2.1 Specimen design and construction
Full-scale loading tests were carried out on a prestressed segmental precast pier modeled after that used in the Hetian Ruoqiang railway. The Hetian Ruoqiang railway is located in Hetian city in the south of the Xinjiang Uygur Autonomous Region. Figure 1 shows the geometry and section details of the prestressed segmental precast pier. The mechanical properties of the materials used to form the prestressed segmental precast pier specimens are listed in Table 1. The prestressed segmental precast pier is divided into three parts: the cap beam, the pier body segments, and the bearing platform. Each segment of the pier body is made separately, and then they are assembled to create the bridge pier when the concrete strength reaches the design strength. 12 high strength and low relaxation self-anchored prestressed tendons with a diameter of 15.2 mm were used to form the prestressed system. The standard tensile strength of each prestressed tendon is fpk = 1,860 MPa. The tension control stress at the anchor is 1,116 MPa.
The construction procedure of the prestressed segmental precast pier is as follows:
(1) The metal bellows were embedded in the bearing platform, the column, and the cap beam, forming the tendon duct during the prefabricating process.
(2) The pier column was placed in position (Fig. 2 (a)).
(3) The cap beam was placed in position (Fig. 2 (b)).
(4) The tendons pass through the metal bellows from the top of the cap beam to the bottom of the column. After self-anchoring the prestressed tendons at the bottom of the column, the prestressed tendons were tensioned at the top of the bent cap. The tension stress of each prestressed tendon was controlled at 1,116 MPa. After the tension stress became stable, the top of the prestressed tendons was anchored (Fig. 2 (c)).
(5) Backfill concrete mortar was poured into the reserved grooves of the bearing platform. Concrete mortar and grout were poured in from the top of the bent cap to the bottom, sealing the metal bellows, preventing the corrosion of the tendons, and ensuring the synchronous deformation of the tendons and each segment of the pier (Fig. 2 (d)).
Table 1. Properties of the prestressed segmental precast pier sample
specimen
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Segmental columns
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Prestressed tendon
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Material: 12 tendons of diameter 15.2 mm
Yielding stress (MPa): 1890; Initial stress (MPa): 1,116
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Longitudinal reinforcement bar
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Material: hot-rolled ribbed bars (HRB) with a diameter of 12 mm, accounting for a reinforcement ratio of approximately 1.0%.
Yielding stress (MPa): 410
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Transverse reinforcement bar
Stirrup spacing: 100 mm (Encrypted area);
Stirrup spacing: 150 mm
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Material: 12 mm diameter
Yielding stress (MPa): 410
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Strength of concrete
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50 MPa
The concrete grade is C50 with the standard value of cubic compressive strength of 50 MPa
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2.2 Test overview
2.2.1 Loading system
A vertical reaction frame was set up to impose a constant vertical load of 2,000 kN on the top of the bent cap to simulate the dead load of a bridge superstructure. As shown in Fig. 3, the steel beam of the reaction frame, placed above the cap beam, is connected to the cushion cap with 32 D32 mm thread steel bars using anchors embedded in the cushion cap. Thus, the loading steel beam, steel bars, and cushion cap form the vertical reaction frame. Four jacks were set between the loading steel beam and the pier bent cap. The axial load, applied at the top of the bent cap, was maintained throughout the experiment to simulate the permanent load acting on the top of the pier. To reduce the influence of the vertical reaction frame on the horizontal displacement of the pier, two metal friction bearings with very small friction coefficients were set between the jacks and the loading steel beam, as shown in Fig. 3.
A horizontal load was applied at the bent cap through a horizontal reaction frame. As shown in Fig. 3, this frame is a 15 m high steel frame with very high stiffness. Sufficient concrete blocks were placed at the bottom of the horizontal reaction frame to ensure that it did not overturn during the loading procedure. The bent cap was connected to the horizontal reaction frame with a prestressed tendon, which passed through a 15 cm diameter hole at the center of the bent cap and was anchored at the side of the cap, while its other end passed through the jack, which was installed on the horizontal reaction frame. The horizontal displacement was imposed at the bent cap through the jack.
2.2.2 Load programs
The horizontal displacement was applied step by step through the jack installed on the horizontal reaction frame, as shown in Fig. 3. The imposed displacement at the cap beam was gradually increased from 5 mm to 130 mm at an increment of 5 mm. During the test, a laser displacement sensor measured the relative displacement between the cap beam and the top surface of the bearing platform.
2.3 Load-displacement curves
The relationship between the horizontal load and the horizontal displacement at the top of the pier is plotted in Fig. 4. Based on the relationship curve, the parameters of the main characteristic points (yield point, peak point, and limit point) were obtained as shown in Table 3. The yield point was determined by the park method (Shim et al. 2008). It can be noted that the maximum bearing capacity is 1,479 kN, and the equivalent yield stiffness is 22,020 kN/m.
Table 2. Key values of the load-displacement curves from the experiment
First crack
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Equivalent yield point
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Maximum horizontal force
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Force
[kN]
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Displacement
[mm]
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Force
Fy /kN
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Displacement
δy /mm
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Force
Fu /kN
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Displacement
δu /mm
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904
|
31.2
|
1090
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49.5
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1479
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122.6
|