Method for predicting spatial and temporal distribution of temperature field of fire under I-shaped composite beam bridge

Abstract

The invention provides a method for predicting spatial and temporal distribution of an under-bridge fire temperature field of an I-shaped composite beam bridge. The method comprises the following steps: S1, establishing an I-shaped composite beam bridge model and setting boundary conditions of the model; s2, setting fire scene working condition parameters; s3, grid division is carried out, and the simulation method is verified; s4, determining fire influence factors and a time-space distribution rule; s5, fire smoke temperature prediction is carried out; aiming at an I-shaped composite beam bridge which is being popularized and used but is easily influenced by a fire, distribution characteristics of an under-bridge fire temperature field under different fire vehicle types, environment wind speeds and under-bridge clear heights are researched through numerical simulation, and by determining influences of different factors on the fire temperature field and further determining a space-time distribution rule of the temperature field, the fire temperature field is determined. Therefore, the proximity between the prediction method and an actual fire scene is further improved, the prediction accuracy and the prediction credibility of the prediction method are further improved, and the method can be used for fire resistance analysis and design of the bridge structure.

Description

Technical Field

[0001] The invention belongs to the technical field of fire temperature prediction under an I-shaped composite beam bridge, and in particular relates to a method for predicting the temporal and spatial distribution of a fire temperature field under an I-shaped composite beam bridge. Background Art

[0002] The steel-concrete composite structure can give full play to the compressive properties of concrete and the high tensile strength of steel, significantly reducing the use of steel while enhancing the rigidity and integrity of the structure. For steel bridges with a span of 40 to 60 meters, the standard structure of I-shaped composite beams should be preferred. Fires of vehicles or piles under bridges occur from time to time, which seriously threatens the safety of bridge structures. The deck of a 40-meter two-way four-lane steel-concrete composite beam bridge is usually composed of a tic-tac-toe spatial structure composed of main beams, cross beams and small longitudinal beams, which affects the spread of smoke and temperature field distribution under the bridge; the steel beams are exposed under the bridge deck. When a fire occurs under the bridge, the steel beams will be directly affected by the high temperature of the fire and are easily damaged or collapsed.

[0003] Determining the actual fire effects on structures is the basis for conducting research on the fire resistance of bridge structures. Existing studies often use standard heating curves such as HC and ISO 834 to simplify the fire effects on bridges, which cannot reflect the uneven distribution characteristics of the temperature field of local fires in semi-open spaces of bridges.

[0004] High-temperature smoke from a fire spreads upwards driven by thermal buoyancy. When the fire source is located below important steel components, it poses the greatest threat to the safety of the bridge structure. The scale of a fire is related to the type of combustibles. The combustibles in bridge fires are mainly vehicles passing on and under the bridge and the accumulations under the bridge. Vehicle fire accidents seriously threaten the safety of bridge structures. Existing studies have mainly focused on large-scale tanker fires. Although the scale of fires in large trucks, buses, and cars is relatively small, the proportion of vehicles is high, and the fire risk to structural safety cannot be ignored.

[0005] At the same time, ambient wind will also change the maximum temperature and spatial temperature distribution of the fire scene, but existing studies have rarely considered the impact of ambient wind, and most of them were conducted on suspension bridges.

[0006] In addition, the smaller the clear height under the bridge, the higher the fire temperature under the bridge deck and the more serious the damage to the structure.

[0007] The temperature distribution prediction model proposed in the prior art based on the research results of bridge fire temperature field under real fire scenarios is often only applicable to specific bridge structure types, and is not applicable to I-beam composite beam bridges with complex spatial structures composed of main beams, cross beams and small longitudinal beams.

[0008] Therefore, there is an urgent need to provide a method for predicting the spatiotemporal distribution of the fire temperature field under an I-shaped composite beam bridge to solve the defects and shortcomings in the above-mentioned prior art. Summary of the invention

[0009] In order to solve the defects and shortcomings in the above-mentioned technology, the present invention provides a method for predicting the spatiotemporal distribution of the temperature field of a fire under an I-shaped composite beam bridge.

[0010] The technical solution provided by the present invention is as follows:

[0011] A method for predicting the temporal and spatial distribution of the temperature field of a fire under an I-shaped composite beam bridge, characterized in that it comprises the following steps:

[0012] S1: Establish an I-shaped composite beam bridge model and set the boundary conditions of the model;

[0013] S2: Set fire scene working condition parameters;

[0014] S3: Perform meshing and verify the simulation method;

[0015] S4: Determine the temporal and spatial distribution patterns of fire influencing factors;

[0016] S5: Predict fire smoke temperature, including:

[0017] S51: Calculate the dimensionless maximum temperature rise data at the bottom of the bridge deck by simulation;

[0018] S52: Predict the variation of smoke temperature rise at the bottom of the bridge deck with fire duration;

[0019] S53: Predict the variation of the longitudinal bridge smoke temperature rise with the distance from the fire center;

[0020] S54: Predict the smoke temperature rise data in the height direction of the main beam affected by fire;

[0021] S55: Predict the temperature value at any location at any time on the bottom of the bridge.

[0022] As a further preferred embodiment of the present invention, in step S1,

[0023] The established I-shaped composite beam bridge model includes a main beam, the bottom of the main beam is supported on the ground, the top of the main beam is fixedly connected to the bridge deck, adjacent main beams are connected by cross beams, a longitudinal beam is arranged between the bottom of the bridge deck and the top of the cross beam, and a stiffening rib is arranged inside the main beam;

[0024] The boundary conditions of the model are as follows: the bridge deck is made of concrete, the main beam, cross beam and longitudinal beam are made of steel, and the surfaces of each material are allowed to exchange heat freely with the air. In addition, the simulation area is surrounded by open boundaries except the ground, so that air and heat can freely enter and exit the simulation area. The initial ambient temperature of the simulation area is 20°C, and the initial ambient pressure is 0.1013Mpa.

[0025] As a further preferred embodiment of the present invention, in step S2, the fire scene operating condition parameters include at least vehicle type, clear height under the bridge and ambient wind speed; and wherein, the vehicle type is set to include small cars, large buses, heavy trucks and tankers; the clear height under the bridge is set to satisfy: the clear height under the bridge is not greater than 22.5m; the ambient wind speed is set to satisfy: the ambient wind speed is not greater than 3m / s.

[0026] As a further preferred embodiment of the present invention, in step S3, according to the characteristic diameter D of the fire source, * Determine the grid size, where the characteristic diameter of the fire source D * Calculate according to the following formula:

[0027]

[0028] Where:

[0029] D * is the characteristic diameter of the fire source, m;

[0030] Q is the heat release rate of the fire source, kW;

[0031] ρ0 is the ambient air density, kg / m 3 ;

[0032] c P is the specific heat of ambient air, kJ / (kg·K);

[0033] T0 is the ambient temperature, K;

[0034] g is the acceleration due to gravity, m / s 2 ;

[0035] And the grid size is between [D * / 16,D * / 4];

[0036] The simulation method is verified by comparing numerical simulation with actual experiments. The simulation method is deemed to have passed the verification if and only if the deviation between the simulated value of the flame temperature at each position away from the fire source and the corresponding test value is lower than the preset threshold.

[0037] As a further preferred embodiment of the present invention, in step S4,

[0038] The influencing factors to be determined include at least the vehicle type, the clear height under the bridge and the ambient wind speed, and they must meet the following requirements:

[0039] The larger the fire size caused by the vehicle type, the smaller the lateral wind speed and the clear height under the bridge, and the higher the fire temperature;

[0040] The smaller the fire size caused by the vehicle type, the greater the lateral wind speed and the clear height under the bridge, and the lower the fire temperature;

[0041] The determined temporal and spatial distribution patterns of fires include at least:

[0042] The temperature of the part directly affected by the flame is the highest, and the temperature of the bottom of the bridge deck above the flame is the second highest;

[0043] As smoke spreads and settles around the bottom of the bridge deck, its temperature gradually decreases;

[0044] The temperature at the bottom of the bridge deck is evenly distributed in each transverse area divided by the main beam and the longitudinal beam, and decays exponentially along the longitudinal direction with the increase of the distance from the center of the fire source.

[0045] As a further preferred embodiment of the present invention, the step S51 includes the following steps:

[0046] S511: Maximum temperature rise ΔT max Defined as the maximum temperature T max The difference between the ambient temperature T0 and ΔT max / T0 is defined as the dimensionless maximum temperature rise ΔT * , the dimensional analysis method is used to establish the theoretical formula of the dimensionless maximum temperature rise and influencing factors at the bottom of the bridge deck:

[0047] ΔT * =f(Q,D,u,h ef ,c p ,ρ0,T0,g)

[0048] Where: ΔT * is the dimensionless maximum temperature rise at the bottom of the bridge deck; Q is the heat release rate of the fire source, kW; D is the equivalent diameter of the fire source, m; u is the ambient wind speed, m / s; h ef is the effective height under the bridge, i.e. the vertical distance between the fuel surface and the bottom of the bridge deck, m; c P is the specific heat of ambient air, kJ / (kg·K); ρ0 is the density of ambient air, kg / m 3 ; T0 is the ambient temperature, K; g is the acceleration due to gravity, m / s 2 ;

[0049] S512: Select [M], [L], [T], [θ] as four basic dimensions, and use ρ0, D, T0, and g as basic physical quantities. The following dimensionless expression is obtained by the π theorem:

[0050]

[0051] π1, π2, π3 and π4 represent 4 dimensional expressions respectively;

[0052] α1-4 , β 1-4 , γ 1-4 and ε 1-4 denote coefficients respectively;

[0053] S513: After dimension consistency calculation, the dimensionless maximum temperature rise calculation formula is obtained:

[0054]

[0055] S514: Combine the dimensionless maximum temperature rise calculation formula to obtain ΔT * The expression is:

[0056]

[0057] S515: When the ambient wind speed is 0, the dimensionless maximum temperature rise at the bottom of the bridge deck and the theoretical formula of the influencing factors are established by dimensional analysis method:

[0058] ΔT * =f(Q,D,h ef ,c p ,ρ0,T0,g)

[0059] S516: Select the three basic dimensions [M], [L], and [T], and take ρ0, D, T0, and g as basic physical quantities. The following dimensionless expression is obtained by the π theorem:

[0060]

[0061] S517: After dimensional consistency calculation, the dimensionless maximum temperature rise calculation formula when there is no wind is obtained:

[0062]

[0063] S518: Combine the dimensionless maximum temperature rise calculation formula when there is no wind to obtain ΔT when there is no wind * The expression is:

[0064]

[0065] S519: The dimensionless maximum temperature rise data of the bottom of the bridge deck under no wind and wind are respectively fitted with the corresponding theoretical formula established to obtain the dimensionless maximum temperature rise calculation formula of the bottom of the bridge deck:

[0066]

[0067] In the formula, u * is the dimensionless ventilation velocity;

[0068] The dimensionless maximum temperature rise data at the bottom of the bridge deck is predicted according to the above formula.

[0069] As a further preferred embodiment of the present invention, the step S52 includes the following steps:

[0070] S521: Divide the temperature time history curve of the bottom of the bridge deck under each fire scenario by the maximum temperature to obtain ratio data;

[0071] S522: normalizing the ratio data in step S521;

[0072] S523: Fit the data to obtain the fitting function formula of the fire temperature rise process:

[0073]

[0074] In the formula,

[0075] t is the fire duration, s;

[0076] ΔT t is the smoke temperature rise at the bottom of the bridge deck at the burning time t seconds, ℃;

[0077] ΔT max is the maximum temperature rise at the bottom of the bridge deck, °C;

[0078] S524: According to the above formula, predict the variation of smoke temperature rise at the bottom of the bridge deck with the duration of the fire.

[0079] As a further preferred embodiment of the present invention, the step S53 includes the following steps:

[0080] S531: normalizing the longitudinal bridge temperature distribution curves under each fire scenario;

[0081] S532: Use exponential function to fit the longitudinal bridge temperature distribution curve to obtain the dimensionless longitudinal temperature prediction formula and the reduction coefficient of the distance between the longitudinal bridge and the fire source center. Calculation formula:

[0082] ΔT x =ΔT max ×e -0.0687x

[0083]

[0084] In the formula,

[0085] x is the distance from the center of the fire source in the longitudinal direction of the bridge, m;

[0086] ΔT x is the smoke temperature rise at a distance x from the fire source in the longitudinal direction;

[0087] is the reduction factor of the distance from the center of the fire source in the longitudinal direction of the bridge;

[0088] S533: According to the dimensionless longitudinal temperature prediction formula, predict the change law of the smoke temperature rise in the longitudinal bridge direction with the distance from the fire source center.

[0089] As a further preferred embodiment of the present invention, the step S54 includes the following steps:

[0090] S541: Divide the temperature rise data in the height direction of the main beam affected by the fire by the maximum temperature at the bottom of the bridge deck;

[0091] S542: Fit the temperature data of the main beam height direction under various fire scenarios to obtain the temperature distribution prediction formula along the main beam height direction and the reduction coefficient of the distance from the bottom of the bridge deck. The calculation formula is:

[0092]

[0093]

[0094] Where:

[0095] z is the vertical distance from the bottom of the bridge deck, m;

[0096] ΔT z is the smoke temperature rise at a vertical distance z from the bottom of the bridge deck;

[0097] is the reduction factor for the distance from the bottom of the bridge deck;

[0098] S543: Predict the smoke temperature rise data in the height direction of the main beam affected by the fire according to the temperature distribution prediction formula along the height direction of the main beam.

[0099] As a further preferred embodiment of the present invention, in step S5, the temperature value of any position at any time on the bottom of the bridge is predicted according to the following formula:

[0100]

[0101] In the formula,

[0102] ΔT (Q,u,hef,x,z,t) is the temperature value at any position at any time on the bottom of the bridge, ℃.

[0103] Compared with the prior art, the beneficial effects achieved by the present invention include:

[0104] 1) The present invention provides a method for predicting the spatiotemporal distribution of the temperature field under an I-shaped composite beam bridge during a fire. For I-shaped composite beam bridges that are being promoted but are susceptible to fire, the distribution characteristics of the temperature field under the bridge during a fire under different types of burning vehicles, environmental wind speeds, and clear height conditions under the bridge are studied through numerical simulation. By determining the impact of different factors on the fire temperature field and further determining the spatiotemporal distribution law of the temperature field, the proximity of the prediction method to the actual fire scene is further improved, thereby further improving the prediction accuracy and prediction credibility of the prediction method.

[0105] 2) The present invention provides a method for predicting the spatiotemporal distribution of the temperature field of a fire under an I-shaped composite beam bridge. On the basis of determining the fire influencing factors and the spatiotemporal distribution laws, a method for predicting the spatiotemporal distribution of the temperature of a fire under an I-shaped composite beam bridge is proposed. The prediction method can predict the temperature value at any time and any position on the I-shaped composite beam bridge under fire scenarios such as different vehicle types, ambient wind speeds and clear heights under the bridge, thereby providing effective simulation prediction and prevention means for the safe use and fire prevention of the I-shaped composite beam bridge. BRIEF DESCRIPTION OF THE DRAWINGS

[0106] Figure 1 It is a flow chart of the steps of the prediction method provided by the present invention.

[0107] Figure 2 It is a schematic diagram of the cross-sectional structure of the I-shaped composite beam bridge provided by the present invention.

[0108] Figure 3 It is a schematic diagram of the flame morphology of the actual test of the present invention.

[0109] Figure 4 It is a schematic diagram of the flame morphology of the numerical simulation of the present invention.

[0110] Figure 5 It is a schematic diagram of temperature distribution in the horizontal direction from the fire source position of the present invention.

[0111] Figure 6 It is a schematic diagram of temperature distribution in the vertical direction from the fire source of the present invention.

[0112] Figure 7 It is the dimensionless maximum temperature rise prediction diagram of the present invention.

[0113] Figure 8 It is a normalized flue gas temperature rise curve diagram of the present invention.

[0114] Fig. 9 It is the dimensionless longitudinal temperature distribution diagram of the present invention.

[0115] Fig.10 It is a temperature distribution diagram along the height direction of the main beam exposed to fire according to the present invention. DETAILED DESCRIPTION

[0116] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. 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 creative work are within the scope of protection of the present invention.

[0117] In the description of the present invention, it should be noted that the terms "upper", "lower", "inner", "outer", "front end", "rear end", "two ends", "one end", "the other end" and the like indicate positions or positional relationships based on the positions or positional relationships shown in the drawings, and are only for the convenience of describing the present invention 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 limiting the present invention. In addition, the terms "first" and "second" are used for descriptive purposes only and cannot be understood as indicating or implying relative importance.

[0118] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "provided with", "connected", etc. should be understood in a broad sense. For example, "connected" 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 it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0119] [First embodiment]

[0120] like Figure 1-10 FIG. 1 is a method for predicting the temporal and spatial distribution of the fire temperature field under an I-shaped composite beam bridge provided by the first embodiment of the present invention. Figure 1 As shown, the following steps are included:

[0121] S1: In the simulation software (the FDS fire dynamic simulator is used in this embodiment), an I-shaped composite beam bridge model is established and the boundary conditions of the model are set; Figure 2 As shown, the I-shaped composite beam bridge model established in this embodiment includes a main beam 1. The main beam 1 in this embodiment includes a first main beam 11 located in the middle position and a second main beam 12 and a third main beam 13 on both sides of the position. Part of the bottom of the main beam 1 is supported on a support, the top of the main beam 1 is fixedly connected to the bridge deck 2, and adjacent main beams are connected by a cross beam 3. A longitudinal beam 4 is arranged between the bottom of the bridge deck and the top of the cross beam, and stiffening ribs 5 are arranged on both sides of the web of the main beam;

[0122] The boundary conditions of the model are as follows: the bridge deck is made of concrete, the main beam, cross beam and longitudinal beam are made of Q370qD steel, and the surfaces of each material are allowed to exchange heat with the air freely. In addition, the simulation area is surrounded by open boundaries except the ground, so that air and heat can freely enter and exit the simulation area. The initial ambient temperature of the simulation area is 20°C, and the initial ambient pressure is 0.1013Mpa.

[0123] S2: Setting fire scenario operating parameters; the fire scenario operating parameters at least include vehicle type, clear height under the bridge and ambient wind speed; wherein, the vehicle types are set to include small cars, large buses, heavy trucks and tankers; the clear height under the bridge is set to satisfy: the clear height under the bridge is not greater than 22.5m; the ambient wind speed is set to satisfy: the ambient wind speed is not greater than 3m / s.

[0124] The combustibles of fires under bridges are mainly passing vehicles. According to the different scales of fires caused by vehicles, the fire sources under bridges can be divided into four categories: small cars, large buses, heavy trucks and tank trucks. The heat release rate, fire source size and heat release rate per unit area of ​​fires of different vehicle types are different. Among them, the heat release rate and heat release rate per unit area of ​​fires caused by small cars are the lowest, while the heat release rate and heat release rate per unit area of ​​fires caused by tank trucks are the highest; the growth time of fires caused by small cars is 10 minutes, and the fire growth coefficient is between medium-speed fire and rapid fire; the growth time of fires caused by other types of vehicles is 15 minutes, and the fire growth coefficient is between rapid fire and ultra-rapid fire.

[0125] Although the location of a fire under a bridge caused by a traffic accident, mechanical failure, etc. is arbitrary, the fire resistance of the bridge structure is mainly controlled by the highest temperature field that the cross-section with the lowest safety reserve (maximum load ratio) may be subjected to. In order to reflect the most adverse impact of the fire on the bridge structure, the longitudinal position of the fire source is set at the mid-span cross-section of the bridge, and the transverse position is set directly below the first main beam 11.

[0126] Ambient wind includes longitudinal wind and transverse wind. Longitudinal wind will cause the cross section where the highest temperature is located to shift longitudinally and reduce the fire temperature, which is beneficial to the structure compared with the windless condition. Transverse wind will change the temperature field distribution of the cross section where the highest temperature is located, and its impact on the structure compared with the windless condition is unknown. Therefore, in order to reflect the most adverse impact of fire on the bridge structure, this embodiment only analyzes the fire temperature field under the action of transverse wind.

[0127] The greater the clear height under the bridge, the farther the bridge superstructure is from the fire source, and the weaker the high temperature effect of the fire. When the wind speed reaches 3m / s and the clear height under the bridge is 22.5m, the maximum temperature of the lower part of the bridge deck will be lower than 300℃. Therefore, in order to reflect the most adverse impact of fire on the bridge structure, the fire scenario operating condition parameters in this embodiment need to be set to meet the following requirements: the wind speed is not greater than 3m / s and the clear height under the bridge is not greater than 22.5m.

[0128] S3: Perform meshing and verify the simulation method;

[0129] In this embodiment, in order to further improve the accuracy of the simulation results, it is necessary to set the grid size between [D * / 16,D * / 4]. Therefore, it is necessary to determine the characteristic diameter of the fire source D * In this embodiment, the characteristic diameter of the fire source is D * Calculate according to the following formula:

[0130]

[0131] Where:

[0132] D * is the characteristic diameter of the fire source, m;

[0133] Q is the heat release rate of the fire source, kW;

[0134] ρ0 is the ambient air density, kg / m 3 ;

[0135] c P is the specific heat of ambient air, kJ / (kg·K);

[0136] T0 is the ambient temperature, K;

[0137] g is the acceleration due to gravity, m / s 2 ;

[0138] When verifying the simulation method, numerical simulation and actual test comparison are used to verify the accuracy of the numerical simulation method. Appropriate actual tests can be selected according to the actual scenario. In this embodiment, the steel-concrete composite bridge fire test proposed by Also-Moya et al. is used to verify the numerical model of this article. Test 1 and Test 2, Test 3 and Test 4 are repeated tests of the same fire scene, so two simulation analyses are required.

[0139] like Figure 3 The actual test flame shape is shown in Figure 4 The figure shows the flame morphology of the numerical simulation. By comparing the two, it is confirmed that the flame morphology is consistent;

[0140] like Figure 5 The figure shows the temperature distribution diagram in the horizontal direction from the fire source. Figure 6 The figure shows a schematic diagram of the temperature distribution in the vertical direction from the fire source. By comparing the two, it is determined that there is a deviation between the simulated values ​​of the flame temperature at each position from the fire source and the corresponding test values, but the deviation is lower than the preset threshold, so the simulation method is verified to be successful; the preset threshold can be set accordingly according to the actual prediction accuracy requirements. In this embodiment, the preset threshold can be set to 15%; the reason for the difference between the simulated value and the corresponding test value may be the influence of weak ambient wind during the test, or it may be that the heat release rate of the oil pool used in the simulation process deviates from the actual value.

[0141] S4: Determine the temporal and spatial distribution patterns of fire influencing factors;

[0142] In this embodiment, the determined influencing factors include at least vehicle type, clear height under the bridge and ambient wind speed, and satisfy the following conditions:

[0143] The larger the fire size caused by the vehicle type, the smaller the lateral wind speed and the clear height under the bridge, and the higher the fire temperature;

[0144] The smaller the fire size caused by the vehicle type, the greater the lateral wind speed and the clear height under the bridge, and the lower the fire temperature;

[0145] For example, when there is no wind, the clear height under the bridge is 5m, and there is a fire in a tanker truck, the temperature of the lower flange of the main beam directly affected by the flame can reach 1200℃, and the temperature of the bottom of the bridge deck can reach 1000℃. When the lateral wind speed is greater than 3m / s or the clear height under the bridge is greater than 22.5m, the temperature of the lower flange of the main beam and the bottom of the bridge deck are both lower than 300℃.

[0146] The determined temporal and spatial distribution patterns of fires include at least:

[0147] The temperature of the part directly affected by the flame is the highest, and the temperature of the bottom of the bridge deck above the flame is the second highest;

[0148] As smoke spreads and settles around the bottom of the bridge deck, its temperature gradually decreases;

[0149] The temperature at the bottom of the bridge deck is evenly distributed in each transverse area divided by the main beam and the longitudinal beam, and decays exponentially along the longitudinal direction with the increase of the distance from the center of the fire source.

[0150] S5: Predict the fire smoke temperature. When a fire occurs under a bridge, the fire smoke moves upwards driven by the thermal buoyancy generated by the fire source, and diffuses to the surroundings after reaching the bottom of the bridge deck. Therefore, the maximum temperature rise at the bottom of the bridge deck is an important parameter for fire resistance analysis of bridge structures. According to the analysis of the distribution characteristics of the temperature field of the fire under the bridge under the above-mentioned different influencing factors and the temporal and spatial distribution data of the temperature field, the maximum temperature rise prediction formula at the bottom of the bridge deck is first proposed. Then, the maximum temperature time history curve at the bottom of the bridge deck, the temperature at each position in the longitudinal direction of the bridge, and the temperature along the height direction of the main beam affected by the fire are divided by the maximum temperature at the bottom of the bridge deck to obtain the dimensionless maximum temperature rise, dimensionless longitudinal temperature distribution, and dimensionless temperature along the height direction of the main beam. Finally, with the maximum temperature at the bottom of the bridge deck as the core indicator, the smoke temperature rise formula at the bottom of the bridge deck when the burning time is t seconds, the temperature reduction coefficient of the distance from the center of the fire source in the longitudinal direction of the bridge, and the reduction coefficient of the distance from the height direction of the main beam affected by the fire to the bottom of the bridge deck are integrated to obtain the temporal and spatial distribution prediction formula of the fire temperature field under the bridge.

[0151] The specific steps include:

[0152] S51: Calculate the dimensionless maximum temperature rise data at the bottom of the bridge deck by simulation;

[0153] The following steps are involved:

[0154] S511: Maximum temperature rise ΔT max Defined as the maximum temperature T max The difference between the ambient temperature T0 and ΔT max / T0 is defined as the dimensionless maximum temperature rise ΔT * , the dimensional analysis method is used to establish the theoretical formula of the dimensionless maximum temperature rise and influencing factors at the bottom of the bridge deck:

[0155] ΔT * =f(Q,D,u,h ef ,c p ,ρ0,T0,g)

[0156] Where: ΔT * is the dimensionless maximum temperature rise at the bottom of the bridge deck; Q is the heat release rate of the fire source, kW; D is the equivalent diameter of the fire source, m; u is the ambient wind speed, m / s; h ef is the effective height under the bridge, i.e. the vertical distance between the fuel surface and the bottom of the bridge deck, m; c P is the specific heat of ambient air, kJ / (kg·K); ρ0 is the density of ambient air, kg / m 3 ; T0 is the ambient temperature, K; g is the acceleration due to gravity, m / s 2 ;

[0157] S512: Select [M], [L], [T], [θ] as four basic dimensions, and use ρ0, D, T0, and g as basic physical quantities. The following dimensionless expression is obtained by the π theorem:

[0158]

[0159] In the formula,

[0160] π1, π2, π3 and π4 represent 4 dimensional expressions respectively;

[0161] α 1-4 , β 1-4 , γ 1-4 and ε 1-4 denote coefficients respectively;

[0162] S513: After dimension consistency calculation, the dimensionless maximum temperature rise calculation formula is obtained:

[0163]

[0164] S514: Combine the dimensionless maximum temperature rise calculation formula to obtain ΔT * The expression is:

[0165]

[0166] S515: When the ambient wind speed is 0, the dimensionless maximum temperature rise at the bottom of the bridge deck and the theoretical formula of the influencing factors are established by dimensional analysis method:

[0167] ΔT * =f(Q,D,h ef ,c p ,ρ0,T0,g)

[0168] S516: Select the three basic dimensions [M], [L], and [T], and take ρ0, D, T0, and g as basic physical quantities. The following dimensionless expression is obtained by the π theorem:

[0169]

[0170] S517: After dimensional consistency calculation, the dimensionless maximum temperature rise calculation formula when there is no wind is obtained:

[0171]

[0172] S518: Combine the dimensionless maximum temperature rise calculation formula when there is no wind to obtain ΔT when there is no wind * The expression is:

[0173]

[0174] S519: The dimensionless maximum temperature rise data of the bottom of the bridge deck under no wind and wind are fitted with the corresponding theoretical formula established to obtain the dimensionless maximum temperature rise calculation formula of the bottom of the bridge deck, such as Figure 7 As shown:

[0175]

[0176] In the formula, u * is the dimensionless ventilation velocity;

[0177] The dimensionless maximum temperature rise prediction diagram is as follows Figure 7 As shown, u * When it is less than 0.19, the ventilation airflow has no effect on the flame plume. At this time, it is believed that the plume mass flow rate will not increase, and the ventilation airflow has little effect on the fire temperature field. In addition, u * When it is greater than 0.19, the flame is deflected, and the ventilation airflow induces additional air to be entrained into the flame plume, causing the plume mass flow to increase with the increase of ventilation speed. At this time, the ventilation airflow significantly affects the fire temperature field. * Determine whether ventilation airflow affects the fire temperature field;

[0178] According to the above formula, the relationship between the dimensionless maximum temperature rise data at the bottom of the bridge deck and various influencing factors is obtained;

[0179] S52: Predicting the variation of smoke temperature rise at the bottom of the bridge deck with the duration of the fire; including the following steps:

[0180] S521: Divide the temperature time history curve of the bottom of the bridge deck under each fire scenario by the maximum temperature to obtain ratio data;

[0181] S522: Normalize the ratio data in step S521 to better observe the law between the data. The normalized flue gas temperature rise curve is as follows: Figure 8 As shown;

[0182] S523: Fit the data to obtain the fitting function formula of the fire temperature rise process:

[0183]

[0184] In the formula,

[0185] t is the fire duration, s;

[0186] ΔT t is the smoke temperature rise at the bottom of the bridge deck at the burning time t seconds, ℃;

[0187] ΔT max is the maximum temperature rise at the bottom of the bridge deck, °C;

[0188] S524: According to the above formula, predict the variation of smoke temperature rise at the bottom of the bridge deck with the duration of the fire.

[0189] S53: Predicting the variation of the smoke temperature rise in the longitudinal direction of the bridge with the distance from the center of the fire source; including the following steps:

[0190] S531: Normalize the longitudinal temperature distribution curves of the bridge under each fire scenario; under each fire scenario, the longitudinal temperature of the bottom of the bridge deck is symmetrically distributed. As the distance from the fire source increases, the overall temperature decreases, and the farther away from the fire source, the slower the temperature decreases. The temperature decay trend conforms to the exponential decay law, such as Fig. 9 As shown;

[0191] S532: Use exponential function to fit the longitudinal bridge temperature distribution curve to obtain a more accurate dimensionless longitudinal temperature prediction formula and the reduction coefficient of the distance between the longitudinal bridge and the fire source center. Calculation formula:

[0192] ΔT x =ΔT max ×e -0.0687x

[0193]

[0194] In the formula,

[0195] x is the distance from the center of the fire source in the longitudinal direction of the bridge, m;

[0196] ΔT x is the smoke temperature rise at a distance x from the fire source in the longitudinal direction;

[0197] is the reduction factor of the distance from the center of the fire source in the longitudinal direction of the bridge;

[0198] S533: According to the dimensionless longitudinal temperature prediction formula, predict the change law of the smoke temperature rise in the longitudinal bridge direction with the distance from the fire source center.

[0199] S54: predicting the smoke temperature rise data in the height direction of the main beam affected by the fire; including the following steps:

[0200] S541: Divide the temperature rise data in the height direction of the main beam affected by the fire by the maximum temperature at the bottom of the bridge deck;

[0201] S542: When the lower flange of the main beam under fire is not out of the direct effect of the flame, the temperature in the height direction of the main beam can be regarded as linear decay away from the lower flange of the main beam, and then evenly distributed after decreasing to the highest temperature at the bottom of the bridge deck. The temperature data in the height direction of the main beam under various fire scenarios are fitted to obtain the temperature distribution prediction formula along the height direction of the main beam and the reduction coefficient of the distance from the bottom of the bridge deck. The calculation formula is as follows: Fig.10 As shown:

[0202]

[0203] Where:

[0204] z is the vertical distance from the bottom of the bridge deck, m;

[0205] ΔT z is the smoke temperature rise at a vertical distance z from the bottom of the bridge deck;

[0206] is the reduction factor for the distance from the bottom of the bridge deck;

[0207] S543: Predict the smoke temperature rise data in the height direction of the main beam affected by the fire according to the temperature distribution prediction formula along the height direction of the main beam.

[0208] S55: Predicting the temperature value of any position at any time at the bottom of the bridge. In this embodiment, the temperature value of any position at any time at the bottom of the bridge is predicted according to the following formula:

[0209]

[0210] In the formula,

[0211] ΔT (Q,u,hef,x,z,t) is the temperature value at any position at any time on the bottom of the bridge, ℃.

[0212] It will be apparent to those skilled in the art that the invention is not limited to the details of the exemplary embodiments described above and that the invention can be implemented in other specific forms without departing from the spirit or essential features of the invention. Therefore, the embodiments should be considered exemplary and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all variations falling within the meaning and scope of the equivalent elements of the claims be included in the invention. Any reference numeral in a claim should not be considered as limiting the claim to which it relates.

Claims

1. A method for predicting the temporal and spatial distribution of the fire temperature field under an I-shaped composite beam bridge, characterized by: The following steps are involved: S1: Establish an I-shaped composite beam bridge model and set the boundary conditions of the model; S2: Set fire scene working condition parameters; S3: Perform meshing and verify the simulation method; S4: Determine the temporal and spatial distribution patterns of fire influencing factors; S5: Predict fire smoke temperature, including: S51: Calculate the dimensionless maximum temperature rise data at the bottom of the bridge deck by simulation; S52: Predict the variation of smoke temperature rise at the bottom of the bridge deck with fire duration; S53: Predict the variation of the longitudinal bridge smoke temperature rise with the distance from the fire center; S54: Predict the smoke temperature rise data in the height direction of the main beam affected by fire; S55: Predict the temperature value at any location at any time on the bottom of the bridge.

2. The method for predicting the temporal and spatial distribution of the fire temperature field under an I-shaped composite beam bridge according to claim 1 is characterized by: In the step S1, The established I-shaped composite beam bridge model includes a main beam, the bottom of the main beam is supported on the ground, the top of the main beam is fixedly connected to the bridge deck, adjacent main beams are connected by cross beams, a longitudinal beam is arranged between the bottom of the bridge deck and the top of the cross beam, and a stiffening rib is arranged inside the main beam; The boundary conditions of the model are as follows: the bridge deck is made of concrete, the main beam, cross beam and longitudinal beam are made of steel, and the surfaces of each material are allowed to exchange heat freely with the air. In addition, the simulation area is surrounded by open boundaries except the ground, so that air and heat can freely enter and exit the simulation area. The initial ambient temperature of the simulation area is 20°C, and the initial ambient pressure is 0.1013Mpa.

3. The method for predicting the temporal and spatial distribution of the fire temperature field under an I-shaped composite beam bridge according to claim 1 is characterized by: In step S2, the fire scene operating condition parameters include at least vehicle type, clear height under the bridge and ambient wind speed; and wherein the vehicle type is set to include small cars, large buses, heavy trucks and tankers; the clear height under the bridge is set to satisfy: the clear height under the bridge is not greater than 22.5m; the ambient wind speed is set to satisfy: the ambient wind speed is not greater than 3m / s.

4. The method for predicting the temporal and spatial distribution of the fire temperature field under an I-shaped composite beam bridge according to claim 1 is characterized by: In step S3, according to the characteristic diameter D of the fire source * Determine the grid size, where the characteristic diameter of the fire source D * Calculate according to the following formula: Where: D * is the characteristic diameter of the fire source, m; Q is the heat release rate of the fire source, kW; ρ0 is the ambient air density, kg / m 3 ; c P is the specific heat of ambient air, kJ / (kg·K); T0 is the ambient temperature, K; g is the acceleration due to gravity, m / s 2 ; And the grid size is between [D * / 16,D * / 4]; The simulation method is verified by comparing numerical simulation with actual experiments. The simulation method is deemed to have passed the verification if and only if the deviation between the simulated value of the flame temperature at each position away from the fire source and the corresponding test value is lower than the preset threshold.

5. The method for predicting the temporal and spatial distribution of the fire temperature field under an I-shaped composite beam bridge according to claim 1 is characterized by: In the step S4, The influencing factors to be determined include at least the vehicle type, the clear height under the bridge and the ambient wind speed, and they must meet the following requirements: The larger the fire size caused by the vehicle type, the smaller the lateral wind speed and the clear height under the bridge, and the higher the fire temperature; The smaller the fire size caused by the vehicle type, the greater the lateral wind speed and the clear height under the bridge, and the lower the fire temperature; The determined temporal and spatial distribution patterns of fires include at least: The temperature of the part directly affected by the flame is the highest, and the temperature of the bottom of the bridge deck above the flame is the second highest; As smoke spreads and settles around the bottom of the bridge deck, its temperature gradually decreases; The temperature at the bottom of the bridge deck is evenly distributed in each transverse area divided by the main beam and the longitudinal beam, and decays exponentially along the longitudinal direction with the increase of the distance from the center of the fire source.

6. The method for predicting the temporal and spatial distribution of the fire temperature field under an I-shaped composite beam bridge according to claim 1 is characterized by: The step S51 includes the following steps: S511: Maximum temperature rise ΔT max Defined as the maximum temperature T max The difference between the ambient temperature T0 and ΔT max / T0 is defined as the dimensionless maximum temperature rise ΔT * , the dimensional analysis method is used to establish the theoretical formula of the dimensionless maximum temperature rise and influencing factors at the bottom of the bridge deck: ΔT * =f(Q,D,u,h ef ,c p ,ρ0,T0,g) Where: ΔT * is the dimensionless maximum temperature rise at the bottom of the bridge deck; Q is the heat release rate of the fire source, kW; D is the equivalent diameter of the fire source, m; u is the ambient wind speed, m / s; h ef is the effective height under the bridge, i.e. the vertical distance between the fuel surface and the bottom of the bridge deck, m; c P is the specific heat of ambient air, kJ / (kg·K); ρ0 is the density of ambient air, kg / m 3 ; T0 is the ambient temperature, K; g is the acceleration due to gravity, m / s 2 ; S512: Select [M], [L], [T], [θ] as four basic dimensions, and use ρ0, D, T0, and g as basic physical quantities. The following dimensionless expression is obtained by the π theorem: π1, π2, π3 and π4 represent 4 dimensional expressions respectively; α 1-4 , β 1-4 , γ 1-4 and ε 1-4 denote coefficients respectively; S513: After dimension consistency calculation, the dimensionless maximum temperature rise calculation formula is obtained: S514: Combine the dimensionless maximum temperature rise calculation formula to obtain ΔT * The expression is: S515: When the ambient wind speed is 0, the dimensionless maximum temperature rise at the bottom of the bridge deck and the theoretical formula of the influencing factors are established by dimensional analysis method: ΔT * =f(Q,D,h ef ,c p ,ρ0,T0,g) S516: Select the three basic dimensions [M], [L], and [T], and take ρ0, D, T0, and g as basic physical quantities. The following dimensionless expression is obtained by the π theorem: S517: After dimensional consistency calculation, the dimensionless maximum temperature rise calculation formula when there is no wind is obtained: S518: Combine the dimensionless maximum temperature rise calculation formula when there is no wind to obtain ΔT when there is no wind * The expression is: S519: The dimensionless maximum temperature rise data of the bottom of the bridge deck under no wind and wind are respectively fitted with the corresponding theoretical formula established to obtain the dimensionless maximum temperature rise calculation formula of the bottom of the bridge deck: In the formula, u * is the dimensionless ventilation velocity; The dimensionless maximum temperature rise data at the bottom of the bridge deck is predicted according to the above formula.

7. The method for predicting the temporal and spatial distribution of the fire temperature field under an I-shaped composite beam bridge according to claim 1 is characterized by: The step S52 includes the following steps: S521: Divide the temperature time history curve of the bottom of the bridge deck under each fire scenario by the maximum temperature to obtain ratio data; S522: normalizing the ratio data in step S521; S523: Fit the data to obtain the fitting function formula of the fire temperature rise process: In the formula, t is the fire duration, s; ΔT t is the smoke temperature rise at the bottom of the bridge deck at the burning time t seconds, ℃; ΔT max is the maximum temperature rise at the bottom of the bridge deck, °C; S524: According to the above formula, predict the variation of smoke temperature rise at the bottom of the bridge deck with the duration of the fire.

8. The method for predicting the temporal and spatial distribution of the fire temperature field under an I-shaped composite beam bridge according to claim 7, characterized in that: The step S53 includes the following steps: S531: normalizing the longitudinal bridge temperature distribution curves under each fire scenario; S532: Use exponential function to fit the longitudinal bridge temperature distribution curve to obtain the dimensionless longitudinal temperature prediction formula and the reduction coefficient of the distance between the longitudinal bridge and the fire source center. Calculation formula: ΔT x =ΔT max ×e -0.0687x In the formula, x is the distance from the center of the fire source in the longitudinal direction of the bridge, m; ΔT x is the smoke temperature rise at a distance x from the fire source in the longitudinal direction; is the reduction factor of the distance from the fire source center in the longitudinal direction of the bridge; S533: According to the dimensionless longitudinal temperature prediction formula, predict the change law of the smoke temperature rise in the longitudinal bridge direction with the distance from the fire source center.

9. The method for predicting the temporal and spatial distribution of the fire temperature field under an I-shaped composite beam bridge according to claim 8, characterized in that: The step S54 includes the following steps: S541: Divide the temperature rise data in the height direction of the main beam affected by the fire by the maximum temperature at the bottom of the bridge deck; S542: Fit the temperature data of the main beam height direction under various fire scenarios to obtain the temperature distribution prediction formula along the main beam height direction and the reduction coefficient of the distance from the bottom of the bridge deck. The calculation formula is: Where: z is the vertical distance from the bottom of the bridge deck, m; ΔT z is the smoke temperature rise at a vertical distance z from the bottom of the bridge deck; is the reduction factor for the distance from the bottom of the bridge deck; S543: Predict the smoke temperature rise data in the height direction of the main beam affected by the fire according to the temperature distribution prediction formula along the height direction of the main beam.

10. The method for predicting the temporal and spatial distribution of the fire temperature field under an I-shaped composite beam bridge according to claim 9, characterized in that: In step S5, the temperature value of any position at any time on the bottom of the bridge is predicted according to the following formula: In the formula, is the temperature value at any position at any time on the bottom of the bridge, ℃.

Images