Coupling analysis method for I-shaped composite beam bridge in local fire

Abstract

The invention provides a coupling analysis method for an I-shaped composite beam bridge in a local fire, which comprises the following steps of: establishing a relational expression among unit area net heat flux, a comprehensive heat transfer coefficient, a heat insulation surface temperature and a solid surface temperature, executing a fire dynamic model of the I-shaped composite beam bridge, and obtaining heat insulation surface temperature data presented in a preset file form; and extracting heat-insulating surface temperature data of each node in the heat-insulating surface temperature data file at a specific time by utilizing a built-in program, outputting the heat-insulating surface temperature data file as a preset format capable of being read by an FE program, and carrying out heat transfer calculation on the I-shaped composite beam bridge by adopting finite element software to obtain a non-uniform temperature field of the structure under the local fire. Bridge structure heat transfer analysis under the action of real fire temperature is realized by means of multi-scale modeling, the accuracy of predicting structural response under local fire is improved, and a reference is provided for bridge fire resistance analysis and fire resistance design.

Description

Technical Field

[0001] The invention belongs to the technical field of numerical simulation research on temperature field distribution considering temperature gradient, and particularly relates to a coupling analysis method for an I-shaped composite beam bridge under local fire. Background Art

[0002] Steel-concrete composite structures can fully utilize the compressive properties of concrete and the high tensile strength of steel, and can be easily standardized and customizable. This significantly reduces steel usage while enhancing structural rigidity and integrity. In recent years, to improve the industrialization and industrialization of steel bridge construction and achieve green, low-carbon, and high-quality development, the proportion of steel-concrete composite beam bridges has increased significantly and has been effectively developed.

[0003] Fires are a major cause of bridge accidents, and the number of bridge collapses due to fires is increasing year by year. Traffic-related fires have become a key factor seriously threatening the safety of bridge structures. Research on bridge fire resistance is still in its early stages, and there are no international fire protection design specifications specifically for bridge structures. Research on bridge fire resistance must first analyze the temperature effects of real fires on bridge structures. Existing research on bridge fire resistance faces the following challenges:

[0004] (1) The use of a standard temperature rise curve based on building fires simplifies the effects of fire. Building fires differ significantly from actual fires in open bridge environments. Furthermore, the effects of ambient wind and the geometry of the fire-affected surface on heat convection and radiation in the fire scene are ignored. Therefore, the standard temperature rise curve is unable to reflect the temperature rise and thermal gradient effects of actual bridge fires.

[0005] (2) The high temperature of a fire is simplified to a time-varying temperature uniformly distributed in space, ignoring the spatial non-uniformity of the high-temperature fire environment. Fires often occur locally, and steel-concrete composite beam bridges develop towards large spans. Under localized fires, thermal gradients exist throughout the space, and the temperature is non-uniformly distributed. Applying the temperature of a single point to the entire fire-affected surface cannot accurately predict the fire response of the structure.

[0006] (3) In addition, due to the complex structure of bridges and the potentially large fire loads, conducting tests is often quite complex and expensive. The fire dynamics software FDS (Fire Dynamics Simulator) and the finite element analysis software ABAQUS are widely used in actual fire temperature field research and structural thermal-mechanical coupling analysis, respectively. To accurately predict the actual response of a structure under localized fire, a key step is to import the actual fire temperature simulated by FDS into ABAQUS. Summary of the Invention

[0007] In order to solve the defects and shortcomings in the above-mentioned technology, the present invention provides a coupling analysis method for an I-shaped composite beam bridge under local fire.

[0008] The technical solutions provided by the present invention are as follows:

[0009] A coupling analysis method for an I-shaped composite beam bridge under localized fire, characterized by comprising the following steps:

[0010] S1: Determine the adiabatic surface temperature as coupling data;

[0011] S2: Establish a fire dynamics model for I-shaped composite beam bridges and calculate adiabatic surface temperature data;

[0012] S3: extracting the adiabatic surface temperature of each node at a specific time in the adiabatic surface temperature data file, and outputting the adiabatic surface temperature data file into a readable preset format;

[0013] S4: Finite element calculation and analysis of the heat transfer of I-beam composite bridges were performed to obtain the non-uniform temperature field of each structure under local fire.

[0014] As a further preferred embodiment of the present invention, in step S1, the following formula is established, and the net heat flux per unit area is obtained by determining the adiabatic surface temperature according to the following formula:

[0015] q tot =h(T AST -T s )

[0016] in,

[0017] q tot is the net heat flux per unit area;

[0018] h is the comprehensive heat transfer coefficient;

[0019] T AST is the adiabatic surface temperature;

[0020] T s is the solid surface temperature.

[0021] As a further preferred embodiment of the present invention, in step S2, a fire dynamics model of an I-shaped composite beam bridge is established based on structural parameters, and the structural parameters include at least the dimensions, material properties and actual layout positions of the main beam, longitudinal beam, transverse beam, bridge deck and vertical stiffening ribs.

[0022] As a further preferred embodiment of the present invention, in step S2, when establishing the fire dynamics model of the I-shaped composite beam bridge, meshing and boundary conditions are performed on it, and the boundary conditions at least include:

[0023] 1) Material selection for bridge deck and columns;

[0024] 2) Material selection for main beams, longitudinal beams and cross beams;

[0025] 3) Heat transfer between the surface of each material and the air is free;

[0026] 4) The simulation area is surrounded by open boundaries, and air and heat can freely enter and exit the simulation area;

[0027] 5) The simulated bridge fire is an open environment with the initial ambient temperature and ambient pressure.

[0028] As a further preferred embodiment of the present invention, step S2 includes the following steps:

[0029] S2.1: Establish a fire dynamics model for I-beam bridges;

[0030] S2.2: Select the export data type;

[0031] S2.3: Implement the I-beam bridge fire dynamics model;

[0032] S2.4: Obtain the non-uniform temperature field and adiabatic surface temperature data of the I-shaped composite beam bridge;

[0033] S2.5: Save the calculated adiabatic surface temperature data in a preset file in the result data folder of the I-beam composite beam bridge fire dynamics model.

[0034] As a further preferred embodiment of the present invention, step S3 includes the following steps:

[0035] S3.1: Call the specified command in the result data folder of the I-beam composite beam bridge fire dynamics model;

[0036] S3.2: Enter the file name with the suffix corresponding to the specified command;

[0037] S3.3: Select the files to be processed;

[0038] S3.4: Select the node to be processed;

[0039] S3.5: Enter the time range of the data to be output;

[0040] S3.6: Select the output direction of the data;

[0041] S3.7: Select the number of output physical quantities;

[0042] S3.8: Input the index number of the physical quantity to be output;

[0043] S3.9: Name the file to which the results are to be output;

[0044] S3.10: Repeat steps S3.6 to S3.9 until data in all directions are extracted;

[0045] S3.11: Merge the extracted data files in the six directions and save them in a readable preset format.

[0046] As a further preferred embodiment of the present invention, step S4 includes the following steps:

[0047] S4.1: Use finite element software to establish a heat transfer model for an I-shaped composite beam bridge with consistent coordinates with the fire dynamics model of the I-shaped composite beam bridge;

[0048] S4.2: Set the model's thermal parameters, boundary conditions, mesh size, and element type;

[0049] S4.3: Use coordinate mapping to import the adiabatic surface temperature data saved in a preset format into the I-beam composite beam bridge heat transfer model;

[0050] S4.4: The surface of the I-beam composite bridge heat transfer model exposed to the fire environment is selected as the fire-exposed surface. Based on the extracted adiabatic surface temperature data, the fire-exposed surface is heated to simulate the time-varying temperature of the internal structure under the action of a real fire. The non-uniform temperature field of each structure under localized fire is solved.

[0051] As a further preferred embodiment of the present invention, in the step S4.1, the heat transfer model of the I-shaped composite beam bridge established includes at least a concrete bridge deck, steel bars, main beams, longitudinal beams, cross beams and vertical stiffening ribs, and each component is connected into a whole using Tie constraints.

[0052] As a further preferred embodiment of the present invention, in step S4.1, being consistent with the coordinates of the I-beam composite beam bridge fire dynamics model includes: achieving coordinate correspondence by establishing the same coordinates for modeling, so that all data calculated from the I-beam composite beam bridge fire dynamics model can be transmitted to the corresponding coordinates of the I-beam composite beam bridge heat transfer model.

[0053] As a further preferred embodiment of the present invention, in step S4.2, the thermal parameters include at least the thermal conductivity, specific heat capacity, thermal expansion coefficient and density of concrete and steel.

[0054] Compared with the prior art, the present invention has the following beneficial effects:

[0055] 1) The present invention provides a coupled analysis method for an I-shaped composite beam bridge under local fire. Compared with the existing standard fire temperature rise curve, which is difficult to reflect the actual fire effect and the uneven temperature field of the I-shaped composite beam bridge fire, the net heat flux per unit area q is established. tot With the comprehensive heat transfer coefficient h, adiabatic surface temperature T AST and the solid surface temperature T s The relationship between the two was used to execute the FDS fire dynamics model of an I-shaped composite beam bridge, obtaining adiabatic surface temperature data in a preset file format (BNDF). A built-in program was used to extract the adiabatic surface temperature data for each node at a specific time in the adiabatic surface temperature data file. The adiabatic surface temperature data file was then exported to a preset format (e.g., ASCII) readable by a FE program. The finite element software ABAQUS was used to perform heat transfer calculations on the I-shaped composite beam bridge, resulting in the non-uniform temperature field of the structure under localized fire. This method provides a method for heat transfer analysis of bridge structures under realistic fire temperatures using multi-scale modeling, improving the accuracy of predicting structural responses under localized fires and providing a reference for bridge fire resistance analysis and design.

[0056] 2) The present invention provides a coupled analysis method for I-shaped composite beam bridges under local fire, which restores the thermal gradient characteristics under real fire scenarios, predicts bridge structure damage in a more accurate manner, and provides a reference for the fire-resistant design of bridge projects. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 It is a flowchart of the steps of the analysis method provided by the present invention.

[0058] Figure 2 This is a standard cross-sectional view of the main beam of the I-shaped composite beam bridge provided by the present invention.

[0059] Figure 3 It is a schematic diagram of the fire dynamics model of the I-shaped composite beam bridge established in step S2 of the present invention.

[0060] Figure 4 It is a step flow chart of step S3 of the present invention.

[0061] Figure 5 It is a schematic diagram of the heat transfer model of the I-shaped composite beam bridge established in step S5 of the present invention.

[0062] Figure 6 This is a bottom view of the uneven temperature distribution of the I-shaped composite beam bridge provided by the present invention after being exposed to fire for 2 hours.

[0063] Figure 7 This is a three-dimensional diagram of the uneven temperature distribution of the I-shaped composite beam bridge provided by the present invention after being exposed to fire for 2 hours.

[0064] Figure 8This is a temperature distribution curve diagram of the main beam exposed to fire 2 hours after being exposed to fire provided by the present invention.

[0065] Figure 9 This is a temperature time history curve diagram of each point in the height direction of the main beam exposed to fire 2 hours after being exposed to fire provided by the present invention.

[0066] Figure 10 This is a temperature distribution curve diagram of the longitudinal beam after being exposed to fire for 2 hours provided by the present invention.

[0067] Figure 11 This is a temperature time history curve diagram of each point in the height direction of the longitudinal beam after being exposed to fire for 2 hours provided by the present invention. DETAILED DESCRIPTION

[0068] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0069] In the description of the present invention, it should be noted that the terms "upper," "lower," "inner," "outer," "front end," "rear end," "both ends," "one end," "the other end," and the like, indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings and are intended solely to facilitate and simplify the description of the present invention. They are not intended to indicate or imply that the devices or components referred to must have, be constructed, or operate in a specific orientation, and therefore should not be construed as limiting the present invention. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0070] In the description of the present invention, it should be noted that, unless otherwise expressly specified or limited, the terms "installed," "provided with," "connected," etc., should be understood in a broad sense. For example, "connected" may refer to a fixed connection, a detachable connection, or an integral connection; it may refer to a mechanical connection or an electrical connection; it may refer to a direct connection or an indirect connection through an intermediate medium; it may refer to internal communication between two components. Those skilled in the art will be able to understand the specific meanings of the above terms in the present invention based on the specific circumstances.

[0071] [First embodiment]

[0072] like Figure 1-11 FIG. 1 shows a coupling analysis method for an I-shaped composite beam bridge under local fire provided by the first embodiment of the present invention. Figure 1 As shown, the following steps are included:

[0073] S1: Determine the adiabatic surface temperature as coupling data;

[0074] In step S1, the following formula is established, and the net heat flux per unit area is obtained by determining the adiabatic surface temperature according to the following formula:

[0075] q tot =h(T AST -T s )

[0076] in,

[0077] q tot is the net heat flux per unit area;

[0078] h is the comprehensive heat transfer coefficient;

[0079] T AST is the adiabatic surface temperature;

[0080] T s is the solid surface temperature;

[0081] At this time, the adiabatic surface temperature is equivalent to the heat source, T s is the temperature of the heated component, h is the comprehensive heat transfer coefficient, as long as the adiabatic surface temperature T under the actual fire scene is known AST , we can restore the true net heat flux per unit area;

[0082] S2: Establish a fire dynamics model for I-shaped composite beam bridges and calculate the adiabatic surface temperature data; Figure 2 As shown, the I-shaped composite beam bridge used in this embodiment includes a main beam 1, in which the main beam 1 and the longitudinal beam 2 jointly support the top bridge deck 4, the longitudinal beam 2 is located between the transverse beam 3 and the bridge deck 4, and vertical stiffening ribs 5 are provided on both sides of the web of the main beam 1; therefore, correspondingly, in this step, a fire dynamics model of the I-shaped composite beam bridge is established based on structural parameters, and the structural parameters include at least the dimensions, material properties, and actual arrangement positions of the main beam, longitudinal beam, transverse beam, bridge deck, and vertical stiffening ribs; by adopting structural parameters close to actual data, the accuracy and reliability of the fire dynamics model of the I-shaped composite beam bridge can be further improved;

[0083] Specifically, in this embodiment, Figure 3As shown, the fire source was located below the main girder. The upper portion of the bridge utilizes a composite concrete deck structure with I-shaped steel beams. The superstructure is 2.2 meters high, with a span arrangement of 3 x 40 meters, resulting in a height-to-span ratio of approximately 1 / 18.2. The deck is constructed of C55 concrete, 0.3 meters high and 12.75 meters wide, with a cantilever length of 1.275 meters and a thickness of 0.25 meters. The deck reinforcement is HRB500 steel, with transverse bars measuring 16 mm in diameter and longitudinal bars measuring 20 mm in diameter. These bars are evenly spaced at 0.125 meter intervals, with a spacing of 0.25 meters between the upper and lower layers. The steel beam components are primarily constructed of Q370qD steel. The main beam consists primarily of an upper flange plate, a web plate, and a lower flange plate. The center-to-center spacing of the main beams is 5.1 meters. The single-piece steel main beam is 1.9 meters high, with upper flange plates measuring 600 mm x 28 mm and lower flange plates measuring 700 mm x 32 mm. The web plate is 20 mm thick. At the pier tops and between spans, 1.1m-high crossbeams are installed every 5m on either side of the main beam, symmetrically centered around the bridge's centerline. The vertical stiffeners of the main beams are welded to the upper and lower flanges. The thickness of the stiffeners at the transverse connection points is 16mm, the thickness of the stiffeners between adjacent crossbeams is 18mm, and the thickness of the stiffeners at the piers is 25mm. A longitudinal beam fixed to the top of the crossbeam is installed between adjacent main beams. The longitudinal beam is 0.3m high, with upper and lower flange plates measuring 300mm x 15mm and a web plate thickness of 10mm.

[0084] When establishing the fire dynamics model of an I-shaped composite beam bridge, meshing and boundary condition settings are performed. The actual fire and smoke diffusion conditions are simulated by discretizing the space into grid cells. The boundary conditions include at least:

[0085] 1) Material selection for the bridge deck and columns; in this embodiment, the material selected for the bridge deck and columns is concrete;

[0086] 2) Material selection for the main beam, longitudinal beam, and cross beam; in this embodiment, the material selected for the main beam, longitudinal beam, and cross beam is steel;

[0087] 3) Heat transfer between the surface of each material and the air is free;

[0088] 4) The simulation area is surrounded by open boundaries (except the ground), and air and heat can freely enter and exit the simulation area;

[0089] 5) The bridge fire simulation was conducted in an open environment with the initial ambient temperature set to 20°C and the ambient pressure set to 0.1013 MPa;

[0090] By setting boundary conditions, it is ensured that the established I-shaped composite beam bridge fire dynamics model is within the threshold range restricted by the boundary conditions, thereby further improving the accuracy and credibility of the established I-shaped composite beam bridge fire dynamics model.

[0091] Specifically, step S2 in this embodiment includes the following steps:

[0092] S2.1: Establish a fire dynamics model for I-beam bridges, such as Figure 3 As shown;

[0093] S2.2: Select the export data type, i.e. Adiabatic Surface Temperature data;

[0094] S2.3: Implement the I-beam bridge fire dynamics model;

[0095] S2.4: Obtain the non-uniform temperature field and adiabatic surface temperature data of the I-shaped composite beam bridge;

[0096] S2.5: Saving the calculated adiabatic surface temperature data in a preset file format in a result data folder of the I-shaped composite beam bridge fire dynamics model. In this embodiment, the preset file format is a BNDF file.

[0097] S3: extract the adiabatic surface temperature of each node at a specific time in the adiabatic surface temperature data file, and output the adiabatic surface temperature data file into a readable preset format; Figure 4 As shown, step S3 specifically includes the following steps:

[0098] S3.1: Using the built-in program, call the specified command fds2ascii in the result data folder of the I-beam composite beam bridge fire dynamics model;

[0099] S3.2: Enter the file name with the suffix corresponding to the specified command, which is .fds in this embodiment;

[0100] S3.3: Select the file to be processed. In this embodiment, the file is a BNDF file containing the adiabatic surface temperature.

[0101] S3.4: Select the nodes to be processed. If no selection is made, all nodes will be processed by default.

[0102] S3.5: Enter the time range of the data to be output. The format of the time range can be set to, for example: time 1 + space + time 2, in seconds.

[0103] S3.6: Select the output direction of the data. Since physical quantities are vector quantities, only one direction can be output at a time. Therefore, you need to select the output direction of the data. Output directions include front-back, up-down, and left-right.

[0104] S3.7: Select the number of physical quantity outputs to meet the output requirements of different physical quantities. In this embodiment, only the adiabatic surface temperature needs to be extracted, so the number of physical quantity outputs is 1;

[0105] S3.8: Enter the index number of the physical quantity to be output. The index number determines the physical quantity to be extracted and output. For example, when a model is divided into 9 grids, each grid will have adiabatic surface temperature data, and the adiabatic surface temperature data in each grid corresponds to a different index number. If you only need to extract the adiabatic surface temperature data in grid 2, you only need to enter the index number corresponding to the adiabatic surface temperature data of grid 2, and you can obtain the adiabatic surface temperature data corresponding to grid 2.

[0106] S3.9: Name the file to be output. The naming method can be the system default naming method or manually edited.

[0107] S3.10: Repeat steps S3.6 to S3.9 until data in all directions is extracted. In this embodiment, data in six directions needs to be extracted.

[0108] S3.11: Merge the extracted data files in the six directions and save them in a preset format that can be read by the front-end development program FE, such as ASCII format.

[0109] S4: Finite element calculation and analysis of the heat transfer of I-beam composite bridges were performed to obtain the non-uniform temperature field of each structure under local fire.

[0110] Step S4 of this embodiment specifically includes the following steps:

[0111] S4.1: Use finite element software, such as ABAQUS, to establish a heat transfer model for an I-shaped composite beam bridge that is consistent with the coordinates of the I-shaped composite beam bridge fire dynamics model;

[0112] Among them, such as Figure 5 As shown in FIG, the heat transfer model of the I-shaped composite beam bridge is established, which includes at least a concrete bridge deck, steel bars 6, a main beam 1, a longitudinal beam 2, a transverse beam 3 and vertical stiffening ribs 5, and each component is connected into a whole by a Tie constraint method;

[0113] The consistency of the coordinates with the I-shaped composite beam bridge fire dynamics model includes: achieving coordinate correspondence by establishing the same coordinates for modeling, so that all data obtained from the I-shaped composite beam bridge fire dynamics model can be transferred to the corresponding coordinates of the I-shaped composite beam bridge heat transfer model; so as to maximize the applicable data range while achieving the accuracy of data transmission;

[0114] S4.2: Set the model's thermal parameters, boundary conditions, mesh size, and element type. In this step, the thermal parameters include at least the thermal conductivity, specific heat capacity, thermal expansion coefficient, and density of concrete and steel. DC3D8 solid elements are used for the concrete deck, main beams, longitudinal beams, transverse beams, and vertical stiffeners, while DC1D2 linear elements are used for the steel bars.

[0115] S4.3: Use coordinate mapping to import the adiabatic surface temperature data saved in a preset format into the I-beam composite beam bridge heat transfer model;

[0116] S4.4: The surface of the I-beam composite bridge heat transfer model exposed to the fire environment is selected as the fire-exposed surface. Based on the extracted adiabatic surface temperature data, the fire-exposed surface is heated to simulate the time-varying temperature of the internal structure under the action of a real fire. The non-uniform temperature field of each structure under localized fire is solved.

[0117] Figure 6 This is a bottom view of the uneven temperature distribution of the I-shaped composite beam bridge provided by the present invention after being exposed to fire for 2 hours. Figure 7 This is a three-dimensional diagram of the uneven temperature distribution of the I-shaped composite beam bridge provided by the present invention after 2 hours of fire. The darker the color block, the higher the temperature. The fire source is located below the deepest color block of the main beam. Figure 6-Figure 7 This shows that this method can well restore the spatial temperature field effect during the simulation of the fire dynamics model of the I-shaped composite beam bridge, reflecting the uneven characteristics of the temperature distribution.

[0118] Figure 8-Figure 9 The present invention provides a temperature distribution curve diagram of the main beam exposed to fire for 2 hours and a temperature time history curve diagram of each point in the height direction of the main beam exposed to fire for 2 hours. Figure 10-11 The present invention provides a temperature distribution curve for the longitudinal beam after 2 hours of fire exposure and a temperature time history curve for each point along the longitudinal beam's height after 2 hours of fire exposure. By obtaining the non-uniform temperature field of the structure under localized fire, multi-scale modeling is used to analyze heat transfer in bridge structures under realistic fire temperatures. This improves the accuracy of predicting structural response under localized fires and provides a reference for analyzing bridge fire resistance performance and fire-resistant design.

[0119] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above and that the invention can be embodied in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as illustrative and non-restrictive, and the scope of the invention is defined by the appended claims, not the foregoing description, and all variations within the meaning and range of equivalents of the claims are intended to be included therein. Any reference sign in a claim should not be construed as limiting the claim to which it relates.

Claims

1. A coupling analysis method for an I-shaped composite beam bridge under local fire, characterized in that: The following steps are involved: S1: Determine the adiabatic surface temperature as coupling data; S2: Establish the fire dynamics model of I-shaped composite beam bridge and calculate the adiabatic surface temperature data; S3: extracting the adiabatic surface temperature of each node at a specific time in the adiabatic surface temperature data file, and outputting the adiabatic surface temperature data file into a readable preset format; S4: Finite element calculation and analysis of the heat transfer of the I-shaped composite beam bridge were performed to obtain the non-uniform temperature field of each structure under local fire.

2. The coupling analysis method of an I-shaped composite beam bridge under local fire according to claim 1, characterized in that: In step S1, the following formula is established, and the net heat flux per unit area is obtained by determining the adiabatic surface temperature according to the following formula: q tot =h(T AST -T s ) in, q tot is the net heat flux per unit area; h is the comprehensive heat transfer coefficient; T AST is the adiabatic surface temperature; T s is the solid surface temperature.

3. The coupling analysis method of an I-shaped composite beam bridge under local fire according to claim 1, characterized in that: In the step S2, a fire dynamics model of an I-shaped composite beam bridge is established based on structural parameters, wherein the structural parameters at least include the size, material properties and actual arrangement position of the main beam, longitudinal beam, cross beam, bridge deck and vertical stiffening ribs.

4. The coupling analysis method for an I-shaped composite beam bridge under local fire according to claim 1, characterized in that: In step S2, when establishing the fire dynamics model of the I-shaped composite beam bridge, meshing and boundary condition setting are performed on it, and the boundary conditions at least include: 1) Material selection for bridge deck and columns; 2) Material selection for main beams, longitudinal beams and cross beams; 3) Each material surface transfers heat freely with the air; 4) The simulation area is surrounded by open boundaries, and air and heat can freely enter and exit the simulation area; 5) The simulated bridge fire is an open environment with initial ambient temperature and ambient pressure.

5. The coupling analysis method for an I-shaped composite beam bridge under local fire according to claim 1, characterized in that: The step S2 includes the following steps: S2.1: Establish a fire dynamics model for I-beam composite bridges; S2.2: Select the export data type; S2.3: Implement the I-beam composite bridge fire dynamics model; S2.4: Obtain the non-uniform temperature field and adiabatic surface temperature data of the I-shaped composite beam bridge; S2.5: Save the calculated adiabatic surface temperature data in the form of a preset file in the result data folder of the I-beam composite beam bridge fire dynamics model.

6. The coupling analysis method for an I-shaped composite beam bridge under local fire according to claim 1, characterized in that: The step S3 includes the following steps: S3.1: Call the specified command in the result data folder of the I-beam composite bridge fire dynamics model; S3.2: Enter the file name with the suffix corresponding to the specified command; S3.3: Select the files to be processed; S3.4: Select the node to be processed; S3.5: Input the time range of the data to be output; S3.6: Select the output direction of the data; S3.7: Select the output number of physical quantities; S3.8: Input the index number of the physical quantity to be output; S3.9: Name the file to which the results are to be output; S3.10: Repeat steps S3.6 to S3.9 until data in all directions are extracted; S3.11: Merge the extracted data files in the six directions and save them in a readable preset format.

7. The coupling analysis method for an I-shaped composite beam bridge under local fire according to claim 1, characterized in that: The step S4 includes the following steps: S4.1: Use finite element software to establish a heat transfer model of an I-shaped composite beam bridge that is consistent with the coordinates of the I-shaped composite beam bridge fire dynamics model; S4.2: Set the thermal parameters, boundary conditions, mesh size and element type of the model; S4.3: Using coordinate mapping, the adiabatic surface temperature data saved in a preset format is imported into the I-shaped composite beam bridge heat transfer model; S4.4: The surface of the I-shaped composite beam bridge heat transfer model exposed to the fire environment is selected as the fire-exposed surface. The fire-exposed surface is heated according to the extracted adiabatic surface temperature data to simulate the change of the internal temperature of the structure with time under the action of a real fire, and the non-uniform temperature field of each structure under local fire is obtained by solving.

8. The coupling analysis method for an I-shaped composite beam bridge under local fire according to claim 7, characterized in that: In the step S4.1, the heat transfer model of the I-shaped composite beam bridge is established, which at least includes a concrete bridge deck, steel bars, a main beam, a longitudinal beam, a cross beam and vertical stiffening ribs, and each component is connected into a whole by a Tie constraint method.

9. The coupling analysis method for an I-shaped composite beam bridge under local fire according to claim 7, characterized in that: In the step S4.1, being consistent with the coordinates of the I-shaped composite beam bridge fire dynamics model includes: achieving coordinate correspondence by establishing the same coordinates for modeling, so that all data calculated by the I-shaped composite beam bridge fire dynamics model can be transmitted to the corresponding coordinates of the I-shaped composite beam bridge heat transfer model.

10. The coupling analysis method for an I-shaped composite beam bridge under local fire according to claim 7, characterized in that: In step S4.2, the thermal parameters include at least the thermal conductivity, specific heat capacity, thermal expansion coefficient and density of concrete and steel.

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