A pre-tensioned prestressed precast beam with an anchor head at one end

Abstract

The present invention relates to a pre-tensioned prestressed precast beam with an anchor head at the end and a manufacturing method thereof. By arranging an anchor at the end, the pre-tensioned prestressed precast beam enables the prestressed steel strands to achieve the same permanent prestress as that in the mid-span within a very short distance at the beam end, increasing the prestress effect within the transfer length of the traditional pre-tensioned prestressed beam. The present invention can solve the problem of insufficient prestress effect within the transfer length range at the end of the traditional pre-tensioned prestressed beam.

Description

Technical Field

[0001] The present invention relates to the field of civil engineering, and particularly to a pretensioned prestressed precast beam with an anchor head at the end and a manufacturing method thereof. Background Art

[0002] At present, the anchorage of the steel strands of pretensioned prestressed beams mainly relies on the bond of concrete to the steel strands. Therefore, there is a transfer length at the end of the precast member. Within this transfer length range, the stress of the steel strands gradually increases from zero to the permanent stress. The transfer length is mainly related to the diameter of the steel strands, the effective prestress value of the prestressed steel bars during tension release, the shape of the prestressed steel bars, and the strength grade of the concrete (Code for Design of Highway Reinforced Concrete and Prestressed Concrete Bridges and Culverts JTG 3362-2018). For large-diameter high-strength prestressed steel strands, this transfer length is very long, about 1.3 m or even longer. For precast beams with a large span and an open cross-section, the shear stress near the support is relatively large (including the shear stress generated by shear force and the shear stress generated by torsion). According to the calculation formula of the principal tensile stress, the principal tensile stress of the concrete near the beam end can be reduced by increasing the precompression stress provided by the prestressed steel bars. However, the prestress effect within the prestress transfer length range of traditional pretensioned members is relatively small. Therefore, reducing the transfer length or changing the transfer method of the steel strands so that the steel strands can quickly exert their precompression stress on the concrete at the beam end plays a very important role in reducing the principal tensile stress near the support.

[0003] Taking a T-beam with a span of 22 m and a beam height of 0.95 m as an example, the support of the beam is 0.3 m away from the beam end. Within a range of 0.5 m on the mid-span side of the support, the shear stress at the intersection of the web and the top plate of the T-beam is relatively large. There is also a normal stress generated by the steel strands within this range. From the previous analysis, it can be seen that the greater the normal stress generated by the steel strands within this range, the smaller the principal tensile stress within this range.

[0004] To solve this problem, the method of setting an anchor at the beam end can be adopted. However, if a traditional wedge anchor is used, the prestress often cannot be applied to the concrete beam due to the retraction of the wedges. According to the literature (Code for Design of Highway Reinforced Concrete and Prestressed Concrete Bridges and Culverts JTG 3362-2018), the retraction amount of the wedges of the wedge anchor is between 4 and 6 mm. If the retraction deformation of the anchor occurs after the concrete hardens, the prestress loss of the steel strands within a range of 1.3 m at the beam end will reach 6 / 1300 * 1.95e5 = 900 MPa. Therefore, special anchors and construction methods must be adopted to minimize the deformation amount of the anchor after the concrete hardens. Summary of the Invention

[0005] The purpose of the present invention is to overcome the defects of the above-mentioned existing technologies and provide a pretensioned prestressed beam with an anchor head at the end and a manufacturing method thereof.

[0006] The object of the present invention can be achieved by the following technical solutions:

[0007] A pretensioned prestressed beam with an anchor head at the end, characterized in that an anchor is provided at the end of the pretensioned prestressed beam, a wedge grip is provided at the inner end of the anchor for clamping the steel strand, and an internal thread is provided at the outer end for connecting the tension rod; the anchor is in close contact with the concrete of the precast member or embedded in the concrete.

[0008] Further, the anchor provided at the end is a wedge grip anchor, or can also be a wedge grip anchor with a nut.

[0009] Further, according to an embodiment of the present invention, the anchor includes an anchor cup, one end of the anchor cup is thicker and provided with an internal thread, the other end is thinner and provided with a wedge grip taper hole, a wedge grip is provided in the wedge grip taper hole, and the thick end and the thin end are connected by an inward concave arc surface.

[0010] Further, according to other embodiments of the present invention, the anchor includes an anchor cup, the anchor cup is a cylinder with the same thickness at both ends, or a cuboid with the same thickness at both ends, an internal thread is provided at one end of the anchor cup, and a wedge grip taper hole is provided at the other end, and a wedge grip is provided in the wedge grip taper hole.

[0011] Further, when the steel strands of two beams are tensioned together, wedge grips are provided at the inner ends of the anchors at both ends of the pretensioned prestressed beam for clamping both ends of the steel strand respectively, internal threads are provided at the outer ends, the outer end of one of the anchors is connected to the tension rod, and the outer side of the other anchor is connected to the steel strand extension rod for connection with the anchor at the end of the other beam.

[0012] Further, the manufacturing method of the pretensioned precast beam includes the following steps:

[0013] The first step: Calculate the stress-free length of the steel strand according to the length of the member, cut the steel strand according to the calculated stress-free length, and install an anchor at the end of the steel strand;

[0014] The second step: Place the steel reinforcement cage and the steel strand in the formwork and position them;

[0015] The third step: Install the tension rod;

[0016] The fourth step: Tension the steel strand;

[0017] The fifth step: Pour the concrete and cure it;

[0018] The sixth step: After meeting the requirements for relaxation, simultaneously relax the tension at both ends of the member;

[0019] The seventh step: Remove the extension rod and the tension rod;

[0020] The eighth step: Lift and store the precast beam.

[0021] The beam end anchor with a clamping piece at one end and an internal thread at the other end adopted by the present invention can greatly reduce the prestress loss caused by the retraction of the clamping piece of the anchor, ensuring that the prestressing force can be applied to the concrete beam within a short distance, and improving the beam end prestress effect of the pretensioned prestressed beam. Compared with the prior art, the pretensioned prestressed beam with an anchor head at the end of the present invention can effectively reduce the principal tensile stress at the beam end under the same structural dimensions of the beam, and improve the crack resistance of the precast beam. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is the elevation view of the pretensioned prestressed beam with an anchor head at the end in Embodiment 1 of the present invention;

[0023] Figure 2 It is the plan view of the pretensioned prestressed beam with an anchor head at the end in Embodiment 1 of the present invention;

[0024] Figure 3 It is the cross-sectional view at the beam end of the pretensioned prestressed beam with an anchor head at the end in Embodiment 1 of the present invention;

[0025] Figure 4 It is the cross-sectional view at the mid-span of the pretensioned prestressed beam with an anchor head at the end in Embodiment 1 of the present invention;

[0026] Figure 5 It is the construction schematic diagram before the stress relaxation of the steel strand during the precast process of the pretensioned prestressed beam with an anchor head at the end in Embodiment 1 of the present invention.

[0027] Figure 6 It is the partial enlarged view at the beam end anchor head during the precast process of the pretensioned prestressed beam with an anchor head at the end in Embodiment 1 of the present invention.

[0028] Figure 7 It is the stress distribution diagram of the prestressing steel strand near the beam end in the pretensioned members of the traditional pretensioning method and the pretensioning method using the anchor of the present invention.

[0029] In the figure, 1 - anchor, 2 - steel strand, 3 - concrete, 4 - tension rod, 5 - reaction column, 6 - tension cross beam, 7 - jack, 8 - end formwork, 9 - bottom formwork, 11 - anchor cup, 12 - clamping piece, 13 - thread. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] The present invention will be described in detail below with reference to the drawings and specific embodiments.

[0031] Embodiment 1

[0032] A pretensioned prestressed concrete rectangular beam with an anchor head at the end, the beam length is 16m, the beam width is 0.3m, the beam height is 1.0m, equipped with 4 steel strands, C50 concrete, and its structure is as Figure 1 shown.

[0033] The anchor device provided at the end of the beam is embedded in the concrete, and the wedge anchor is adopted as the anchor device. A wedge is provided at the inner end of the anchor device for clamping the steel strand, and the outer end is provided with an internal thread for connecting the tension rod; the anchor device is closely attached to the concrete of the precast member or embedded in the concrete. The anchor cup of the anchor device can have a shape with one end thick and one end thin, or can have a shape with the same dimensions at both ends, and can have a circular cross-section or a rectangular cross-section.

[0034] The main construction steps of the pretensioned prestressed precast beam are as follows:

[0035] The first step: Calculate the stress-free length of the steel strand according to the length of the member, cut the steel strand according to the calculated stress-free length, and install the anchor device at the end of the steel strand.

[0036] The second step: Place the steel reinforcement cage and the steel strand in the formwork and position them.

[0037] The third step: Install the tension rod.

[0038] The fourth step: Tension the steel strand.

[0039] The fifth step: Pour the concrete and cure it.

[0040] The sixth step: After meeting the requirements for relaxation of tension, simultaneously relax the tension at both ends of the member.

[0041] The seventh step: Remove the tension rod.

[0042] The eighth step: Lift and store the precast beam.

[0043] Taking a T-beam with a span of 22 m and a beam height of 0.95 m as an example, and taking the distance between the supports of the beam from the beam end as 0.3 m, the anchor device of the present invention is arranged at a position about 5 cm from the beam end. Figure 7 The stress distribution 1 and the stress distribution 2 in the figure respectively correspond to the stress distributions in the prestressed steel strands near the beam end in the traditional pretensioning method and the pretensioning method using the anchor device of the present invention. Obviously, the prestress effect of the present invention near the beam end is greatly improved, so the principal tensile stress of the concrete beam at the beam end will also be greatly reduced.

[0044] The above description of the embodiments is for the convenience of those of ordinary skill in the art to understand and use the invention. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative labor. Therefore, the present invention is not limited to the above embodiments, and the improvements and modifications made by those skilled in the art according to the disclosure of the present invention without departing from the scope of the present invention should be within the protection scope of the present invention.

Claims

1. A manufacturing method of a pretensioned prestressed beam with an anchor at the end, characterized in that An anchor is provided at the end of the pretensioned prestressed beam. A wedge is provided at one end of the anchor for clamping the steel strand, and an internal thread is provided at the other end for connecting the tension rod. The anchor is in close contact with the concrete of the precast member or embedded in the concrete. The manufacturing method includes the following construction steps: Step 1: Calculate the stress-free length of the steel strand according to the length of the member, cut the steel strand according to the calculated stress-free length, and install an anchor at the end of the steel strand. Step 2: Place the steel reinforcement cage and the steel strand in the formwork and position them. Step 3: Install the tension rod. Step 4: Tension the steel strand. Step 5: Pour the concrete and cure it. Step 6: After meeting the requirements for tension release, tension release is carried out simultaneously at both ends of the member. Step 7: Remove the extension rod and the tension rod. Step 8: Lift and store the precast beam.

2. The manufacturing method of a pre-tensioned prestressed beam with an anchor at the end according to claim 1, characterized in that, The anchor provided at the end is a wedge anchor or a wedge anchor with a nut.

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