A method and device for designing the middle tower of a three-tower suspension bridge

Abstract

The present invention relates to the technical field of bridge construction, and more specifically to a method and apparatus for designing the middle tower of a three-tower suspension bridge. The method comprises the following steps: establishing an equivalent bridge calculation model that treats the middle tower as a spring; determining the longitudinal stiffness range of the middle tower through simulation calculations using the equivalent bridge calculation model in accordance with design specifications for the vertical stiffness of the entire bridge and the anti-slip safety factor of the main cable; and determining the material and cross-sectional dimensions of the middle tower based on the longitudinal stiffness range. This method addresses the problem of existing solutions for three-tower suspension bridges, where the design parameters of the middle tower are determined through extensive trial calculations using a full-bridge model, resulting in a high workload and low design efficiency.

Description

Technical Field

[0001] The present invention relates to the technical field of bridge construction, and in particular to a method and device for designing a middle tower of a three-tower suspension bridge. Background Art

[0002] Due to the scarcity of bridge resources, multifunctional bridge construction has become a trend. In order to adapt to higher navigation requirements and complex riverbed evolution, three-tower suspension bridges are a solution worth considering. When the above two requirements are met at the same time, dual-use three-tower suspension bridges for road and rail come into being.

[0003] Many factors influence the vertical stiffness of a three-tower suspension bridge and the anti-slip safety factor of the main cables. Extensive computational analysis revealed that the key lies in determining the longitudinal stiffness of the middle tower. Compared with previous three-tower highway suspension bridges, dual-use highway-rail suspension bridges have large live loads and require high vertical stiffness for the entire bridge. This, to a certain extent, compresses the reasonable range of the longitudinal stiffness of the middle tower, increasing the difficulty of middle tower design accordingly.

[0004] The existing technology proposes an analytical method to determine the reasonable range of the longitudinal stiffness of the middle tower. However, due to the many influencing factors, the complex calculation process, and the limited operating conditions, its practicality needs to be improved. Some bridges determine the design parameters of the middle tower through large-scale trial calculations of the full-bridge model, which requires a large amount of calculation and analysis work, and the design efficiency needs to be improved. Summary of the Invention

[0005] In response to the defects existing in the prior art, the purpose of the present invention is to provide a method and device for designing the middle tower of a three-tower suspension bridge, which can solve the problems in the prior art that the design parameters of the middle tower are determined through large-scale trial calculations of the entire bridge model, resulting in a large workload of calculation and analysis and low design efficiency.

[0006] In order to achieve the above purpose, the technical solution adopted by the present invention is:

[0007] The present invention provides a method for designing a middle tower of a three-tower suspension bridge, comprising the following steps:

[0008] Establish an equivalent bridge calculation model that treats the middle tower as a spring;

[0009] Based on the design specifications for the vertical stiffness of the entire bridge and the anti-sliding safety factor of the main cables, the longitudinal stiffness range of the middle tower was determined through simulation calculations using an equivalent bridge calculation model.

[0010] Based on the longitudinal stiffness range of the middle tower, the material and cross-sectional dimensions of the middle tower are determined.

[0011] In some optional solutions, the longitudinal stiffness range of the middle tower is determined by simulation calculation using an equivalent bridge calculation model based on the requirements of the design specifications for the vertical stiffness of the entire bridge and the anti-sliding safety factor of the main cable, including:

[0012] Adjust the longitudinal stiffness of the spring and simulate the equivalent bridge calculation model to obtain the deflection-span ratio and main cable anti-slip safety factor corresponding to various spring longitudinal stiffnesses;

[0013] Determine whether the deflection-span ratio and main cable anti-slip safety factor corresponding to various spring longitudinal stiffnesses meet the design specification requirements for the vertical stiffness of the entire bridge and the main cable anti-slip safety factor;

[0014] The longitudinal stiffness range of the middle tower is determined based on all the longitudinal stiffness requirements of the design specifications for the vertical stiffness of the entire bridge and the anti-sliding safety factor of the main cable.

[0015] In some optional solutions, the longitudinal stiffness range of the middle tower is determined based on all longitudinal stiffness requirements of the design specifications for the vertical stiffness of the entire bridge and the anti-sliding safety factor of the main cable, including:

[0016] The minimum value of all longitudinal stiffnesses that meet the design specifications for the vertical stiffness of the entire bridge and the anti-sliding safety factor of the main cables will be reduced by a set ratio, and the maximum value will be increased by a set ratio;

[0017] The interval in which the minimum value of all longitudinal stiffnesses is reduced by a set ratio and the maximum value is magnified by a set ratio is taken as the longitudinal stiffness interval of the middle tower.

[0018] In some optional solutions, determining the material and cross-sectional dimensions of the middle tower based on the longitudinal stiffness range of the middle tower includes:

[0019] Determine the preliminary material and cross-sectional dimensions of the middle tower based on the tower top reaction force and engineering experience;

[0020] Determine whether the longitudinal stiffness corresponding to the preliminary material and cross-sectional dimensions of the middle tower is within the middle tower longitudinal stiffness range. If not, reselect the material and cross-sectional dimensions of the middle tower until the obtained middle tower longitudinal stiffness is within the middle tower longitudinal stiffness range.

[0021] Determine whether the nominal stress of the middle tower, crack width, vertical stiffness of the entire bridge, and anti-slip safety factor of the main cables meet the design requirements. If not, reselect the material and cross-sectional dimensions of the middle tower until the nominal stress, crack width, vertical stiffness of the entire bridge, and anti-slip safety factor of the main cables meet the design requirements.

[0022] Determine whether the stability safety factor meets the design requirements. If not, reselect the material and cross-sectional dimensions of the middle tower until the obtained stability safety factor meets the design requirements.

[0023] In some optional schemes, when judging whether the longitudinal stiffness corresponding to the preliminary material and cross-sectional dimensions of the middle tower is within the middle tower longitudinal stiffness range, the preliminary material and cross-sectional dimensions of the middle tower are substituted into the middle tower model for calculation to obtain the middle tower longitudinal stiffness.

[0024] In some optional schemes, when judging whether the nominal stress of the middle tower, the crack width, the vertical stiffness of the entire bridge, and the anti-slip safety factor of the main cable meet the design requirements, a full-bridge model is established, and the selected middle tower material and cross-sectional dimensions are substituted into the full-bridge model for calculation to obtain the nominal stress of the middle tower, the crack width, the vertical stiffness of the entire bridge, and the anti-slip safety factor of the main cable.

[0025] In some optional solutions, when judging whether the stability safety factor meets the design requirements, the stability safety factor is also calculated based on the full-bridge model.

[0026] In some optional schemes, when determining the preliminary material and cross-sectional dimensions of the middle tower based on the tower top reaction force and engineering experience, the longitudinal stiffness corresponding to the preliminary material and cross-sectional dimensions of the middle tower is biased towards the upper limit value of the longitudinal stiffness range of the middle tower.

[0027] In some optional schemes, in the equivalent bridge calculation model, the main cables are simulated by cable units, the suspenders are simulated by rod units, the bridge towers, stiffening beams and saddles are simulated by beam units, the stiffening beams are vertically and horizontally constrained at the middle towers, elastic cables are provided longitudinally, and vertical and horizontal constraints are adopted at the side towers and auxiliary piers.

[0028] In another aspect, the present invention further provides a device for designing a middle tower of a three-tower suspension bridge, comprising:

[0029] An equivalent model building module is used to build an equivalent bridge calculation model in which the middle tower is equivalent to a spring;

[0030] The longitudinal stiffness interval determination module is used to determine the longitudinal stiffness interval of the middle tower based on the design specifications for the vertical stiffness of the entire bridge and the anti-sliding safety factor of the main cables, through simulation calculations using an equivalent bridge calculation model;

[0031] The middle tower parameter determination module is used to determine the material and cross-sectional dimensions of the middle tower based on the longitudinal stiffness range of the middle tower.

[0032] Compared with the existing technology, the advantages of the present invention are: by establishing an equivalent bridge calculation model that treats the middle tower as a spring, according to the design specifications for the vertical stiffness of the entire bridge and the anti-slip safety factor of the main cable, the longitudinal stiffness range of the middle tower is determined through simulation calculation using the equivalent bridge calculation model. This can accurately and quickly determine the reasonable range of the longitudinal stiffness of the middle tower, narrow the selection range, and improve design efficiency. The present invention is particularly suitable for the design of the middle tower of a three-tower suspension bridge for both road and rail. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0034] Figure 1 Flowchart of a method for designing a middle tower of a three-tower suspension bridge according to an embodiment of the present invention;

[0035] Figure 2 Schematic diagram of an equivalent bridge calculation model in an embodiment of the present invention;

[0036] Figure 3 Schematic diagram of a full-bridge model according to an embodiment of the present invention;

[0037] Figure 4 This is a flow chart for determining the design parameters of the middle tower in an embodiment of the present invention. DETAILED DESCRIPTION

[0038] To make the purpose, technical solutions, and advantages of the embodiments of this application more clear, the technical solutions in the embodiments of this application will be clearly and completely described below in conjunction with the drawings in the embodiments of this application. Obviously, the described embodiments are part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0039] The embodiments of the present invention are further described in detail below with reference to the accompanying drawings.

[0040] like Figure 1 As shown, the present invention provides a method for designing a middle tower of a three-tower suspension bridge, comprising the following steps:

[0041] S1: Establish an equivalent bridge calculation model in which the middle tower is equivalent to a spring.

[0042] In some optional embodiments, in the equivalent bridge calculation model, the main cables are simulated by cable units, the suspenders are simulated by rod units, the side towers, stiffening beams and saddles are simulated by beam units, the stiffening beams are vertically and horizontally constrained at the middle towers, elastic cables are provided longitudinally, and vertical and horizontal constraints are adopted at the side towers and auxiliary piers.

[0043] In this embodiment, if Figure 2 As shown in Figure 2, the equivalent bridge calculation model for calculating the reasonable range of the longitudinal stiffness of the middle tower is used. The spring stiffness is taken longitudinally at the bottom of the middle main cable saddle, and consolidation is applied in other directions.

[0044] For a dual-use highway-railway suspension bridge with a main span of 2×1120m, the main loads considered in the calculation include dead load, live load, system temperature difference, wind load, and seismic action.

[0045] Dead load: The first phase dead load is calculated based on the dead weight of the structure, and the second phase dead load totals 350kN / m.

[0046] Live load: Highway live load is loaded as Class I 6-lane highway; railway live load is 2 lanes ZK and 2 lanes ZC, and the loading length is 550 meters.

[0047] System temperature difference: concrete temperature rise and fall is 23℃, steel structure temperature rise and fall is 33℃.

[0048] Wind load: The wind speed on the bridge deck with vehicles is 25m / s, and the extreme wind speed is V10=28.9m / s.

[0049] Earthquake action: The peak acceleration of the seismic motion for E1 earthquake action is 0.11g, the peak acceleration of the seismic motion for E2 earthquake action is 0.15g, and the characteristic period of the response spectrum is 0.55s.

[0050] S2: Based on the design specifications for the vertical stiffness of the entire bridge and the anti-slip safety factor of the main cables, the longitudinal stiffness range of the middle tower is determined through simulation calculation using an equivalent bridge calculation model.

[0051] In some optional embodiments, step S2 includes:

[0052] S21: Adjust the longitudinal stiffness of the spring and simulate the calculation using an equivalent bridge model to obtain the deflection-span ratio and main cable anti-slip safety factor corresponding to various spring longitudinal stiffnesses.

[0053] In this example, the longitudinal spring stiffness was adjusted to calculate the corresponding deflection-span ratio and main cable anti-slip safety factor. As shown in Table 1, increasing the longitudinal spring stiffness increases the overall vertical stiffness of the bridge and decreases the main cable anti-slip safety factor. The range of the mid-tower longitudinal stiffness that meets both the overall vertical stiffness and main cable anti-slip requirements is [70,000, 100,000] kN / m. To account for the differences between the equivalent model and the actual model, the stiffness range is appropriately increased. In this paper, the upper limit is increased by 10% and the lower limit is decreased by 10%, resulting in a mid-tower longitudinal stiffness range of [63,000, 110,000] kN / m.

[0054] Table 1 Influence of spring longitudinal stiffness on the vertical stiffness of the entire bridge and the anti-slip safety factor of the main cable

[0055]

[0056] When the vertical stiffness requirements for the entire bridge are high, the longitudinal stiffness range of the middle tower may not exist. This problem can be addressed in two ways: first, determining the friction coefficient through anti-slip testing or taking auxiliary measures to increase the friction coefficient, such as installing friction plates; second, studying the necessity of using a main cable anti-slip safety factor of 2. Eurocode 3 recommends a main cable anti-slip safety factor of 1.65, which allows for some adjustment.

[0057] S22: Determine whether the deflection-span ratio and main cable anti-slip safety factor corresponding to the longitudinal stiffness of various springs meet the design specification requirements for the vertical stiffness of the entire bridge and the main cable anti-slip safety factor.

[0058] S23: Determine the longitudinal stiffness range of the middle tower based on all longitudinal stiffness requirements of the design specifications for the vertical stiffness of the entire bridge and the anti-sliding safety factor of the main cables.

[0059] S3: Based on the longitudinal stiffness range of the middle tower, determine the material and cross-sectional dimensions of the middle tower.

[0060] In this embodiment, the longitudinal stiffness of the bridge tower is judged before the full bridge model is calculated, which narrows the selection range and improves the design efficiency.

[0061] In some optional embodiments, step S3 includes:

[0062] S31: Determine the preliminary material and cross-sectional dimensions of the middle tower based on the tower top reaction force and engineering experience.

[0063] In this embodiment, when determining the preliminary material and cross-sectional dimensions for the mid-tower based on the tower top reaction force and engineering experience, the longitudinal stiffness corresponding to these preliminary materials and cross-sectional dimensions is set toward the upper limit of the mid-tower longitudinal stiffness range. This reduces the number of adjustments required to select the mid-tower material and cross-sectional dimensions.

[0064] S32: determining whether the longitudinal stiffness corresponding to the preliminary material and cross-sectional dimensions of the middle tower is within the middle tower longitudinal stiffness range; if not, reselecting the material and cross-sectional dimensions of the middle tower until the obtained middle tower longitudinal stiffness is within the middle tower longitudinal stiffness range;

[0065] In this embodiment, when determining whether the middle tower longitudinal stiffness is within the middle tower longitudinal stiffness range, the material and cross-sectional dimensions of the middle tower are brought into the middle tower model for calculation to obtain the middle tower longitudinal stiffness.

[0066] S33: Determine whether the nominal stress of the middle tower, crack width, vertical stiffness of the entire bridge, and anti-slip safety factor of the main cable meet the design requirements. If not, reselect the material and cross-sectional dimensions of the middle tower until the nominal stress of the middle tower, crack width, vertical stiffness of the entire bridge, and anti-slip safety factor of the main cable meet the design requirements.

[0067] In this embodiment, when judging whether the nominal stress of the middle tower, the crack width, the vertical stiffness of the whole bridge and the anti-sliding safety factor of the main cable meet the design requirements, a whole bridge model is established, such as Figure 3 As shown in Figure 3, the material and cross-sectional dimensions of the middle tower are brought into the full-bridge model for calculation to obtain the nominal stress of the middle tower, crack width, vertical stiffness of the entire bridge, and anti-slip safety factor of the main cable.

[0068] S34: Determine whether the stability safety factor meets the design requirements. If not, reselect the material and cross-sectional dimensions of the middle tower until the obtained stability safety factor meets the design requirements.

[0069] In this embodiment, when determining whether the stability safety factor meets the design requirements, the stability safety factor is also calculated based on the full bridge model. The stability safety factor includes a first-class stability safety factor and a second-class stability safety factor. This solution adopts the first-class stability safety factor.

[0070] In addition, after determining whether the stability safety factor meets the design requirements, the seismic verification step is carried out to determine whether the seismic requirements are met. If not, the material and cross-sectional dimensions of the middle tower are reselected until the seismic requirements obtained meet the design requirements.

[0071] A specific implementation method for selecting the middle tower material and cross-sectional dimensions is given below.

[0072] From the perspective of economy and durability, the medium tower forms mainly consider I-shaped concrete tower, A-shaped concrete tower, and A-shaped mixed tower. Figure 4 The process is to design the middle tower. In the figure, Kz is the longitudinal stiffness of the middle tower. Only the I-shaped concrete tower and the A-shaped hybrid tower can meet all the requirements at the same time. As a comparison, the calculation results of the A-shaped concrete tower are still listed, as shown in Table 2.

[0073] A-shaped hybrid tower: The 101.7m downward from the top of the tower is a steel structure made of Q420qE, of which 71.7m is a merged section and 30m is a limb. The merged section is 9~20m long longitudinally and 9m wide transversely. The limb is 10m long longitudinally and 9m wide transversely. The side walls and partitions are 50mm thick, and the vertical ribs are 500mm wide and 50mm thick. The steel limbs are box-shaped concrete limbs downward. This section is 124.5m long and made of C60 concrete. It is 10~11m long longitudinally and 9~10m wide transversely, with a wall thickness of 1~2m.

[0074] I-shaped concrete tower: The bridge tower is made of C60 concrete, the tower column is a box-shaped section, the longitudinal length is 10~35m, the transverse width is 10~15m, the wall thickness is 1.3~1.9m, and the tower height is 226.2m.

[0075] A-shaped concrete tower: The bridge tower is made of C60 concrete, with a combined section length of 51.7m, a box-shaped cross-section, a longitudinal length of 9~20m, a transverse width of 9m, and a wall thickness of 1.3~1.65m. The branch length is 174.5m, a box-shaped cross-section, a longitudinal length of 10m, a transverse width of 9~10m, and a wall thickness of 1.3~2.1m.

[0076] The A-shaped concrete tower has the highest stiffness, but the main cable's anti-sliding safety factor does not meet regulatory requirements. The I-shaped concrete tower has large longitudinal and transverse dimensions, making shrinkage cracks during construction difficult to manage. This results in the lowest vertical stiffness for the entire bridge, resulting in a poorer landscape. The A-shaped hybrid tower's performance lies between the other two tower types, and its landscape effect is relatively good. Therefore, the A-shaped hybrid tower was selected. The maximum tensile stress in the concrete is 5.6 MPa, as shown in Table 2. The crack width can be controlled through reinforcement.

[0077] The longitudinal stiffness of the A-shaped hybrid tower is 103,636 kN / m, which is greater than the upper limit of the longitudinal stiffness of the bridge tower before adjustment, but less than the upper limit after adjustment. The calculated results for the vertical stiffness of the entire bridge and the main cable anti-sliding safety factor also differ from those in Table 1, indicating that the difference between the equivalent model and the actual model is objective. The initial stiffness range is a reference range, and adjustment to the initial stiffness range is necessary. When initially determining the design parameters of the middle tower, the longitudinal stiffness of the middle tower can be biased towards the upper limit, and subsequent optimization can be carried out to reduce the number of adjustments.

[0078] Table 2 Main calculation results of the middle tower

[0079]

[0080] The stability safety factor for the A-type hybrid tower is shown in Table 3. Its value, 12.1, is significantly greater than 4, meeting regulatory requirements. Empirical calculations show that the energy demand ratios for all parts of the A-type hybrid tower under E1 and E2 earthquakes are greater than 1, demonstrating satisfactory seismic performance. In summary, the A-type hybrid tower design is reasonable and feasible.

[0081] Table 3 Type A mixing tower stability safety factor

[0082]

[0083] In addition, the present invention also provides a design device for the middle tower of a three-tower suspension bridge, including: an equivalent model establishment module, a longitudinal stiffness interval determination module and a middle tower parameter determination module, wherein the equivalent model establishment module is used to establish an equivalent bridge calculation model in which the middle tower is equivalent to a spring; the longitudinal stiffness interval determination module is used to determine the longitudinal stiffness interval of the middle tower through simulation calculation of the equivalent bridge calculation model according to the requirements of the design specifications for the vertical stiffness of the entire bridge and the anti-slip safety factor of the main cable; the middle tower parameter determination module is used to determine the material and cross-sectional size of the middle tower based on the longitudinal stiffness interval of the middle tower.

[0084] In summary, by establishing an equivalent bridge calculation model that treats the middle tower as a spring, and according to the design specifications for the vertical stiffness of the entire bridge and the anti-slip safety factor of the main cable, the longitudinal stiffness range of the middle tower is determined through simulation calculation using the equivalent bridge calculation model. This allows for accurate and rapid determination of the reasonable range of the longitudinal stiffness of the middle tower, narrowing the selection range and improving design efficiency. This approach is particularly suitable for the design of middle towers in dual-use three-tower suspension bridges for highway and rail transport. In addition, this method reveals that when the longitudinal stiffness of the middle tower increases, the vertical stiffness of the entire bridge increases, while the anti-slip safety factor of the main cable decreases. When the vertical stiffness requirements for the entire bridge are high, the longitudinal stiffness range of the middle tower may not exist. In this case, the rationality of the values ​​of the friction coefficient and the anti-slip safety factor can be studied. Hybrid towers have the advantages of both steel and concrete, with a wide stiffness adjustment range, easy compliance of the tower column stress with specification requirements, moderate cost, and good durability. These hybrid towers have certain advantages in the selection of middle towers.

[0085] In the description of this application, it should be noted that the terms "upper" and "lower" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on this application. Unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or an indirect connection through an intermediate medium, or it can be internal communication between two elements. For those of ordinary skill in the art, the specific meanings of the above terms in this application can be understood according to the specific circumstances.

[0086] It should be noted that, in this application, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, an element defined by the sentence "comprising a ..." does not exclude the presence of other identical elements in the process, method, article or device comprising the element.

[0087] The foregoing is merely a list of specific embodiments of the present application, intended to enable those skilled in the art to understand or implement the present application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application is not limited to the embodiments shown herein, but is intended to conform to the broadest scope consistent with the principles and novel features of the present application.

Claims

1. A method for designing the middle tower of a three-tower suspension bridge, characterized in that: The following steps are involved: Establish an equivalent bridge calculation model that treats the middle tower as a spring; Based on the design specifications for the vertical stiffness of the entire bridge and the anti-sliding safety factor of the main cables, the longitudinal stiffness range of the middle tower was determined through simulation calculations using an equivalent bridge calculation model, including: Adjust the longitudinal stiffness of the spring and simulate the equivalent bridge calculation model to obtain the deflection-span ratio and main cable anti-slip safety factor corresponding to various spring longitudinal stiffnesses; Determine whether the deflection-span ratio and main cable anti-slip safety factor corresponding to various spring longitudinal stiffnesses meet the design specification requirements for the vertical stiffness of the entire bridge and the main cable anti-slip safety factor; Determine the longitudinal stiffness range of the middle tower based on all longitudinal stiffness requirements of the design specifications for the vertical stiffness of the entire bridge and the anti-sliding safety factor of the main cables; Based on the longitudinal stiffness range of the middle tower, the material and cross-sectional dimensions of the middle tower are determined.

2. The method for designing the middle tower of a three-tower suspension bridge according to claim 1, wherein: The longitudinal stiffness range of the middle tower is determined based on all longitudinal stiffness requirements of the design specifications for the vertical stiffness of the entire bridge and the anti-sliding safety factor of the main cable, including: The minimum value of all longitudinal stiffnesses that meet the design specifications for the vertical stiffness of the entire bridge and the anti-sliding safety factor of the main cables will be reduced by a set ratio, and the maximum value will be increased by a set ratio; The interval in which the minimum value of all longitudinal stiffnesses is reduced by a set ratio and the maximum value is magnified by a set ratio is taken as the longitudinal stiffness interval of the middle tower.

3. The method for designing the middle tower of a three-tower suspension bridge according to claim 1, wherein: The determination of the material and cross-sectional dimensions of the middle tower based on the longitudinal stiffness range of the middle tower includes: Determine the preliminary material and cross-sectional dimensions of the middle tower based on the tower top reaction force and engineering experience; Determine whether the longitudinal stiffness corresponding to the preliminary material and cross-sectional dimensions of the middle tower is within the middle tower longitudinal stiffness range. If not, reselect the material and cross-sectional dimensions of the middle tower until the obtained middle tower longitudinal stiffness is within the middle tower longitudinal stiffness range. Determine whether the nominal stress of the middle tower, crack width, vertical stiffness of the entire bridge, and anti-slip safety factor of the main cables meet the design requirements. If not, reselect the material and cross-sectional dimensions of the middle tower until the nominal stress, crack width, vertical stiffness of the entire bridge, and anti-slip safety factor of the main cables meet the design requirements. Determine whether the stability safety factor meets the design requirements. If not, reselect the material and cross-sectional dimensions of the middle tower until the obtained stability safety factor meets the design requirements.

4. The method for designing the middle tower of a three-tower suspension bridge according to claim 3, wherein: When judging whether the longitudinal stiffness corresponding to the preliminary material and cross-sectional dimensions of the middle tower is within the range of the longitudinal stiffness of the middle tower, the preliminary material and cross-sectional dimensions of the middle tower are brought into the middle tower model for calculation to obtain the longitudinal stiffness of the middle tower.

5. The method for designing the middle tower of a three-tower suspension bridge according to claim 3, wherein: When judging whether the nominal stress of the middle tower, crack width, vertical stiffness of the entire bridge, and anti-slip safety factor of the main cable meet the design requirements, a full-bridge model is established, and the selected middle tower material and cross-sectional dimensions are substituted into the full-bridge model for calculation to obtain the nominal stress of the middle tower, crack width, vertical stiffness of the entire bridge, and anti-slip safety factor of the main cable.

6. The method for designing the middle tower of a three-tower suspension bridge according to claim 4, wherein: When judging whether the stability safety factor meets the design requirements, the stability safety factor is also calculated based on the full-bridge model.

7. The method for designing the middle tower of a three-tower suspension bridge according to claim 3, wherein: When determining the preliminary material and cross-sectional dimensions of the middle tower based on the tower top reaction force and engineering experience, the longitudinal stiffness corresponding to the preliminary material and cross-sectional dimensions of the middle tower is biased towards the upper limit of the longitudinal stiffness range of the middle tower.

8. The method for designing the middle tower of a three-tower suspension bridge according to claim 1, wherein: In the equivalent bridge calculation model, the main cables are simulated by cable units, the suspenders are simulated by rod units, the bridge towers, stiffening beams and saddles are simulated by beam units, the stiffening beams are vertically and horizontally constrained at the middle tower, elastic cables are provided longitudinally, and vertical and horizontal constraints are applied at the side towers and auxiliary piers.

9. A device for designing the middle tower of a three-tower suspension bridge, characterized in that: include: An equivalent model building module is used to build an equivalent bridge calculation model in which the middle tower is equivalent to a spring; The longitudinal stiffness range determination module is used to determine the longitudinal stiffness range of the middle tower based on the design specifications for the vertical stiffness of the entire bridge and the anti-sliding safety factor of the main cables, through simulation calculations using an equivalent bridge calculation model. It includes: Adjust the longitudinal stiffness of the spring and simulate the equivalent bridge calculation model to obtain the deflection-span ratio and main cable anti-slip safety factor corresponding to various spring longitudinal stiffnesses; Determine whether the deflection-span ratio and main cable anti-slip safety factor corresponding to various spring longitudinal stiffnesses meet the design specification requirements for the vertical stiffness of the entire bridge and the main cable anti-slip safety factor; Determine the longitudinal stiffness range of the middle tower based on all longitudinal stiffness requirements of the design specifications for the vertical stiffness of the entire bridge and the anti-sliding safety factor of the main cables; The middle tower parameter determination module is used to determine the material and cross-sectional dimensions of the middle tower based on the longitudinal stiffness range of the middle tower.

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