectangular shape. For a more accurate comparison, a connection with circular deformed HMSB should be tested. However, even for the HMSB with a plain surface and rectangular shape, the connection with HMSBs showed better seismic performance. The effect of the axial load on the proposed RC connection was investigated by a series of FEAs. The results showed that the displacement ductility of the analysis model with HMSBs was higher than that with conventional reinforcing bars for all considered axial load ratios. The displacement ductility reached its maximum in the range from
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
This study was supported by the Research Grant from Kangwon National University.