A top-mounted beam portal frame structure and a method for modifying the top beam of cast-in-place beams.

Abstract

This invention discloses a gantry structure for lifting beams and a method for modifying the top beam of cast-in-place beams, belonging to the field of bridge construction technology. It includes a first and a second gantry structure for lifting beams. The first gantry structure comprises a bottom frame and a support structure located on the bottom frame. The bottom frame surrounds the bridge body of the first pier. The support structure is located near the cast-in-place beam and fixed to the bottom frame. The support structure includes a first main beam, a first telescopic sleeve assembly, and a first hydraulic cylinder assembly. The second gantry structure is a Π-shaped structure, comprising a second main beam, a second telescopic sleeve assembly, a second lower support beam, and a second hydraulic cylinder assembly. This invention, through the cooperation of the first and second gantry structures, can effectively lift cast-in-place beams with inconsistent pier heights. Simultaneously, the first and second telescopic sleeve assemblies facilitate adjustment after lifting.

Description

Technical Field

[0001] This invention belongs to the field of bridge construction technology, and in particular relates to a top-drop beam portal frame structure and a method for modifying the top beam of a cast-in-place beam. Background Technology

[0002] The Jianqu Tashiwu Middle Bridge, located in Jiande City, Zhejiang Province, is 62.515m long. It features 2-24m single-track simply supported box girders and spans from JQDK2+793.76 to +856.275. The bridge passes under the Hangzhou-Huangshan Railway at an angle of 39 degrees. The bottom elevation of the Hangzhou-Huangshan Railway is 84.216m, while the elevation of the track surface of the Jianqu underpass connecting line is 76.599m, a difference of 7.617m.

[0003] The completed cast-in-place beam is a 24m single-line simply supported box girder, with a beam length of 24.6m, a top width of 7.6m, a bottom width of 3.0m, a support centerline distance of 0.75m from the beam end, and a support transverse width of 2.3m. Figure 1 The image shows the cast-in-place beams, the first pier, and the second pier of the Tashiwu Middle Bridge. The first pier is a single-column pier, and the second pier is a stepped base adjacent to the road surface.

[0004] The existing cast-in-place beams require jacking and modification work, involving beam jacking, pier and abutment modification, and beam lowering. Bridge jacking generally adopts an overall hydraulic synchronous lifting scheme, which utilizes the original pier load-bearing structure without damaging the original bridge deck pavement. First, the superstructure of the bridge is supported by a hydraulic jacking device. Then, the columns under the cap beams of each pier and abutment are cut off, and the structure is raised to the design height using the hydraulic jacking device again. Finally, the column reinforcement is extended, formwork is erected, and the second phase of concrete is poured.

[0005] However, this method is applicable to this... Figure 1 For the Tashiwu Bridge on display, it is difficult to bear the weight of the bridge piers themselves; at the same time, it is difficult to adapt to the situation where the heights of the two piers are different. When one pier is higher, it is difficult to install a hydraulic jacking device; in addition, the installation of the hydraulic jacking device can easily affect the installation and dismantling of the pier formwork. Summary of the Invention

[0006] The purpose of this invention is to provide a top-drop beam portal frame structure and a method for modifying the top beam of a cast-in-place beam, so as to solve the problems existing in the background art.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] A gantry structure for lifting cast-in-place beams is provided. The cast-in-place beams have a first pier and a second pier at their bottom. The length of the first pier is greater than the length of the second pier. The structure includes a first gantry and a second gantry. The first gantry includes a bottom frame and a support structure located on the bottom frame. The bottom frame surrounds the bridge body of the first pier. The support structure is located on the side close to the cast-in-place beam and is fixed to the bottom frame. The support structure includes a first main beam, a first telescopic sleeve assembly, and a first hydraulic cylinder assembly.

[0009] The second jacking beam gantry is a Π-shaped structure. The second jacking beam gantry is located on both sides of the abutment of the second pier. The second jacking beam gantry includes a second main beam, a second telescopic sleeve assembly, a second lower pad beam, and a second hydraulic cylinder assembly.

[0010] Furthermore, the bottom frame is located on the abutment of the first pier. The bottom frame includes a first lower support beam, an anchor beam, a column, and a protruding beam. A first diagonal brace is provided between the anchor beam and the first lower support beam, a second diagonal brace is provided between the protruding beam and the first lower support beam, and a third diagonal brace is provided between the first main beam and the protruding beam.

[0011] Furthermore, the axis of the first main beam is perpendicular to the axis of the cast-in-place beam, and the two first telescopic sleeve assemblies are located on both sides of the first main beam, with one side of each first telescopic sleeve assembly corresponding to a first hydraulic cylinder assembly.

[0012] Furthermore, the second telescopic sleeve assembly includes a second telescopic inner sleeve and a second telescopic outer sleeve. The second telescopic inner sleeve is fixed to the lower end of the second main beam, and the second telescopic outer sleeve is fixed to the second lower pad beam. The second telescopic outer sleeve is also provided with a second outer sleeve extension section. The second outer sleeve extension section is a split structure. The split sections of the second outer sleeve extension section are connected by bolts and sleeved outside the second telescopic inner sleeve. The second outer sleeve extension section is connected to the second telescopic outer sleeve by bolts.

[0013] Furthermore, the first telescopic sleeve assembly has the same structure as the second telescopic sleeve assembly, and the first telescopic sleeve assembly includes a first telescopic inner sleeve, a first telescopic outer sleeve, and a first outer sleeve extension section.

[0014] Furthermore, the two sets of second lower support beams are located on both sides of the second pier, and the axial direction of the second lower support beam is the same as that of the cast-in-place beam. A high-level diagonal brace is provided between the second main beam and the second lower support beam.

[0015] Furthermore, the axis of the second main beam is perpendicular to the axis of the cast-in-place beam, and two second telescopic sleeve assemblies are located on both sides of the second main beam. The two ends of the second telescopic sleeve assembly are connected to the second main beam and the second lower pad beam, respectively. Each side of the second telescopic sleeve assembly corresponds to a second hydraulic cylinder assembly.

[0016] A method for modifying the top beam of a cast-in-place beam includes the following steps:

[0017] S1: Use the slope excavation method to excavate the backfill soil that is above the top of the first and second pier caps;

[0018] S2: Simultaneously install the first and second top beam gantry frames;

[0019] S3: Trial jacking. Remove all bolts between all supports and the embedded plates of the cast-in-place beams. Control the first hydraulic cylinder assembly and the second hydraulic cylinder assembly to perform trial jacking. First, jack the two gantry frames 10mm and then stop for 5 minutes to observe.

[0020] S4: Formal lifting, control the first hydraulic cylinder assembly and the second hydraulic cylinder assembly to lift synchronously to 200mm, the lifting speed is 0.2m / min, and add a set of outer extension sections to the telescopic inner sleeve exposed during lifting;

[0021] S5: Repeat step S4, lift the main beam of the gantry by 600mm, then retract the hydraulic cylinder. The cast-in-place beam is supported by the extended section of the outer sleeve and the telescopic outer sleeve. Add a pad to the bottom of the hydraulic cylinder.

[0022] S6: Repeat steps S4 and S5 to complete the lifting to the designed height;

[0023] S7: Pier body renovation, abutment top and abutment cap renovation;

[0024] S8: Beam lowering operation;

[0025] S9: Backfill and compact the foundation area of ​​the first and second piers with soil.

[0026] Furthermore, step S7, the pier modification, includes sub-steps S71-S74:

[0027] S71: Remove the concrete within 3.3m below the top of the existing pier cap, and chisel away the concrete within 70cm below.

[0028] S72: Clean and rinse the reinforcing bars, weld the main reinforcing bars and tie the stirrups;

[0029] S73: Install the pier cap formwork, tie the pier cap reinforcement and the embedded pad stone reinforcement, and pour the pier cap concrete;

[0030] S74: Pier cap maintenance, foundation stone concrete formwork erection and pouring.

[0031] Furthermore, the modification of the platform top and platform cap in step S7 includes sub-steps S75-S77:

[0032] S75: Remove the existing concrete of the platform top and cap;

[0033] S76: Clean and rinse the main reinforcement bars, install the formwork for the top and cap of the platform, weld and connect the main reinforcement bars, tie the reinforcement bars, and embed the pad stone reinforcement bars;

[0034] S77: Pour concrete for the cap and top of the platform, cure it, and erect and pour the concrete for the foundation stone.

[0035] The beneficial effects of this invention are:

[0036] 1) The combination of the first and second jacking beam gantry structures can be used to lift cast-in-place beams with different pier heights. At the same time, the first and second telescopic sleeve components can be used to facilitate adjustment after lifting.

[0037] 2) The first jacking beam gantry is heightened and supported by the bottom frame, which facilitates the lifting of the cast-in-place beam of the first pier accessory by the first main beam, the first telescopic sleeve assembly and the first hydraulic cylinder assembly in the support structure. At the same time, the support structure is only located on the side of the bottom frame close to the cast-in-place beam, which does not affect the formwork installation and dismantling construction.

[0038] 3) The extended beams are connected to the first lower support beam and the first main beam through the second and third diagonal braces, respectively, to enhance the ability of the first top beam gantry to resist longitudinal horizontal loads while supporting the cast-in-place beams. Attached Figure Description

[0039] Figure 1 This is a schematic diagram of a cast-in-place beam in the background art of this invention;

[0040] Figure 2 This is a schematic diagram of the operation of the present invention when lifting is performed using a jacking beam gantry structure;

[0041] Figure 3 This is a front view of the first top-drop beam gantry in this invention;

[0042] Figure 4 This is a side view of the first top-drop beam gantry in this invention;

[0043] Figure 5 This is a mid-plane view of the first top-drop beam gantry in this invention;

[0044] Figure 6 This is a front view of the second top-drop beam gantry in this invention;

[0045] Figure 7This is a side view of the second top-drop beam gantry in this invention;

[0046] Figure 8 This is a top view of the second telescopic inner sleeve and the second outer sleeve extension section in this invention;

[0047] In the diagram, 1-cast-in-place beam, 2-first pier, 3-second pier, 4-first jacking beam gantry, 41-first lower support beam, 42-column, 43-anchor beam, 44-extended beam, 45-first main beam, 46-first diagonal brace, 47-second diagonal brace, 48-third diagonal brace, 49-first hydraulic cylinder assembly, 5-second jacking beam gantry, 51-second lower support beam, 52-second main beam, 53-high-level diagonal brace, 54-second hydraulic cylinder assembly, 6-second telescopic sleeve assembly, 61-second telescopic inner sleeve, 62-second outer sleeve extension section, 63-second telescopic outer sleeve, 7-first telescopic sleeve assembly. Detailed Implementation

[0048] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0049] like Figure 1 As shown, the bottom of the cast-in-place beam 1 is supported by the first pier 2 and the second pier 3. The first pier 2 is a single-column pier, and the second pier 3 is a stepped base adjacent to the road surface. The overall height of the second pier 3 is much lower than the overall height of the first pier 2. Therefore, the traditional jacking method cannot be used for jacking.

[0050] Example 1

[0051] See Figures 2-8 This invention provides a technical solution: a top-drop beam portal frame structure, used for... Figure 1 The lifting of the cast-in-place beam 1 includes a first lifting and lowering gantry 4 and a second lifting and lowering gantry 5. The first lifting and lowering gantry 4 includes a bottom frame and a support structure located on the bottom frame. The bottom frame surrounds the bridge body of the first pier 2. The support structure is located on the side close to the cast-in-place beam 1 and is fixed to the bottom frame. The support structure includes a first main beam 45, a first telescopic sleeve assembly 7 and a first hydraulic cylinder assembly 49.

[0052] The second top-drop beam gantry 5 has a Π-shaped structure. The second top-drop beam gantry 5 is located on both sides of the abutment of the second pier 3. The second top-drop beam gantry 5 includes a second main beam 52, a second telescopic sleeve assembly 6, a second lower pad beam 51, and a second hydraulic cylinder assembly 54.

[0053] Through the above technical solution, the first jacking and lowering gantry 4, with its heightened and supported bottom frame, facilitates the lifting of the cast-in-place beam 1 attached to the first pier 2 by the first main beam 45, the first telescopic sleeve assembly 7, and the first hydraulic cylinder assembly 49 in the support structure. Simultaneously, the support structure is located only on the side of the bottom frame closest to the cast-in-place beam 1, without affecting the formwork installation and dismantling. The second pier 3 has a stepped structure. To facilitate the lifting of the cast-in-place beam 1 attached to the second pier 3, the second jacking and lowering gantry 5 adopts a Π-shaped structure arranged on both sides of the bridge body.

[0054] The first main girder 45 and the second main girder 52 are located below the cast-in-place beam 1, respectively, and support the cast-in-place beam 1. The first telescopic sleeve assembly 7 and the second telescopic sleeve assembly 6 below lift the first main girder 45 and the second main girder 52, thereby raising the cast-in-place beam 1. The cooperation of the first jacking and lowering gantry frame 4 and the second jacking and lowering gantry frame 5 can effectively lift the cast-in-place beam 1 with different pier heights. At the same time, the first telescopic sleeve assembly 7 and the second telescopic sleeve assembly 6 can facilitate adjustment after lifting.

[0055] Furthermore, the bottom frame is located on the abutment of the first pier 2. The bottom frame includes a first lower support beam 41, an anchor beam 43, a column 42, and a protruding beam 44. The first lower support beam 41, the anchor beam 43, and the column 42 form a frame structure, which is supported and fixed by the first diagonal brace 46. The protruding beam 44 is connected to the first lower support beam 41 and the first main beam 45 by the second diagonal brace 47 and the third diagonal brace 48, respectively, which strengthens the ability of the first top beam gantry 4 to resist the longitudinal horizontal load during the support of the cast-in-place beam 1.

[0056] Furthermore, the axis of the first main beam 45 is perpendicular to the axis of the cast-in-place beam 1, and the two first telescopic sleeve assemblies 7 are located on both sides of the first main beam 45, with each side of the first telescopic sleeve assembly 7 corresponding to a first hydraulic cylinder assembly 49.

[0057] The second telescopic sleeve assembly 6 includes a second telescopic inner sleeve 61 and a second telescopic outer sleeve 63. The second telescopic inner sleeve 61 is fixed to the lower end of the second main beam 52, and the second telescopic outer sleeve 63 is fixed to the second lower pad beam 51. The second telescopic outer sleeve 63 is also provided with a second outer sleeve extension section 62. The second outer sleeve extension section 62 is a split structure. The split parts of the second outer sleeve extension section 62 are connected by bolts and sleeved outside the second telescopic inner sleeve 61. The second outer sleeve extension section 62 is connected to the second telescopic outer sleeve 63 by bolts.

[0058] Based on the above technical solutions, see [link / reference] Figures 6-8The second telescopic inner sleeve 61 is fixed to the lower part of the second main beam 52, and the second telescopic outer sleeve 63 is fixed to the second lower pad beam 51 and sleeved outside the second telescopic inner sleeve 61. When the second main beam 52 is lifted a certain distance by the second hydraulic cylinder assembly 54, the second telescopic inner sleeve 61 extends a certain distance from the second telescopic outer sleeve 63. After being connected by bolts through the split second outer sleeve extension section 62, it is sleeved on the second telescopic inner sleeve 61. Then, the added second outer sleeve extension section 62 is fixed to the lower second telescopic outer sleeve 63 until it is lifted to the appropriate position. Then, the uppermost second outer sleeve extension section 62 is fixed to the second main beam 52. At this time, the second main beam 52, the second outer sleeve extension section 62 and the second lower pad beam 51 are fixed as one unit.

[0059] The first telescopic sleeve assembly 7 has the same structure as the second telescopic sleeve assembly 6. The first telescopic sleeve assembly 7 includes a first telescopic inner sleeve, a first telescopic outer sleeve, and a first outer sleeve extension section. The working principle of the first telescopic sleeve assembly 7 is the same as above.

[0060] Furthermore, the two sets of second lower support beams 51 are located on both sides of the second pier 3, and the axial direction of the second lower support beams 51 is the same as the axial direction of the cast-in-place beam 1. A high-level diagonal brace 53 is provided between the second main beam 52 and the second lower support beams 51.

[0061] Furthermore, the axis of the second main beam 52 is perpendicular to the axis of the cast-in-place beam 1, and the two second telescopic sleeve assemblies 6 are located on both sides of the second main beam 52 respectively. The two ends of the second telescopic sleeve assembly 6 are connected to the second main beam 52 and the second lower pad beam 51 respectively. Each side of the second telescopic sleeve assembly 6 corresponds to a second hydraulic cylinder assembly 54.

[0062] With the above technical solution, the second lower support beam 51 is located on the pier platform of the second pier 3. The second lower support beam 51, the second expansion sleeve assembly 6 and the second main beam 52 form a support structure. At the same time, the second main beam 52 and the second lower support beam 51 are connected and supported by the high-level diagonal brace 53.

[0063] Example 2

[0064] A method for modifying the top beam of a cast-in-place beam includes the following steps:

[0065] S1: Use the slope excavation method to remove the backfill soil within the range above the top of the first pier 2 and the second pier 3; at the same time, the existing concrete railings can be removed using the skylight points to facilitate subsequent construction.

[0066] S2: The installation of the first top beam gantry 4 and the second top beam gantry 5 can be carried out with reference to the structure of Embodiment 1;

[0067] The first top-drop gantry frame 4 adopts a support structure of a bottom frame + upper single beam double vertical legs. The top surface of the bottom frame is 20cm from the pier removal line, and the bottom rests on the pier cap. Longitudinally, it is wedged and anchored to the pier body via the frame's longitudinal beams and precision-rolled threaded steel. In the space above the pier removal line and below the heightened pier cap, the support structure is located on the side with the beam; no gantry frame components are arranged on the other three sides, facilitating pier removal / chiseling. Within this area, the side of the pier with the beam has only two leg columns 42 and their outer diagonal braces. The leg columns 42 are 1m from the pier cap, which is sufficient for formwork installation and dismantling. The first hydraulic cylinder assembly 49 consists of a total of four 100t hydraulic cylinders.

[0068] The second top beam gantry 5 has a Π-shaped structure. The second lower support beam 51 is anchored on the bearing platform. The second hydraulic cylinder assembly 54 consists of a total of 4 hydraulic cylinders weighing 63t each. The upper end of the hydraulic cylinder is connected to the ear plates on both sides of the beam end of the second main beam 52, and the lower end is supported on the second lower support beam 51 / support pier.

[0069] S3: Trial jacking. Remove all bolts between all supports and the embedded plates of the supports of the cast-in-place beam 1. Control the first hydraulic cylinder assembly 49 and the second hydraulic cylinder assembly 54 to perform trial jacking. The two gantry frames are first jacked up by 10mm and then stopped for 5 minutes for observation.

[0070] S4: Formal lifting, control the first hydraulic cylinder assembly 49 and the second hydraulic cylinder assembly 54 to lift synchronously to 200mm, the lifting speed is 0.2m / min, and add a set of outer extension sections to the telescopic inner sleeve exposed during lifting;

[0071] S5: Repeat step S4, lift the main beam of the gantry by 600mm, then retract the hydraulic cylinder. The cast-in-place beam 1 is supported by the extended section of the outer sleeve and the telescopic outer sleeve. A pad is added to the bottom of the hydraulic cylinder.

[0072] S6: Repeat steps S4 and S5 to complete the lifting to the designed height;

[0073] S7: Pier body renovation, abutment top and abutment cap renovation;

[0074] The pier modification includes sub-steps S71-S74:

[0075] S71: Remove the concrete within 3.3m below the top of the existing pier cap, and chisel away the concrete within 70cm below. When chiseling, first use an ink line to mark the lowest elevation line to be chiseled on the surface of the pier body, and then use a cutting machine to cut 3-5mm deep around the perimeter to reduce damage to the joint surface concrete during chiseling.

[0076] S72: Clean and rinse the reinforcing bars, weld the main reinforcing bars and tie the stirrups;

[0077] S73: Install the pier cap formwork, tie the pier cap reinforcement and the pre-embedded pad stone reinforcement, pour the pier cap concrete, and use the shaped steel formwork for the pier body. The inner surface of the formwork is evenly coated with the formwork. The formwork is installed in sections and fixed with bolts in the upper and lower and left and right. The longitudinal direction and the surrounding area are tied with high-strength threaded steel. Double-sided tape is pasted flush with the inner surface of the steel formwork at the joint to ensure that the steel formwork joint is tight and does not leak grout.

[0078] S74: Pier cap maintenance, foundation stone concrete formwork erection and pouring.

[0079] The modification of the stage top and cap includes sub-steps S75-S77:

[0080] S75: Remove the existing concrete of the platform top and cap;

[0081] S76: Clean and rinse the main reinforcement bars, install the formwork for the top and cap of the platform, weld and connect the main reinforcement bars, tie the reinforcement bars, and embed the pad stone reinforcement bars;

[0082] S77: Pour concrete for the cap and top of the platform, cure it, and erect and pour the concrete for the foundation stone.

[0083] S8: Beam lowering operation; lower the cast-in-place beam 1 onto the three-dimensional beam lowering device. The beam lowering operation is the reverse of the beam jacking operation. Install the three-dimensional beam lowering device inside the pier and abutment pad stone. Install one 200t lifting jack as a vertical support structure in each three-dimensional beam lowering device. The two sets of support structures on the top of one pier are set as two independent support points, and the two sets of support structures on the top of the other pier are a balance support point. The four support points are a four-point support and three-point balance support method.

[0084] The three-dimensional beam lowering device has two independent support points. Two vertical jacks on one pier are supplied with oil via their inlet and outlet ports to a high-pressure oil pump. The piston's extension height is determined based on the design elevation. A hydraulically controlled check valve is installed on the jack's oil inlet line to maintain the jack's extension position, thus maintaining the lifting pressure. The device also has a balancing support point. Two jacks on the left and right sides of another pier are connected by a T-junction, and a high-pressure oil pump supplies oil to both, ensuring balanced operation of the two jacks. The piston's extension height is determined by an external load, maintaining balanced load support between the two jacks. The connecting pipelines between the two jacks on either side of the pier are connected by steel pipes. A hydraulically controlled check valve is installed on the main T-junction to maintain the jack's extension position, thus maintaining the lifting pressure. The high-pressure oil pumps for the independent and balancing support points are placed on the two piers of the span to be erected.

[0085] The three-dimensional beam lowering device's transverse movement system is equipped with 50t jacks along both the longitudinal and transverse directions of the bridge as driving mechanisms for longitudinal and transverse translation. The longitudinal and transverse movement strokes of the transverse movement system are limited by frames in all directions, thus eliminating the need for additional limiting devices. The horizontal reaction force of the jacks acts on the outer wall of the device, constituting an internal force; therefore, no additional embedded parts are required between the three-dimensional beam lowering device and the bridge pier. When moving the beam forward along the bridge direction, the longitudinal jacks in the two front three-dimensional beam lowering devices are used to push the concrete box girder forward. When moving the beam left or right in the transverse direction, the transverse jacks in the left or right three-dimensional beam lowering devices are used to push the concrete box girder left or right.

[0086] After the box girder is lowered onto the three-dimensional girder lowering device via the girder lowering gantry, the vertical, longitudinal, and transverse jacks of the three-dimensional girder lowering device are used to adjust the vertical, longitudinal, and transverse positions of the box girder to ensure it is in place. Then, the support grouting operation is carried out according to the conventional girder erection procedure. After the grout has filled the gaps and reached the design allowable strength, the three-dimensional girder lowering device is removed.

[0087] S9: Use soil to backfill and compact the area of ​​the foundation of the first pier 2 and the second pier 3.

[0088] The above description is merely a preferred embodiment of the present invention. It should be understood that the present invention is not limited to the forms disclosed herein and should not be construed as excluding other embodiments. It can be used in various other combinations, modifications, and environments, and can be altered within the scope of the concept described herein through the above teachings or related technologies or knowledge. Modifications and variations made by those skilled in the art that do not depart from the spirit and scope of the present invention should be within the protection scope of the appended claims.

Claims

1. A jacking beam portal frame structure for jacking a cast-in-place beam (1), wherein the bottom of the cast-in-place beam (1) is provided with a first pier (2) and a second pier (3), the length of the first pier (2) being greater than the length of the second pier (3), characterized in that: It includes a first top beam gantry (4) and a second top beam gantry (5). The first top beam gantry (4) includes a bottom frame and a support structure located on the bottom frame. The bottom frame surrounds the bridge body of the first pier (2). The support structure is located on the side close to the cast-in-place beam and is fixed to the bottom frame. The support structure includes a first main beam (45), a first telescopic sleeve assembly (7), and a first hydraulic cylinder assembly (49). The second top-drop beam gantry (5) is a Π-shaped structure. The second top-drop beam gantry (5) is located on both sides of the abutment of the second pier (3). The second top-drop beam gantry (5) includes a second main beam (52), a second telescopic sleeve assembly (6), a second lower pad beam (51), and a second hydraulic cylinder assembly (54). The first main beam 45 and the second main beam 52 are respectively located below the cast-in-place beam 1; The axis of the first main beam (45) is perpendicular to the axis of the cast-in-place beam (1). The two first telescopic sleeve assemblies (7) are located on both sides of the first main beam (45), and each side of the first telescopic sleeve assembly (7) corresponds to a first hydraulic cylinder assembly (49). The axis of the second main beam (52) is perpendicular to the axis of the cast-in-place beam (1). Two second telescopic sleeve assemblies (6) are located on both sides of the second main beam (52). The two ends of the second telescopic sleeve assembly (6) are connected to the second main beam (52) and the second lower pad beam (51) respectively. Each side of the second telescopic sleeve assembly (6) corresponds to a second hydraulic cylinder assembly (54). The second telescopic sleeve assembly (6) includes a second telescopic inner sleeve (61) and a second telescopic outer sleeve (63). The second telescopic inner sleeve (61) is fixed to the lower end of the second main beam (52), and the second telescopic outer sleeve (63) is fixed to the second lower pad beam (51). The second telescopic outer sleeve (63) is also provided with a second outer sleeve extension section (62). The second outer sleeve extension section (62) is a split structure. The split parts of the second outer sleeve extension section (62) are connected by bolts and sleeved outside the second telescopic inner sleeve (61). The second outer sleeve extension section (62) is connected to the second telescopic outer sleeve (63) by bolts.

2. The top-drop beam portal frame structure according to claim 1, characterized in that: The bottom frame is located on the pier of the first pier (2). The bottom frame includes a first lower support beam (41), an anchor beam (43), a column (42) and a protruding beam (44). A first diagonal brace (46) is provided between the anchor beam (43) and the first lower support beam (41). A second diagonal brace (47) is provided between the protruding beam (44) and the first lower support beam (41). A third diagonal brace (48) is provided between the first main beam (45) and the protruding beam (44).

3. The top-drop beam portal frame structure according to claim 1, characterized in that: The first telescopic sleeve assembly (7) has the same structure as the second telescopic sleeve assembly (6). The first telescopic sleeve assembly (7) includes a first telescopic inner sleeve, a first telescopic outer sleeve, and a first outer sleeve extension section.

4. The top-drop beam portal frame structure according to claim 1, characterized in that: Two sets of second lower support beams (51) are located on both sides of the second pier (3). The axial direction of the second lower support beam (51) is the same as that of the cast-in-place beam (1). A high-level diagonal brace (53) is provided between the second main beam (52) and the second lower support beam (51).

5. A method for modifying the top beam of a cast-in-place beam, using the top-drop beam portal frame structure as described in any one of claims 1-4, characterized in that: Includes the following steps: S1: Use the slope excavation method to excavate the backfill soil within the range above the top of the first pier (2) and the second pier (3); S2: Simultaneously install the first top beam gantry (4) and the second top beam gantry (5); S3: Trial jacking, remove all bolts between all supports and embedded plates of the cast-in-place beam (1), control the first hydraulic cylinder assembly (49) and the second hydraulic cylinder assembly (54) to perform trial jacking, the two gantry frames are first jacked 10mm and then stopped for 5 minutes for observation; S4: Formal lifting, control the first hydraulic cylinder assembly (49) and the second hydraulic cylinder assembly (54) to lift synchronously to 200mm, the lifting speed is 0.2m / min, and add a set of outer extension sections to the telescopic inner sleeve exposed during lifting; S5: Repeat step S4, lift the main beam of the gantry by 600mm, then retract the hydraulic cylinder. The cast-in-place beam (1) is supported by the extended section of the outer sleeve and the telescopic outer sleeve. A pad is added to the bottom of the hydraulic cylinder. S6: Repeat steps S4 and S5 to complete the lifting to the designed height; S7: Pier body renovation, abutment top and abutment cap renovation; S8: Beam lowering operation; S9: Use soil to backfill and compact the foundation area of ​​the first pier (2) and the second pier (3).

6. The method for modifying the top beam of a cast-in-place beam according to claim 5, characterized in that: Step S7, the pier modification, includes sub-steps S71-S74: S71: Remove the concrete within 3.3m below the top of the existing pier cap, and chisel away the concrete within 70cm below. S72: Clean and rinse the reinforcing bars, weld the main reinforcing bars and tie the stirrups; S73: Install the pier cap formwork, tie the pier cap reinforcement and the embedded pad stone reinforcement, and pour the pier cap concrete; S74: Pier cap maintenance, foundation stone concrete formwork erection and pouring.

7. The method for modifying the top beam of a cast-in-place beam according to claim 5, characterized in that: Step S7, the modification of the platform top and cap, includes sub-steps S75-S77: S75: Remove the existing concrete of the platform top and cap; S76: Clean and rinse the main reinforcement bars, install the formwork for the top and cap of the platform, weld and connect the main reinforcement bars, tie the reinforcement bars, and embed the pad stone reinforcement bars; S77: Pour concrete for the cap and top of the platform, cure it, and erect and pour the concrete for the foundation stone.

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