Method for preventing cracks at the end of a precast box girder caused by post-tensioning

Abstract

The application discloses a method for preventing cracks at the beam end of a precast box girder caused by the post-tensioning method, and comprises the following steps: setting movable bottom webs at the two ends of the outer formwork of the precast box girder, wherein the movable bottom web comprises a central movable component arranged in the middle, two side movable components arranged at the two sides and chamfer movable components arranged at the two corner regions; before pouring the precast box girder, keeping each movable component on the movable bottom web flush with the outer formwork of the precast box girder and supporting and locking the position of each movable component; before tensioning the precast box girder, releasing the position locking of each movable component of the movable bottom web, and enabling each movable component of the movable bottom web to have a movable space in the vertically downward direction. The method has the advantages that the movable bottom web and the chamfer plate arranged at the two ends of the box girder outer formwork can prevent cracks at the box girder end caused by the extrusion of the beam end and the formwork when the pre-stressed steel strand tensioning the box girder arches upward.

Description

Technical Field

[0001] This invention belongs to the field of precast box girder construction technology, specifically relating to a method for preventing cracks at the beam ends of precast box girders caused by post-tensioning. Background Technology

[0002] Bridge construction requires a large number of formworks. Due to their frequent use, steel structure formwork is often used in bridge construction. With the rapid development of my country's railway industry, precast box girders are widely used in the construction of high-speed railways.

[0003] In the prefabrication of post-tensioned prestressed concrete simply supported box girders for high-speed railways, reinforcement binding platforms, girder fabrication platforms, and girder storage platforms are set up in the prefabrication yard. The girder fabrication platforms are assembled from girder formwork and are responsible for the following procedures: concrete pouring, prestressing pretensioning, and initial prestressing tensioning. The girder can only be lifted out of the fabrication platform after the initial prestressing tensioning is completed. Figure 8 As shown, the beam formwork is mostly composed of bottom and side formwork. During the prestressing tensioning stage of the prestressed steel strands 9, a is the tensioning direction. Due to the application of prestress, the beam body of the precast box girder 7 will arch upward, as shown in the box girder arching curve c. The stress at the bottom and chamfer of the beam ends increases, and downward pressure is generated at the beam ends as shown in b. This pressure is squeezed against the beam formwork, which can easily cause cracks d at the beam ends. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of the prior art by providing a method for preventing cracks at the ends of precast box girders caused by post-tensioning. This method involves setting movable bottom webs at both ends of the outer formwork of the precast box girder. Each movable bottom web includes a centrally located movable component, two movable components on either side, and chamfered movable components at two corner areas. Before casting the precast box girder, each movable component on the movable bottom web is kept flush with the outer formwork and its position is supported and locked. Before tensioning the precast box girder, the position locking of each movable component on the movable bottom web is released, allowing each movable component of the movable bottom web to have vertically downward movable space.

[0005] The objective of this invention is achieved through the following technical solutions:

[0006] A method for preventing cracks at the ends of precast box girders caused by post-tensioning, characterized by the following steps: A movable bottom web is installed at both ends of the outer formwork of the precast box girder; the movable bottom web includes a central movable component, two movable components on either side, and chamfered movable components at two corner areas; before casting the precast box girder, each movable component on the movable bottom web is flush with the outer formwork of the precast box girder and its position is supported and locked; before tensioning the precast box girder, the position locking of each movable component on the movable bottom web is released, allowing each movable component of the movable bottom web to have vertically downward movable space.

[0007] The central movable component includes a central movable plate, a central movable plate support, a central limiting plate, a central support bolt, a central limiting nut one, and a central limiting nut two. The central movable plate support supports the central movable plate located above it. The central limiting plate is located below the central movable plate support and is connected to the overall support steel frame of the precast box girder to maintain a fixed position. The central limiting plate is H-shaped. The central support bolt passes through the web of the central limiting plate from below and supports the central movable plate support. To increase the stability of the central movable plate, the central limiting nut one is welded to the bottom of the central movable plate support for screwing in the central support bolt. The central limiting nut two is correspondingly assembled on the central support bolt and located between the central limiting plate and the central movable plate support. The movement distance of the central movable plate can be adjusted by adjusting the central support bolt.

[0008] Before casting the precast box girder, the central support bolt is screwed upwards to fit tightly against the web of the central limiting plate and screwed into the central limiting nut to fix the central support bolt so that it supports the central movable plate support and keeps its position fixed. Before tensioning the precast box girder, the central support bolt is screwed outwards by a preset distance so that the central movable plate support and the central movable plate have space to move downwards.

[0009] The two movable components include two movable plates, two movable plate supports, two limiting plates, two support bolts, two limiting nuts (first and second). The two movable plate supports support the two movable plates located above them. The two limiting plates are located below the two movable plate supports and are connected to the overall support steel frame of the precast box girder to maintain a fixed position. The two limiting plates are H-shaped. The two support bolts pass through the web of the two limiting plates from below and support the two movable plate supports. To increase the stability of the two movable plates, two limiting nuts (first) are welded to the bottom of the two movable plate supports for screwing in the two support bolts. Two limiting nuts (second) are correspondingly assembled on the two support bolts and located between the web of the two movable plate supports and the two limiting plates. The movement distance of the two movable plates can be adjusted by adjusting the two support bolts.

[0010] Before casting the precast box girder, the two side support bolts are screwed upwards to fit tightly against the web of the two side limiting plates, and screwed into the two side limiting nuts to fix the two side support bolts so that they support the two side movable plate supports and keep their positions fixed. Before tensioning the precast box girder, the two side support bolts are screwed outwards by a preset distance so that the two side movable plate supports and the two side movable plates have space to move downwards.

[0011] The chamfering movable assembly includes a chamfering movable plate, a chamfering limiting plate, a chamfering support bolt, and a chamfering limiting nut. The chamfering limiting plate is arranged on the outside of the chamfering movable plate and is connected to the integral support steel frame of the precast box girder to maintain a fixed position. The chamfering limiting nut is welded and fixed to the outer surface of the chamfering movable plate. The chamfering support bolt passes through the chamfering limiting plate and is screwed into the chamfering limiting nut.

[0012] Before casting the precast box girder, the chamfer support bolt is rotated into the chamfer limiting nut on the chamfer movable plate to fix and support the chamfer movable plate and keep it in a fixed position; before tensioning the precast box girder, the chamfer support bolt is unscrewed from the chamfer limiting nut and screwed outward by a preset distance so that the chamfer movable plate has space to move downward.

[0013] The advantages of this invention are:

[0014] (1) The movable bottom web and chamfer plate set at both ends of the box girder’s outer formwork can prevent cracks from forming at the box girder end due to the squeezing of the beam end and the formwork when the prestressed steel strands are tensioned to arch the box girder.

[0015] (2) The ends of the box girder are intact, which reduces the prestress loss of the prestressed steel strands in the box girder and makes the bridge safer in use;

[0016] (3) By setting the movable bottom web and chamfer plate at both ends of the outer formwork of the box girder, the stress at the beam end of the box girder is redistributed, the stress is more reasonable, the precamber is easier to meet the requirements, and the bridge structure is safer.

[0017] (4) Before the prestressed steel strand is tensioned, the end template is lowered slightly by adjusting the bolts. This does not change the stress on the template during pouring, thus preventing cracks at the beam end. The device has a reasonable structure, simple construction procedures, high construction efficiency, and saves costs. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the external formwork for a precast box girder with a movable bottom web to prevent beam end cracks in this invention.

[0019] Figure 2 For the present invention Figure 1 Detailed view of point A in the image;

[0020] Figure 3 For the present invention Figure 1 Detailed view of section B in the image;

[0021] Figure 4 For the present invention Figure 1 Detailed view of point C in the image;

[0022] Figure 5 This is a top view of the precast box girder outer formwork in this invention;

[0023] Figure 6 This is a longitudinal sectional view of the precast box girder outer formwork in this invention;

[0024] Figure 7 For the present invention Figure 6 Detailed view of point D in the image;

[0025] Figure 8 A schematic diagram illustrating the cracks at the ends of a box girder caused by traditional methods;

[0026] Figure 9 This is a schematic diagram of the workflow of the central active component in this invention. Detailed Implementation

[0027] The features and other related features of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments, so as to facilitate understanding by those skilled in the art:

[0028] like Figures 1-9 The markings in the diagram are as follows:

[0029] 1. Central movable component, 2. Side movable components, 3. Chamfered movable component, 4. Overall supporting steel frame, 5. Outer formwork, 6. Box girder to be precast, 7. Precast box girder, 9. Prestressed steel strand, a. Tensioning direction, b. Pressure, c. Box girder arch curve, d. Crack;

[0030] 11. Central movable plate; 12. Central movable plate support; 13. Central limiting plate; 14. Central support bolt; 15. Central limiting nut one; 16. Central limiting nut two;

[0031] 21. Movable plates on both sides; 22. Support components for movable plates on both sides; 23. Limiting plates on both sides; 24. Support bolts on both sides; 25. Limiting nut one on both sides; 26. Limiting nut two on both sides.

[0032] 31. Chamfered movable plate, 32. Chamfered limiting plate, 33. Chamfered support bolt, 34. Chamfered limiting nut.

[0033] Example: Figures 1-9 As shown, this embodiment specifically relates to a method for preventing cracks at the ends of precast box girders caused by post-tensioning. The method involves setting movable bottom webs at both ends of the outer formwork 5 of the box girder 6 to be precast. The movable bottom webs include a central movable component 1, two movable components 2 on either side, and chamfered movable components 3 at two corner areas. Before casting the box girder 6, each movable component on the movable bottom web is kept flush with the outer formwork 5 and its position is supported and locked. Before tensioning the precast box girder 7, the position locking of each movable component on the movable bottom web is released, allowing each movable component of the movable bottom web to have movable space in the vertically downward direction.

[0034] like Figure 1 As shown, the bottom formwork of the precast box girder outer formwork 5 is supported by the overall support steel frame 4.

[0035] like Figure 1 , 2As shown in Figure 5, the central movable component 1 consists of a central movable plate 11, a central movable plate support 12, a central limiting plate 13, a central support bolt 14, a central limiting nut 15, and a central limiting nut 16. Before concrete pouring, the central movable plate 11 is kept flush with the bottom outer formwork 5. The central movable plate support 12 is located below the central movable plate 11 and supports it. Multiple central limiting plates 13 are arranged at the bottom of the central movable support 12. The specific number of central limiting plates 13 is selected based on the dimensions of the central movable support 12. The central limiting plates 13 are fixedly connected to the overall support steel frame 4 to ensure their position remains fixed. The central limiting plates 13 are H-shaped steel... The material has screw holes on its web. A central support bolt 14 passes through the screw holes on the web from below the central limiting plate 13 and supports the central movable plate support 12 located above. To increase the stability of the central movable plate 11, a central limiting nut 15 is welded to the bottom of the central movable plate support 12 for screwing in the central support bolt 14. A corresponding central limiting nut 16 is fitted on the central support bolt 14. The central limiting nut 16 is located between the web of the central limiting plate 13 and the central movable plate support 12. By adjusting the vertical position of the central support bolt 14 on the central limiting plate 13, the vertical movement distance of the central movable plate support 12 (central movable plate 11) can be controlled. Figure 9 As shown, before casting the precast box girder 7, the central support bolt 14 is screwed to the web of the central limiting plate 13 to fix the central support bolt 14 so that it supports the central movable plate support member 12 and keeps its position fixed; before tensioning the precast box girder 7, the central support bolt 14 is screwed outward by a preset distance so that the central movable plate support member 12 and the central movable plate 11 have space to move downward.

[0036] like Figure 1 , 3As shown in Figure 5, the two movable components 2 include two movable plates 21, two movable plate supports 22, two limiting plates 23, two supporting bolts 24, two limiting nuts 25, and two limiting nuts 26. The two movable plates 21 are kept flush with the bottom outer formwork 5 before concrete is poured. The two movable plate supports 22 are located below the two movable plates 21 and support them. Multiple limiting plates 23 are arranged below the two movable plate supports 22. The specific number of the two limiting plates 23 is selected based on the size of the two movable plate supports 22. The two limiting plates 23 are fixedly connected to the overall support steel frame 4 to ensure that their positions remain fixed. The two side limiting plates 23 are H-shaped steel with screw holes on their webs. The two side support bolts 24 pass through the screw holes on their webs from bottom to top and support the two side movable plate support members 22 located above them. To increase the stability of the two side movable plates 21, the two side limiting nuts 25 are welded to the bottom of the two side movable plate support members 22 for screwing in the two side support bolts 24. The two side limiting nuts 26 are correspondingly installed on the two side support bolts 24. The two side limiting nuts 26 are located between the two side limiting plates 23 and the two side movable plate support members 22. By adjusting the vertical position of the two side support bolts 24 on the two side limiting plates 23, the vertical movement distance of the two side movable plate support members 22 (two side movable plates 21) can be controlled. Before pouring the precast box girder 6, screw the two side support bolts 24 onto the web of the two side limiting plates 23 to fix the two side support bolts 24 so that they support the two side movable plate support members 22 and keep their positions fixed; before tensioning the precast box girder 7, screw the two side support bolts 24 outward by a preset distance so that the two side movable plate support members 22 and the two side movable plates 21 have space to move downward.

[0037] like Figure 1 , 4As shown in Figures 5, 6, and 7, the chamfering movable assembly 3 includes a chamfering movable plate 31, a chamfering limiting plate 32, a chamfering support bolt 33, and a chamfering limiting nut 34. The chamfering movable plate 31 has a corner structure and remains flush with the chamfering template portion of the outer template 5. The chamfering limiting plate 32 is arranged on the outside of the chamfering movable plate 31 and is fixedly connected to the overall support steel frame 4 to ensure that its position remains fixed. The chamfering limiting nut 34 is welded to the outer surface of the chamfering movable plate 31. The chamfering limiting plate 32 has a screw hole. The chamfering support bolt 33 passes through the screw hole on the chamfering limiting plate 32 from the outside to the inside and is screwed into the chamfering limiting nut 34. The movement distance of the chamfering movable plate 31 can be adjusted by rotating the chamfering support bolt 33. For example, before grouting, the chamfering support bolt 33 is screwed into the chamfering limiting nut 34 to maintain support for the chamfering movable plate 31. During tensioning, the chamfering support bolt 33 is unscrewed from the chamfering limiting nut 34 to create space for the chamfering movable plate 31 to move, allowing the end formwork to drop slightly and reducing the stress on the end. Before pouring the precast box girder 6, the chamfering support bolt 33 is rotated into the chamfering limiting nut 34 on the chamfering movable plate 31 to fix and support the chamfering movable plate 31 and keep it in a fixed position. Before tensioning the precast box girder 7, the chamfering support bolt 33 is unscrewed from the chamfering limiting nut 34 and screwed outward by a predetermined distance to allow the chamfering movable plate to move downward.

[0038] The beneficial effects of this embodiment are:

[0039] (1) The movable bottom web and chamfer plate set at both ends of the box girder’s outer formwork can prevent cracks from forming at the box girder end due to the squeezing of the beam end and the formwork when the prestressed steel strands are tensioned to arch the box girder.

[0040] (2) The ends of the box girder are intact, which reduces the prestress loss of the prestressed steel strands in the box girder and makes the bridge safer in use;

[0041] (3) By setting the movable bottom web and chamfer plate at both ends of the outer formwork of the box girder, the stress at the beam end of the box girder is redistributed, the stress is more reasonable, the precamber is easier to meet the requirements, and the bridge structure is safer.

[0042] (4) Before the prestressed steel strand is tensioned, the end template is lowered slightly by adjusting the bolts. This does not change the stress on the template during pouring, thus preventing cracks at the beam end. The device has a reasonable structure, simple construction procedures, high construction efficiency, and saves costs.

[0043] Inspired by the preferred embodiments of the invention described above, and based on the above description, those skilled in the art can make various changes and modifications without departing from the technical spirit of the invention. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A method for preventing cracks at the ends of precast box girders caused by post-tensioning, characterized in that... The method includes the following steps: setting movable bottom webs at both ends of the outer formwork of the precast box girder, the movable bottom webs including a central movable component, two movable components on both sides, and chamfered movable components in two corner areas; before pouring the precast box girder, keeping each movable component on the movable bottom webs flush with the outer formwork of the precast box girder and locking its position; before tensioning the precast box girder, releasing the position locking of each movable component on the movable bottom webs, so that each movable component on the movable bottom webs has movable space in the vertically downward direction; The central movable component includes a central movable plate, a central movable plate support, a central limiting plate, a central support bolt, a central limiting nut one, and a central limiting nut two. The central movable plate support supports the central movable plate located above it. The central limiting plate is located below the central movable plate support and is connected to the overall support steel frame of the precast box girder to maintain a fixed position. The central limiting plate is H-shaped. The central support bolt passes through the web of the central limiting plate from below and supports the central movable plate support. To increase the stability of the central movable plate, the central limiting nut one is welded to the bottom of the central movable plate support for screwing in the central support bolt. The central limiting nut two is correspondingly assembled on the central support bolt and located between the central limiting plate and the central movable plate support. The movement distance of the central movable plate can be adjusted by adjusting the central support bolt. Before casting the precast box girder, the central support bolt is screwed upwards to fit tightly against the web of the central limiting plate, and then screwed into the central limiting nut to fix the central support bolt so that it supports the central movable plate support and keeps its position fixed. Before tensioning the precast box girder, the central support bolt is screwed outwards by a preset distance so that the central movable plate support and the central movable plate have space to move downwards. The two movable components include two movable plates, two movable plate supports, two limiting plates, two supporting bolts, two limiting nuts (first and second). The two movable plate supports support the two movable plates located above them. The two limiting plates are located below the two movable plate supports and are connected to the overall supporting steel frame of the precast box girder to maintain a fixed position. The two limiting plates are H-shaped. The two supporting bolts pass through the web of the two limiting plates from below and support the two movable plate supports. To increase the stability of the two movable plates, the two limiting nuts (first) are welded to the bottom of the two movable plate supports for screwing in the two supporting bolts. The two limiting nuts (second) are correspondingly assembled on the two supporting bolts and located between the web of the two movable plate supports and the two limiting plates. The movement distance of the two movable plates can be adjusted by adjusting the two supporting bolts. Before casting the precast box girder, the two side support bolts are screwed upwards to fit tightly against the web of the two side limiting plates, and screwed into the two side limiting nuts to fix the two side support bolts so that they support the two side movable plate supports and keep their positions fixed; before tensioning the precast box girder, the two side support bolts are screwed outwards by a preset distance so that the two side movable plate supports and the two side movable plates have space to move downwards; The chamfering movable assembly includes a chamfering movable plate, a chamfering limiting plate, a chamfering support bolt, and a chamfering limiting nut. The chamfering limiting plate is arranged on the outside of the chamfering movable plate and is connected to the integral support steel frame of the precast box girder to maintain a fixed position. The chamfering limiting nut is welded and fixed to the outer surface of the chamfering movable plate. The chamfering support bolt passes through the chamfering limiting plate and is screwed into the chamfering limiting nut.

2. The method for preventing cracks at the ends of precast box girders caused by post-tensioning as described in claim 1, characterized in that... Before casting the precast box girder, the chamfer support bolt is rotated into the chamfer limiting nut on the chamfer movable plate to fix and support the chamfer movable plate and keep it in a fixed position; before tensioning the precast box girder, the chamfer support bolt is unscrewed from the chamfer limiting nut and screwed outward by a preset distance so that the chamfer movable plate has space to move downward.

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