Steel-UHPC prefabricated combined box girder with UHPC bridge deck slab integrally poured and manufacturing method of steel-UHPC prefabricated combined box girder

Abstract

The invention discloses a UHPC bridge deck slab integrally poured steel-UHPC prefabricated combined box girder which comprises a plurality of segments, and each segment comprises a steel box girder and a UHPC bridge deck slab integrally poured on the top face of the steel box girder; the steel box girder comprises a top plate, a bottom plate, inclined webs, side webs, middle webs, small longitudinal beams and transverse partition plates of a steel structure. The top plate is arranged on the side webs, the middle webs, the small longitudinal beams and the transverse partition plates and is welded and fixed; a double-layer reinforcing mesh is laid in the UHPC bridge deck slab, and after the UHPC bridge deck slab is integrally poured and formed on the top plate, the UHPC bridge deck slab and the steel box girder form an integral stress structure through connecting pieces; the UHPC bridge deck slab is locally thickened at the positions corresponding to the joints of the top plate, the side webs, the middle webs and the small longitudinal beams. The steel box girder top plate is not only a stress structure, but also a formwork for pouring the UHPC bridge deck slab. The UHPC bridge deck slab is integrally formed through pouring, wet joints between traditional prefabricated slabs are avoided, high-altitude operation is avoided through factory pouring, and the integrity, durability and engineering quality of the bridge deck structure are greatly improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of bridge engineering structure, and particularly relates to a steel-UHPC prefabricated composite box girder of UHPC bridge deck slab integral pouring and a manufacturing method thereof. BACKGROUND

[0002] In the existing girder structure, the orthotropic plate steel box girder is widely applied in long-span bridge structure due to the advantages of light self-weight, high bearing capacity and convenient construction. However, due to the large flexibility of the orthotropic bridge deck slab, fatigue cracking is prone to occur at the welding seam where the longitudinal rib, the transverse rib and the bridge deck slab are connected under the repeated action of vehicle load.

[0003] Compared with the orthotropic plate steel box girder, the steel-concrete composite girder combines the steel girder and the concrete bridge deck slab into a whole through shear connectors, and the steel and the concrete are stressed together, so that the material properties of the steel and the concrete are fully utilized, the structural stiffness is improved, and certain advantages are achieved. However, with the continuous increase of the bridge span, the application of the traditional steel-concrete composite girder faces severe challenges: when applied to continuous system or long-span flexible system bridge, the concrete bridge deck slab in the negative bending moment area bears tensile stress, and the tensile strength of the ordinary concrete is low, which easily leads to cracking, and the effect of the conventional anti-cracking measures is limited. Meanwhile, in order to bear the horizontal component of the stay cable and meet the stress requirement, the average thickness of the concrete bridge deck slab of the traditional composite girder is often greater than 26 cm in the long-span cable-stayed bridge, which leads to excessive self-weight of the main girder. The weight of the bridge deck slab accounts for more than 70% of the total weight of the main girder, and the excessive self-weight makes the composite girder scheme completely uneconomical and difficult to be hoisted and installed in segments. A large number of engineering practices at home and abroad show that the steel-concrete composite girder bridge generally has the problems of excessive self-weight of the main girder and easy cracking of the concrete bridge deck slab, which has long plagued the engineering field and become the main technical bottleneck restricting the further development of the traditional steel-concrete composite girder.

[0004] The ultra-high performance concrete (UHPC) has excellent mechanical properties, high elastic modulus and compressive and tensile strength, and excellent anti-cracking performance, and the post-shrinkage of the UHPC is zero after high-temperature steam curing, which basically eliminates the cracking risk of the concrete bridge deck slab in the negative bending moment area.

[0005] CN201811136656 discloses a steel-concrete composite girder structure in which the bridge deck slab is prefabricated in blocks and then spliced into shape through wet joints. However, the interface performance of the new and old concrete at the wet joint is weak, and especially for the UHPC material, the steel fibers inside the UHPC are discontinuous at the joint, which leads to a significant reduction in the tensile strength of the region, and the region easily becomes the starting point of cracking, which seriously affects the durability and integrity of the structure.

[0006] Therefore, the engineering community urgently needs a new type of steel-concrete composite beam structure and corresponding manufacturing method that can fully utilize the advantages of high-performance materials, effectively reduce the self-weight of the structure, and systematically solve the problem of bridge deck cracking. Summary of the Invention

[0007] Purpose of the invention: The first purpose of this invention is to provide a steel-UHPC precast composite box girder with integrally cast UHPC bridge deck; the second purpose is to provide a method for preparing the above-mentioned steel-UHPC precast composite box girder with integrally cast UHPC bridge deck.

[0008] Technical solution: The steel-UHPC precast composite box girder with integrally cast UHPC bridge deck proposed in this invention includes multiple segments, each segment including a steel box girder and an integrally cast UHPC bridge deck on the top surface of the steel box girder; The steel box girder includes a top plate, bottom plate, inclined web plates, side web plates, middle web plates, small longitudinal beams, and transverse diaphragms. The transverse diaphragms are evenly distributed on the bottom plate along the bridge direction. The inclined web plates are located at the two edges of the bottom plate, the side web plates are located at the two edges of the inclined web plates, and the middle web plates are located at 1 / 6 to 1 / 3 of the inner length of the steel box girder on both sides. The inclined web plates, side web plates, and middle web plates are perpendicular to the transverse diaphragms. The small longitudinal beams are located in the middle of the transverse diaphragms. The top plate is attached to the side web plates, middle web plates, small longitudinal beams, and transverse diaphragms and is welded and fixed. The UHPC bridge deck is internally laid with a double-layer steel mesh, which is cast integrally into the top slab and then connected to the steel box girder to form an integral load-bearing structure. The UHPC bridge deck is locally thickened at the locations corresponding to the connections between the top plate and the side webs, the middle webs, and the small longitudinal beams.

[0009] Furthermore, the thickness of the UHPC bridge deck is 10~25cm, and the thickness of the top plate of the steel box girder is 6~20mm.

[0010] Furthermore, the bottom surface of the top plate is provided with top plate stiffening ribs, and the top surface is provided with connectors. The top plate stiffening ribs are U-shaped ribs, L-shaped ribs, or V-shaped ribs. The connectors include studs and perforated plate connectors, which connect the UHPC bridge deck and the top plate.

[0011] Furthermore, the UHPC bridge deck is horizontally provided with a double-layer steel mesh, which consists of an upper steel mesh and a lower steel mesh, and is connected by erected steel bars to form a whole; The spacing between the upper and lower steel mesh is 1 / 3 to 1 / 2 of the thickness of the UHPC bridge deck; the net distance between the lower steel mesh and the top plate of the steel box girder is 1 / 6 to 1 / 3 of the thickness of the UHPC bridge deck.

[0012] Furthermore, the top plate is recessed at the corresponding positions where it connects with the side web, the middle web, and the small longitudinal beam to form a groove structure, and the UHPC bridge deck is locally thickened at the corresponding recessed positions. The thickness of the locally thickened section of the UHPC bridge deck is 1.2 to 1.5 times the standard thickness, and the width of the locally thickened section is 50 to 200 cm. The top plate of the steel box girder is provided with studs and perforated plate connectors on the top surface of the locally thickened section of the UHPC bridge deck. The perforated plate connectors are provided with openings, and bent-up steel bars are provided at the openings. The two ends of the bent-up steel bars are bent upward and tied to the double-layer steel mesh.

[0013] Furthermore, the hole spacing of the perforated plate connector is the same as the mesh spacing of the double-layer steel mesh.

[0014] Furthermore, the UHPC bridge deck is provided with longitudinal prestressed connectors, which are prestressed steel bars or prestressed steel strands. The prestressed connectors are arranged in the middle of the double-layer steel mesh and anchored at both ends with flat anchor heads; the horizontal spacing of the prestressed connectors is 2 to 4 times the thickness of the UHPC bridge deck.

[0015] Furthermore, the UHPC in the bridge deck has a compressive strength of ≥120MPa, a cracking stress of ≥10MPa, an elastic modulus of ≥55GPa, and a total shrinkage strain of ≤350×10⁻⁶ days. -6 .

[0016] The preparation method of the above-mentioned steel-UHPC precast composite box girder with integrally cast UHPC bridge deck includes the following steps: S1: The steel box girder is fabricated in the factory, and the connecting parts are welded on the top plate. At the same time, the double-layer steel mesh is fabricated in the factory and the longitudinal prestressed ducts are pre-embedded. S2: Set up a formwork frame, fix the steel box girder on the formwork frame, use the top plate of the steel box girder as the bottom formwork, and at the same time set up the side formwork. The double-layer steel mesh is laid on the top plate using Y-shaped support members. S3: Cast the UHPC bridge deck as a whole, and then perform standard curing or steam curing until the design strength is reached; S4: Tensioning and anchoring of longitudinal prestressed connectors in the sections of the pre-embedded prestressed ducts; S5: Transport the prefabricated composite beam segments to the hoisting position for hoisting and connection.

[0017] Furthermore, the UHPC bridge deck is cast in one go or in two stages. The two-stage casting involves first casting the UHPC bridge decks on both sides of the transverse bridge, and then casting the UHPC bridge decks in the central median area.

[0018] This method is based on the shrinkage characteristic curve of UHPC and makes full use of the characteristic that UHPC can complete more than 80% shrinkage within 48 hours after pouring. This means that most of the shrinkage of the first-poured part is completed before the central strip is poured, thereby significantly reducing the shrinkage constraint stress of UHPC and fundamentally improving the crack resistance of the bridge deck.

[0019] Beneficial effects: Compared with the prior art, the significant advantages of this invention are: 1. The UHPC bridge deck is cast as a whole in the factory, which completely eliminates the risk of cracking and quality problems caused by wet joints in the existing technology, and greatly improves the integrity and durability of the structure. 2. The top slab serves as both a formwork and a structural load-bearing element, simplifying the construction process and increasing the level of industrialization; 3. By using high-performance concrete materials, the thickness of the UHPC bridge deck can be further optimized and reduced, thereby reducing the structural weight and expanding the applicable span. 4. For tension zones, additional steel mesh or prestressing is applied to effectively control cracks and improve the tensile strength of the bridge deck. 5. Except for the segmental connection of the bridge site, all processes are completed in the factory, ensuring quality control, shortening the construction period, and being environmentally friendly and energy-saving, which is in line with the development direction of industrialized construction. 6. By incorporating locally thickened UHPC bridge deck structures at the joints of the side webs, middle webs, and top of the small longitudinal beams in the steel box girder under complex stress conditions, and combining these with a connection system of studs, perforated plates, or a combination of both, the shear force transfer capacity of the steel-UHPC interface and the negative bending moment resistance of this area are significantly enhanced. The organic combination of bent-up reinforcing bars, perforated connectors, and double-layer steel mesh forms a three-dimensional force transmission path, effectively preventing interface failure caused by stress concentration. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of a steel-UHPC precast composite box girder with integrally cast UHPC bridge deck according to the present invention; Figure 2 This is a schematic diagram of the internal structure of the steel-UHPC precast composite box girder with integrally cast UHPC bridge deck according to the present invention; Figure 3 This is a schematic diagram of the UHPC bridge deck section of the steel-UHPC precast composite box girder, which is integrally cast using the UHPC bridge deck of the present invention. Figure 4 This is a schematic diagram of the cross-section of the steel-UHPC precast composite box girder with integrally cast UHPC bridge deck according to the present invention, and an enlarged view of the locally thickened part; Figure 5 for Figure 4 Schematic diagram of the structure of the locally thickened section; Figure 6This is a schematic diagram of the steel box (without top plate) of the steel-UHPC precast composite box girder with integral casting of UHPC bridge deck according to the present invention; Figure 7 This is a schematic diagram of the top slab of the steel-UHPC precast composite box girder with integrally cast UHPC bridge deck according to the present invention. Figure 8 for Figure 2 Enlarged view of a portion of point A in the middle; Figure 9 This is a schematic diagram of the flat anchor head of the steel-UHPC precast composite box girder with integral casting of UHPC bridge deck according to the present invention; Figure 10 This is a schematic diagram of the two-stage casting method for the steel-UHPC precast composite box girder with integral casting of UHPC bridge deck according to the present invention. Figure 11 This is a UHPC shrinkage curve diagram of the steel-UHPC precast composite box girder with integrally cast UHPC bridge deck according to the present invention. Detailed Implementation

[0021] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0022] This embodiment describes a steel-UHPC precast composite box girder with integrally cast UHPC bridge deck, such as... Figures 1-10 .

[0023] like Figure 1 , 2 As shown, the steel-UHPC precast composite box girder with integrally cast UHPC bridge deck consists of multiple segments, each segment including a steel box girder 1 and a UHPC bridge deck 2, with the UHPC bridge deck 2 being integrally cast.

[0024] The steel box girder 1 includes a bottom plate 101, an inclined web plate 102, a side web plate 103, a middle web plate 104, a small longitudinal beam 105, a transverse diaphragm 106, and a top plate 107.

[0025] Transverse diaphragms 106 are evenly distributed on the bottom plate 101 along the bridge direction. Inclined web plates 102 are located at the two sides of the bottom plate 101. Side web plates 103 are located at the two sides of the inclined web plates 102. Middle web plates 104 are located at 1 / 6 to 1 / 3 of the inner sides of the steel box girder 1. The inclined web plates 102, side web plates 103, and middle web plates 104 are perpendicular to the transverse diaphragms 106. Small longitudinal beams 105 are located in the middle of the transverse diaphragms 106. The top plate 107 is located on the side web plates 103, middle web plates 104, small longitudinal beams 105, and transverse diaphragms 106, and is welded and fixed to form a stable stiffening system.

[0026] like Figure 3 and 4As shown, the top plate 107 is located on the top of the steel box girder and is welded and fixed to the inclined web 102, the intermediate web 104, the small longitudinal beams 105, and the transverse diaphragms 106. Its bottom surface is provided with top plate stiffening ribs 109, and its top surface is provided with connectors 108. The top plate stiffening ribs 109 are U-shaped, L-shaped, or V-shaped ribs, significantly improving the local stiffness of the top plate. The connectors 108 include studs 108-A and perforated plate connectors 108-B, used to ensure shear force transfer between the steel and the UHPC. The outer layer of the connectors is coated with epoxy resin to prevent steel corrosion.

[0027] Figure 3 This is a schematic diagram of the cross-section of the monolithically cast UHPC bridge deck. The material for UHPC bridge deck 2 is coarse aggregate UHPC concrete. The UHPC compressive strength is above 120 MPa, the cracking stress is above 10 MPa, the elastic modulus is above 55 GPa, and the total shrinkage strain after 90 days is ≤350 × 10⁻⁶. -6 .

[0028] The thickness of the UHPC bridge deck is 10~25cm, and 15cm is used in this embodiment. The thickness of the top plate of the steel box girder is 6~20mm, and 8mm is used in this embodiment.

[0029] The concrete mix design for a monolithically cast UHPC bridge deck comprises, by weight: 600-750 parts cement, 150-200 parts silica fume, 1000-1200 parts quartz sand, 400-600 parts coarse aggregate, 25-40 parts water-reducing agent, 150-200 parts steel fiber, and 130-160 parts water. The coarse aggregate is basalt or granite crushed stone with a particle size of 5mm-10mm. Preferably, 20-50 parts of magnesium oxide expanding agent are added to the UHPC mix design to reduce shrinkage of the UHPC bridge deck.

[0030] The bridge deck is equipped with a double-layer steel mesh 202 to bear the service load. The double-layer steel mesh 202 is laid on the top slab 107 via Y-shaped supports 110. Stirring bars 204 are provided between the double-layer steel mesh 202 to fix and support them, ensuring they are in their designed positions. The UHPC 201 is cast into the top slab 107 in one piece in the factory and forms an integral load-bearing structure with the top slab 107 via connectors 108.

[0031] In this embodiment, the top plate 107 is 8mm thick, the UHPC 201 is 15cm thick, and the double-layer steel mesh 202 uses HRB400 grade ordinary steel bars or epoxy steel bars with a diameter of 10mm and a mesh spacing of 180mm.

[0032] The spacing between the upper and lower reinforcing mesh is 1 / 3 to 1 / 2 of the thickness of the UHPC bridge deck 2; the net distance between the lower reinforcing mesh and the top plate 107 of the steel box girder is 1 / 6 to 1 / 3 of the thickness of the UHPC bridge deck 2. In this embodiment, the spacing between the upper and lower reinforcing mesh of the double-layer reinforcing mesh 202 is 7cm, and the net distance between the lower reinforcing mesh and the top plate 107 is 2.5cm.

[0033] Figure 4 and Figure 5 The structure is designed for the partial thickening of the integrally cast UHPC bridge deck 2. Figure 5 The double-layer steel mesh 202 in the middle section has been concealed. A local thickening structure for the UHPC bridge deck is provided in the top region of the web 104 of the steel box girder. The specific features of this structure are as follows: In this region, the top plate 107 is concave downwards, forming a localized thickening, which significantly enhances the stiffness and load-bearing capacity of the steel top plate in this area. Correspondingly, the thickness of the UHPC bridge deck 2 formed after the UHPC is poured in this region is also increased to 1.2 to 1.5 times the standard thickness. In this embodiment of the invention, the standard thickness of the UHPC bridge deck 2 is 15 cm, so it is thickened to 24 cm here. The increased concrete thickness provides a larger moment of inertia of the section, effectively improving the bending resistance of the section.

[0034] In this locally thickened area, a hybrid connection system is installed on the top surface of the top plate 107. This includes: Stud 108-A, as a basic anti-lifting connector, is evenly distributed; The perforated plate connector 108-B, as the main shear-resistant connector, has regularly spaced perforations on its web. The bent-up rebar 207 passes through the opening in the perforated plate connector 108-B, with both ends bent upwards and tied to the double-layer steel mesh 202 inside the UHPC. This forms a three-dimensional spatial truss force transmission system with the bent-up rebar 207, the double-layer steel mesh 202, and the perforated plate connector 108-B.

[0035] When subjected to stress, the shear force is transmitted through the concrete tenon at the opening of the connector and the bent steel bar. The force flow is smoothly introduced into the steel skeleton of the entire bridge deck through the bent steel bar, avoiding stress concentration and greatly improving the toughness and load-bearing capacity of the connection interface.

[0036] The local thickening structure in this embodiment systematically solves the technical problem of easy damage to the steel-UHPC interface of composite beams in the negative bending moment zone at the support by adopting four measures: "thickening of steel top plate + thickening of UHPC + hybrid connection of perforated connectors and studs + integration of bent-up bars". This provides a reliable guarantee for the safety of the structure under complex stress conditions.

[0037] Figure 6 This is a schematic diagram of a steel box girder without a top plate. Figure 6 As shown, the steel box girder consists of a bottom plate 101, an inclined web plate 102, a middle web plate 104, small longitudinal beams 105, and transverse diaphragms 106, forming a stable skeleton structure.

[0038] like Figure 6 As shown in the enlarged view, the steel box girder structure has been specially optimized in the top area of ​​the web 104: the steel top plate 107 at the top of the web 104 is designed to be partially concave, and the thickness of the top plate in the concave area is increased to 1.2 to 1.5 times the thickness of the standard top plate; the corresponding diaphragm 106 has a matching notch reserved in this area, the shape of which fits the concave top plate 107 to ensure the continuous connection between the diaphragm and the thickened top plate.

[0039] Figure 7 This is a schematic diagram of the top plate 107. Figure 5 As shown, the top plate 107 is provided with a connector 108 and a top plate stiffening rib 109. The connector 108 is used to bond with the concrete, and the top plate stiffening rib 109 is used to improve the rigidity of the top plate.

[0040] Figure 8 and Figure 9 This is a schematic diagram of the flat anchor head 206 according to an embodiment of the present invention. In areas of the bridge deck with high longitudinal tensile stress, such as the negative bending moment area of ​​a continuous beam, flat corrugated metal pipes 205 are provided between the double-layer steel mesh 202. Each flat corrugated metal pipe 205 has three longitudinal prestressed connectors 203, which can be prestressed steel bars or prestressed steel strands. The two ends of the longitudinal prestressed connectors 203 are anchored by the flat anchor head 206. The horizontal spacing of the prestressed connectors 203 is 2 to 4 times the thickness of the UHPC bridge deck 2. In this embodiment, the spacing of each group of longitudinal prestressed connectors 203 is 600 mm. The flat anchor head 206 can be completely embedded within the thickness range of the UHPC bridge deck without increasing the structural height, thus maintaining the flatness of the bridge deck.

[0041] Figure 10 This is a schematic diagram of the two-stage casting method for UHPC bridge decks. The core technology lies in controlling the time interval between the two stages of casting, which is based on the shrinkage performance test of the UHPC material used.

[0042] Figure 11 The UHPC shrinkage curve used in the two-stage casting method of UHPC bridge deck in this embodiment shows that the shrinkage strain of the UHPC used can develop to more than 80% of the final shrinkage value within 48 hours after casting, that is, most of the shrinkage is completed rapidly within this time window.

[0043] Based on this material property, the specific steps of this embodiment are as follows: First, the steel box girder 1 is fabricated in the factory, and the connector 108 is welded on the top plate 107. At the same time, the double-layer steel mesh 202 is fabricated in the factory and longitudinal prestressed ducts are pre-embedded. A formwork was set up, and the steel box girder 1 was fixed on the formwork. The top plate 107 of the steel box girder served as the bottom formwork, and the side formwork was erected simultaneously. Double-layer steel mesh 202 was laid on the top plate 107 through Y-shaped supports 110. Subsequently, the UHPC bridge decks on both sides of the transverse bridge were poured first, and a waiting period of at least 48 hours was allowed. During this interval, the strength of the first-poured UHPC slabs increased rapidly, and most of their shrinkage was released freely, resulting in a significant dissipation of internal shrinkage stress. Finally, the UHPC bridge decks in the central median 3 area were poured.

[0044] The two-stage casting method divides the large bridge deck into sections and casts them at different times. The shrinkage deformation of the top plates on both sides that are cast first can occur earlier and relatively independently, thereby significantly reducing the constraint stress on the central dividing strip 3 of the later-cast concrete and effectively preventing the generation of shrinkage cracks.

[0045] The pre-embedded prestressed duct sections are tensioned and anchored with longitudinal prestressed connectors. By tensioning the prestress in the factory, prestress is actively established in the bridge deck to completely offset or significantly reduce the tensile stress generated in the area, thus fundamentally preventing cracking.

[0046] The steel-UHPC precast composite box girder, with its integrally cast UHPC bridge deck, employs factory-based, standardized, intelligent, and assembly-based construction techniques. All fabrication work for the composite girder is completed in the factory, including steel box girder fabrication, top slab installation, bridge deck casting, and prestressing tensioning. Except for the bridge site connection, all main girder fabrication is completed to a high standard in the factory, significantly reducing on-site operational difficulties, effectively improving efficiency, shortening the construction period, and enhancing the industrialization level of composite girder construction.

[0047] This invention employs a structural form combining integrally cast UHPC bridge deck and steel box girder, completely eliminating wet joints, reducing structural weight, and effectively solving the cracking problem in the negative bending moment zone through prestressing technology, enabling the steel-concrete composite beam to meet the needs of bridges with larger spans. Furthermore, this composite beam structure allows all main beam fabrication, except for the docking at the bridge site, to be completed in the factory with high quality, exhibiting a high degree of factory production, aligning with the development trend of industrialized construction, and contributing to environmental sustainability.

Claims

1. A steel-UHPC precast composite box girder with integrally cast UHPC bridge deck, characterized in that: It includes multiple segments, each segment including a steel box girder (1) and a UHPC bridge deck (2) integrally cast on the top surface of the steel box girder; The steel box girder (1) includes a top plate (107), a bottom plate (101), inclined web plates (102), side web plates (103), a middle web plate (104), small longitudinal beams (105), and transverse diaphragms (106) of a steel structure; the transverse diaphragms (106) are evenly distributed on the bottom plate (101) along the bridge direction, the inclined web plates (102) are located at the two sides of the bottom plate (101), and the side web plates (103) are located at the two sides of the inclined web plates (102). At the location, the middle web plate (104) is set at 1 / 6 to 1 / 3 of the inner sides of the steel box girder (1), and the inclined web plate (102), side web plate (103), and middle web plate (104) are perpendicular to the transverse diaphragm (106); the small longitudinal beam (105) is set in the middle of the transverse diaphragm (106); the top plate (107) is set on the side web plate (103), middle web plate (104), small longitudinal beam (105) and transverse diaphragm (106) and is welded and fixed. The UHPC bridge deck (2) is internally laid with a double-layer steel mesh (202), and after being cast into the top plate (107), it forms an integral load-bearing structure with the steel box girder through the connector (108); The UHPC bridge deck (2) is locally thickened at the corresponding positions where the top plate (107) connects with the side web plate (103), the middle web plate (104) and the small longitudinal beam (105).

2. The steel-UHPC precast composite box girder with integrally cast UHPC bridge deck as described in claim 1, characterized in that: The thickness of the UHPC bridge deck (2) is 10~25cm, and the thickness of the top plate (107) of the steel box girder is 6~20mm.

3. The steel-UHPC precast composite box girder with integrally cast UHPC bridge deck as described in claim 1, characterized in that: The bottom surface of the top plate (107) is provided with a top plate stiffening rib (109), and the top surface is provided with a connector (108). The top plate stiffening rib (109) is a U-shaped rib, an L-shaped rib, or a V-shaped rib. The connector (108) includes a stud (108-A) and a perforated plate connector (108-B) to connect the UHPC bridge deck (2) and the top plate (107).

4. The steel-UHPC precast composite box girder with integrally cast UHPC bridge deck as described in claim 1, characterized in that: The UHPC bridge deck (2) is horizontally provided with a double-layer steel mesh (202), which is composed of an upper steel mesh and a lower steel mesh, and is connected to form a whole by erecting steel bars (204); The spacing between the upper and lower steel mesh is 1 / 3 to 1 / 2 of the thickness of the UHPC bridge deck (2); the net distance between the lower steel mesh and the top plate (107) of the steel box girder is 1 / 6 to 1 / 3 of the thickness of the UHPC bridge deck (2).

5. The steel-UHPC precast composite box girder with integrally cast UHPC bridge deck as described in claim 3, characterized in that: The top plate (107) is recessed at the corresponding position of the connection between the side web plate (103), the middle web plate (104) and the small longitudinal beam (105) to form a groove structure, and the UHPC bridge deck (2) is locally thickened at the corresponding recessed position. The thickness of the locally thickened part of the UHPC bridge deck (2) is 1.2 to 1.5 times the standard thickness, and the width of the locally thickened part is 50 to 200 cm. The top plate (107) of the steel box girder is provided with studs (108-A) and perforated plate connectors (108-B) on the top surface of the locally thickened part of the UHPC bridge deck (1). The perforated plate connectors (108-B) are provided with openings, and bent-up steel bars (207) are provided at the openings. The bent-up steel bars (207) are bent upward at both ends and tied to the double-layer steel mesh (202).

6. The steel-UHPC precast composite box girder with integrally cast UHPC bridge deck as described in claim 5, characterized in that: The opening spacing of the perforated plate connector (108-B) is the same as the steel mesh spacing of the double-layer steel mesh (202).

7. The steel-UHPC precast composite box girder with integrally cast UHPC bridge deck as described in claim 1, characterized in that: The UHPC bridge deck (2) is provided with a longitudinal prestressed connector (203), which is a prestressed steel bar or a prestressed steel strand; The prestressed connector (203) is arranged in the middle of the double-layer steel mesh (202) and is anchored at both ends by flat anchor heads (206); the horizontal spacing of the prestressed connector (203) is 2 to 4 times the thickness of the UHPC bridge deck (2).

8. The steel-UHPC precast composite box girder with integrally cast UHPC bridge deck as described in claim 1, characterized in that: The UHPC bridge deck (2) has a compressive strength of ≥120MPa, a cracking stress of ≥10MPa, an elastic modulus of ≥55GPa, and a total shrinkage strain of ≤350×10⁻⁶ days. -6 .

9. The method for preparing a steel-UHPC precast composite box girder with integrally cast UHPC bridge deck as described in any one of claims 1-8, characterized in that, Includes the following steps: S1: The steel box girder (1) is fabricated in the factory and the connector (108) is welded on the top plate (107). At the same time, the double-layer steel mesh (202) is fabricated in the factory and longitudinal prestressed ducts are pre-embedded. S2: Set up a formwork frame, fix the steel box girder (1) on the formwork frame, use the top plate (107) of the steel box girder (1) as the bottom formwork, and set up the side formwork at the same time. The double-layer steel mesh (202) is laid on the top plate (107) using Y-shaped support members (110). S3: Cast the UHPC bridge deck as a whole (1), and then perform standard curing or steam curing until the design strength is reached; S4: Tensioning and anchoring of longitudinal prestressed connectors in the sections of the pre-embedded prestressed ducts; S5: Transport the prefabricated composite beam segments to the hoisting position for hoisting and connection.

10. The method for preparing a steel-UHPC precast composite box girder with integrally cast UHPC bridge deck according to claim 9, characterized in that: The UHPC bridge deck (1) is cast in one go or in two stages. The two-stage casting is to first cast the UHPC bridge deck on both sides of the transverse bridge and then cast the UHPC bridge deck in the central median (3) area.

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