Prediction method for progressive collapse resistance of space grid structure in fire

Abstract

The invention discloses a method for predicting the progressive collapse resistance of a space grid structure in a fire. The method comprises the following steps: establishing a finite element model of the space grid structure; performing modal analysis on the finite element model; calculating an equivalent external load, and applying the gravity and the equivalent external load to the structure model; establishing a temperature field database; a heating load setting step: applying a temperature field as a time-varying unit load to each unit, and performing heating analysis; and in each sub-step of the load step, extracting a unit solution, calculating a failure critical value of the component at the current temperature, performing failure judgment on each rod piece, and continuously and circularly calculating and determining the failure of the component and the structure by finely utilizing a restart and life-death unit technology. By means of the method, the progressive collapse performance of the space grid structure in the fire disaster can be comprehensively analyzed and predicted, and a scientific basis is provided for engineering design and fire disaster safety assessment.

Description

Technical Field

[0001] The invention relates to the fields of structural engineering and engineering disaster prevention and mitigation, and in particular to a method for predicting the progressive collapse resistance of a space grid structure under fire. Background Art

[0002] In the field of building structures, large-span space steel structures have been widely used due to their excellent spatial performance and aesthetics. Such structures are commonly found in civil and industrial buildings such as gymnasiums, warehouses, industrial plants, and large public facilities. However, despite the excellent mechanical properties and construction efficiency of steel structures, their fire resistance is relatively poor.

[0003] When a steel structure encounters a fire, the basic mechanical properties of the steel, such as strength and elastic modulus, will drop sharply at high temperatures. This degradation of material properties will cause the steel structure to be easily damaged locally in a fire, and may even cause the continuous collapse of the entire structure. Continuous collapse refers to the failure of a structure to be damaged locally due to accidental loads (such as fire, explosion, earthquake, etc.), which will cause continuous damage to components connected to the failed components, ultimately leading to a disproportionate collapse damage that is too large in scale relative to the initial local damage. This collapse phenomenon will not only cause serious loss of life and property, but also have a great negative impact on society.

[0004] In order to deal with this problem, the engineering and academic communities have been working hard to explore and study methods to improve the progressive collapse resistance of steel structures. However, existing studies mostly use the component removal method to simulate the collapse process of the structure. This method ignores the causes of component failure and does not consider the progressive collapse process of spatial grid structures under fire conditions. Therefore, this method has certain limitations in predicting the progressive collapse resistance of spatial grid structures under fire. In addition, factors such as the uneven distribution of temperature fields, geometric and material nonlinearities, and dynamic effects during the fire process will also have a significant impact on the progressive collapse resistance of the structure. These factors make the prediction of the progressive collapse resistance of spatial grid structures under fire more complicated and difficult.

[0005] In summary, in order to more effectively evaluate the impact of fire on spatial grid structures and improve the progressive collapse resistance of structures, a more accurate and comprehensive prediction method is needed. Summary of the invention

[0006] The main purpose of the present application is to provide a method for predicting the progressive collapse resistance of a spatial grid structure under fire, aiming to solve the problem in the prior art that its progressive collapse resistance cannot be accurately predicted.

[0007] To achieve the above objectives, the present application provides a method for predicting the progressive collapse resistance of a spatial grid structure under fire, comprising the following steps:

[0008] S1. Establish a finite element model of the spatial grid structure;

[0009] S2. performing modal analysis on the finite element model to determine the first-order natural frequency and the second-order natural frequency of the structure, and calculating the corresponding Rayleigh damping coefficient;

[0010] S3. Calculate the equivalent external load and apply gravity and the equivalent external load to the structural model;

[0011] S4. Establish a temperature field database to store the temperature-time history data of each component during the fire process;

[0012] S5, set the heating load step i, apply the temperature field as a time-varying unit load to each unit, and perform heating analysis;

[0013] S6. Under substep j of load step i, calculate the critical failure value of the component at the current temperature;

[0014] S7. In each sub-step of the load step, the unit solution is extracted and the failure judgment is performed on each rod, including calculating the axial tensile stress and axial compressive stress of the rod and adjusting the material properties according to the temperature;

[0015] S8. Determine failure of components and structures by evaluating:

[0016] a) If no component is damaged in sub-step j of load step i, increase the number of sub-steps j and return to step S6 to continue the analysis;

[0017] b) If component failure is detected in sub-step j of load step i, the failed component is removed, the model is updated, and the analysis is restarted, the load step number i is increased, and the sub-step number j=1 is reset, and the analysis is returned to step S5 to continue;

[0018] c) The analysis ends if the specified fire duration is reached or the structure experiences global failure.

[0019] Preferably, the calculation formulas of the Rayleigh damping coefficients τ1 and τ2 in step S2 are as follows:

[0020]

[0021] Among them, ω1 is the first-order natural frequency, ω2 is the second-order natural frequency, ζ1 and ζ2 are the damping ratios at the corresponding frequencies respectively.

[0022] Preferably, the equivalent external load in step S3 includes a dead load and a live load, and is equivalent to a node load applied to the upper chord node of the structural model.

[0023] Preferably, in step S4, the temperature field database is established by iteratively calculating the empirical formula for air temperature rise in a practical large-space building fire and the formula for temperature rise of components without fire protection to obtain temperature data of the structure that changes with space and time.

[0024] Preferably, the practical empirical formula for air temperature rise in large-space building fires is:

[0025] T (x，z，t) =T z [1-0.8exp(-βt)-0.2exp(-0.1βt)][η

[0026] +(1-η)exp(-(xb) / μ)]+T0

[0027] Among them, T (x，z，t) is the air temperature at the horizontal distance x from the center of the fire source and the vertical distance z from the ground at the corresponding time t (℃);

[0028] T z is the maximum air temperature at a vertical distance z from the center of the fire source (°C);

[0029] β is the power of the fire source and αt 2 The shape coefficient of the temperature rise curve determined by the growth type fire source is 0.0018;

[0030] η is the temperature attenuation coefficient at the horizontal distance x from the center of the fire source;

[0031] b is the distance from the center of the fire source to the outermost edge of the fire source (m);

[0032] T0 is the ambient temperature before the fire occurs, which is 20℃.

[0033] Preferably, the temperature rise formula without fire protection components is:

[0034]

[0035] γ=γ c +γ r

[0036]

[0037] Among them, T g , T s are the temperatures of air and steel at time t (℃);

[0038] Δt is the time step (s), which should not exceed 5s;

[0039] ΔT s is the temperature rise of the steel component within the time (t, t+Δt) (℃);

[0040] ρ s is the density of steel (kg / m3);

[0041] c s is the specific heat of steel [J / (kg·℃)];

[0042] F / V is the cross-sectional shape factor of the steel member without fire protection;

[0043] γ is the comprehensive heat transfer coefficient [W / (m2·℃)];

[0044] γ c is the convection heat transfer coefficient [W / (m2·℃)];

[0045] γ r is the thermal radiation heat transfer coefficient [W / (m2·℃)];

[0046] ε r is the comprehensive radiation rate;

[0047] σ is the Stefan-Boltzmann constant, which is 5.67×10-8W / (m 2 ℃ 4 ).

[0048] Preferably, the calculation step of the failure critical value in step S6 is specifically as follows:

[0049] For tension members:

[0050]

[0051] Among them, f u , T is the ultimate strength of steel at temperature T, f y , T is the yield strength of steel at temperature T;

[0052] For compression members:

[0053]

[0054] Among them, f c is the critical axial compressive stress, f yc is the buckling stress of steel, K is the calculated length coefficient of the member, L is the unsupported length, r is the section radius of gyration, and E is the elastic modulus of steel.

[0055] Preferably, the specific steps of performing failure determination on each rod in step S7 are as follows:

[0056] For tension rods, it is considered that the component will fail when the stress reaches the ultimate strength. The failure judgment formula is:

[0057]

[0058] Among them, I a is the critical tensile failure criterion, σ t is the axial tensile stress;

[0059] For compressive rods, it is considered that instability failure occurs when the stress of the component reaches the critical axial compressive stress. The judgment formula is:

[0060]

[0061] Among them, I b is the critical instability criterion, σ c is the axial compressive stress.

[0062] Preferably, after each component failure determination in step S8, the following operations are further performed:

[0063] If the maximum deflection of the structure in the simulated fire scenario exceeds 1 / 30 of the short span, the structure is considered to be deformed too much, the structure loses its bearing capacity and is completely destroyed, and the program terminates the calculation;

[0064] If the maximum deflection of the structure in the simulated fire scenario does not exceed 1 / 30 of the short span, check whether the specified fire resistance verification time t has been reached. max If the specified fire resistance verification time t is not reached max , then return to step S5; if the specified fire resistance verification time t has been reached at this time max , the analysis ends.

[0065] Preferably, the finite element model is established using ANSYS finite element analysis software, and the method for predicting the progressive collapse resistance of the spatial grid structure is implemented through an APDL command stream program.

[0066] Through the above technical scheme, the beneficial effects of the present invention are as follows: the present application provides a method for predicting the continuous collapse performance of a spatial grid structure under fire, which can accurately predict the continuous collapse performance of a spatial grid structure under fire by considering the uneven distribution of the fire temperature field, the geometric and material nonlinearity of the structure, and the dynamic effect. Through the implementation of this method, it can provide an important reference basis for the design and fire safety assessment of spatial grid structures, and has important engineering application value and broad market prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0067] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention, and the embodiments in the drawings do not constitute any limitation to the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0068] Figure 1 A schematic diagram of a flow chart of a method for predicting the progressive collapse resistance of a spatial grid structure under fire provided in one embodiment of the present application;

[0069] Figure 2 A schematic diagram of a finite element model of a spatial grid structure provided in one embodiment of the present application;

[0070] Figure 3 A schematic diagram of the working conditions of the F1-F5 fire sources provided in one embodiment of the present application;

[0071] Figure 4 A time-maximum vertical deformation diagram under the F1-F5 fire source conditions provided in one embodiment of the present application;

[0072] Figure 5 A distribution diagram of rod damage positions under an F1 fire source condition provided in an embodiment of the present application;

[0073] Figure 6 This is the final deformation diagram under the F1 fire source condition provided by an embodiment of the present application;

[0074] Figure 7 A distribution diagram of rod damage positions under an F2 fire source condition provided in an embodiment of the present application;

[0075] Figure 8 A diagram of collapse failure morphology under F2 fire source conditions provided in one embodiment of the present application;

[0076] Fig. 9 A distribution diagram of rod damage positions under an F3 fire source condition provided in an embodiment of the present application;

[0077] Fig.10 A diagram of collapse failure morphology under F3 fire source conditions provided in one embodiment of the present application;

[0078] Fig.11 A distribution diagram of rod damage positions under an F4 fire source condition provided in an embodiment of the present application;

[0079] Fig.12 A diagram of collapse failure morphology under F4 fire source conditions provided in one embodiment of the present application;

[0080] Fig.13A distribution diagram of rod damage positions under an F5 fire source condition provided in an embodiment of the present application;

[0081] Fig.14 This is the final deformation diagram under the F5 fire source condition provided by an embodiment of the present application;

[0082] Fig.15 A schematic flow chart of a method for predicting the progressive collapse resistance of a spatial grid structure under fire provided in another embodiment of the present application.

[0083] The realization of the purpose, functional features and advantages of this application will be further explained in conjunction with embodiments and with reference to the accompanying drawings. DETAILED DESCRIPTION

[0084] It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application. On the contrary, these embodiments are provided to make the present disclosure more thorough and complete, and to fully convey the scope of the present disclosure to those skilled in the art.

[0085] The embodiments of the present application provide a method for predicting the anti-progressive collapse performance of a spatial grid structure under fire. The terms "first", "second", "third", "fourth", etc. (if any) 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 data used in this way can be interchanged where appropriate, so that the embodiments described herein can be implemented in an order other than that illustrated or described herein. In addition, the terms "including" or "having" and any variations thereof are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units that are clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0086] For ease of understanding, the specific process of the embodiment of the present application is described below. Figure 1 In the embodiment of the present application, an embodiment of the method for predicting the progressive collapse resistance of a spatial grid structure under fire includes:

[0087] S1. Establish a finite element model of the spatial grid structure;

[0088] S2. Perform modal analysis on the finite element model to determine the first-order natural frequency and the second-order natural frequency of the structure, and calculate the corresponding Rayleigh damping coefficient;

[0089] S3. Calculate the equivalent external load and apply gravity and the equivalent external load to the structural model;

[0090] S4. Establish a temperature field database to store the temperature-time history data of each component during the fire process;

[0091] S5, set the heating load step i, the initial value of i is 1, apply the temperature field as a time-varying unit load to each unit, and perform heating analysis;

[0092] S6. Under substep j of load step i, calculate the critical failure value of the component at the current temperature;

[0093] S7. In each sub-step of the load step, the unit solution is extracted and the failure judgment is performed on each rod, including calculating the axial tensile stress and axial compressive stress of the rod and adjusting the material properties according to the temperature;

[0094] S8. Determine failure of components and structures by evaluating:

[0095] a) If no component is damaged in sub-step j of load step i, increase the number of sub-steps j and return to step S6 to continue the analysis;

[0096] b) If component failure is detected in sub-step j of load step i, the failed component is removed, the model is updated, and the analysis is restarted, the load step i is increased, and the number of sub-steps j=1 is reset, and the analysis is returned to step S5 to continue;

[0097] c) The analysis ends if the specified fire duration is reached or the structure experiences global failure.

[0098] It is understandable that the execution subject of the present application may be a prediction system for the progressive collapse resistance of a spatial grid structure under fire, or a terminal or a server, which is not specifically limited here. The present application embodiment is described by taking a server as the execution subject as an example.

[0099] Specifically, the finite element model in this application is established using ANSYS finite element analysis software, and the prediction method for the anti-continuous collapse performance of the spatial grid structure is implemented through the APDL command flow program. ANSYS software has a rich unit library that can accurately simulate the mechanical behavior of various rods and nodes in the spatial grid structure. Its material model library covers various properties of common materials. When simulating fire scenes, it can accurately consider the temperature-dependent properties of materials such as steel at high temperatures, such as elastic modulus and yield strength, providing a basis for accurate analysis. The powerful meshing function can generate high-quality meshes according to the geometric shape of the structure and analysis requirements to ensure calculation accuracy and efficiency.

[0100] For example, when establishing a finite element model of a spatial grid structure in step S1, first, create nodes and units in ANSYS based on the geometric dimensions of the actual spatial grid structure. For an upright tetrahedral grid structure, it is necessary to accurately input the plane dimensions, height, grid size and other parameters of the grid, define the connection relationship of each rod, and construct a geometric model that matches the actual structure. Then, according to the selected material, set its thermal expansion coefficient, Poisson's ratio, elastic modulus, yield strength and other parameters in the material property module, and define the curves of these parameters changing with temperature to reflect the impact of high temperature in fire on material properties. Finally, set boundary conditions according to the actual support conditions of the structure, such as multi-point fixed hinge supports in the periphery, accurately limit the degrees of freedom of the corresponding nodes, and complete the establishment of the finite element model.

[0101] In a specific embodiment, Figure 2 As shown in the figure, a finite element model of a space grid structure is established, with the upright tetrahedral grid structure as the research object. The design size of the grid is 45m×72m in plane, 2.8m in height, and the grid size is set to 3m×3m. The steel material is Q345 steel, and the cross section of the upper chord is Selection of lower chord cross section The cross section of the diagonal web is supported by perimeter multi-point fixed hinge supports with a support spacing of 9m.

[0102] Next, the location of the fire source and its related parameters were set. The fire source was placed below the grid structure, with the upper chord of the grid at a height of 12m from the ground. Considering the large plane area of ​​the grid, in order to study the local high temperature under extreme conditions, the fire source parameters were set: temperature T z The temperature is 800℃, the fire source power is 30MW, and the heat release rate per unit area reaches 250kW / m 2 The fire source growth coefficient β is selected as 0.0018 according to the standard of rapid fire, and other parameters are calculated by interpolation.

[0103] In order to comprehensively analyze the influence of different fire source positions on the grid structure, the position of the fire source on the plane was changed, and a total of five fire source conditions (F1-F5) were considered, such as Figure 3 As shown. These working conditions are achieved by dividing 1 / 4 of the grid into four equal parts and setting the dividing points on the edge to different positions corresponding to the center of the fire source. In the two span directions of the grid, a total of five fire source positions are set, namely: (-36, 0), (-18, 0), (0, 0), (0, 11.25), (0, 22.5), where the center point of the grid is represented by the coordinate (0, 0), which is used as the origin of the coordinate system. It is worth noting that the fire sources in all fire source working conditions are 12m away from the upper chord in the vertical direction, that is, the fire sources are all located on the ground.

[0104] Through the above settings, this embodiment provides a detailed and comprehensive model basis for the subsequent analysis of the impact of fire on the grid structure.

[0105] In step S2, the Rayleigh damping coefficients τ1 and τ2 are calculated using the formula: [C] = τ1[M] + τ2[K], where [C] is the damping matrix. The calculation formulas for the Rayleigh damping coefficients τ1 and τ2 are as follows:

[0106]

[0107] Where ω1 is the first-order natural frequency, ω2 is the second-order natural frequency, ζ1 and ζ2 are the damping ratios at the first-order and second-order natural frequencies, respectively.

[0108] In actual operation, the damping characteristics of the structure are considered. The actual structure will consume energy during vibration due to factors such as internal friction of the material and friction of component connections, which is manifested as damping. The introduction of the Rayleigh damping coefficient can make the finite element analysis closer to the dynamic response of the actual structure and make the calculation results more accurate. In subsequent dynamic analysis, such as the vibration analysis of the structure under the action of fire, considering damping can simulate the dissipation of structural vibration energy and more accurately predict the dynamic response and deformation of the structure.

[0109] In the embodiment of the present application, the equivalent external load in step S3 includes a dead load and a live load. First, the dead load is the fixed load that the structure itself bears for a long time. In this example, it is set to 0.5 kN / m 2 , including the self-weight of structural components, decorations, fixed equipment and other non-variable loads. Secondly, live load refers to the variable load that the structure may bear during use, such as personnel, furniture, mobile equipment, etc. In this example, the live load is also set to 0.5kN / m 2 This value represents the maximum variable load condition that the structure may face and is used to ensure that the structure has sufficient margin in terms of safety performance.

[0110] When these two loads are equivalent to node loads, they are evenly distributed and act on the upper chord node. The advantage of this is that complex surface loads or line loads can be simplified into node loads that are easy to calculate and process, thereby greatly improving the efficiency and accuracy of the analysis. At the same time, since the upper chord node is the key stress point of the grid structure, applying the load there can also better simulate the stress conditions of the structure in actual use.

[0111] In step S4, establishing a temperature field database is a crucial step, which aims to store the time-varying data of temperature rise of each component in a fire. Due to the unique characteristics of large-span spatial structures such as large space, their temperature fields usually show obvious non-uniformity when a fire occurs. In order to be closer to the actual situation, this example uses a non-uniform temperature field for simulation.

[0112] Specifically, this example uses a practical large-space building fire air temperature rise empirical formula when simulating the air temperature field. This formula can comprehensively consider multiple factors such as the fire source location, fire source power, time, and spatial location (including the horizontal distance from the center of the fire source and the vertical distance from the ground), thereby more accurately reflecting the changes in the air temperature of the fire.

[0113] Among them, the practical empirical formula for air temperature rise in large-space building fires is:

[0114] T (x，z，t) =T z [1-0.8exp(-βt)-0.2exp(-0.1βt)][η

[0115] +(1-η)exp(-(xb) / μ)]+T0

[0116] Among them, T (x，z，t) is the air temperature at the horizontal distance x from the center of the fire source and the vertical distance z from the ground at the corresponding time t (℃);

[0117] T z is the maximum air temperature at a vertical distance z from the center of the fire source (°C);

[0118] β is the power of the fire source and αt 2 The shape coefficient of the temperature rise curve determined by the growth type fire source is 0.0018;

[0119] η is the temperature attenuation coefficient at the horizontal distance x from the center of the fire source;

[0120] b is the distance from the center of the fire source to the outermost edge of the fire source (m);

[0121] T0 is the ambient temperature before the fire occurs, which is 20℃.

[0122] At the same time, in order to calculate the temperature rise of the components, this example also uses the temperature rise formula for components without fire protection given in the "Technical Code for Fire Protection of Building Steel Structures" for iterative calculation. This formula can take into account multiple factors such as the duration of the fire, the material of the component, the cross-sectional shape, and the heat transfer coefficient, so as to obtain the temperature change of the component in the fire. Among them, the temperature rise formula for components without fire protection is:

[0123]

[0124] γ=γ c +γ r

[0125]

[0126] Among them, T g , T s are the temperatures of air and steel at time t (℃);

[0127] Δt is the time step (s), which should not exceed 5s;

[0128] ΔT s is the temperature rise of the steel component within the time (t, t+Δt) (℃);

[0129] ρ s is the density of steel (kg / m3);

[0130] c s is the specific heat of steel [J / (kg·℃)];

[0131] F / V is the cross-sectional shape factor of the steel member without fire protection;

[0132] γ is the comprehensive heat transfer coefficient [W / (m2·℃)];

[0133] γ c is the convection heat transfer coefficient [W / (m2·℃)];

[0134] γ r is the thermal radiation heat transfer coefficient [W / (m2·℃)];

[0135] ε r is the comprehensive radiation rate;

[0136] σ is the Stefan-Boltzmann constant, which is 5.67×10-8W / (m 2 ℃ 4 ).

[0137] Through the above operation process, for each component in the structure, calculations are performed at different spatial positions and time points, so as to obtain detailed temperature data of the structure changing with space and time, such as Figure 4 As shown, these data are stored in the temperature field database in an orderly manner, providing accurate and comprehensive temperature load information for subsequent structural analysis, thereby laying a foundation for accurately evaluating the progressive collapse resistance of spatial grid structures under fire.

[0138] In step S6, the calculation steps of the failure critical value are as follows:

[0139] For tension members, the ultimate strength is taken as:

[0140]

[0141] Among them, f u , T is the ultimate strength of steel at temperature T, f y , T is the yield strength of steel at temperature T;

[0142] For compression rods, the critical stress is determined according to the stability judgment method for circular steel tube axial compression members given by the International Standards Organization:

[0143]

[0144] Among them, f c is the critical axial compressive stress, f yc is the buckling stress of steel, K is the calculated length coefficient of the member, L is the unsupported length, r is the section radius of gyration, and e is the elastic modulus of steel.

[0145] After obtaining the critical failure value of the member and the member stress at time t, calculate the critical tensile failure criterion I a And critical instability criterion I b , to determine the failure of each rod. The specific steps are as follows:

[0146] For tension rods, it is considered that the component will fail when the stress reaches the ultimate strength. The failure judgment formula is:

[0147]

[0148] Among them, I a is the critical tensile failure criterion, σ t is the axial tensile stress;

[0149] For compressive rods, it is considered that instability failure occurs when the stress of the component reaches the critical axial compressive stress. The judgment formula is:

[0150]

[0151] Among them, I b is the critical instability criterion, σ c is the axial compressive stress.

[0152] When no component failure occurs at the corresponding moment of substep j of load step i, substep j is incremented by 1 to continue to check the status of the subsequent substep members. This is a step-by-step refinement analysis process. In each substep, the members are strictly checked according to the established failure judgment criteria to ensure that the safety assessment of the structure at each time node is sufficiently accurate. If no member failure is detected in all substeps within load step i, it means that the structure as a whole is in a stable state during the time period of this load step. At this time, load step i+1 and substep j are reset to 1, and return to step S5 for a new round of temperature rise calculation. This simulates the staged changes in the structural state during the continuous development of the fire, ensuring that the calculation can continue to follow up the impact of the fire process on the structure.

[0153] Once component damage is found in a sub-step of load step i, decisive measures are taken immediately. By "killing" the failed component, that is, using the life and death unit technology to remove the component in the finite element model, the structural model is then updated to reflect the change in structural topology. Set the restart analysis and set i=2 to return to step S5 for recalculation. This operation can timely capture the internal force redistribution and deformation changes caused by component failure. This dynamic adjustment mechanism enables the calculation to adapt to the damage evolution of the structure in real time during a fire, and effectively simulates the process from local damage to possible continuous collapse of the structure. For specific simulation data in this embodiment, see Figure 5-14 Schematic diagram of the continuous collapse process under the F1-F5 fire source conditions shown.

[0154] The above process is automatically performed through the APDL cycle. Obviously, if the analysis end condition is not set, the calculation will continue. Therefore, after each component failure determination, the following operations are also performed:

[0155] Set the structural fire resistance verification time t max and the allowable deflection of the structure [w];

[0156] Get the time t corresponding to substep j of load step i, if t ≥ t max , the analysis is finished;

[0157] Get the maximum deflection w of the structure at time t. If w ≥ [w], it is judged that the entire structure has collapsed and the analysis ends.

[0158] In actual operation, if no component fails at time t, set j = j + 1 and return to continue the analysis; when j> j imax , that is, when the maximum value of sub-step j under load step i is exceeded, a new load step i=i+1 is set, and a restart judgment is performed at time t and the heating analysis is returned;

[0159] If a component fails at time t, record the failed member number, set a new load step i=i+1, restart the analysis at time t, kill the failed member and return to the heating analysis.

[0160] In a specific embodiment, if the maximum deflection of the structure in the simulated fire scenario exceeds 1 / 30 of the short span, it is considered that the structural deformation is too large, the structure loses its bearing capacity and is completely destroyed, and the program terminates the calculation;

[0161] If the maximum deflection of the structure in the simulated fire scenario does not exceed 1 / 30 of the short span, check whether the specified fire resistance verification time t has been reached. max If the specified fire resistance verification time t is not reached max , then return to step S5; if the specified fire resistance verification time t has been reached at this time max , the analysis ends.

[0162] Through the above steps, this application can comprehensively analyze and predict the progressive collapse performance of spatial grid structures under fire, providing a scientific basis for engineering design and fire safety assessment.

[0163] The technical solution of the present application can be applied to various types of double-layer space grid structures, including but not limited to upright tetrahedral pyramid grid structures. By adjusting the finite element model and analysis parameters, the present method can adapt to different structural forms and fire conditions.

[0164] In addition, the method of the present application can also be used in combination with other structural analysis methods, such as structural seismic analysis, wind load analysis, etc., to provide a more comprehensive structural safety assessment.

[0165] Finally, the technical solution of the present invention is not limited to a specific fire source location or support form, but can be flexibly applied to different fire scenarios and structural configurations to meet different engineering needs.

[0166] In summary, this application provides a method for predicting the continuous collapse performance of a spatial grid structure under fire. This method can accurately predict the continuous collapse performance of a spatial grid structure under fire by considering the uneven distribution of the fire temperature field, the geometric and material nonlinearity of the structure, and the dynamic effect. Through the implementation of this method, it can provide an important reference for the design and fire safety assessment of spatial grid structures, and has important engineering application value and broad market prospects.

[0167] Finally, it should be noted that the above embodiments are only specific implementation methods of the present application, which are used to illustrate the technical solutions of the present application, rather than to limit them. The protection scope of the present application is not limited thereto. Although the present application is described in detail with reference to the above embodiments, ordinary technicians in the field should understand that any technician familiar with the technical field can still modify the technical solutions recorded in the above embodiments within the technical scope disclosed in the present application, or can easily think of changes, or make equivalent replacements for some of the technical features therein; and these modifications, changes or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application, and should be included in the protection scope of the present application. Therefore, the protection scope of the present application shall be based on the protection scope of the claims.

[0168] In addition, although the operations of the method of the present application are described in a specific order in the drawings, this does not require or imply that the operations must be performed in this specific order, or that all the operations shown must be performed to achieve the desired results. Additionally or alternatively, some steps may be omitted, multiple steps may be combined into one step, and / or one step may be decomposed into multiple steps.

Claims

1. A method for predicting the progressive collapse resistance of a spatial grid structure under fire, characterized in that: The steps include: S1. Establish a finite element model of the spatial grid structure; S2. performing modal analysis on the finite element model to determine the first-order natural frequency and the second-order natural frequency of the structure, and calculating the corresponding Rayleigh damping coefficient; S3. Calculate the equivalent external load and apply gravity and the equivalent external load to the structural model; S4. Establish a temperature field database to store the temperature-time history data of each component during the fire process; S5, set the heating load step i, apply the temperature field as a time-varying unit load to each unit, and perform heating analysis; S6. Under substep j of load step i, calculate the critical failure value of the component at the current temperature; S7. In each sub-step of the load step, the unit solution is extracted and the failure judgment is performed on each rod, including calculating the axial tensile stress and axial compressive stress of the rod and adjusting the material properties according to the temperature; S8. Determine failure of components and structures by evaluating: a) If no component is damaged in sub-step j of load step i, increase the number of sub-steps j and return to step S6 to continue the analysis; b) If component failure is detected in sub-step j of load step i, the failed component is removed, the model is updated, and the analysis is restarted, the load step number i is increased, and the sub-step number j=1 is reset, and the analysis is returned to step S5 to continue; c) The analysis ends if the specified fire duration is reached or the structure experiences global failure.

2. According to claim 1, a method for predicting the progressive collapse resistance of a spatial grid structure under fire, characterized in that: The calculation formulas of the Rayleigh damping coefficients τ1 and τ2 in step S2 are as follows: Among them, ω1 is the first-order natural frequency, ω2 is the second-order natural frequency, ζ1 and ζ2 are the damping ratios at the corresponding frequencies respectively.

3. The method for predicting the progressive collapse resistance of a spatial grid structure under fire according to claim 1, characterized in that: The equivalent external load in step S3 includes a dead load and a live load, and is equivalent to a node load applied to the upper chord node of the structural model.

4. The method for predicting the progressive collapse resistance of a spatial grid structure under fire according to claim 1, characterized in that: In step S4, the temperature field database is established by iteratively calculating the practical large-space building fire air temperature rise empirical formula and the temperature rise formula of the non-fire protection component to obtain the temperature data of the structure changing with space and time.

5. The method for predicting the progressive collapse resistance of a spatial grid structure under fire according to claim 4, characterized in that: The practical empirical formula for air temperature rise in large-space building fires is: T (x ,z,t)=T z [1-0.8exp(*-βt)-0.2exp(-0.1βt)][η+(1-η)exp(-(x-B) / μ)]+T0 Among them, T (x，z，t) is the air temperature at the horizontal distance x from the center of the fire source and the vertical distance z from the ground at the corresponding time t (℃); T z is the maximum air temperature at a vertical distance z from the center of the fire source (°C); β is the power of the fire source and αt 2 The shape coefficient of the temperature rise curve determined by the growth type fire source is 0.0018; η is the temperature attenuation coefficient at the horizontal distance x from the center of the fire source; b is the distance from the center of the fire source to the outermost edge of the fire source (m); T0 is the ambient temperature before the fire occurs, which is 20℃.

6. A method for predicting the progressive collapse resistance of a spatial grid structure under fire according to claim 5, characterized in that: The temperature rise formula for the non-fire protection component is: c = c c +g r Among them, T g , T s are the temperatures of air and steel at time t (℃); Δt is the time step (s), which should not exceed 5s; ΔT s is the temperature rise of the steel component within the time (t, t+Δt) (℃); ρ s is the density of steel (kg / m3); c s is the specific heat of steel [J / (kg·℃)]; F / V is the cross-sectional shape factor of the steel member without fire protection; γ is the comprehensive heat transfer coefficient [W / (m2·℃)]; γ c is the convection heat transfer coefficient [W / (m2·℃)]; γ r is the thermal radiation heat transfer coefficient [W / (m2·℃)]; ε r is the comprehensive radiation rate; σ is the Stefan-Boltzmann constant, which is 5.67×10-8W / (m 2 ℃ 4 ).

7. The method for predicting the progressive collapse resistance of a spatial grid structure under fire according to claim 1, characterized in that: The calculation steps of the failure critical value in step S6 are specifically as follows: For tension members: Among them, f u , T is the ultimate strength of steel at temperature T, f y , T is the yield strength of steel at temperature T; For compression members: Among them, f c is the critical axial compressive stress, f yc is the buckling stress of steel, K is the calculated length coefficient of the member, L is the unsupported length, r is the section radius of gyration, and E is the elastic modulus of steel.

8. The method for predicting the progressive collapse resistance of a spatial grid structure under fire according to claim 7, characterized in that: The specific steps of performing failure determination on each rod in step S7 are as follows: For tension rods, it is considered that the component will fail when the stress reaches the ultimate strength. The failure judgment formula is: Among them, I a is the critical tensile failure criterion, σ t is the axial tensile stress; For compressive rods, it is considered that instability failure occurs when the stress of the component reaches the critical axial compressive stress. The judgment formula is: Among them, I b is the critical instability criterion, σ c is the axial compressive stress.

9. The method for predicting the progressive collapse resistance of a spatial grid structure under fire according to claim 1, characterized in that: After each component failure determination, step S8 also performs the following operations: If the maximum deflection of the structure in the simulated fire scenario exceeds 1 / 30 of the short span, the structure is considered to be deformed too much, the structure loses its bearing capacity and is completely destroyed, and the program terminates the calculation; If the maximum deflection of the structure in the simulated fire scenario does not exceed 1 / 30 of the short span, check whether the specified fire resistance verification time t has been reached. max If the specified fire resistance verification time t is not reached max , then return to step S5; if the specified fire resistance verification time t has been reached at this time max , the analysis ends.

10. A method for predicting the progressive collapse resistance of a spatial grid structure under fire according to any one of claims 1 to 9, characterized in that: The finite element model is established by using ANSYS finite element analysis software, and the method for predicting the anti-progressive collapse performance of the spatial grid structure is implemented by using an APDL command stream program.

Images