Method for analyzing and evaluating safety of suspension bridge under LNG tank truck fire

Abstract

The application discloses a kind of LNG oil tank truck under fire suspension bridge safety analysis and evaluation method, first establish suspension bridge fire analysis model to carry out analysis to key influence parameter of fire, determine the most unfavorable working condition of suspension bridge LNG oil tank truck under fire;Establish suspension bridge fire analysis model, calculate the wall temperature of cable system under the most unfavorable fire scene;Establish suspension bridge finite element model, calculate the cross-sectional temperature distribution and stiffness reduction of main cable and sling;Establish suspension bridge full-bridge finite element model, the performance reduction of main cable and sling is regarded as material parameter after fire and is applied in suspension bridge full-bridge finite element model, and the maximum displacement of suspension bridge girder, the distribution of main cable axial force and cable force is calculated, finally whether suspension bridge is in dangerous state is judged.This method can evaluate the risk of suspension bridge structure under different fire conditions, provide reliable technical support for identifying fire weak parts and improving bridge fire resistance.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of bridge engineering, and particularly relates to a method for analyzing and evaluating the safety of a suspension bridge under LNG tanker fire. BACKGROUND

[0002] According to statistical analysis of relevant agencies, about 3% of the bridge accidents in the United States after 1951 were caused by fire, which is slightly higher than the proportion of earthquakes. In recent years, several serious vehicle fires have occurred on famous cable system bridges at home and abroad, which have seriously damaged the bridges and even caused the bridges to collapse, causing huge economic losses to the local area. At present, there are a large number of suspension bridges in China, and most of them are the key routes to ensure the smooth transportation of highways. Under the situation of increasing demand for the transportation of hazardous chemicals, the possibility of large-scale fire on the suspension bridges is significantly increased. In order to protect the fire safety of the suspension bridges under the new situation, scholars at home and abroad have started to study the fire resistance of the suspension bridges to protect the structural safety of the bridges under fire and reduce the economic losses and maintenance costs caused by fire. However, most of the existing studies on tanker fires are focused on fires caused by traditional oil or gasoline, and there are few studies on LNG tanker fires. Unlike traditional oil or gasoline fires, the main component of liquefied natural gas (LNG) is methane, which is a class 1 flammable gas and a class A fire hazard. The explosion limit is 5% to 15% (volume fraction), the minimum ignition energy is only 0.28 mJ, the combustion speed is fast, the combustion heat value is high (about 33440 kJ / m 3 ), and it is extremely easy to burn, explode, and spread. If an explosion occurs, the fire will spread extremely quickly. If the traditional oil or gasoline fire temperature rise curve is used for research, the development trend of the flame will be seriously underestimated, and the calculation accuracy of the existing research based on the simplified "thermal-force simulation method" based on the temperature rise curve and the finite element method still has a lot of room for improvement. In addition, due to the lack of industry standards for bridge fire resistance design, the environmental impact parameters are mostly determined based on experience, which cannot fully represent the most unfavorable conditions of the fire on the bridge.

[0003] Fire analysis on the suspension bridge is an important prerequisite to ensure the safety and operation of the bridge structure. At present, the existing information on bridge fires does not include research on LNG tanker fires. In addition, the structural model of the cable under fire in the existing research only considers the local effect and ignores the overall cooperation of the structure, and the internal force of the isolated structural model is seriously deviated from the actual structural stress, which will bring great safety hazards to the overall structure. SUMMARY

[0004] In order to solve the technical problems mentioned in the above background, the present application provides a method for analyzing and evaluating the safety of a suspension bridge under LNG tanker fire, which uses numerical simulation to calculate the mechanical response of the bridge structure under LNG tanker fire, evaluates the safety of the overall bridge structure under fire, and identifies the weak fire-resistant parts to ensure the safety of the bridge operation.

[0005] In order to achieve the above technical purposes, the technical scheme of the present application is as follows:

[0006] A method for analyzing and evaluating the safety of a suspension bridge under LNG tanker fire, comprising the following steps: establishing a suspension bridge fire analysis model, analyzing key fire influence parameters using the fire analysis model, and obtaining the most unfavorable working condition of the suspension bridge under LNG tanker fire; establishing a suspension bridge fire analysis model according to the most unfavorable working condition of the suspension bridge under LNG tanker fire, calculating the wall temperature of the cable system under the most unfavorable fire scenario; establishing a suspension bridge finite element model, taking the wall temperature of the cable system under the most unfavorable working condition as the first type of temperature boundary condition, calculating the cross-sectional temperature distribution and stiffness reduction of the main cable and the sling, establishing a suspension bridge finite element model, applying the performance reduction of the main cable and the sling as the material parameters after fire in the suspension bridge full-bridge finite element model, and calculating the maximum displacement of the main girder of the suspension bridge, the axial force of the main cable and the distribution of the cable force, to determine whether the suspension bridge structure is in a dangerous state.

[0007] Preferably, the control equation set formula of the suspension bridge finite element model and the suspension bridge fire analysis model is expressed as follows:

[0008]

[0009]

[0010]

[0011]

[0012]

[0013] p=ρRT (19)

[0014] e=c v T (20)

[0015] In the formula, ρ, u, v, w, p, e, and T are seven flow field parameters, respectively representing density, velocity components in x-axis, y-axis, and z-axis directions, pressure, viscosity, and temperature.

[0016] Preferably, the analysis of key fire influence parameters specifically includes:

[0017] The position of the fire starting point, according to the bridge type arrangement and the vehicle passing scheme, the mechanical response of the bridge structure when the LNG tanker fire occurs at the key points such as the midspan, the tower, the long suspension cable, the short suspension cable, the anchor, and the 1 / 4 of the main span is analyzed to determine the relatively most unfavorable position of the fire starting point;

[0018] The height of the fire surface, according to the relative distance between the main cable and the bridge surface, the relatively most unfavorable height of the fire surface is determined by analyzing the fire surface at the top and middle of the tanker and the fire surface on the bridge surface;

[0019] The environmental wind speed, according to the overall design and monitoring data, the influence degree of the flame height, the flame shape, and the flame trend on the cable system under different environmental wind speeds is analyzed to determine the relatively most unfavorable environmental wind speed;

[0020] The fire holding time, according to the specific needs and actual situation of fire fighting, the mechanical response of the bridge structure under different fire durations is analyzed to determine the relatively most unfavorable fire holding time.

[0021] Preferably, the most unfavorable working condition suspension bridge fire analysis model includes mass conservation equation, momentum conservation equation, energy conservation equation and material state equation;

[0022] The mass conservation equation formula is expressed as follows:

[0023]

[0024] In the formula, ρ is the gas density, and u is the velocity vector;

[0025] The momentum conservation equation formula is expressed as follows:

[0026]

[0027] In the formula, p is the pressure, f b is the external force vector other than the weight, τ ij is the viscous stress tensor;

[0028] The energy conservation equation formula is expressed as follows:

[0029]

[0030] In the formula, h s is the specific enthalpy, is the unit volume heat release rate, q″′ b is the energy transferred by droplet evaporation, is the heat flux vector;

[0031] The material state equation formula is expressed as follows:

[0032]

[0033] wherein p is the gas density, p is the pressure, T is the gas temperature, R is the ideal gas constant, and M is the molecular weight of the gas mixture.

[0034] Preferably, the calculation of the stiffness reduction of the cable system under fire specifically refers to: dividing the main cable section into a plurality of micro units, taking the average temperature of all nodes in the unit as the equivalent temperature of the unit, and calculating the stiffness reduction coefficient of the unit at the corresponding temperature according to the interpolation calculation of the steel stiffness parameters at different temperatures specified in the European standard EC3, and then weighting and averaging the stiffness reduction coefficients of all units in the section to obtain the stiffness reduction coefficient of the whole section of the main cable, so as to evaluate the overall stiffness degradation of the main cable section, which is expressed as follows:

[0035]

[0036] wherein Ψ is the overall stiffness reduction coefficient of the section; E is the elastic modulus of steel at room temperature, A is the sectional area; n is the number of sectional units; is the stiffness reduction coefficient of the i-th unit, which is obtained by looking up the table according to the average temperature of all nodes in the unit; E i is the elastic modulus of the i-th unit at the corresponding temperature, a i is the area of the i-th unit.

[0037] Preferably, after calculating the maximum displacement of the main girder of the suspension bridge, the displacement of the top of the main tower, the axial force of the main cable and the distribution of the cable force in the finite element model of the suspension bridge, the carrying capacity is checked, including the deflection check of the main girder and the breaking force check of the cable, which is expressed as follows:

[0038]

[0039] wherein [F max ] is the breaking force, with the unit of kN; f pk is the standard value of the tensile strength of steel wire; A is the sectional area of the main cable; and γ is the safety factor, γ≥2.5.

[0040] Preferably, the judgment basis for whether the suspension bridge structure is dangerous is as follows: when the bridge structure form is simply supported or continuous truss or simply supported or continuous plate girder, the calculated deflection value of the main girder of the suspension bridge is less than or equal to when the bridge structure form is the cantilever end of the beam, the calculated deflection value of the main girder of the suspension bridge is less than or equal to when the bridge structure form is the main girder of the cable-stayed bridge, the calculated deflection value of the main girder of the suspension bridge is less than or equal to when the bridge structure form is the stiffened girder of the suspension bridge, the calculated deflection value of the main girder of the suspension bridge is less than or equal to wherein l is the calculated span, and l1 is the cantilever length.

[0041] The beneficial effects brought by the above technical scheme are:

[0042] The application provides a safety analysis and evaluation method of a suspension bridge under LNG oil tank truck fire, which determines the most unfavorable working condition of the suspension bridge under the LNG oil tank truck fire by analyzing the influence of different environmental factors on the LNG oil tank truck fire through finite element numerical simulation analysis. The wall surface temperature of the main cable under the most unfavorable fire working condition calculated by a fire dynamics simulation software is applied as a first type of temperature boundary condition in the finite element model, the mechanical response of the bridge structure is analyzed, and the bearing capacity is checked according to relevant industry specifications. The weak fire-resistant position and the most unfavorable fire working condition are determined, which is an important prerequisite for ensuring the operation safety of the bridge and an important guarantee for the fire safety of the suspension bridge under the new situation. The application has a wide application range and can be applied to general long-span suspension bridges and can be popularized to other cable-supported system bridges.

[0043] The application adopts a modulus-based "weighted average" concept to calculate the stiffness reduction method of the main cable under fire, can evaluate the overall stiffness degradation of the main cable section, and simplifies the calculation method of the bridge fire. Compared with the prior art, the method is more objective and accurate, and has the advantage of avoiding excessive conservatism of the stiffness reduction. BRIEF DESCRIPTION OF DRAWINGS

[0044] Figure 1 is a safety analysis and evaluation method flowchart of a suspension bridge under LNG oil tank truck fire of the application;

[0045] Figure 2 is a temperature time history curve diagram of the bottom of the main cable under different fire surface heights of the application;

[0046] Figure 3 is a temperature time history curve diagram of the bottom of the main cable under different environmental wind speeds of the application;

[0047] Figure 4 is a temperature time history curve diagram of the bottom of the main cable under different fire holding times of the application;

[0048] Figure 5 is a temperature time history curve diagram of the main cable under the most unfavorable fire working condition of the application;

[0049] Figure 6 is a temperature distribution diagram of the main cable section under the most unfavorable fire working condition of the application;

[0050] Figure 7 is a temperature time history curve diagram of the fire cable sling under the most unfavorable fire working condition of the application;

[0051] Figure 8 is a temperature distribution diagram of the fire cable sling section under the most unfavorable fire working condition of the application. DETAILED DESCRIPTION

[0052] The technical solutions of the application will be described in detail below with reference to the drawings.

[0053] The application discloses a method for analyzing and evaluating safety of a suspension bridge under LNG oil tank truck fire, and first establishes a fire analysis model of the suspension bridge, analyzes key influence parameters of the fire by using the fire analysis model, and obtains the most unfavorable working condition of the suspension bridge LNG oil tank truck fire; the most unfavorable working condition of the suspension bridge LNG oil tank truck fire is used to establish the fire analysis model of the suspension bridge, the wall surface temperature of the cable system under the most unfavorable fire scene is calculated, a finite element model of the suspension bridge is established, the wall surface temperature of the cable system under the most unfavorable working condition is taken as a first type of temperature boundary condition, the cross section temperature distribution and stiffness reduction of the main cable and the sling are calculated, a full bridge finite element model of the suspension bridge is established, the performance reduction of the main cable and the sling is taken as a material parameter after the fire and is applied to the full bridge finite element model of the suspension bridge, and the maximum displacement of the main beam of the suspension bridge, the axial force of the main cable and the distribution of the cable force are calculated; if all indexes meet the requirements of the specification, the bridge structure is considered to be in a safe state; if an index exceeds the requirements of the specification, the bridge structure is considered to be in a dangerous state, and relevant measures need to be taken. The method comprises the following specific steps:

[0054] 1) a fire analysis model of the suspension bridge is established, and key influence parameters of the fire are analyzed by using the fire analysis model. The fire dynamics simulation software FDS is prepared based on a field model as a theoretical basis in the application, and the control equation group is as follows:

[0055]

[0056]

[0057]

[0058]

[0059]

[0060] p = pRT (32)

[0061] e = c v T (33)

[0062] The control equation group contains seven partial differential equations, and seven flow field parameters, namely p, u, v, w, p, e and T, can be obtained by solving the equations ((27)~((33)), so that the equation group is a closed equation group.

[0063] 2) key influence parameters of the fire are analyzed, including a fire starting position, a fire starting surface height, an environmental wind speed, a fire holding time and other environmental factors, and the most unfavorable working condition of the suspension bridge LNG oil tank truck fire is determined by comprehensive comparison and selection:

[0064] a) The location of the fire ignition point. According to the bridge type arrangement and the vehicle passing scheme, the mechanical response of the bridge structure is analyzed when the LNG tanker fire occurs at the midspan, the tower, the long suspension cable, the short suspension cable, the anchorage, the 1 / 4 of the main span, etc. to determine the relatively most unfavorable location of the fire ignition point.

[0065] b) The height of the fire surface. According to the relative distance between the main cable and the bridge deck, the mechanical response of the bridge structure is analyzed when the fire surface is at the top and middle of the tanker (i.e. the vehicle body is on fire) and the fire surface is on the bridge deck (i.e. the bridge deck is on fire) to determine the relatively most unfavorable height of the fire surface.

[0066] c) The environmental wind speed. According to the overall design and monitoring data, the influence of the flame height, flame shape and flame trend on the cable system under different environmental wind speeds is analyzed to determine the relatively most unfavorable environmental wind speed.

[0067] d) The fire duration. According to the specific needs and actual situation of fire fighting, the mechanical response of the bridge structure under different fire durations is analyzed to determine the relatively most unfavorable fire duration.

[0068] 3) The numerical simulation model of the most unfavorable fire condition is established in the fire dynamics simulation software. The basic equations to be solved mainly include the mass conservation equation (continuity equation), the momentum conservation equation (i.e. Navier-Stokes equation), the energy conservation equation and the state equation. The differential form of the mass conservation equation is

[0069]

[0070] wherein ρ is the gas density and u is the velocity vector.

[0071] The momentum conservation equation is

[0072]

[0073] wherein p is the pressure, f b is the external force vector other than the gravity, τ ij is the viscous stress tensor.

[0074] The energy conservation equation is

[0075]

[0076] wherein h s is the specific enthalpy, is the unit volume heat release rate, q″ b is the energy transferred by droplet evaporation, is the heat flux vector.

[0077] The state equation of the substance is

[0078]

[0079] In the formula, R is the ideal gas constant, and M is the molecular weight of the gas mixture.

[0080] 4) A finite element model of the suspension bridge is established, the wall temperature of the main cable calculated by the fire dynamics simulation software is applied as a first type of temperature boundary condition in the finite element model, and the temperature distribution of the main cable and the sling is calculated.

[0081] 5) The stiffness reduction of the cable system under fire is calculated. The main cable stiffness calculation method based on the modulus "weighted average" concept is adopted to reflect the stiffness degradation of the main cable section, the main cable section is divided into a plurality of micro units, the average temperature of all nodes in the unit is taken as the equivalent temperature of the unit, and the stiffness reduction coefficient of the unit at the corresponding temperature is calculated by interpolation according to the steel stiffness parameters at different temperatures specified in the European standard EC3, and then the stiffness reduction coefficients of all units in the section are weighted and averaged to obtain the stiffness reduction coefficient of the whole section of the main cable, so as to evaluate the overall stiffness degradation of the main cable section, and the calculation formula is as follows:

[0082]

[0083] In the formula, Ψ is the overall stiffness reduction coefficient of the section, E is the elastic modulus of the steel at room temperature, A is the section area, n is the number of section division units, and Ei is the stiffness reduction coefficient of the i-th unit, which is obtained by looking up the table according to the average temperature of all nodes in the unit. i Ei is the elastic modulus of the i-th unit at the corresponding temperature, and a i is the area of the i-th unit.

[0084] 6) A full-bridge finite element model of the suspension bridge is established, the stiffness reduction of the main cable and the sling after the fire is reflected in the finite element model through material property parameters, and the mechanical response of the full-bridge structure under the fire of the LNG tank truck is calculated.

[0085] 7) The maximum displacement of the main beam, the displacement of the top of the main tower, the axial force of the main cable and the distribution of the cable force are calculated respectively, and the bearing capacity is checked according to the relevant specifications, mainly including the deflection checking of the main beam and the breaking force checking of the cable. Among them, the calculated deflection value of the main beam should not exceed the limit value specified in Table 1.

[0086] Table 1 Limit value of vertical deflection of bridge

[0087]

[0088] Note: 1. In the table, l is the calculated span, and l1 is the cantilever length.

[0089] 2. When the load acts on a span, it is possible to cause positive and negative deflection of the span, and the calculated deflection should be the sum of the absolute values of the positive and negative deflection.​

[0090] 3. Deflection is calculated on the basis of the gross cross-section.

[0091] The breaking force of the main cable and the sling is calculated according to the following formula:

[0092]

[0093] In the formula, [F max ] is the breaking force (kN); f pk is the standard value of the tensile strength of the steel wire; A is the cross-sectional area of the main cable; γ is the safety factor, which should not be less than 2.5.

[0094] a) If all the indicators meet the requirements of the relevant industry standards, it is considered that the bridge structure is still in a safe state.

[0095] b) If one of the indicators exceeds the requirements of the standards, it is considered that the bridge structure is in a dangerous state, and relevant measures need to be taken.

[0096] Implementation case: Taking a large-span suspension bridge in China as an example, the bridge passes 136 types of hazardous chemical vehicles, and it is estimated that 54 oil and chemical vehicles and 550 liquefied natural gas vehicles pass through the bridge every day. Under this background, the number of hazardous chemical transport vehicles passing through the suspension bridge has increased significantly, and the possibility of large-scale fire has also increased significantly, which brings a major safety hazard to the bridge structure and unprecedented pressure to the bridge management department. If the weak parts of the bridge against fire are not clearly defined or the risk assessment of the suspension bridge under the fire of LNG tank vehicles is ignored, the fire safety of the bridge structure under the new situation cannot be guaranteed.

[0097] A finite element model and a fire analysis model of the suspension bridge are established, and the key influence parameters of the fire are analyzed. The temperature time-history curve of the bottom of the main cable under different fire surface heights is shown in Figure 2 , the temperature time-history curve of the bottom of the main cable under different wind speeds is shown in Figure 3 , and the temperature time-history curve of the bottom of the main cable under different fire holding times is shown in Figure 4 . Based on the above analysis results and combined with the actual situation and traffic policy of the bridge, the most unfavorable fire working condition scenario is determined as the fire of 2 LNG tank vehicles at the midspan position, the environmental wind speed is 4.3 m / s, and the fire duration is 90 min. The numerical simulation model of the most unfavorable fire working condition is established in the fire dynamics simulation software, the calculated wall surface temperature of the main cable under fire is applied as the first type of temperature boundary condition in the finite element model, and the temperature time-history curve of the main cable under the most unfavorable fire working condition is analyzed and obtained as shown in Figure 5 , the temperature distribution of the cable cross-section is shown in Figure 6 , the temperature time-history curve of the sling under fire is shown in Figure 7 , and the temperature distribution of the sling cross-section under fire is shown in Figure 8The performance of the main cable and the sling is reduced as the material parameters after the fire, which are applied in the finite element model, to calculate the mechanical response of the whole bridge structure under the fire of the LNG tank truck. In the most unfavorable fire condition, the maximum vertical displacement of the main beam and the maximum longitudinal displacement of the tower top are less than the limit value specified in the relevant industry standard. The risk assessment method and results of the implementation case are shown in Table 2.

[0098] Table 2 Risk assessment of the suspension bridge under the main fire condition of the suspension bridge

[0099]

[0100]

[0101] Those skilled in the art will understand that the embodiments of the present application can be provided as methods, systems, or computer program products. Therefore, the present application can take the form of an entirely hardware embodiment, an entirely software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present application can take the form of a computer program product implemented on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROMs, optical storage, etc.) containing computer-usable program code. The solutions in the embodiments of the present application can be implemented in various computer languages, such as the object-oriented programming language Java and the interpreted scripting language JavaScript.

[0102] The present application is described with reference to flowcharts and / or block diagrams according to the methods, devices (systems), and computer program products of the embodiments of the present application. It should be understood that each flow and / or block in the flowcharts and / or block diagrams, and the combination of flows and / or blocks in the flowcharts and / or block diagrams can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing apparatus to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing apparatus produce a device that implements the functions specified in the flowcharts and / or block diagrams. Figure 1 one or more flows and / or blocks Figure 1 an apparatus that carries out the functions specified in one or more blocks.

[0103] These computer program instructions can also be stored in a computer-readable memory that can direct the computer or other programmable data processing apparatus to work in a specific manner, so that the instructions stored in the computer-readable memory produce a manufactured product including instruction apparatus, which implements the functions specified in the flowcharts and / or block diagrams. Figure 1 one or more flows and / or blocks Figure 1 an apparatus that carries out the functions specified in one or more blocks.

[0104] These computer program instructions can also be loaded into a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart block or blocks. Figure 1 Figure 1

[0105] Although preferred embodiments of the application have been described herein, substitutions and modifications of these preferred embodiments made by those skilled in the art are to be considered within the scope of the application. Therefore, it is intended that the appended claims be construed as including all such substitutions and modifications as fall within the true spirit and scope of the application.

[0106] Obviously, numerous modifications and variations of the present application are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims and their equivalents, the application can be practiced otherwise than as specifically described.​​

Claims

1. A method for analyzing and evaluating the safety of a suspension bridge under LNG tank truck fire, characterized in that, The method comprises the following steps: A suspension bridge fire analysis model is established, key fire influence parameters are analyzed by using the fire analysis model, and a most unfavorable working condition of the suspension bridge under LNG oil tank truck fire is obtained; A numerical simulation model of the most unfavorable fire working condition is established according to the most unfavorable working condition of the suspension bridge under LNG oil tank truck fire, the wall surface temperature of the cable system under the most unfavorable fire scene is calculated, a suspension bridge finite element model is established, the wall surface temperature of the cable system under the most unfavorable working condition is taken as a first type of temperature boundary condition, the cross section temperature distribution and stiffness reduction of the main cable and the sling are calculated, a suspension bridge full bridge finite element model is established, the performance reduction of the main cable and the sling is taken as a material parameter after fire and is applied to the suspension bridge full bridge finite element model, and the maximum displacement of the suspension bridge main beam, the axial force of the main cable and the distribution of the cable force are calculated to determine whether the suspension bridge structure is in a dangerous state; The analysis of the key fire influence parameters specifically comprises: A fire starting point position, according to the bridge type arrangement and the vehicle passing scheme, the mechanical response of the bridge structure when the LNG oil tank truck fire occurs at the key point positions of the midspan, the bridge tower, the long sling, the short sling, the anchorage and the main span 1 / 4 is analyzed to determine the relatively most unfavorable fire starting point position; A fire surface height, according to the relative distance between the main cable and the bridge surface, the relatively most unfavorable fire surface height is determined under the conditions that the fire surface is at the top and the middle of the oil tank truck and the fire surface is on the bridge surface; An environmental wind speed, according to the overall design and the monitoring data, the influence degree of the flame height, the flame shape and the flame trend on the cable system under different environmental wind speeds is analyzed to determine the relatively most unfavorable environmental wind speed; a fire holding time, according to the specific requirements and the actual situation of fire fighting, the mechanical response of the bridge structure under different fire durations is analyzed by trial calculation to determine the relatively most unfavorable fire holding time; The calculation of the stiffness reduction specifically refers to that the main cable cross section is divided into a plurality of micro units, the average value of the temperatures of all nodes in the unit is taken as the equivalent temperature of the unit, the stiffness reduction coefficient of the unit at the corresponding temperature is calculated according to the interpolation calculation of the steel stiffness parameters at different temperatures specified in the European standard EC3, then the stiffness reduction coefficients of all units in the cross section are weighted and averaged to obtain the stiffness reduction coefficient of the main cable whole cross section, so as to evaluate the overall stiffness degradation of the main cable cross section, and the formula is expressed as follows: In the formula, Ψ is the overall stiffness reduction coefficient of the cross section; E is the elastic modulus of steel at room temperature, A is the cross-sectional area; n is the number of cross-sectional division units; Ψi is the stiffness reduction coefficient of the i-th unit, which is obtained by looking up the table according to the average temperature of all nodes of the unit; E i Ei is the elastic modulus of the i-th unit at the corresponding temperature, a i A is the area of the i-th unit.

2. The method for analyzing and evaluating the safety of a suspension bridge under LNG tank truck fire according to claim 1, characterized in that, The control equation set formula of the suspension bridge finite element model and the suspension bridge fire analysis model is expressed as follows: p=ρRT (7) e = c v T (8) In the formula, ρ, u, v, w, p, e and T are seven flow field parameters, which are respectively density, velocity components in x-axis, y-axis and z-axis directions, pressure, viscosity and temperature.

3. The method for analyzing and evaluating the safety of a suspension bridge under LNG tank truck fire according to claim 1, characterized in that, The numerical simulation model of the most unfavorable fire working condition comprises a mass conservation equation, a momentum conservation equation, an energy conservation equation and a material state equation; The mass conservation equation is expressed as follows: In the formula, ρ is the gas density, and u is the velocity vector. The momentum conservation equation is expressed as follows: where p is pressure, f b is the external force vector other than the weight, τ ij is the viscous stress tensor; The energy conservation equation is expressed as follows: where h s is the apparent enthalpy, is the heat release rate per unit volume, q" b is the energy transferred by droplet evaporation, is the heat flux vector; The material state equation is expressed as follows: In the formula, ρ is the gas density, p is the pressure, T is the gas temperature, R is the ideal gas constant, and M is the gas mixture molecular weight.

4. The method for analyzing and evaluating the safety of a suspension bridge under LNG tank truck fire according to claim 1, characterized in that, After the maximum displacement of the main girder, the displacement of the top of the main tower, the axial force of the main cable and the distribution of the cable force of the suspension bridge are calculated by the finite element model of the suspension bridge, the carrying capacity checking is carried out, including the deflection checking of the main girder and the breaking force checking of the cable, which is expressed by the following formula: [F max ]=f pk ×A (13) In the formula, [F max ] is the breaking force, in kN; f pk is the standard value of the tensile strength of the steel wire; A is the cross-sectional area of the main cable; and γ is the safety factor, γ≥2.

5.

5. The method for analyzing and evaluating the safety of a suspension bridge under LNG tank truck fire according to claim 1, characterized in that, The judgment basis of whether the suspension bridge structure is dangerous is as follows: when the bridge structure form is simple support or continuous truss or continuous plate beam, the calculated deflection value of the suspension bridge main beam is less than or equal to When the bridge structure form is the cantilever end of the beam, the calculated deflection value of the suspension bridge main beam is less than or equal to When the bridge structure form is the main beam of the cable-stayed bridge, the calculated deflection value of the suspension bridge main beam is less than or equal to When the bridge structure form is the stiffening beam of the suspension bridge, the calculated deflection value of the suspension bridge main beam is less than or equal to Wherein, l is the calculated span, and l1 is the cantilever length.

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