Seismic behavior of steel tube confined reinforced concrete double-column bridge bents

Journal of Constructional Steel Research 166 (2020) 105919

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Journal of Constructional Steel Research

Seismic behavior of steel tube confined reinforced concrete double-column bridge bents Haicui Wang a,b, Xuhong Zhou a,b, Jiepeng Liu a,b, Xuanding Wang a,b,⁎, Y. Frank Chen a a b

School of Civil Engineering, Chongqing University, Chongqing 400045, China Key Laboratory of New Technology for Construction of Cities in Mountain Area (Chongqing University), Ministry of Education, Chongqing 400045, China

a r t i c l e

i n f o

Article history: Received 24 August 2019 Received in revised form 23 December 2019 Accepted 23 December 2019 Available online xxxx Keywords: Double-column bent Bridge pier Steel tube confined reinforced concrete Seismic behavior Lateral resistance

a b s t r a c t A novel type of composite bridge bent consisting of slender steel tube confined reinforced concrete (STCRC) double-column connected by binding steel plates is proposed. Its main advantage is the convenient construction process that eliminates the embedment of a steel tube into the concrete foundation. Four STCRC double-column bent specimens were tested under the pseudo-static loading considering the key parameters of the axial load ratio and the disconnection mode of circular steel tube. All test specimens showed the mixed failure mode of column bending and binding steel plate buckling. As the axial load ratio increased from 0.12 to 0.24, the lateral load capacity and flexural stiffness of the specimens were improved. Although the disconnection mode of the steel tube changed the layout of plastic hinges, it had a limited influence on the seismic behavior of STCRC doublecolumn bents. The ultimate limit state of the double-column STCRC bridge bent could be simplified as a frame model with plastic hinges, by which the lateral resistances were well predicted. © 2019 Elsevier Ltd. All rights reserved.

1. Introduction Large cross sections are commonly used in reinforced concrete (RC) piers to resist lateral seismic loads, resulting in heavy structural weight and wasting material. For bridges with high RC piers, the excess structural weight drives up the foundation cost significantly [1,2]. Concrete filled steel tube (CFST) piers, by contrast, have higher strength, and better ductility benefitted from the composite effect between the steel tube and the concrete [3–5]. The reduced cross-section and self-weight of piers contributes to a saving in construction material and labor. However, the application of CFST piers is often limited by the inherent difficulty of connections with the foundation or RC cap beam. The steel tubes at the column base need a sufficient embedment depth to meet the anchorage requirement, which would be difficult for largediameter CFST piers [6,7]. What is more, the deeply-embedded steel tube will cause a complicated layout of reinforcement in the foundation [8–10]. Another composite construction that uses the thin-walled steel tube as the transverse confinement rather than the direct load-carrying component, known as “Steel Tube Confined Reinforced Concrete (STCRC)”, has attracted widespread attention in building structures in China recently [11]. The steel tube in an STCRC column is discontinuous at the beam-column joints, so that simplifies the connections of the CFST column to beams [12]. ⁎ Corresponding author at: School of Civil Engineering, Chongqing University, Building 2, Campus B, No.174, Shanzheng Road, Shapingba District, Chongqing 400044, China. E-mail address: [email protected] (X. Wang).

https://doi.org/10.1016/j.jcsr.2019.105919 0143-974X/© 2019 Elsevier Ltd. All rights reserved.

In this study, a novel type of composite bridge bents consisting of slender STCRC double-column connected by binding steel plates is proposed and investigated. As shown in Fig. 1, the steel tube in the STCRC column is disconnected at the column ends with gaps in place, while reinforcing bars are embedded into the column and extended into the RC cap beam and foundation to resist bending moments. The binding steel plates are designed to connect to the STCRC column using additional casing steel tubes, which are intended to dissipate seismic energy. Compared to traditional CFST piers, the novel STCRC double-column bents have the following advantages: (1) avoid the complexity of inserting steel tube into foundation as the steel tube can be installed after completing the foundation construction; (2) alleviate the excessive flexural demand on the connections of CFST piers; and (3) reduce the possibility of local buckling of a thin-walled steel tube at the column base and maximize the confinement, since there is no direct axial load carried by the steel tube. The STCRC is originated from a retrofit method for non-ductile RC piers in which a steel jacket is added to the potential plastic hinge region, thus known as steel-jacketed reinforced concrete (SJRC) or tubed reinforced concrete (TRC). Priestley et al. (1985) [13] studied the seismic behavior of steel tube reinforced concrete bridge piles with continuous and discontinuous steel tubes. The results showed that the discontinuous steel tube-reinforced concrete bridge piles exhibited outstanding seismic performance. Priestley et al. [14,15] conducted comparative seismic tests on four circular RC piers and four retrofitted piers with steel tubes not extended into the cap beam and foundation. Aboutaha et al. [16] combined drilling bolts with rectangular steel

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H. Wang et al. / Journal of Constructional Steel Research 166 (2020) 105919

Fig. 1. Steel tube confined reinforced concrete double-column bridge bent.

jackets to enhance the confinement effect of non-ductile RC columns. Xiao et al. [17] developed a retrofit method using welded rectilinear steel jackets and stiffeners, which significantly improved the strength and ductility of square and rectangular RC columns. Lubiewski et al. [18] proposed a method to increase the ductility of column-bent cap connections of Alaska's bridges by disconnecting the semi-embedded steel tubes from the bent cap with a 51 mm wide gap and by enlarging the bent caps in the joint region. Montejo et al. [19] investigated the seismic performance of circular RC-filled steel tube columns in multispan bridge bents, where the steel tubes were extended into the foundation at the bottom but terminated at the top connection with a cap beam. Stephens et al. [20] proposed some CFST piers-prefabricated RC cap beam connections for rapid bridge construction, in which the connection with steel tube terminated at the cap beam is similar to STCRC. Such connections are superior due to their reliable performance and construction convenience. Tomii et al. [21] first investigated STCRC columns to prevent shear failure and to improve the ductility of short columns in RC frames and boundary columns of shear walls. Han et al. [22] conducted a test on thin-walled steel tube confined concrete column to RC beam joints subjected to cyclic loading, which showed a good seismic performance. Guo et al. [23] studied the behavior of stainless

Fig. 2. Details of the STCRC double-column bridge bent specimens (unit:mm).

H. Wang et al. / Journal of Constructional Steel Research 166 (2020) 105919

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Table 1 Details of specimens. Specimens

L (mm)

h (mm)

h1 (mm)

D (mm)

tc (mm)

ls (mm)

hs (mm)

ts (mm)

hj (mm)

tj (mm)

LR

TR

Disconnection mode

n

N (kN)

STCRC1 STCRC2 STCRC3 STCRC4

1800 1800 1800 1800

800 800 800 800

600 600 600 600

200 200 200 200

2 2 2 2

400 400 400 400

150 150 150 150

4 4 4 4

200 200 200 200

4 4 4 4

4Φ10 4Φ10 4Φ10 4Φ10

Φ8@100 Φ8@100 Φ8@100 Φ8@100

Mode A Mode A Mode B Mode B

0.12 0.24 0.12 0.24

282 565 282 565

steel tube confined concrete and proposed a model to predict the axial compressive strength. Zhou and Liu et al. [24–26] conducted a series of experimental and theoretical investigations on the behavior of STCRC members and their beam-to-column joints and proposed a basic design theory. In this study, pseudo-static tests were conducted on four STCRC double-column bridge bents, considering the key influencing parameters of axial load ratio (0.12, 0.24) and the disconnection mode of steel tube (disconnected at the binding steel plates or not). The loaddisplacement hysteresis curves, mechanical properties, failure modes, and stress development of STCRC double-column bridge bents are described and discussed. Based on the test results, a simplified lateral resistance model for the STCRC double-column bridge bent is proposed.

steel tube, ls, hs, and ts are respectively the length, height, and thickness of the binding steel plate, hj and tj are respectively the height and wall-thickness of the joint casing steel tube, LR and TR respectively represent the longitudinal and transverse reinforcement in the STCRC column (4Φ10 = four 10 mm-diameter longitudinal reinforcing bars; Φ8@100 = 8 mm-diameter stirrups spaced evenly at 100 mm), and n is the axial load ratio determined by Eq. (1). n ¼ N=ð f co Ac Þ

ð1Þ

where N is the axial load, fco is the axial compressive strength of the concrete, Ac is the cross-sectional area of the specimen. 2.2. Material properties

2. Test program 2.1. Specimens Four STCRC double-column bent specimens were prepared and tested under the combined loading of constant axial load and cyclic lateral load. Fig. 2 shows the details of the specimens. Each specimen has a bottom RC foundation block of 1800 mm (length) × 800 mm (width) × 600 mm (height) with eight 40 mm diameter vertical holes for fixing and a top RC large beam of 1000 mm (length) × 400 mm (width) × 400 mm (height) with four 40 mm-diameter transverse holes for connecting the loading devices. The circular steel tubes were fabricated by butt welding after rolling the steel plate, and they were used as the formwork for columns when pouring the concrete. The double STCRC columns were connected by two binding steel plates at the midheight and top end of the columns (Fig. 2). Each binding plate was welded to the joint casing steel tubes, which were jacketed with the column steel tubes and connected by fillet welds. The reinforcement cages were placed inside the steel tube and embedded into the foundation and cap beam with a sufficient anchorage length. Note that the circular steel tubes were not inserted into the foundation and cap beam, and two disconnection modes for the steel tube were included (i.e., Mode A and Mode B, as shown in Fig. 2). For the specimens with Mode A, each circular steel tube was only disconnected at both ends of the column with a 10 mm wide gap. For the specimens with Mode B, except for the end gaps, three additional 10 mm wide strips were cut off from each circular steel tube near the joint of a binding plate. To better describe the test results presented below, all gaps from the top to bottom of the SRCRC column are referred to as G1~G5, respectively (Fig. 2). The specimen details are listed in Table 1, where L is the length of the STCRC pier, h is the external edge height of STCRC double-column bridge bent section, h1 is the distance between the centers of two circular tubes, D and tc are respectively the diameter and wall-thickness of the circular

Tensile coupons were tested according to the Chinese Standard GB50010/T228.1–2010 [27] to determine the mechanical properties of the steel plates and reinforcements used in the specimens, as indicated in Table 2. All specimens were cast using the same batch of ready-mixed concrete, and the layered concrete pouring process was adopted to ensure a more uniform and densified concrete finish. Concrete cubes (150 mm × 150 mm × 150 mm) and prisms (150 mm × 150 mm × 300 mm) were prepared and cured under the same condition to

Reaction frame 2500kN Jack

Steel hinge Top beam

N

Steel rods

P Steel hinge

Specimen Triangle corbel

Fig. 3. Test set-up.

Table 2 Material properties of the steel components. Steel type

Longitudinal reinforcement Stirrups Steel plate

Steel plate thickness (mm)

Diameter (mm)

Nominal

Measured

Nominal

Measured

– – 2.00 4.00

– – 1.90 3.92

10.0 8.00 – –

9.85 7.65 – –

Yield strength (MPa)

Ultimate strength (MPa)

474 456 355 361

581 – 523 534

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H. Wang et al. / Journal of Constructional Steel Research 166 (2020) 105919

Displacement-control

Lateral displacement Δ/Δy

Load-control 16

2500 kN hydraulic jacks were installed to the reaction frame and connected to the top beam of the specimen by steel hinges to simulate the axial load (N) and lateral load (P). The cyclic lateral loads (P) were applied after reaching the target axial load(N). The cyclic lateral loading history consisted of an elastic stage and an inelastic stage (JGJ/T 101–2015) [30]. The elastic stage was conducted under the load-control method till yielding load, with an interval of 1/3 yielding load. After the yielding load, the displacement-control method was adopted in the inelastic stage (Fig. 4), and each displacement level was cycled twice until the specimen failed or the bearing capacity of the specimens reduced to less than 85% of the peak bearing capacity [30]. The load was applied at a rate of 5 kN/s and 1 mm/s in the elastic and inelastic stages, respectively.

12 8 4 0 -4 -8 -12 -16

0

2

4

6

8

10

12

14

16

18

2.4. Measuring system

Load control history

Load cells were mounted inline with the jacks to measure the applied loads. Three linear variable differential transducers (LVDT) were employed to measure specimens' lateral displacements, in which LVDT-1 was set at the top of the specimen where the lateral load was applied, LVDT-2 was placed at the top of the STCRC double-column pier, LVDT-3 was set at the middle of the pier, and LVDT-4 was placed at the specimen foundation (Fig. 5). The measured results from LVDT4 indicated that the displacement of the specimen foundation relative to the self-balancing frame was generally less than 1 mm during the whole loading process. The strains at 10 measuring points of the binding steel plate including 5 at the top binding steel plate (P1–P5) and 5 at the middle binding steel plate (P6–P10) were recorded using strain rosettes (Fig. 5). Six pairs (P11–P16) of perpendicularly arranged strain gauges were employed to measure the strains of the circular steel tubes at the

Fig. 4. Cyclic loading history.

determine the average cubic concrete strength (fcu,150 = 49.3 MPa) and elastic modulus (Ec = 41,237 MPa) [28]. Based on the cubic strength, the average axial compressive strength of concrete fco = 39.92 MPa large top in the calculation, which was obtained according to the Model Code 1990 (CEB-FIP 1991) [29]. 2.3. Test set-up A self-balancing frame was used for the quasi-static seismic loading, as shown in Fig. 3. The bottom foundation of the specimen was fixed to the frame by eight 38 mm-diameter steel rods and triangle corbels. Two

N

P LVDT-1 P1

P4

P3

Strain rosette (steel plate)

LVDT-2 Transverse

strain

P5

P2

Strain gauge(rebar) gauge

Longitudinal strain gauge (tube) Strain rosette ( steel plate)

10

P6

P9

P8

10 LVDT-3 10

90

10

=1 80 0

P10

P7

P11-P13

P14-P16

10 @ 50 P11

P13

LVDT-4

P16

P14

P12 Fig. 5. Layout of LVDTs and strain gauges.

P15

H. Wang et al. / Journal of Constructional Steel Research 166 (2020) 105919

Fig. 6. The ultimate failure characteristics of specimens with Mode A.

bottom section (Fig. 5). Sixteen strain gauges were arranged to the left and right outmost bars at 100 mm intervals to measure the strain development of the reinforcements at the bottom and middle of the piers (Fig. 5).

Table 3 Main damage characteristics of the specimens at different loading drift. Specimens with Mode A

3. Results and discussion

Observations

STCRC1 STCRC2

3.1. Test observations and failure modes Figs. 6 and 7 show the observed test phenomena of the specimens, and Table 3 lists the main damage characteristics at different loading stages where Δ is the displacement measured by LVDT-1 (Fig. 5). All the STCRC double-column bridge bents showed a combined failure mode of column bending and binding steel plate buckling. During the whole loading process, concrete cracks at the STCRC column ends where the gaps of steel tube were located, yielding of longitudinal reinforcement, visible buckling of binding steel plates, and tearing of the joint casing steel tube were observed in turn. After the test, the steel tubes were removed to examine the damage of the core concrete. The concrete crushing was seen at the column ends, and the main difference between the specimens with Mode A and those with Mode B (Fig. 2) lied in the location of the top plastic hinges of the STCRC columns:

1%

0.75%

3% 4%

1.5% 2.25%

5%

4.5%

Specimens with Mode B

Horizontal concrete cracks at G1 and G5 (Fig. 2); Outermost rebar yielded Peak load; Steel tube yielded at the pier bottom Buckling of top and middle binding steel plates; Tearing of top and middle joint casing steel tube 85% of the peak load; Concrete crushing at the pier bottom Observations

STCRC3 STCRC4 1% 2% 3% 4%

1% 2% 3% 4%

5%

5%

Horizontal concrete cracks at G2 and G5(Fig. 2) Outermost rebar yielded Peak load; Steel tube yielded at the pier bottom Buckling of middle binding steel plates; Tearing of middle joint casing steel tube 85% of the peak load; Concrete crushing at G2 and G5

Fig. 7. The ultimate failure characteristics of specimens with Mode B.

5

6

H. Wang et al. / Journal of Constructional Steel Research 166 (2020) 105919

80

(1) For specimens with Mode A (STCRC1 and STCRC2), the top plastic hinges of the columns were located at the gaps above the top binding steel plate (G1) and significant warping was observed in the top binding steel plate; and (2) For specimens with Mode B (STCRC3 and STCRC4), the top plastic hinges of the columns were located at the gaps below the top binding steel plate (G2) and the deformation of top binding steel plate was much smaller than that of a specimen with Mode A. Note that the core concrete at the column mid-height of all test specimens had very minor damage, even for the specimens with Mode B, no significant concrete cracks were observed at G3 and G4 (Fig. 2). The measured strains of the reinforcement at the column mid-height also indicated that the bars there maintained a relatively low-stress level (about 0.2fy). Besides, the foundation and the top cap beam were robust enough despite a few cracks were found on their surfaces.

STCRC1 STCRC2 STCRC3 STCRC4

P (kN)

40

0

-40

-80 -8

-6

-4

-2

0 2 Drift (%)

4

6

8

3.2. Load versus displacement hysteresis curves and comparative analysis

Fig. 9. Comparison of load versus displacement skeleton curves.

Fig. 8 shows the lateral load (P) versus lateral displacement (Δ) hysteresis curves of the specimens, where the points corresponding to the yield of outmost longitudinal rebars and the tearing of joint steel tube are marked. All the curves show a good deformability and energy dissipation ability, as displayed by the full hysteresis loops. The pinch effect was observed, caused by the opening and closure of concrete cracks and the relative slip between the steel tube and the concrete. The disconnection mode of the external steel tube had limited influence on the hysteresis curves, indicating that the seismic performance of STCRC double-column bridge bents was mainly determined by the plastic hinges at the column ends.

Fig. 9 shows the envelop curves obtained from the P-Δ hysteretic curves. Table 4 lists the average mechanical properties calculated from the envelope curves (averaged over the positive and negative lateral loading directions), wherePy is the yield load, Δyc is the yield displacement, Pu is the peak load, Δu is the peak displacement,P0.85is the ultimate load, Δ0.85 is the ultimate displacement, μ is the displacement ductility factor (μ = Δ0.85/Δy), and φ is the ultimate drift ratio (φ = Δ0.85/L). The yield of the specimens was determined by the geometric graphic method [31], the ultimate state is defined as the moment when the lateral load reduces to 85% of Pu. According to the comparison of the principal indexes of the envelop curves (Fig. 10), it can be concluded that:

80 60

Yield of reinforcement Tube tearing

60

40

40

20

20

P(kN)

P(kN)

80

Yield of reinforcement Tube tearing

0

-20

0

-20

-40

-40

-60

-60

-80

-80 -8

-6

-4

-2 0 2 Drift (%)

4

6

8

-8

-6

-4

(a) STCRC1 80

80

6

8

4

6

8

Yield of reinforcement Tube tearing

60

40

40

20

20

P(kN)

P(kN)

4

(b) STCRC2

Yield of reinforcement Tube tearing

60

-2 0 2 Drift (%)

0

0

-20

-20 -40

-40

-60

-60 -80

-80 -8

-6

-4

-2 0 2 Drift (%)

(c) STCRC3

4

6

8

-8

-6

-4

-2 0 2 Drift (%)

(d) STCRC4

Fig. 8. Lateral load (P) versus lateral displacement (Δ) hysteretic curves of the specimens.

H. Wang et al. / Journal of Constructional Steel Research 166 (2020) 105919

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Table 4 Characteristic values of all specimens. Specimen

n

λ

Py (kN)

Δyc (mm)

Pu (kN)

Δu (mm)

P0.85 (kN)

Δ0.85 (mm)

μ

φ

Kel (kN/mm)

STCRC1 STCRC2 STCRC3 STCRC4

0.12 0.24 0.12 0.24

2.5 2.5 2.5 2.5

53.81 62.73 51.17 61.43

21.35 15.85 20.01 19.73

62.85 72.62 63.31 71.00

50.99 27.12 50.95 33.83

53.42 61.73 53.81 60.35

88.68 61.87 86.87 87.32

4.15 3.90 4.34 4.43

4.43% 3.09% 4.34% 4.37%

2.52 3.96 2.56 3.11

(1) The lateral load capacities of the specimens with the axial load ratio of 0.24 are larger by more than 10% than those of the specimens with an axial load ratio of 0.12, due to the improvement of bending strength of the plastic hinges in STCRC columns with the increasing axial compression in this range. (2) Increasing the axial load ratio improves the lateral elastic stiffness of test (Kel) of the STCRC double-column bridge bent to a certain extent, but weakens its deformability. For specimens with Mode A, the displacement ductility factor and ultimate drift ratio under the axial load ratio of 0.12 increase by 6.4% and 43.4% respecitively than those of the specimens under the axial load ratio of 0.24. (3) The disconnection mode of the steel tube has an insignificant influence on the load capacity of the STCRC double-column bridge bent. The displacement ductility factor and ultimate drift ratio of Specimen STCRC4 (Mode B, n = 0.24) increase by 13.5% and 41.4% respectively than those of Specimen STCRC2 (Mode A, n = 0.24).

3.3. Load versus stress curves of steel plate and circular steel tube Based on the recorded strains of steel plate and circular steel tube, the stress development was studied using the elastic-plastic analysis

method [12]. Fig. 11 shows the steel plate analysis results of a typical specimen, where σv, σh, στ, and σm represent the longitudinal stress, transverse stress, shear stress, and von Mises stress, respectively. Note that: (1) σv and σh are positive in tension and negative in compression; and (2) the directions of σv and σh in the steel tube correspond to the tube height and tube circumference respectively and the counterparts in the binding steel plate correspond to the plate width and plate length respectively; (3) P6–P8 and P11–P13 indicate the measuring points (Fig. 5); and (4) k0 is a coefficient defined in Reference [12]. At the ends of binding steel plate, the transverse tensile and compressive stresses (Fig. 11a and Fig. 11c) developed rapidly with the loading. At the peak load, the transverse stresses generally reached the yield strength of steel, showing a bending mechanism. Diagonal shear stresses were observed in the binding plate ends and middle region. However, compared to the transverse stresses, the shear stresses were relatively small and were not dominant in the ultimate strength of the binding plate. For the circular steel tube, the longitudinal and transverse stresses at the column end increased due to the accumulation of bonding/friction (there is distance between the location of the longitudinal strain gauge and the tube gap) and the concrete dilation, respectively. The transverse stresses of the steel tube generally remained at a high level and provided an effective confinement to the concrete core. At the measurement point P11,

Fig. 10. Comparison of the principal indexes of the envelop curves.

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the steel tube was approximately yielded transversely at the final state of the loading.

4. Lateral resistance model for STCRC double-column bridge bents

80 60 40 20 0 -20 -40 -60 -80 -1.2 -0.8 -0.4

v

P(kN)

P(kN)

Based on the test results, the ultimate limit state of the STCRC double-column bridge bent can be simplified as a frame model with column plastic hinges, as shown in Fig. 12, for predicting the lateral resistance. This simplified model has the following distinct features:

(1) The locations of column plastic hinges are determined according to the failure modes observed from the test, where the top column hinges of specimens with Mode A are located at the first gap of the steel tube (G1) and those of specimens with Mode B are located at the second gap of the steel tube (G2). (2) Considering that the bending stresses at the ends of a binding steel plate are much larger than the diagonal tensile stresses at the middle of the plate observed during the test, the binding steel plate may be simplified as a flexural member rather than a diagonal tension member. Furthermore, the elastic section modulus (Wel) was adopted when calculating the bending

h

m

0.0 0.4 / k0 fy

0.8

1.2

80 60 40 20 0 -20 -40 -60 -80 -1.2 -0.8 -0.4

80 60 40 20 0 -20 -40 -60 -80 -1.2 -0.8 -0.4

v

h

m

0.0 0.4 / k0 fy

0.8

1.2

80 60 40 20 0 -20 -40 -60 -80 -1.2 -0.8 -0.4

m

0.0 0.4 /k0 fy

0.8

1.2

v

h m

0.0 0.4 / k0 fy

0.8

1.2

(e) P12

v

P(kN)

P(kN)

(b) P8

80 60 40 20 0 -20 -40 -60 -80 -1.2 -0.8 -0.4

h

(d) P11

P(kN)

P(kN)

(a) P6

v

h

m

0.0 0.4 / k0 fy

0.8

1.2

80 60 40 20 0 -20 -40 -60 -80 -1.2 -0.8 -0.4

(c) P7

v

h m

0.0 0.4 / k0fy

(f) P13 Fig. 11. Lateral load versus stress curves of a typical specimen.

0.8

1.2

H. Wang et al. / Journal of Constructional Steel Research 166 (2020) 105919

N/2 Internal force details and simplifications

Mp

N

N

Δ

P

P V / 2=P/2

Vs

9

Ms

Vs

Mp

Ms

Mp Ms

Mp

Mp

L

Ms

Ms

L-hj

Ms

Ms

Ms

V / 2=P/2 Mp

Mp

Mp

N/2+2Vs ≈ N/2

(a) Specimens with Mode A

Mp

Mp

(b) Specimens with Mode B

Fig. 12. Lateral resistance model for STCRC double-column bridge bents.

moment of the binding plate (Ms). As for specimens under a low axial load ratio of 0.12, the elastic section modulus was modified by a plastic development factor (γu) of 1.2. (3) Both the lateral load (P) and the axial compression (N) are assumed to be evenly distributed to the two STCRC columns in the ultimate limit state. In addition, the influence of shear forces in the binding plates on the axial forces at the bottom column hinges is ignored considering their anti-symmetric nature. The plastic bending capacity (Mp) of the circular STCRC column hinge corresponding to the axial compression (N/2) can be estimated by the axial force versus bending moment interaction curves proposed in Reference [32]. (4) The N-Δ effect should be considered in this model. The lateral displacement of the top column hinge is assumed to be equal to that at the loading point on the cap beam. Based on the above-simplified model, the following equations can be derived from the bending equilibrium to estimate the lateral load capacity of STCRC double-column bridge bents.

Vu ¼

8 4Mp þ 4M s −NΔu > > > < L

Mode A

> 4Mp þ 2M s −NΔu > > : L−h j

Mode B

ð2Þ

2

Ms ¼ γ u f ysp W el ¼ γ u f ysp t s hs =6

Lateral resistance (kN)

100

Pu Vu

80 60

ð3Þ

R=Vu/Pu R=1.03

R=1.05 R=0.99

R=0.92

40 20 0 RC1

STC

RC2

STC

RC3 STC

RC4

STC

Fig. 13. Comparison of the predictions by the simplified model and the test results.

where γu=1.2 for specimens under axial load ratio of 0.12 and γu=1.0 for specimens axial load ratio of 0.24. Fig. 13 shows the comparison between the lateral load capacities predicted by the simplified model and the test results, which demonstrates a good agreement.

5. Conclusions In this study, four STCRC double-column bridge bents were tested under pseudo-static loads, considering the key influencing parameters of axial load ratio (0.12 and 0.24) and the disconnection mode of the steel tube (Mode A and Mode B, Fig. 2). The loaddisplacement hysteresis curves, mechanical properties, failure modes, and stress development of STCRC double-column bridge bents are described and discussed in detail. Based on the test results, a simplified analytical model for STCRC double-column bridge bents is proposed. The main conclusions are as follows: (1) The failure mode of the tested STCRC double-column bridge bents was characterized by the column bending accompanied by the flexural buckling of binding steel plates. As the disconnection pattern of the steel tube changed from Mode A to Mode B, the position of the top STCRC plastic hinge changed from the gap region above the binding steel plate (G1, Fig. 2) to the gap region below the binding steel plate (G2, Fig. 2). (2) Increasing the axial load ratio from 0.12 to 0.24 could improve the lateral load capacity and flexural stiffness of the STCRC double-column bridge bent to a certain extent but reduced the peak and ultimate displacement. (3) Except that the specimens with Mode B appeared to be superior to the specimens with Mode A in ductility under the axial load ratio of 0.24, the disconnection mode of steel tube generally had limited influence on the seismic behavior of STCRC doublecolumn bridge bents. (4) The lateral load capacities of the STCRC double-column bridge bent specimens were well predicted by the proposed simplified frame model with plastic hinges in the STCRC columns and binding steel plates. (5) Because of the effective confinement of the circular steel tube, the STCRC double-column bridge bents generally performed a relatively good seismic behavior with limited damages at the column ends. However, the binding steel plates in this system had some issues, such as series buckling and joint tearing. Therefore,

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H. Wang et al. / Journal of Constructional Steel Research 166 (2020) 105919

the connection between the two STCRC columns needs to be further studied. Notation Ac D fcc fco fy fys fysp h h1 hs hj Kel k0 L Le ls Mp Ms N n Py Pu P0.85 R tc ts tj Vf V Vs Wel μ σy σh σv στ σm Δy Δu Δ0.85 φ γu

interest that represents a conflict of interest in connection with the work submitted. Acknowledgments

cross-sectional area of the specimen diameter of the circular steel tube confined concrete strength unconfined concrete strength yield strength of longitudinal reinforcement yield strength of steel tube yield strength of binding steel plate external edge height of STCRC double-column bridge bent section distance between centers of two circular tubes in the STCRC double-column bridge bents section height of binding steel plate height of joint casing steel tube lateral elastic stiffness of test steel material constant length of specimen distance between the lateral loading point and the top surface of the foundation length of binding steel plate full plastic moment of single circular STCRC corresponding the applied axial load (N/2) full plastic moment of binding steel plate axial load axial load ratio yield load peak load ultimate load V to Pu ratio the thickness of circular steel tube the thickness of binding steel plate wall-thickness of joint casing steel tube shear of STCRC double-column bridge bents without N-Δ effect shear of STCRC double-column bridge bents shear of binding steel plate elastic section modulus displacement ductility factor transverse stress distribution of binding steel plate transverse stress of steel longitudinal stress of steel shear stress of steel Von Misses stress of steel yield displacement peak displacement ultimate displacement ultimate drift ratio plastic development factor

Author statement Prof. Xuhong Zhou and Prof. Jiepeng Liu conceived and designed the structure of the manuscript. Ms. Haicui Wang and Dr. Xuanding Wang conducted the experimental and theoretical analyses and written the first draft. Prof. Y. Frank Chen and Dr. Xuanding Wang polished the language of the manuscript. All the authors have given approval to the final version of the manuscript. Declaration of Competing Interest The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative

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