Suspension bridge double-layer different-sag four-main-cable extension structure and extension method

Abstract

The invention discloses a suspension bridge double-layer different-sag four-main-cable extension structure and an extension method. The extension structure is characterized in that a lower-layer extension stiffening beam and an existing suspension bridge stiffening beam are arranged in parallel up and down and are connected through web members to form a double-layer truss structure; the lower-layer extension stiffening beam transmits a load to the newly-added main cable through the beam bottom transverse suspension cable cross beam and the newly-added sling; the newly-added main cables are located on the side, far away from the center line of the bridge axis, of the existing main cable and arranged side by side, the newly-added main cables are vertically supported on the expanded bridge tower through newly-added vertical main cable saddles, and pulling force is transmitted to the newly-added second anchors through side span newly-added second cable saddles and anchoring systems. The midspan point of the newly-added main cable is located below the upper-layer bridge floor, the vertical span ratio is larger than that of an existing main cable, and the number of materials of the newly-added main cable is reduced. The newly-added slings and the existing slings are longitudinally arranged at intervals and are different in longitudinal and transverse positions, so that the problem that the local fatigue stress of the steel structure at the anchoring positions of the slings and the steel beams is large is solved; the sags of the four main cables are different, so that the dynamic stability of the suspension bridge is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of civil engineering bridges, and particularly relates to a four-main-cable expansion structure with double-layer different sag and an expansion method for a suspension bridge. Background Technique

[0002] The statements in this part only provide background technical information related to the present invention and do not necessarily constitute prior art.

[0003] For the engineering examples of the reconstruction and expansion of long-span suspension bridges, there are very few. There are only some unpublished and unlicensed patents. For example, a four-main-cable suspension bridge expansion structure for single-layer to double-layer traffic transformation disclosed in the publication number CN119491461A. The newly added main cable is vertically stacked with the existing main cable, and the anchoring positions of the newly added suspenders and the existing suspenders are at the same position. The welding residual stress of the anchoring steel plate at this place is relatively large, and the suspender anchoring steel plate bears dynamic loads, resulting in relatively large fatigue stress at this place, and it is easy to have problems of structural damage and fatigue failure. At the same time, in terms of the main cable design, for the vertically stacked main cables, the sag-span ratio of the newly added main cable is relatively small, so that the tension of the newly added main cable is relatively large under the same vertical load, resulting in a large amount of steel used for the main cable and high costs. Moreover, the distance between the newly added suspenders and the existing suspenders is relatively small, and the long suspenders are prone to large deformations and collisions under the action of strong winds, which is not conducive to the durability of the bridge. Summary of the Invention

[0004] In view of the above problems, the present invention provides a four-main-cable expansion structure with double-layer different sag for a suspension bridge and an expansion method. By making the longitudinal and transverse anchoring positions of the newly added suspenders and the existing suspenders different, the fatigue problem at the suspender anchoring place is solved; at the same time, the distance between the suspenders is relatively large, avoiding the risk of suspender collision under the action of strong winds; the sag-span ratio of the newly added main cable is relatively large, controlling the internal force of the main cable and reducing the cost of the main cable; the sag-span ratios of the newly added main cable and the existing main cable are different, improving the dynamic stability of the bridge structure.

[0005] In order to achieve the above object, the present invention is realized by the following technical solutions:

[0006] In the first aspect, the present invention provides a four-main-cable expansion structure with double-layer different sag for a suspension bridge, including:

[0007] The stiffening girder of the existing suspension bridge, the lower-layer expanded stiffening girder, the existing main cable, the newly added main cable, the existing main saddle, the expanded main saddle, the existing bridge tower and the expanded tower column, the first anchor and the second anchor.

[0008] The stiffening girder of the existing suspension bridge and the lower-layer extended stiffening girder are arranged parallel to each other vertically, and are connected by vertical web members and diagonal web members to form a double-deck truss structure; the beam segments of the stiffening girder of the existing suspension bridge and the double-deck steel truss stiffening girder after extension at the bridge tower are both connected to the bottom cross beam or column of the existing bridge tower or the extended bridge tower; the stiffening girder of the existing suspension bridge is connected to the existing main cable saddle at the top of the existing bridge tower through two existing main cables; the lower-layer extended stiffening girder is connected to the extended main cable saddle at the top of the extended tower column through two newly added main cables; the sag-span ratio of the existing main cable is less than that of the newly added main cable, and the cable force of the newly added main cable is less than that of the existing main cable under the same vertical load; the end of the existing main cable is fixedly connected to the first anchor through the first cable dispersion saddle, and the end of the newly added main cable is fixedly connected to the second anchor through the second cable dispersion saddle.

[0009] As a further implementation method, the stiffening girder of the existing suspension bridge adopts a steel box girder or a steel plate girder. A first connection joint plate is fixedly arranged at the bottom of the stiffening girder of the existing suspension bridge, and a force transmission structure plate is arranged at a position corresponding to the first connection joint plate inside the stiffening girder of the existing suspension bridge; the lower-layer extended stiffening girder adopts a steel box girder or a steel plate girder. A second connection joint plate is fixedly arranged at the top of the lower-layer extended stiffening girder, and a force transmission structure plate is also arranged at a position corresponding to the second connection joint plate inside the lower-layer extended stiffening girder;

[0010] The vertical web members and diagonal web members are both arranged on both sides of the lower-layer extended stiffening girder; and are used to connect the stiffening girder of the existing suspension bridge and the lower-layer extended stiffening girder; the vertical web members and diagonal web members are arranged at intervals along the axial direction of the stiffening girder of the existing suspension bridge; the top ends of the vertical web members and diagonal web members are both fixedly connected to the first connection joint plate, and the bottom ends of the vertical web members and diagonal web members are both fixedly connected to the second connection joint plate.

[0011] As a further implementation method, suspension beams are arranged at intervals along the length direction on the lower-layer extended stiffening girder. The length of the suspension beam is greater than the width of the lower-layer extended stiffening girder, and both ends of the suspension beam protrude from both side surfaces of the lower-layer extended stiffening girder and are used to connect the newly added main cables.

[0012] As a further implementation method, a main cable saddle is arranged at the top of the existing bridge tower. The main cable saddle includes two existing main cable saddles and two extended main cable saddles; the two existing main cable saddles are arranged on the tower columns of the existing bridge tower, and the extended main cable saddles are arranged side by side with the existing main cable saddles and are respectively arranged on the side far from the bridge axis of the two existing main cable saddles; the existing main cables are connected to the existing main cable saddles, and the newly added main cables are connected to the extended main cable saddles.

[0013] As a further implementation method, the existing bridge tower has a portal structure, and the extended tower column is realized by increasing the cross-sectional dimensions on the longitudinal and transverse outer sides of the existing bridge tower, which is used to increase the size of the existing bridge tower to improve its bearing capacity;

[0014] There are two tower bridges, which are respectively arranged at both ends of the mid-span of the stiffening girder of the existing suspension bridge.

[0015] As a further implementation method, the first anchor and the second anchor are respectively arranged on both sides of the connection structure of the existing suspension bridge; the second anchor is arranged outside the first anchor and is arranged near one end of the existing bridge tower.

[0016] As a further implementation method, existing suspenders are arranged between the existing main cable and the stiffening girder of the existing suspension bridge, and new suspenders are arranged between the new main cable and the lower-layer extended stiffening girder.

[0017] As a further implementation method, one end of the existing suspender is connected to the existing main cable through a cable clamp, and the existing suspender and the cable clamp are hinged; the other end is connected to the stiffening girder of the existing suspension bridge through a connecting piece; one end of the new suspender is connected to the new main cable through a cable clamp, and the new suspender and the cable clamp are hinged; the other end is connected to the suspension beam of the lower-layer extended stiffening girder through a connecting piece.

[0018] As a further implementation method, one straight web member corresponds to the lower end of each existing suspender, and one straight web member corresponds to each suspension beam, which is used to reduce the local tensile stress of the suspender on the suspension bridge or the extended bridge.

[0019] In the second aspect, the present invention also provides an extension method for an extension structure of a double-layer different sag four-main-cable suspension bridge, including the following steps:

[0020] S1. New bored cast-in-place pile foundations and new bearing platforms are added on the lateral far-bridge-axis side of the existing bridge tower foundation;

[0021] S2. On the basis of the new bearing platform, the extended tower column adds a new cross-section and installs a new main cable saddle;

[0022] S3. New anchor foundation is added, and a new dispersion cable saddle and an anchoring system are set; during the construction process of this step and the previous steps, the existing suspension bridge can operate normally;

[0023] S4. During the construction process of this step and subsequent steps, the existing suspension bridge needs to be closed for construction; relying on the existing bridge tower and the existing main cable, the catwalk of the new main cable is installed, the new main cable is erected; the new suspenders are installed at the set distance;

[0024] S5. Adopt a split self - propelled cable - borne crane located on the newly added main cable, and symmetrically hoist the lower - layer expansion part of the double - layer steel truss girder from both sides of the bridge tower towards the mid - span. The web members of the expanded part of the double - layer steel truss girder are temporarily connected to the existing upper - layer stiffening girder, and the newly added suspenders are tensioned.

[0025] S6. After the mid - span closure of the expanded part of the double - layer steel truss girder, finely adjust the cable forces of the newly added suspenders so that the newly added main cable reaches the completed - bridge alignment. Finely adjust the elevation at the interface between the web members of the expanded double - layer steel truss girder and the existing upper - layer stiffening girder, and adjust the temporary connection to a welded or high - strength bolt connection.

[0026] S7. Wind the newly added main cable with wire ropes for protection; then remove the catwalk; complete the closure project of the newly added dispersion saddles, anchorage systems and newly added main saddles; finally, conduct acceptance and open to traffic for operation.

[0027] Compared with the prior art, the advantages and positive effects of the present invention are as follows:

[0028] The present invention cleverly divides all the loads of the expanded double - layer steel truss stiffening girder into two parts. One part is transmitted to the existing main cable through the suspenders corresponding to the existing main cable, and the other part is directly transmitted to the newly added main cable through the suspenders corresponding to the newly added main cable. The existing suspenders and the newly added suspenders are longitudinally arranged at intervals, avoiding the concentrated arrangement of the anchorage steel plates of the suspenders on the upper - layer stiffening girder, where the residual welding stress and fatigue stress of the anchorage steel plates are relatively large, and it is easy to have structural damage and fatigue failure problems. By setting the sag - span ratio of the existing main cable to be less than that of the newly added main cable and using the mechanical relationship between the sag - span ratio and the tension, the tensions of the existing main cable and the newly added main cable used for the lower - layer expanded stiffening girder are effectively reduced, thereby reducing the steel consumption of the main cable and realizing cost control on the premise of ensuring structural safety. Through reasonable structural arrangement and taking advantage of the longitudinal interval arrangement of the existing suspenders and the newly added suspenders, the collision problem between long suspenders under strong wind is avoided. At the same time, the different vertical deflections of the existing main cable and the newly added main cable increase the system damping and improve the dynamic stability of the suspension bridge.

[0029] In the present invention, by designing the connection between the existing suspenders and the cable clips, and between the newly added suspenders and the cable clips as hinged joints, and connecting the other ends to the bridge through connectors, good adaptability of the suspenders is given, effectively avoiding abnormal deformation of the suspenders caused by large longitudinal displacements of the double - layer steel truss stiffening girder. Through the design that each lower end of the existing suspenders corresponds to a straight web member and each suspender cable beam corresponds to a straight web member, the load transmitted from the stiffening girder to the suspenders is smoothly transmitted, significantly reducing the local stress of the suspenders on the double - layer truss girder. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The specification drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation to the present invention.

[0031] Figure 1 Elevation schematic diagram of the double-layer different sag four-main cable expansion structure of the suspension bridge of the present invention;

[0032] Figure 2 Top view of the double-layer different sag four-main cable expansion structure of the suspension bridge of the present invention;

[0033] Figure 3 Partial three-dimensional structure diagram of the double-layer different sag four-main cable expansion structure of the suspension bridge of the present invention;

[0034] Figure 4 Schematic diagram of the arrangement of the main cable saddles of the double-layer different sag four-main cable expansion structure of the suspension bridge of the present invention;

[0035] Figure 5 Cross-sectional structure diagram at the sling cantilever beam of the double-layer different sag four-main cable expansion structure of the suspension bridge of the present invention;

[0036] Figure 6 Cross-sectional structure diagram at the existing sling of the double-layer different sag four-main cable expansion structure of the suspension bridge of the present invention.

[0037] In the figure: 1. New main cable; 2. Existing main cable; 3. New sling; 4. Existing sling; 5. Existing stiffening girder of the suspension bridge; 6. Lower-layer expansion stiffening girder; 7. Existing bridge tower; 8. Expansion tower column; 9. Expansion main cable saddle; 10. Existing main cable saddle; 11. Tower-girder connection device; 12. First cable saddle; 13. First anchor; 14. Second cable saddle; 15. Second anchor; 16. Suspension beam; 17. Straight web member; 18. Diagonal web member. Detailed implementation manners

[0038] It should be noted that the following detailed description is exemplary and is intended to provide further illustration of the present invention. Unless otherwise specified, all technical and scientific terms used in the present invention have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention belongs.

[0039] It should be noted that the terms used herein are only for describing specific implementation manners and are not intended to limit the exemplary embodiments according to the present invention. As used herein, unless the present invention otherwise clearly indicates, the singular forms are also intended to include the plural forms. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they specify the presence of features, steps, operations, devices, components, and / or combinations thereof;

[0040] At present, there are very few engineering examples of the double-layer expansion of suspension bridges; for the disclosed patents, new main cables are vertically stacked with the existing main cables, and the new main cables are located directly above the existing main cables; there are two problems with this solution. First, the new vertical-to-span ratio is small, resulting in a greater tensile force in the main cable. To withstand a greater tensile force, more steel is required for the main cable, leading to an increase in the steel consumption of the new main cable. Second, the suspenders of the new main cable and the suspenders of the existing main cable have the same anchoring position on the upper stiffening girder, where the welding residual stress and fatigue stress are relatively large, and fatigue failure is likely to occur.

[0041] Embodiment 1

[0042] This embodiment provides a double-layer and different sag four-main-cable expansion structure for a suspension bridge, as Figures 1-6 shown, including: the existing stiffening girder 5 of the suspension bridge, the lower-layer expansion stiffening girder 6, the existing main cable 2, the new main cable 1, the existing main saddle 10, the expansion main saddle 9, the existing bridge tower 7 and the expansion tower column 8, the first anchor 13, and the second anchor 15;

[0043] The existing stiffening girder 5 of the suspension bridge, the existing main cable 2, the existing suspenders 4, the existing bridge tower 7, the existing main saddle 10, and the first anchor 13 constitute the existing operating suspension bridge; the existing operating suspension bridge generally adopts a double-tower single-span type.

[0044] The existing stiffening girder 5 of the suspension bridge and the lower-layer expansion stiffening girder 6 are arranged parallel to each other up and down and are connected by straight web members 17 and diagonal web members 18; both the existing stiffening girder 5 of the suspension bridge and the lower-layer expansion stiffening girder 6 are connected to the bottom cross beam or the bottom column of the existing bridge tower 7 and the expansion tower column 8; the existing stiffening girder 5 of the suspension bridge is connected to the existing main saddle 10 at the top of the existing bridge tower 7 through two existing main cables 2; the lower-layer expansion stiffening girder 6 is connected to the expansion main saddle 9 at the top of the expansion tower column 8 through two new main cables 1; the vertical-to-span ratio of the existing main cable 2 is smaller than that of the new main cable 1, and the cable force of the new main cable 1 is smaller than that of the existing main cable 2 under the same vertical load; the new main cable 1 is vertically supported and its alignment is smoothly changed by the expansion tower column 8 through the expansion main saddle 9, and the new anchor provides anchoring support through the new saddle; the double-layer truss structure formed by the upper stiffening girder and the lower-layer expansion stiffening girder 6 is vertically supported by the tower-girder connection device 11 adjusted to be below the lower-layer expansion stiffening girder 6 at the existing bridge tower 7, and the stiffening girder load is transmitted to the existing bridge tower 7; the end of the existing main cable 2 is fixedly connected to the first anchor 13 through the first saddle 12, and the end of the new main cable 1 is fixedly connected to the second anchor 15 through the second saddle 14.

[0045] As a further implementation method, the existing stiffening girder 5 of the suspension bridge adopts a steel box girder or a steel plate girder. A first connection node plate is fixedly arranged at the bottom of the existing stiffening girder 5 of the suspension bridge, and a force transmission structure plate is arranged at a position corresponding to the first connection node plate inside the existing stiffening girder 5 of the suspension bridge; the lower-layer extended stiffening girder 6 adopts a steel box girder or a steel plate girder, and a second connection node plate is fixedly arranged at the top of the lower-layer extended stiffening girder 6. A force transmission structure plate is also arranged at a position corresponding to the second connection node plate inside the lower-layer extended stiffening girder 6; to avoid the interference of the upper-layer stiffening girder on the newly added main cable 1 and the newly added suspender 3, the upper-layer stiffening girder can be modified to cancel the wind fairing, etc.

[0046] Both the vertical web member 17 and the diagonal web member 18 are arranged on both sides of the lower-layer extended stiffening girder 6; they are used to connect the existing stiffening girder 5 of the suspension bridge and the lower-layer extended stiffening girder 6; the vertical web member 17 and the diagonal web member 18 are arranged at intervals along the axial direction of the existing stiffening girder 5 of the suspension bridge; the top ends of the vertical web member 17 and the diagonal web member 18 are fixedly connected to the first connection node plate, and the bottom ends of the vertical web member 17 and the diagonal web member 18 are fixedly connected to the second connection node plate.

[0047] As a further implementation method, suspension beams 16 are arranged at intervals along the length direction on the lower-layer extended stiffening girder 6. The length of the suspension beam 16 is greater than the width of the lower-layer extended stiffening girder 6, and both ends of the suspension beam 16 protrude from both side surfaces of the lower-layer extended stiffening girder 6 and are used to connect the newly added main cable 1.

[0048] As a further implementation method, a main cable saddle is arranged at the top of the existing bridge tower 7. The main cable saddle includes two existing main cable saddles 10 and two extended main cable saddles 9; the two existing main cable saddles 10 are arranged on the tower columns of the existing bridge tower 7, and the extended main cable saddles 9 are arranged side by side with the existing main cable saddles 10 and are respectively arranged on the side away from the bridge axis of the two existing main cable saddles 10; the existing main cable 2 is connected to the existing main cable saddle 10, and the newly added main cable 1 is connected to the extended main cable saddle 9; the existing main cable saddle 10 is used for the vertical support of the existing main cable 2 and the smooth change of the alignment direction; the extended main cable saddle 9 is used for the vertical support of the newly added main cable 1 and the smooth change of the alignment direction.

[0049] As a further implementation method, the existing bridge tower 7 has a portal structure. The extended tower columns 8 are arranged on both sides of the existing bridge tower 7. The extended tower columns 8 are realized by increasing the cross-sectional dimensions on the longitudinal and transverse outer sides of the existing bridge tower 7 and are used to increase the size of the existing bridge tower 7 to improve the bearing capacity of the existing bridge tower 7;

[0050] There are two tower bridges, which are respectively arranged at both ends of the mid-span of the existing stiffening girder 5 of the suspension bridge.

[0051] As a further implementation method, the first anchor 13 and the second anchor 15 are respectively arranged on the transverse sides of the existing suspension bridge connection structure (not shown in the figure); to avoid interference between the newly added main cable 1, the existing main cable 2 and the connection driving space; the second anchor 15 is arranged outside the first anchor 13 and is arranged near one end of the existing bridge tower 7.

[0052] As a further implementation method, an existing suspender 4 is arranged between the existing main cable 2 and the existing stiffening girder 5 of the existing suspension bridge, and a newly added suspender 3 is arranged between the newly added main cable 1 and the lower-layer extended stiffening girder 6; the existing suspender 4 and the newly added suspender 3 are arranged at intervals along the longitudinal direction of the upper-layer stiffening girder to avoid interference during construction and operation.

[0053] As a further implementation method, one end of the existing suspender 4 is connected to the existing main cable 2 through a cable clamp, and the existing suspender 4 and the cable clamp are hinged, and the other end is connected to the existing stiffening girder 5 of the existing suspension bridge through a connecting piece; one end of the newly added suspender 3 is connected to the newly added main cable 1 through a cable clamp, and the newly added suspender 3 and the cable clamp are hinged, and the other end is connected to the suspension girder 16 of the lower-layer extended stiffening girder 6 through a connecting piece; it can adapt to the longitudinal dislocation deformation between the main cable and the stiffening girder when the double-deck steel truss stiffening girder has a large longitudinal displacement.

[0054] As a further implementation method, one straight web member 17 corresponds to the lower end of each existing suspender 4, and one straight web member 17 corresponds to each suspension girder 16, which is used to realize the smooth transfer of the stiffening girder load to the suspender and reduce the local stress of the double-deck steel truss bridge on the upper or lower layer caused by the suspender.

[0055] Embodiment 2

[0056] This embodiment provides an expansion method for an expansion structure of a double-layer different sag four-main-cable suspension bridge, including the following steps:

[0057] S1. New bored pile foundations and new bearing platforms are added on the side far from the bridge axis horizontally on the foundation of the existing bridge tower 7.

[0058] S2. On the basis of the newly added bearing platform, the tower column 8 is expanded with a new section and a newly added main saddle is installed.

[0059] S3. A new anchor foundation is added, and a new cable saddle and an anchoring system are set; during the construction process of this step and the previous steps, the existing suspension bridge can operate normally.

[0060] S4. During the construction process of this step and later, the existing suspension bridge needs to be closed for construction; relying on the existing bridge tower 7 and the existing main cable 2, the catwalk of the newly added main cable 1 is installed, the newly added main cable 1 is erected; the newly added suspenders 3 are installed at a set distance.

[0061] S5. Use a split self-propelled cable-suspended crane located on the newly added main cable 1 to symmetrically hoist the lower-layer expansion part of the double-deck steel truss girder from both sides of the existing bridge tower 7 towards the mid-span. The web members of the expanded part of the double-deck steel truss girder are temporarily connected to the existing upper stiffening girder, and the newly added suspension cables 3 are tensioned.

[0062] S6. After the mid-span closure of the expanded part of the double-deck steel truss girder, finely adjust the cable forces of the newly added suspension cables 3 to make the newly added main cable 1 reach the completed bridge alignment. Finely adjust the elevation at the interface between the web members of the expanded part of the double-deck steel truss girder and the existing upper stiffening girder, and adjust the temporary connection to a welded or high-strength bolt connection.

[0063] S7. Wind and protect the newly added main cable 1. Then remove the catwalk. Complete the closure project of the newly added dispersion saddles and anchoring systems and the newly added main saddles. Finally, conduct acceptance and open to traffic for operation.

[0064] Although the specific implementation manners of the present invention are described above in conjunction with the accompanying drawings, it is not a limitation on the protection scope of the present invention. Those skilled in the art should understand that based on the technical solutions of the present invention, various modifications or deformations that can be made by those skilled in the art without creative efforts are still within the protection scope of the present invention.

Claims

1. A double-layer different sag four-main cable expansion structure for a suspension bridge, characterized in that, Including: The existing stiffening girder of the suspension bridge, the lower-layer extended stiffening girder, the existing main cable, the newly added main cable, the existing main saddle, the extended main saddle, the existing bridge tower, the extended tower column, the first anchor and the second anchor; The existing stiffening girder of the suspension bridge and the lower-layer extended stiffening girder are arranged parallel to each other up and down, and are connected by vertical web members and diagonal web members to form a double-layer truss structure; the beam sections of the existing stiffening girder of the suspension bridge and the extended double-layer steel truss stiffening girder at the bridge tower are both connected to the cross beam or column at the bottom of the existing bridge tower and the extended tower column; the existing stiffening girder of the suspension bridge is connected to the existing main saddle at the top of the existing bridge tower through two existing main cables; the lower-layer extended stiffening girder is connected to the extended main saddle at the top of the extended tower column through two newly added main cables; the sag-span ratio of the existing main cable is smaller than that of the newly added main cable, and the cable force of the newly added main cable is smaller than that of the existing main cable under the same vertical load; the end of the existing main cable is fixedly connected to the first anchor through the first cable saddle, and the end of the newly added main cable is fixedly connected to the second anchor through the second cable saddle.

2. The double-layer different sag four main cable expansion structure of a suspension bridge as claimed in claim 1, wherein The existing stiffening girder of the suspension bridge is made of a steel box girder or a steel plate girder, and a first connection joint plate is fixedly arranged at the bottom of the existing stiffening girder of the suspension bridge, and a force transmission structure plate is arranged at a position corresponding to the first connection joint plate inside the existing stiffening girder of the suspension bridge; the lower-layer extended stiffening girder is made of a steel box girder or a steel plate girder, and a second connection joint plate is fixedly arranged at the top of the lower-layer extended stiffening girder, and a force transmission structure plate is also arranged at a position corresponding to the second connection joint plate inside the lower-layer extended stiffening girder; The vertical web members and diagonal web members are both arranged on both sides of the lower-layer extended stiffening girder; used to connect the existing stiffening girder of the suspension bridge and the lower-layer extended stiffening girder; the vertical web members and diagonal web members are arranged at intervals along the axis direction of the existing stiffening girder of the suspension bridge; the top ends of the vertical web members and diagonal web members are both fixedly connected to the first connection joint plate, and the bottom ends of the vertical web members and diagonal web members are both fixedly connected to the second connection joint plate.

3. The double-layer different sag four main cable expansion structure of a suspension bridge as claimed in claim 1, wherein, Suspension beams are arranged at intervals along the length direction on the lower-layer extended stiffening girder, the length of the suspension beam is greater than the width of the lower-layer extended stiffening girder, and both ends of the suspension beam protrude from both side surfaces of the lower-layer extended stiffening girder for connecting the newly added main cable.

4. A double-layer different sag four-main cable expansion structure for a suspension bridge as described in claim 3, characterized in that, A main saddle is arranged at the top of the existing bridge tower, and the main saddle includes two existing main saddles and two extended main saddles; the two existing main saddles are arranged on the tower columns of the existing bridge tower, and the extended main saddles are arranged side by side with the existing main saddles, and are respectively arranged on the side away from the bridge axis of the two existing main saddles; The existing main cable is connected to the existing main saddle, and the newly added main cable is connected to the extended main saddle.

5. A double-layer different sag four-main cable expansion structure of a suspension bridge according to claim 4, characterized in that, The existing bridge tower is in a portal structure, and the extended tower column is realized by increasing the cross-sectional dimensions on the longitudinal and transverse outer sides of the existing bridge tower, and is used to increase the size of the existing bridge tower to improve the bearing capacity of the existing bridge tower; There are two tower bridges, which are respectively arranged at both ends of the mid-span of the existing stiffening girder of the suspension bridge.

6. A double-layer different sag four-main cable expansion structure of a suspension bridge as claimed in claim 1, wherein, The first anchor and the second anchor are respectively arranged on both sides of the existing suspension bridge connection structure; the second anchor is arranged outside the first anchor and is arranged near one end of the existing bridge tower.

7. A double-layer different sag four-main cable expansion structure of a suspension bridge according to claim 5, characterized in that There are existing suspension cables between the existing main cable and the stiffening girder of the existing suspension bridge, and there are new suspension cables between the new main cable and the lower-layer extended stiffening girder.

8. A double-layer different sag four-main cable expansion structure of a suspension bridge according to claim 7, characterized in that, One end of the existing suspension cable is connected to the existing main cable through a cable clamp, and the existing suspension cable and the cable clamp are hinged; the other end is connected to the stiffening girder of the existing suspension bridge through a connecting piece; one end of the new suspension cable is connected to the new main cable through a cable clamp, and the new suspension cable and the cable clamp are hinged; the other end is connected to the suspension beam of the lower-layer extended stiffening girder through a connecting piece.

9. A double-layer different sag four-main cable expansion structure of a suspension bridge according to claim 8, characterized in that, One straight web member corresponds to the lower end of each existing suspension cable, and one straight web member corresponds to each suspension beam, which is used to reduce the local tensile stress of the suspension cable on the suspension bridge or the extended bridge.

10. The expansion method of an expansion structure of a double-layer different sag four-main cable of a suspension bridge according to any one of claims 1-9, characterized in that, It includes the following steps: S1. New bored pile foundations and new bearing platforms are added on the lateral far-bridge-axis side of the existing bridge tower foundation; S2. On the basis of the new bearing platform, the cross-section of the extended tower column is increased and a new main cable saddle is installed; S3. A new anchor foundation is added, and a new cable spreader saddle and an anchoring system are set; during this step and the previous construction process, the existing suspension bridge can operate normally; S4. During this step and subsequent construction processes, the existing suspension bridge needs to be closed for construction; relying on the existing bridge tower and the existing main cable, a catwalk for the new main cable is installed, and the new main cable is erected; new suspension cables are installed at set distances; S5. A split self-propelled cable-carrying crane located on the new main cable is used to symmetrically hoist the lower-layer extended part of the double-deck steel truss girder from both sides of the existing bridge tower side to the mid-span; the web members of the extended part of the double-deck steel truss girder are temporarily connected to the upper-layer existing stiffening girder, and the new suspension cables are tensioned. S6. After the mid-span of the extended part of the double-deck steel truss girder is closed, the cable forces of the new suspension cables are finely adjusted so that the new main cable reaches the as-built alignment; the elevation of the interface between the extended web members of the double-deck steel truss girder and the upper-layer existing stiffening girder is finely adjusted, and the temporary connection is adjusted to a welded or high-strength bolt connection. S7. The new main cable is wire-wound and protected; then the catwalk is removed; the closed works of the new cable spreader saddle and the anchoring system and the new main cable saddle are completed; finally, acceptance and opening to traffic for operation are carried out.

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