Method for evaluating damage of concrete box girder under fire high-temperature action

Abstract

The invention provides a concrete box girder damage assessment method under fire high temperature, and belongs to the technical field of road and bridge detection, and the method comprises the steps: determining the combustion time and fire scene temperature of a combustion source based on site fire investigation information and concrete apparent state; according to the combustion time and the fire scene temperature, the highest concrete temperature, the highest steel bar temperature and the highest steel strand temperature of the concrete box girder are determined; calculating the elasticity modulus and compressive strength of the concrete after the high temperature according to the highest temperature of the concrete; and determining the actual elastic modulus of the steel bar according to the highest temperature of the steel bar, and determining the actual elastic modulus of the steel strand according to the highest temperature of the steel strand. According to the method for evaluating the damage of the concrete box girder under the high-temperature action of the fire disaster, the fire scene condition can be evaluated more accurately, and then a scientific and systematic basis is provided for reinforcement and repair of the concrete box girder structure after the high temperature.

Description

Technical Field

[0001] The present application relates to road and bridge inspection technology, and in particular to a method for assessing damage to concrete box girders under high temperature in a fire. Background Art

[0002] Under the high temperature of fire, concrete box girders will experience quality problems such as concrete spalling, reduced strength of concrete and steel bars, and deterioration of steel strand performance after high temperature, causing serious damage to the concrete box girder structure. In extreme cases, the structure may be damaged or collapsed, bringing serious impact on line operations.

[0003] The material and structural properties of concrete box girder structures deteriorate after being exposed to high temperatures in fire. The relevant technologies are not accurate enough in judging the damage status of concrete box girders after fire, and there is no systematic method to evaluate the structural status of concrete box girders. As a result, there is a lack of technical support for the repair and reinforcement of concrete box girder structures after being exposed to high temperatures in fire, which affects the normal repair and reinforcement work. Summary of the invention

[0004] The present application provides a method for assessing the damage of a concrete box girder under high temperature in a fire, so as to solve the technical problem in the related art that the damage state of the concrete box girder after a fire is not accurately judged.

[0005] The present application provides a method for assessing damage of a concrete box girder under high temperature in a fire, comprising:

[0006] Determine the burning time and fire temperature of the combustion source based on on-site fire investigation information and the apparent state of concrete;

[0007] Determine the maximum temperature of concrete, the maximum temperature of steel bars and the maximum temperature of steel strands of the concrete box girder according to the combustion time and the fire scene temperature;

[0008] Calculate the elastic modulus and compressive strength of the concrete after high temperature according to the maximum temperature of the concrete;

[0009] The actual elastic modulus of the steel bar is determined according to the maximum temperature of the steel bar, and the actual elastic modulus of the steel strand is determined according to the maximum temperature of the steel strand.

[0010] In a possible implementation, the on-site fire investigation information includes the cause of the fire, the characteristics of the main combustion materials, the duration of combustion, the characteristics and duration of the explosion, the fire extinguishing method, and the residual state of the fire;

[0011] The concrete appearance state includes the appearance state of the concrete box beam structure after high temperature, and the color and breakage of the surface of the concrete box beam structure after high temperature.

[0012] In a possible implementation manner, the elastic modulus of the high-temperature concrete is calculated according to the maximum temperature of the concrete, and is calculated by the following formula:

[0013] E cT / E c =0.83-0.0011θ,60℃≤θ<700℃

[0014] Where, θ is the temperature; E cT is the actual elastic modulus of concrete after high temperature; E c It is the design elastic modulus of concrete when it is not subjected to high temperature.

[0015] In a possible implementation manner, the compressive strength of the concrete after high temperature is calculated according to the maximum temperature of the concrete, and is calculated by the following formula:

[0016]

[0017] Where θ is the temperature; f cT is the actual compressive strength of concrete after high temperature, f c It is the design compressive strength of concrete when it is not subjected to high temperature.

[0018] In a possible implementation, the actual elastic modulus of the steel bar is determined according to the maximum temperature of the steel bar, and the actual elastic modulus of the steel strand is determined according to the maximum temperature of the steel strand, which is calculated by the following formula:

[0019]

[0020] Where, θ is the maximum temperature of the steel bar / strand; E sT E is the elastic modulus of the steel bar / strand after high temperature; s It is the design elastic modulus of the steel bar / strand when it has not experienced high temperature.

[0021] In a possible implementation, it further includes:

[0022] Determine the yield strength decay curve of steel bars / strands after high temperature;

[0023] Substitute the maximum temperature of the steel bar / steel strand into the yield strength attenuation curve of the steel bar / steel strand after high temperature to determine the yield strength coefficient of the steel bar / steel strand after high temperature.

[0024] In a possible implementation, it further includes:

[0025] Performing structural state evaluation calculation on the concrete box girder structure after high temperature, the structural state evaluation calculation includes an apparent characteristic inspection unit, a crack state inspection unit after high temperature, a concrete strength inspection unit, a protective wall steel bar strength inspection unit and a steel strand strength inspection unit, and each inspection unit corresponds to a structural state damage coefficient;

[0026] Determine a dynamic weighting coefficient, and perform dynamic weighting calculations in sequence according to the dynamic weighting coefficient and the structural state damage coefficient to obtain a plurality of structural state assessment results; wherein the dynamic weighting coefficients corresponding to different inspection units have different proportions;

[0027] A structural damage level is determined according to a plurality of the structural state assessment results.

[0028] In a possible implementation, the dynamic weighted coefficients corresponding to the surface characteristic inspection unit, the post-high temperature crack state inspection unit, the concrete strength inspection unit, the protective wall steel bar strength inspection unit and the steel strand strength inspection unit respectively account for 10%, 10%, 20%, 20% and 30%.

[0029] In a possible implementation, it further includes:

[0030] Establishing a finite element model of the concrete box girder structure, and determining a theoretical deflection value and a theoretical strain value of the concrete box girder that has not been subjected to high temperature;

[0031] Determine the maximum deflection test value and maximum strain test value of the concrete box girder structure after high temperature;

[0032] The safety factor of the concrete box girder is calculated based on the theoretical deflection value, theoretical strain value, maximum deflection test value and maximum strain test value, and the comprehensive damage state of the concrete box girder is obtained by referring to the factor and structural damage level.

[0033] In a possible implementation, a preset load is linearly applied step by step to the concrete box girder structure after high temperature to test and obtain the maximum deflection test value and the maximum strain test value of the concrete box girder structure after high temperature.

[0034] The damage assessment method for concrete box girders under high temperature in fire provided by this application, through careful on-site investigation, combined with the intuitive damage of concrete, accurately estimates the extreme conditions of the fire scene, and then scientifically evaluates the performance degradation of materials under high temperature; in addition, using accurate temperature data, the mechanical property changes of concrete and steel bars are calculated to ensure the comprehensiveness and accuracy of the assessment. Therefore, this application not only accelerates the decision-making process of post-disaster recovery and optimizes resource allocation, but also strengthens the scientific management of bridge structure safety, provides a solid scientific basis for ensuring the stable operation of transportation infrastructure, and effectively ensures the use of structures and personnel safety. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present application and, together with the description, serve to explain the principles of the present application.

[0036] Figure 1 This is a schematic flow chart of the steps of a method for assessing damage of a concrete box girder under high temperature in a fire in an embodiment of the present application;

[0037] Figure 2 This is a schematic cross-sectional view of a concrete box girder structure in an embodiment of the present application;

[0038] Figure 3 This is a schematic diagram of the key inspection area of ​​the concrete box girder structure in the embodiment of the present application;

[0039] Figure 4 A schematic diagram of the position of applied loads for the concrete box beam structure in the embodiment of the present application;

[0040] Figure 5 It is a schematic diagram of the finite element model of the concrete box girder structure in the embodiment of the present application.

[0041] The above drawings have shown clear embodiments of the present application, which will be described in more detail later. These drawings and text descriptions are not intended to limit the scope of the present application in any way, but to illustrate the concept of the present application to those skilled in the art by referring to specific embodiments. DETAILED DESCRIPTION

[0042] In order to make the purpose, technical scheme and advantages of the present application clearer, the technical scheme in the embodiment of the present application will be described in more detail below in conjunction with the drawings in the preferred embodiments of the present application. In the drawings, the same or similar reference numerals throughout represent the same or similar parts or parts with the same or similar functions. The described embodiments are part of the embodiments of the present application, not all of the embodiments. The embodiments described below with reference to the drawings are exemplary and are intended to be used to explain the present application, and should not be construed as limitations on the present application. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present application. The embodiments of the present application are described in detail below in conjunction with the drawings.

[0043] In the description of this application, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, or it can be an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0044] In the description of the present application, it should be understood that the terms "upper", "lower", "front", "back", "left", "right", etc. indicate directions or positional relationships based on the directions or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific direction, be constructed and operated in a specific direction, and therefore should not be understood as a limitation on the present application.

[0045] The terms "first", "second", "third", "fourth", etc. in the specification and claims of the present application and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the numbers used in this way can be interchanged where appropriate, so that the embodiments of the present application described herein can, for example, be implemented in an order other than those illustrated or described herein.

[0046] In addition, the terms "comprises," "comprising," and "having," and any variations thereof, are intended to cover non-exclusive inclusions. For example, a process, method, system, product, or apparatus that includes a series of steps or elements is not necessarily limited to those steps or elements explicitly listed, but may include other steps or elements not explicitly listed or inherent to such process, method, product, or apparatus.

[0047] As mentioned in the background technology, in the field of road and bridge engineering, when faced with damage to concrete box girder structures caused by extreme events such as fire, traditional damage assessment methods often rely on empirical judgment and limited on-site observations, which are limited in accuracy and scientificity. In the past, due to the lack of a systematic assessment process, it was difficult to accurately assess the specific impact of high temperature on the elastic modulus, compressive strength, and performance of steel bars and strands of concrete, resulting in inaccurate repair decisions, which could underestimate structural damage and increase safety hazards, or lead to excessive repairs and waste of resources.

[0048] In addition, the fire temperature estimation is mostly based on general models, which do not fully consider the complexity of the specific conditions on site, such as the diversity of materials and the non-uniform heat conduction effect of the structure. For the performance changes of concrete and metal materials at high temperatures, it is usually necessary to rely on the simple extrapolation of standard experimental data, ignoring the interaction of multiple factors under actual fire conditions.

[0049] The concrete box girder damage assessment method under high temperature fire provided in the embodiment of the present application realizes a closed loop from the burning time to the accurate calculation of the maximum temperature of the material, and then to the scientific evaluation of the mechanical properties by combining detailed on-site investigation with in-depth analysis of the high temperature response of the material. This method not only takes into account the microscopic changes of the structure, but also integrates the thermal behavior of the structure as a whole, ensuring the comprehensiveness and accuracy of the assessment. In addition, the method can also provide customized damage assessment, provide solid data support for subsequent repair strategies, effectively shorten the bridge recovery cycle, and reduce maintenance costs.

[0050] Based on the above related description, an embodiment of the present application provides a method for assessing damage of a concrete box girder under high temperature in a fire, which is described below with reference to the accompanying drawings.

[0051] like Figure 1 and Figure 2 As shown, the method for assessing damage of a concrete box girder under high temperature in a fire provided by an embodiment of the present application comprises the following steps:

[0052] Step 100, determining the burning time of the combustion source and the fire scene temperature based on the on-site fire investigation information and the apparent state of the concrete;

[0053] Step 200, determining the maximum temperature of concrete, the maximum temperature of steel bars and the maximum temperature of steel strands of the concrete box girder according to the combustion time and the fire scene temperature;

[0054] Step 300, calculating the elastic modulus and compressive strength of the concrete after high temperature according to the maximum temperature of the concrete;

[0055] Step 400, determining the actual elastic modulus of the steel bar according to the maximum temperature of the steel bar, and determining the actual elastic modulus of the steel strand according to the maximum temperature of the steel strand.

[0056] From the above description, it can be seen that the damage assessment method for concrete box girders under high temperature in fire provided by this application, through careful on-site investigation, combined with the intuitive damage of concrete, accurately estimates the extreme conditions of the fire scene, and then scientifically evaluates the performance degradation of materials under high temperature; in addition, using accurate temperature data, the mechanical property changes of concrete and steel bars are calculated to ensure the comprehensiveness and accuracy of the assessment. Therefore, this application not only accelerates the decision-making process of post-disaster recovery and optimizes resource allocation, but also strengthens the scientific management of bridge structure safety, provides a solid scientific basis for ensuring the stable operation of transportation infrastructure, and effectively ensures the use of structures and personnel safety.

[0057] In the above step 100, the on-site fire investigation information includes the cause of the fire, the characteristics of the main combustible materials, the duration of combustion, the characteristics and duration of the explosion, the fire extinguishing method, and the residual state of the fire; the concrete surface state includes the appearance state of the concrete box girder structure after high temperature, and the color and crushing of the surface of the concrete box girder structure after high temperature.

[0058] Here, the above-mentioned fire investigation information and data on the apparent state of concrete are partly obtained through naked eye observation and partly obtained through detection by professional equipment and instruments, and this embodiment of the present application does not make an absolute limitation on this.

[0059] In the above step 200, the maximum temperature of concrete, the maximum temperature of steel bars and the maximum temperature of steel strands of the concrete box girder are determined according to the combustion time and the fire scene temperature. Specifically, the method may be as follows:

[0060] Obtain the relationship table between the apparent state of concrete and high temperature, and preliminarily obtain the fire temperature and burning time of the concrete box girder structure based on the relationship table and fire investigation information;

[0061] Based on the fire temperature and burning time, a finite element model was established, and the temperature propagation method was used to analyze and determine the maximum temperature T1 of concrete in the box girder structure, the maximum temperature T2 of steel bars, and the maximum temperature T3 of steel strands.

[0062] Here, the finite element model can be obtained using three-dimensional drawing software in the relevant technology, which will not be described in detail in the embodiments of the present application.

[0063] The corresponding relationship between concrete surface state and temperature is shown in Table 1.

[0064] Table 1 Relationship between concrete surface characteristics and temperature

[0065]

[0066] For different concrete surface characteristics, the corresponding temperature range can be selected. The maximum temperature value of the concrete can be flexibly adjusted within the temperature range. Generally, the minimum value, midpoint value or maximum value of the temperature range is selected according to the surface characteristics of the concrete.

[0067] Furthermore, in step 300, the elastic modulus of the concrete after high temperature is calculated according to the maximum temperature of the concrete, which is calculated by the following formula:

[0068] E c T / E c =0.83-0.0011θ,60℃≤θ<700℃

[0069] Where, θ is the temperature; E cT is the actual elastic modulus of concrete after high temperature; Ec It is the design elastic modulus of concrete when it is not subjected to high temperature.

[0070] In step 300, the compressive strength of the concrete after high temperature is calculated according to the maximum temperature of the concrete, which is calculated by the following formula:

[0071]

[0072] Where θ is the temperature; f cT is the actual compressive strength of concrete after high temperature, f c It is the design compressive strength of concrete when it is not subjected to high temperature.

[0073] In addition, in the embodiment of the present application, in step 400, the actual elastic modulus of the steel bar is determined according to the maximum temperature of the steel bar, and the actual elastic modulus of the steel strand is determined according to the maximum temperature of the steel strand, which is calculated by the following formula:

[0074]

[0075] Where, θ is the maximum temperature of the steel bar / strand; E sT E is the elastic modulus of the steel bar / strand after high temperature; s It is the design elastic modulus of the steel bar / strand when it has not experienced high temperature.

[0076] In the above formula, it is necessary to calculate the actual elastic modulus of the steel bars after high temperature based on the maximum temperature of the steel bars and the design elastic modulus of the steel bars when they have not experienced high temperature, and to calculate the actual elastic modulus of the steel strands after high temperature based on the maximum temperature of the steel strands and the design elastic modulus of the steel strands when they have not experienced high temperature.

[0077] In some embodiments, the method for assessing damage of a concrete box girder under high temperature in a fire further includes:

[0078] Determine the yield strength decay curve of steel bars / strands after high temperature;

[0079] Substitute the maximum temperature of the steel bar / steel strand into the yield strength attenuation curve of the steel bar / steel strand after high temperature to determine the yield strength coefficient of the steel bar / steel strand after high temperature.

[0080] Specifically, the reduction in yield strength of the steel bar after high temperature is determined according to Table 2. The maximum temperature T2 of the steel bar is brought into the table, and the strength f of the steel bar after high temperature is determined by linear interpolation method. sT .

[0081] Table 2 Reduction coefficient of yield strength of steel bars after high temperature cooling

[0082]

[0083] HRB335 and HRB400 in the above Table 2 correspond to two different types of hot-rolled steel bars in the concrete box girder structure. For other types of hot-rolled steel bars, corresponding adjustments can be made by referring to Table 2.

[0084] Similarly, the reduction in yield strength of the steel strand after high temperature is determined according to Table 3. The maximum temperature T3 of the steel strand is brought into the table, and the strength f of the steel strand after high temperature is determined by linear interpolation method. pT .

[0085] Table 3 Yield strength reduction factor of steel strand after high temperature cooling

[0086] Temperature(℃) Strength reduction factor normal temperature 1 100 0.98 200 0.98 300 0.98 400 0.89 500 0.72 600 0.55 700 0.38

[0087] In the above embodiment, by calculating the actual elastic modulus and actual compressive strength of the concrete box girder structure after high temperature, and calculating the actual elastic modulus and respective yield strength of the steel bars and steel strands after high temperature, targeted repairs can be carried out according to the guidance of the above accurate data, avoiding excessive or insufficient repair work, ensuring safety and saving costs. In addition, the actual elastic modulus and actual compressive strength of the concrete box girder structure after high temperature, the actual elastic modulus and actual yield strength of the steel bars and steel strands after high temperature, and other data are directly related to the load-bearing capacity and stability assessment of the structure. Therefore, by determining the actual elastic modulus and actual compressive strength, they can be scientifically compared with the designed actual elastic modulus and designed compressive strength, thereby ensuring that the structure is safe and reliable before resuming use.

[0088] In some embodiments, the method for assessing damage of a concrete box girder under high temperature in a fire of the present application further includes the following steps:

[0089] Step 100', performing structural state evaluation calculation on the concrete box girder structure after high temperature, wherein the structural state evaluation calculation includes an appearance characteristic inspection unit, a crack state inspection unit after high temperature, a concrete strength inspection unit, a protection wall steel bar strength inspection unit and a steel strand strength inspection unit, and each inspection unit corresponds to a structural state damage coefficient;

[0090] Step 200', determining a dynamic weighting coefficient, and performing dynamic weighting calculations in sequence according to the dynamic weighting coefficient and the structural state damage coefficient to obtain a plurality of structural state assessment results; wherein the dynamic weighting coefficients corresponding to different inspection units have different proportions;

[0091] Step 300': determining a structural damage level according to the plurality of structural state assessment results.

[0092] In the above step 100', the structural state damage coefficient corresponding to the surface feature inspection unit is calculated by the following formula:

[0093]

[0094] Among them, R1 is the structural state damage coefficient corresponding to the apparent characteristic inspection unit, T1 is the upper temperature limit corresponding to the apparent color of concrete, T2 is the damaged area in the key inspection area, M1 is the limit temperature value corresponding to the color of the concrete box girder after high temperature, and M2 is the total area of ​​the key inspection area.

[0095] The key inspection area in the above formula refers to the area that is easily damaged in a high temperature fire scene. This area is sensitive to damage and needs to be inspected in a focused manner. The setting of the key inspection area is related to the location of the combustion source and the concrete box girder structure, and there is no absolute limitation on this in the embodiments of the present application.

[0096] In the above step 100', the structural state damage coefficient corresponding to the post-high temperature crack state inspection unit is calculated by the following formula:

[0097]

[0098] Among them, R2 is the structural state damage coefficient corresponding to the crack state inspection unit, H1 is the maximum longitudinal length of the crack in the key inspection area, H2 is the maximum width of the crack in the key inspection area, K1 is the total longitudinal length of the overall inspection area of ​​the box girder structure, and K2 is the overall crack width limit of the box girder structure.

[0099] In the above step 100', the structural state damage coefficient corresponding to the concrete strength inspection unit is calculated by the following formula:

[0100]

[0101] Among them, R3 is the structural damage coefficient corresponding to the concrete strength inspection unit; fc is the design compressive strength of concrete in the key inspection area when it has not experienced high temperature; Ec is the design elastic modulus of concrete in the key inspection area when it has not experienced high temperature; fcT is the actual compressive strength of concrete in the key inspection area when it encounters high temperature; EcT is the actual elastic modulus of concrete in the key inspection area when it encounters high temperature.

[0102] In the above step 100', the structural state damage coefficient corresponding to the protective wall reinforcement strength inspection unit is calculated by the following formula:

[0103]

[0104] Among them, R4 is the structural damage coefficient corresponding to the reinforcement strength inspection unit of the protective wall; f s In order to focus on checking the design tensile strength of the regional steel bars without high temperature, E s Design elastic modulus for steel bars in the key inspection area that are not exposed to high temperatures, fsT In order to focus on checking the actual tensile strength of the regional steel bars under high temperature conditions, E sT To focus on checking the actual elastic modulus of steel bars in areas exposed to high temperatures.

[0105] Similarly, in the above step 100', the structural state damage coefficient corresponding to the steel strand strength inspection unit is calculated by the following formula:

[0106]

[0107] Among them, R5 is the structural damage coefficient corresponding to the steel strand strength inspection unit; f p In order to focus on checking the design tensile strength of the regional steel strands without high temperature, E p Design elastic modulus for the steel strands in the key inspection area without high temperature, f pT In order to focus on checking the actual tensile strength of the regional steel strands under high temperature conditions, E pT To focus on checking the actual elastic modulus of the steel strands in areas exposed to high temperatures.

[0108] Here, it should be noted that the key inspection area in the above formula refers to the area that is easily damaged in the high temperature scene of the fire. This part of the area is sensitive to damage and needs to be checked. The setting of the key inspection area is related to the location of the combustion source and the concrete box girder structure. Generally, the high temperature fire occurs at the bottom of the concrete box girder structure. Therefore, for example, Figure 3 The area marked by the dotted box is the key inspection area.

[0109] After calculating the structural state damage coefficient of each inspection unit, in step 200', the dynamic weighted coefficients corresponding to different inspection units have different proportions. The weights are weighted according to the influence of each inspection unit on the damage degree of the box girder structure after encountering high temperature in fire. The weights of each system are as shown in Table 4:

[0110] Table 4

[0111]

[0112] Thus, according to the dynamic weighted coefficient and the structural state damage coefficient, dynamic weighted calculation is performed in sequence to obtain multiple structural state assessment results, which are calculated by the following formula:

[0113]

[0114] Among them, Xi is the dynamic weighting coefficient corresponding to each detection unit, and the weighting coefficient is used to connect the influence of the key indicators of each detection content on the structural state; Ri is the structural state damage coefficient corresponding to each detection unit.

[0115] In step 300', the structural damage level is determined according to the multiple structural state assessment results. For example, Table 5 lists five factors that need to be considered in the state assessment of the concrete box girder structure after encountering high temperature in a fire. According to the interval range of the theoretical maximum value and the theoretical minimum value of the final result G and the degree of damage, the final state result is divided into five damage levels: slight, medium, heavy, severe and extremely severe.

[0116] Specifically, the structural state of concrete box beams after encountering high temperature in fire is divided into four levels: A, B, C, and D. Level A is further divided into two levels: AA and A1. Level D is mild, level C is moderate, level B is severe, level A1 is severe, and level AA is extremely severe. The evaluation and measures involved in each degradation level are shown in Table 5:

[0117] Table 5

[0118]

[0119] The method for evaluating the structure of the box girder after encountering high temperature in a fire proposed in the present invention above shows that the structural performance is at level B, C, or D, which can be directly determined and relevant repairs can be carried out. However, since the structural performance is at level A, the following field tests need to be carried out to verify the state of the structure.

[0120] The specific steps include the following:

[0121] Establishing a finite element model of the concrete box girder structure, and determining a theoretical deflection value and a theoretical strain value of the concrete box girder that has not been subjected to high temperature;

[0122] Determine the maximum deflection test value and maximum strain test value of the concrete box girder structure after high temperature;

[0123] The safety factor of the concrete box girder is calculated based on the theoretical deflection value, theoretical strain value, maximum deflection test value and maximum strain test value, and the comprehensive damage state of the concrete box girder is obtained by referring to the factor and structural damage level.

[0124] Among them, in the above steps, a loading test is performed on the concrete box girder structure after high temperature by means of a loading system, and a preset load is linearly applied step by step to the concrete box girder structure after high temperature, so as to test and obtain the maximum deflection test value and the maximum strain test value of the concrete box girder structure after high temperature. The loading system preferably uses a load-carrying vehicle or a concrete block for loading, and the load and displacement in the field test process are measured by the measurement system.

[0125] Figure 4 The schematic diagram of the field test device for the concrete box girder after high temperature in fire proposed by the present invention is given, which includes the box girder structure, the load system, the test system, etc. In the above field test, the performance test is carried out using the concrete box girder after high temperature in fire. Among them, the loading system is preferably loaded by a heavy-load vehicle, and the loading efficiency should not be less than 0.8.

[0126] Figure 4 The test contents of the concrete box girder in the field test after the concrete box girder encounters high temperature include displacement measurement points and concrete strain measurement points, and the number of measurement points can be adjusted according to actual needs. Figure 4 The downward arrow (↓) indicates the direction of the vertical load applied to the concrete box girder structure; the vertex of the triangle symbol (▲) indicates the corresponding concrete strain measurement point; the special-shaped symbol (Φ) indicates the vertical displacement measurement point. Strain and displacement tests are mandatory items for the model test of the present invention, and vibration characteristics and other measurement points can also be added according to the structural characteristics of the box girder.

[0127] Table 6 shows the loading priority process of the field test of the concrete box girder structure after high temperature fire of the present invention. The load level during the loading process of the field specimen can be divided by considering the design load under different load combination conditions. The loading process is designed to be 5 cycles, the first cycle is the initial loading cycle; the maximum values ​​of the second and third cycles are the maximum values ​​in the test loads, which are used to evaluate the stability of the structural state.

[0128] The specific measured data are shown in Table 6:

[0129] Table 6

[0130]

[0131] The crack observation, displacement and strain tests in Table 6 are the test process data collection tasks that are completed first during the field test loading process. If it is inconvenient to observe the crack status on site, the observation can be carried out at the maximum value of each cyclic load.

[0132] like Figure 5 The finite element model shown in the figure represents the specific aspects of the concrete box girder structure. The finite element model is established to calculate the mechanical properties of the concrete box girder that has not experienced high temperature in fire, and the deflection value L of the beam under the test load is obtained. sT and strain value σ ST .

[0133] The structural characteristics obtained from the field test and the calculated characteristics are comprehensively analyzed to obtain the safety factor of the concrete box girder after being exposed to high temperature;

[0134]

[0135] Where n is the safety factor; L sT is the maximum deflection test value of the beam under the test load; σ ST is the maximum strain test value under the test load; L s is the theoretical value of the maximum deflection of the beam under the test load in the simulation analysis; σ S is the theoretical value of the maximum strain under the test load in the simulation analysis.

[0136] According to the safety factor obtained, referring to the structural performance evaluation level given in Table 7, the structural damage state of the concrete box girder structure after encountering high temperature is obtained.

[0137] Table 7 shows the damage status evaluation table of the concrete box girder structure after high temperature fire of the present invention.

[0138] Table 7

[0139]

[0140] It can be seen that the damage assessment method for concrete box beams under high temperature in fire provided by the embodiment of the present application not only takes into account the microscopic changes of the structure, but also integrates the thermal behavior of the entire structure, ensuring the comprehensiveness and accuracy of the assessment. In addition, the method can also provide customized damage assessment, provide solid data support for subsequent repair strategies, effectively shorten the bridge recovery cycle, and reduce maintenance costs.

[0141] Those skilled in the art will readily appreciate other embodiments of the present application after considering the specification and practicing the invention disclosed herein. The present application is intended to cover any modification, use or adaptation of the present application, which follows the general principles of the present application and includes common knowledge or customary techniques in the art that are not disclosed in the present application. The specification and examples are intended to be exemplary only, and the true scope and spirit of the present application are indicated by the following claims.

[0142] It should be understood that the present application is not limited to the precise structures that have been described above and shown in the drawings, and that various modifications and changes may be made without departing from the scope thereof. The scope of the present application is limited only by the appended claims.

Claims

1. A method for assessing damage of concrete box beams under high temperature in fire, characterized in that: include: Determine the burning time and fire temperature of the combustion source based on on-site fire investigation information and the apparent state of concrete; Determine the maximum temperature of concrete, the maximum temperature of steel bars and the maximum temperature of steel strands of the concrete box girder according to the combustion time and the fire scene temperature; Calculate the elastic modulus and compressive strength of the concrete after high temperature according to the maximum temperature of the concrete; The actual elastic modulus of the steel bar is determined according to the maximum temperature of the steel bar, and the actual elastic modulus of the steel strand is determined according to the maximum temperature of the steel strand.

2. The method for assessing damage of concrete box beams under high temperature in fire according to claim 1 is characterized in that: The on-site fire investigation information includes the cause of the fire, the characteristics of the main combustibles, the duration of combustion, the characteristics and duration of the explosion, the fire extinguishing method, and the residual state of the fire; The concrete appearance state includes the appearance state of the concrete box beam structure after high temperature, and the color and breakage of the surface of the concrete box beam structure after high temperature.

3. The method for assessing damage of concrete box beams under high temperature in fire according to claim 1 is characterized in that: The elastic modulus of the high-temperature concrete is calculated according to the maximum temperature of the concrete, and is calculated by the following formula: E cT / E c =0.83-0.0011θ,60℃≤θ<700℃ Where, θ is the temperature; E cT is the actual elastic modulus of concrete after high temperature; E c It is the design elastic modulus of concrete when it is not subjected to high temperature.

4. The method for assessing damage of concrete box beams under high temperature in fire according to claim 1 is characterized in that: The compressive strength of the concrete after high temperature is calculated according to the maximum temperature of the concrete, and is calculated by the following formula: Where θ is the temperature; f cT is the actual compressive strength of concrete after high temperature, f c It is the design compressive strength of concrete when it is not subjected to high temperature.

5. The method for assessing damage of concrete box beams under high temperature in fire according to claim 1 is characterized in that: The actual elastic modulus of the steel bar is determined according to the maximum temperature of the steel bar, and the actual elastic modulus of the steel strand is determined according to the maximum temperature of the steel strand, which is calculated by the following formula: Where, θ is the maximum temperature of the steel bar / strand; E sT E is the elastic modulus of the steel bar / strand after high temperature; s It is the design elastic modulus of the steel bar / strand when it has not experienced high temperature.

6. The method for assessing damage of concrete box beams under high temperature in fire according to claim 1 is characterized in that: Also includes: Determine the yield strength decay curve of steel bars / strands after high temperature; Substitute the maximum temperature of the steel bar / steel strand into the yield strength attenuation curve of the steel bar / steel strand after high temperature to determine the yield strength coefficient of the steel bar / steel strand after high temperature.

7. The method for assessing damage of concrete box beams under high temperature in fire according to claim 1 is characterized in that: Also includes: Performing structural state evaluation calculation on the concrete box girder structure after high temperature, the structural state evaluation calculation includes an apparent characteristic inspection unit, a crack state inspection unit after high temperature, a concrete strength inspection unit, a protective wall steel bar strength inspection unit and a steel strand strength inspection unit, and each inspection unit corresponds to a structural state damage coefficient; Determine a dynamic weighting coefficient, and perform dynamic weighting calculations in sequence according to the dynamic weighting coefficient and the structural state damage coefficient to obtain a plurality of structural state assessment results; wherein the dynamic weighting coefficients corresponding to different inspection units have different proportions; A structural damage level is determined according to a plurality of the structural state assessment results.

8. The method for assessing damage of concrete box beams under high temperature in fire according to claim 7 is characterized in that: The dynamic weighted coefficients corresponding to the surface characteristic inspection unit, post-high temperature crack state inspection unit, concrete strength inspection unit, protective wall steel bar strength inspection unit and steel strand strength inspection unit respectively account for 10%, 10%, 20%, 20% and 30%.

9. The method for assessing damage of concrete box beams under high temperature in fire according to claim 8, characterized in that: Also includes: Establishing a finite element model of the concrete box girder structure, and determining a theoretical deflection value and a theoretical strain value of the concrete box girder that has not been subjected to high temperature; Determine the maximum deflection test value and maximum strain test value of the concrete box girder structure after high temperature; The safety factor of the concrete box girder is calculated based on the theoretical deflection value, theoretical strain value, maximum deflection test value and maximum strain test value, and the comprehensive damage state of the concrete box girder is obtained by referring to the factor and structural damage level.

10. The method for assessing damage of concrete box beams under high temperature in fire according to claim 9, characterized in that: The preset loads are linearly applied step by step to the concrete box girder structure after high temperature to test and obtain the maximum deflection test value and the maximum strain test value of the concrete box girder structure after high temperature.

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