1. Introduction
With the rapid development of national high-speed railways and expressway projects, prestressed structures have been increasingly widely used. Various new high-strength prestressed members have been designed, and various new anchorages are being widely used, among which the most important application is for various new bridges. The new 2000 MPa parallel steel wire stay cable used for the Hutong Yangtze River Bridge was specifically designed with a supporting anchorage in the research and development stage, and the stress and deformation in the anchorage were analyzed using the finite element method, the results of which verified that the strength and stiffness of the anchorage met the requirements [
1]. Similarly, in the research and development of a 1960 MPa main cable steel wire and cable strand for the Humen Second Bridge, the corresponding anchorage was specifically designed, the design drawing was created, and finite element analysis was performed for verification [
2]. In the construction of the Wuhan Qingshan Yangtze River Bridge, the anchorage with a 1860 MPa stay cable was specifically designed, and 42CrMo as selected as the anchorage material. The anchorage size was studied and checked using finite element analysis [
3]. During the construction of the Third Yangtze River Bridge in Wuhu, a DM upsetting anchorage was used to stretch the steel wire, and a series of measurements were recorded to avoid quality problems such as fractures caused by eccentric stress at the pier head [
4]. In the construction of the Wuhe Dinghuai Huaihe River Bridge, rotary cables in the same direction were adopted, a special anchorage system was designed, and specific construction technology was studied [
5].
The development of special anchorages is sometimes aimed at specific materials; for example, when FRP composite materials have been used to strengthen prestressed members, many scholars have designed matching anchorages. Yang Zeying et al. [
6] designed a clip-bonded anchorage with a jack to tension CFRP bars. Zhang Baojing et al. [
7] designed a set of anchorages for tensioning and strengthening CFPR plates and tested the mechanical properties of strengthened beams. Guo Rong et al. [
8] designed a new anchorage for bonded prestressed CFRP plates, which were used to strengthen RC beams for a fatigue performance test. Shang Shouping et al. [
9] adopted a self-developed CFRP plate tensioning anchorage device for an experimental study on the creep performance of strengthened beams on bridges, which could prevent the prestress loss caused by anchorage slip. Viktor Gribniak et al. [
10] designed a frictional anchorage system for flat CFRP ribbon strips and developed an anchorage prototype optimizing the geometry for compact spiral distribution to ensure the gripping system’s specific traction ratio. A new split wedge anchorage system was proposed for FRP cables by Damiani et al. [
11], which has a cable capacity of 257 kN. Shakiba et al. [
12] set handmade mat anchorages between concrete and GFRP bars to enhance bonding. Bernd Zwingmann et al. [
13], Xiao Xinhui et al. [
14,
15], Deng Enfeng et al. [
16], and Masoud Abedini et al. [
17] also performed related studies.
After the anchorage design is completed, experimental studies of its mechanical properties are usually required. Zhong Xingu et al. [
18] studied the tangential stiffness between different anchorages and backing plates through field tests and theoretical analyses. Shi Long et al. [
19] designed a complete set of anchorage systems, including an anchor backing plate and a clip, to cope with the tension of 2000 MPa prestressed steel strands and performed numerical simulations and static load testing to study the anchorage’s performance. In the study of long-span prestressed beamless floor slabs, Zhang Caigang et al. [
20] used clip anchorage to tension prestressed tendons, described the tensioning process and key points, and optimized the design.
Many scholars have studied anchorages matched with some special materials. Du Yunxing and Tang Ziyun [
21] designed a new type of inorganic adhesive-impregnated carbon fiber bundle clamp anchorage, determined the appropriate anchorage size, and tested the anchorage performance through static tensile testing. Sun Shengjiang et al. [
22] designed a straight-tube bonded anchorage for composite steel bars composed of basalt fibers to test the tensile properties of the composite steel bars. Fei Hanbing et al. [
23] designed a new anchorage matched with filled epoxy-coated steel strand, determined the most suitable anchorage size design through finite element simulation analysis, and conducted static load tests to verify its performance.
The above results show that researchers have extensively studied anchorages in various application scenarios, and a variety of new anchorages have been designed. At present, prestress is usually applied to a post-tensioned hollow slab beam or thin concrete structure in China by directly tensioning prestressed flat anchorages with single-hole prestress tensioning equipment, as shown in
Figure 1 [
24]. However, in the traditional method, because the structure of the flat anchorage is compact, and the outer edge of the clip in the hole on the anchorage is exposed outside the end face, the actual spacing of each hole is small. As such, in the butt joint process between the tension jack and the flat anchorage, the clip on the adjacent hole position of the butt joint hole and the front-end base of the jack may contact and squeeze. Sometimes, the base at the front end of the jack may even touch the adjacent steel strand, which results in the outer end of the clip on the adjacent hole position extruding and deforming, which can easily cause the sliding of the tensioned steel strand or the inclination of the tension direction of the jack, and the steel strand may even break. To avoid these problems, simply reducing the thickness or length of the front base of the jack seriously affects the use strength of the jack, causing the jack to deform during use and the wires to become slippery. If serious, the clip can fly out due to slippery wires, which poses safety risks.
Due to the use of circular limit plates in circular anchorages, the above risks do not arise during the construction process; however, owing to the structural size limitations, flat anchorages are not equipped with a better limit device. To avoid the abovementioned safety problems in the tensioning process of flat anchorages, we used the idea of a circular anchorage limit plate as a reference, designed a flat anchorage limit plate, analyzed the mechanical properties of the new type of limit device in the tensioning process with finite element software, and initially applied the proposed limit plate on a construction site. The results showed that the limit plate avoids the aforementioned safety problems, providing a solution for reducing the tension risk of flat anchorages for similar projects.
3. Establishment of Numerical Model of Flat Anchorage
According to the flat anchorage limit plate described in the previous section, a numerical analysis model was established using ANSYS finite element software, and the hexahedron element in the solid element was adopted. The model included a flat anchorage, a clip, a steel strand, and the newly designed limit plate, as shown in
Figure 5. The conventional flat anchorage model without a limit plate for a comparative study is also shown in
Figure 5a.
Figure 5b shows the combined model of the limit plate and flat anchorage,
Figure 5c shows a model of the clip,
Figure 5d shows the steel strand model, and
Figure 5e shows the limit plate model. As the main component materials involved were all steel, the structural steel simulation in the material library was directly selected in the software, and the elastic model was selected as the constitutive model. The material parameters involved in the numerical simulation are detailed in
Table 1.
The model dimensions were as follows: the outer contour size of the flat anchorage was 185 mm (length) × 48 mm (width) × 50 mm (height), the diameter of the bottom hole was 18 mm, the diameter of the top taper hole was 27 mm, the taper of the hole was 14°, the center distance between holes was 34 mm, and the edge of the side hole was 24.5 mm away from the short side of anchorage and 10.5 mm away from the long sides of both sides. The diameter of the steel strand was 12 mm, the height of the clip was 54 mm, the diameter of the bottom was 18.9 mm, and that of the top is 27.72 mm. The length and width of the limit plate were the same as that of the flat anchorage, with a height of 50 mm; the bottom cylindrical hole had a diameter of 12 mm, and the upper cylindrical hole had a diameter of 29 mm and a depth of 8 mm.
The boundary conditions and internal contact settings of the model were as follows: the bottom surface of the flat anchorage was arranged as a fixed support; the contact surface between the top surface of the flat anchorage and the bottom of the limit plate was arranged to be only supported by compression; friction contact was arranged between the clip and the steel strand and between the clip and the anchorage. No contact occurred between the limit plate and the steel strand.
5. Construction Site Application
On the basis of previous design and research, a batch of flat anchorage limit plates were manufactured using 45# steel, as shown in
Figure 12a. For the dimensions, we referred to the flat anchorage design; the dimensions were basically consistent with those mentioned in the numerical simulation. As shown in
Figure 12b, the extended clip was placed in the second limit hole of the limit plate, and the steel strand passed through the first limit hole, finally forming the shape shown in
Figure 12c. As such, the limit plate adequately protected the flat anchorage, the top surface was a plane, no clips were protruding, the hole spacing increased, and the jack tension construction was convenient.
The device was used in the construction of narrow areas of the Huainan Huaishang Huaihe River Highway Bridge and Chizhou Yangtze River Bridge. The use position is as described in
Section 2.1, mainly as the upper plate structure of the bridge box girder.
Figure 13a,b show the construction site where workers used the new flat anchorage limit plate for tensioning, and the member next to the jack in the figure is the limit plate. After the application of the newly designed limit plate, no safety problems occurred at the construction site, such as sliding wires or squeezing deformation of clip by jack, and the construction efficiency was increased, which verified the feasibility of the design. Because of the low cost of the limit plate, this technology is convenient for popular use.
6. Conclusions
(1) A limit plate for use with flat anchorages was designed, which changes the jack tension work into a butt limit plate, increases the hole distance, and protects the clips. In field practice, the plate prevents safety hazards such as slippery wires and clips being squeezed and deformed by jacks, as well as increases the tension efficiency.
(2) Through numerical simulation, we found that the stress concentration of flat anchorages in the tensioning process was effectively alleviated using the limit plate, and the extreme stress value decreased by 10–13%. Because the limit plate is a replaceable member and the flat anchorage is a permanent member, the adverse effects of tensioning force are passed on to the replaceable limit plate, thus improving the reliability of flat anchorages.
(3) By simulating the five-hole tensioning process, we found that the ultimate extreme stress value of the anchorage differed little under different tensioning sequences, and the standard deviation of the extreme stress value after adding the limit plate was only 8.37 kPa. The symmetrical tensioning scheme represented by the sequence tensioning of holes 1, 4, 2, 5, and 3 should be adopted, as the ultimate stress extreme was the smallest at 685.55 kPa, being 22.42 kPa lower than the maximum value in other various schemes.
At present, although a numerical simulation was conducted on the proposed prestressed tensioning flat anchorage system including a limit plate, we found that it could effectively alleviate the stress concentration in the tensioning construction process, and we confirmed that the new flat anchorage system can avoid safety hazards such as sliding wires and clip deformation through a field application. However, more systematic research needs to be conducted, mainly including the determination of the stress and strain of the limit plate in the actual tensioning process, the manufacturing materials of the limit plate, and a sensitivity analysis of the design parameters, to verify and improve the safety of the limit plate.