A large-diameter cable test model and its enclosed space fire resistance test method

Abstract

The present invention discloses a large-diameter cable test model and a closed-space fire resistance test method thereof. The model comprises a cable test model composed of multiple strands, the dimensions of which are calculated by a predetermined formula, the strands being reduced in proportion to the predetermined formula, and an annular steel clamp at each end of the model. The model also includes pre-embedded thermocouples. During model fabrication, the number of strands to be welded with thermocouples is counted according to predetermined target measuring points, and the thermocouples are welded to the surface of the strand measuring points. During model fabrication, the strands with thermocouples are positioned in a predetermined order according to the measuring point locations, the thermocouple wires are led out through the strand gaps, and steel gaskets are installed on the exposed thermocouple surfaces of the model to isolate the flame. The model surface is coated with a fireproof and anti-corrosion layer, and the model is placed in a test furnace for fire resistance testing. The present invention primarily studies the effects of different fire conditions, fire protection, and fire exposure time on heat transfer and thermal damage of large-diameter cables, and verifies the actual fire resistance performance of fireproof materials.

Description

Technical Field

[0001] The present invention relates to the technical field of fire testing of large-diameter cables for bridges, and in particular to a large-diameter cable test model and a closed space fire resistance test method thereof. Background Art

[0002] With economic development, the construction of transportation infrastructure, such as long-span bridges, has rapidly expanded. This increase in urban traffic has also led to an increase in vehicles transporting flammable and explosive materials, making bridge fire safety hazards more prominent. Cables are critical force-transmitting components of bridge structures and are typically composed of steel wire bundles and strands. High-strength steel wires and strands are highly susceptible to heat damage in fires, making them difficult to repair and replace. Therefore, fire safety in bridge cables presents significant challenges.

[0003] Cables for large-span bridges generally have a large diameter. Currently, the largest bridge cable in service has a diameter of more than 1,500 mm. Therefore, when studying the heat transfer process and fire resistance of cables during fire, the influence of cross-sectional size should be considered, which helps to accurately simulate the actual fire conditions of large-diameter bridge cables. In addition, when a fire occurs on a bridge, a large temperature gradient will be generated during the heat transfer process of the cable as the cross-sectional depth increases, further affecting the stress distribution inside the cable. Large-diameter cables will also produce more realistic evolution mechanisms and intuitive destruction mechanisms under the action of fire. At present, the evaluation of the fire resistance of large-diameter cables under fire is not accurate. Therefore, it is necessary to study the disaster of large-diameter cables under fire in enclosed spaces and establish a large-diameter cable and fire temperature field monitoring test system. The construction of this test system mainly involves the following issues:

[0004] Currently, there is a lack of systematic and mature testing methods for enclosed space fire testing of large-diameter cables. Conventional testing methods typically involve reducing the cable size. If the reduction ratio is too small, it is impossible to physically simulate the fire temperature field of a full-scale cable. Therefore, studying the heat transfer process of large-diameter cables is more meaningful. If the reduction ratio is too large, such as when the cable model diameter exceeds 1000mm, cable restraint during model production becomes difficult, increasing the risk. Existing testing conditions are difficult to meet testing requirements.

[0005] The porosity of the model significantly influences the physical heat transfer of cables, and strand size is a key factor influencing the porosity. Furthermore, for large-diameter cables, a large number of thermocouples must be placed within the model cross-section to accurately measure the cross-sectional temperature field during fire exposure. This increase in thermocouples leads to a decrease in the model porosity. Therefore, the placement and positioning of the thermocouples within the cable, as well as the strand size, need to be optimized to ensure accurate porosity.

[0006] Currently, large-span bridge cable assembly uses large-scale cable extrusion machines and cable clamps for constrained molding. Due to the size of conventional fire furnaces in the laboratory, the length of cable fire-resistant models is generally short, with a maximum of about 2 meters. There is currently no dedicated cable extrusion and assembly equipment and supporting cable clamps, making it difficult to constrain and mold large-diameter cables in the laboratory. Summary of the Invention

[0007] In view of the above-mentioned technical deficiencies, the purpose of the present invention is to provide a large-diameter cable test model and a closed space fire resistance test method thereof, which can not only meet the accuracy of the physical simulation of the fire temperature field of the large-diameter cable, but also achieve convenience and safety in the assembly process of the large-diameter cable.

[0008] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0009] In a first aspect, the present invention provides a large-diameter cable test model, comprising:

[0010] A cable test model of assembled strands, whose cross-sectional diameter is proportionally reduced from the full-scale cable, is composed of multiple strands of mutually parallel, equal length, and aligned ends, secured by a plurality of annular steel clamps. The strand reduction ratio is consistent with that of the cable test model, and the strands are formed by extruding multiple bundles of fine steel wire into round shapes and are secured at both ends and in the middle with staggered steel tie straps.

[0011] Thermocouples are welded to the measuring point surfaces of several strands inside the cable test model and led out through the gaps between the strands. The strands welded with thermocouples are positioned in sequence within the cable test model according to the target measuring point locations. Steel gaskets are installed on the thermocouple surfaces exposed outside the cable test model to isolate the flame.

[0012] Steel wire, wrapped around the surface of the cable test model.

[0013] In a second aspect, the present invention further provides a closed space fire resistance test method for a large diameter cable test model, comprising the following steps:

[0014] Step 1: Determine the diameter of the cable test model and use fine steel wire to make an appropriate number of cable strands. Tie each cable strand several times with steel tie bands at both ends and in the middle to round and secure the fine steel wire. The steel tie bands are staggered, and the tying positions of different cable strands are staggered.

[0015] Step 2: Place several annular auxiliary devices in the same straight line at both ends and the middle of the cable test model to be made. The annular auxiliary devices at both ends are placed on the outside, and the annular auxiliary devices in the middle ensure that the cable test model is straight as a whole.

[0016] Step 3: Count the number of cable strands that need to be welded with thermocouples according to the preset target measuring point positions, and then weld the thermocouples to the surfaces of the cable strand measuring points;

[0017] Step 4: Using the annular auxiliary device as a mold, the cable strands are arranged in layers. At the same time, the cable strands welded with thermocouples are positioned in a preset order according to the target measurement point locations, ensuring that the strands are parallel and aligned at both ends. The thermocouple wires are led out of the cable test model through the gaps between the strands.

[0018] Step 5: After the cable strands are installed, fix both ends with annular steel clamps. At the same time, remove the completed cable test model from the annular auxiliary device to complete the production of the cable test model.

[0019] Step 6: Wrap steel wire around the surface of the cable test model, and install steel gaskets on the surface of the thermocouple exposed outside the cable test model to isolate the flame;

[0020] Step 7: Place the two steel-concrete refractory supports in a closed test furnace and adjust the placement direction and the distance between the two steel-concrete refractory supports;

[0021] Step 8: hoist and install the cable test model on the steel-concrete fire-resistant support, and use aluminum silicate wool to cover the ends of the cable test model, the steel-concrete fire-resistant support, and the surface of the thermocouple for fireproof and heat insulation protection;

[0022] Step 9: After completing the fire protection of the cable test model end and the thermocouple wire, close the furnace body and connect the thermocouple wire to the data acquisition instrument. The test is ready and the test is completed according to the specified fire environment and target fire exposure time.

[0023] Preferably, in step seven, the steel-concrete refractory support is made of square steel pipes and concrete, and includes an I-shaped bottom support, two columns are provided on the bottom support, and a crossbeam is fixedly connected between the two columns.

[0024] Preferably, in step eight, a fireproof expansion end is used to fireproof both ends of the cable test model. The fireproof expansion end is a sealed shell and its bottom is also adapted to be equipped with a steel-concrete bracket fireproof protection body that wraps the steel-concrete fire-resistant bracket. A steel gasket is installed on the surface of the thermocouple exposed outside the cable test model to isolate the flame, and the thermocouple wire of the thermocouple is led out of the furnace from the fireproof expansion end and the inside of the steel-concrete bracket fireproof protection body.

[0025] Preferably, in step 2, the annular auxiliary device includes a base steel plate, the base steel plate is welded and fixed to the vertical annular cut steel plate, and a plurality of stiffening ribs are provided between the base steel plate and the vertical annular cut steel plate; the length of the base steel plate is greater than the outer diameter of the cable test model, and the width of the base steel plate is half of its length;

[0026] The vertical ring-cut steel plate is a square steel plate, and its side length is equal to the length of the base steel plate. The middle part of the vertical ring-cut steel plate is ring-cut into a hole according to the size of the cable test model. The upper part of the vertical ring-cut steel plate is opened from the center of the side length, and the opening size is larger than the outer diameter of the cable strand.

[0027] Preferably, in step five, the annular steel hoop is composed of two semicircular steel plates fixedly connected, and the cable test model has a plurality of annular steel hoops symmetrically arranged at both ends thereof, and two adjacent annular steel hoops on the same side are arranged at a preset distance.

[0028] Preferably, in step six, when a cable fire resistance test is required, a removable fireproof and anti-corrosion layer is applied to the outer surface of the steel wire, and the removable fireproof and anti-corrosion layer is made of high-temperature resistant aerogel felt.

[0029] Preferably, different fire environments and different target fire exposure times are set in the test to meet different fire conditions, and the test termination conditions include the steel surface temperature reaching a preset temperature and the test duration reaching the target fire exposure time.

[0030] The beneficial effects of the present invention are:

[0031] 1. This invention constructs a model by scaling down the strands to match the overall model and arranges the strands in layers, achieving accurate reconstruction of the full-scale cable temperature field while ensuring the correct diameter. Steel ties are used to stagger the ends and middle of the strands, and the spatial staggered distribution of the different strand binding points ensures that the model porosity approaches the actual porosity.

[0032] 2. Thermocouples are welded to the surface of the cable strands at the measuring points and properly led out. Steel gaskets are installed on the exposed parts to isolate the flame. At the same time, fireproof and heat-insulating protection is provided on both ends of the cable test model, the steel-concrete fire-resistant bracket and the thermocouple to prevent heat from entering from the ends and burning the thermocouple wires, thus ensuring the accuracy of temperature data collection.

[0033] 3. The cable test model is equipped with a removable fire and corrosion resistant layer, which can be applied when the cable fire resistance test is required, thus enhancing the protection performance of the model and the diversity of the test. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0035] Figure 1 A schematic diagram of a large-diameter cable test model experiment provided by an embodiment of the present invention;

[0036] Figure 2 A schematic structural diagram of a large-diameter cable test model provided by an embodiment of the present invention when placed on a steel-concrete fire-resistant support;

[0037] Figure 3 A schematic cross-sectional view of a cable test model provided by an embodiment of the present invention;

[0038] Figure 4 A schematic structural diagram of a ring-shaped auxiliary device provided in an embodiment of the present invention;

[0039] Figure 5 A schematic structural diagram of an annular steel clamp provided in an embodiment of the present invention;

[0040] Figure 6 A schematic diagram of the structure of a cable test model produced on a ring-shaped auxiliary device according to an embodiment of the present invention;

[0041] Figure 7 A schematic structural diagram of a steel-concrete refractory support provided in an embodiment of the present invention.

[0042] Description of reference numerals:

[0043] 1-cable test model; 2-annular auxiliary device; 3-cable strand; 4-annular steel clamp; 5-steel wire; 6-thermocouple; 7-stiffening rib; 8-base steel plate; 9-vertical ring-cut steel plate; 10-bolt; 11-steel-concrete fire-resistant support; 12-bottom support; 13-column; 14-beam; 15-test furnace; 16-fireproof expansion end; 17-steel-concrete support fire protection body; 18-removable fireproof and anti-corrosion layer. DETAILED DESCRIPTION

[0044] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0045] like Figures 1 to 3 As shown, this embodiment provides a large diameter cable test model, hereinafter referred to as the model, comprising:

[0046] A cable test model 1 of assembled strands is a scaled-down version of a full-scale, long-span bridge cable. The cable test model 1 is composed of multiple parallel strands 3 of equal length and aligned at both ends, secured by a plurality of annular steel clamps 4. The strands 3 are scaled down to the same scale as the cable test model 1. The strands 3 are formed by extruding multiple bundles of fine steel wire and are secured at both ends and in the middle with staggered steel ties to reduce the porosity of the cable test model 1 to approximate the porosity of an actual cable.

[0047] Thermocouples 6 are welded to the surfaces of the strands 3 at the measuring points and are led out through the gaps between the strands 3. Steel gaskets are installed on the surfaces of the thermocouples 6 exposed outside the cable test model 1 to isolate the flame. Thermocouples 6 are welded to the surfaces of several strands 3 inside the cable test model 1. The strands 3 with thermocouples 6 are positioned in sequence within the cable test model 1 according to the locations of the target measuring points.

[0048] The steel wire 5 is wound around the surface of the cable test model 1 to prevent the flame from directly contacting the cable test model 1 .

[0049] Assume that the cable prototype diameter adapted by the large-diameter cable test model of this embodiment is R, 80 mm ≤ R ≤ 1500 mm.

[0050] An embodiment of the present invention further provides a closed space fire resistance test method for a large-diameter cable test model, comprising the following steps:

[0051] Step 1: Determine the diameter of the cable test model 1, and use fine steel wire to make an appropriate number of cable strands 3. Each cable strand 3 is tied several times with steel tie bands at both ends and in the middle to round and secure the fine steel wire. The steel tie bands are staggered, and the tying positions of different cable strands 3 are staggered to reduce porosity.

[0052] Step 2: Place several annular auxiliary devices 2 in the same straight line at both ends and the middle of the cable test model 1 to be made. The annular auxiliary devices 2 at both ends are placed outwards to facilitate removal after the cable is formed, and the annular auxiliary devices 2 in the middle ensure that the cable test model is straight as a whole.

[0053] See also Figure 4 The annular auxiliary device 2 includes a base steel plate 8, which is welded to a vertical ring-cut steel plate 9, and a plurality of stiffening ribs 7 for reinforcement are provided between the base steel plate 8 and the vertical ring-cut steel plate 9; the length of the base steel plate 8 is greater than the outer diameter of the cable test model 1, and the width of the base steel plate 8 is half of its length to prevent the annular auxiliary device 2 from tilting;

[0054] The vertical ring-cut steel plate 9 is a square steel plate, and its side length is equal to the length of the base steel plate 8. The middle part of the vertical ring-cut steel plate 9 is ring-cut into a hole according to the size of the cable test model 1. The upper part of the vertical ring-cut steel plate 9 is opened from the center of the side length, and the opening size is larger than the outer diameter of the cable strand 3.

[0055] Step 3: Count the number of strands 3 that need to be welded with thermocouples 6 according to the preset target measuring point positions, and then weld the thermocouples 6 to the surfaces of these strands 3 at the measuring points;

[0056] Step 4: Using the annular auxiliary device 2 as a mold, the cable strands 3 are arranged in layers. At the same time, the cable strands 3 welded with thermocouples 6 are positioned in a preset order according to the target measurement point positions, ensuring that the cable strands 3 are parallel and aligned at both ends. The thermocouple wires of the thermocouples 6 are led out of the cable test model 1 through the gaps between the cable strands 3.

[0057] Step 5: After the cable strands 3 are installed, the ends are fixed with annular steel clamps 4. At the same time, the completed cable test model 1 is taken out of the annular auxiliary device 2 to complete the main body of the cable test model 1.

[0058] See also Figure 5-Figure 6 The annular steel hoop 4 is composed of two semicircular steel plates fixedly connected. The cable test model 1 has two groups of annular steel hoops 4 symmetrically arranged at both ends and fastened with bolts 10, which are used to extrude and fix the cable strands 3. Each group is provided with several annular steel hoops 4 and a preset distance between two adjacent annular steel hoops 4 is used to adapt to the lifting equipment.

[0059] Step 6: Wrap the steel wire 5 around the surface of the cable test model 1, and install a steel gasket on the surface of the thermocouple 6 exposed outside the cable test model 1 to isolate the flame;

[0060] Step 7: Place the two steel-concrete refractory supports 11 in the closed test furnace 15, and adjust the placement direction and the distance between the two steel-concrete refractory supports 11 to ensure that the cable test model 1 can be installed on the crossbeam 14 of the steel-concrete refractory supports 11;

[0061] See also Figure 7 The steel-concrete refractory support 11 is made of square steel pipes and concrete, and includes an I-shaped bottom support 12. Two columns 13 are provided on the bottom support 12, and a crossbeam 14 is fixedly connected between the two columns 13. When the crossbeam 14 supports the cable test model 1, it can ensure that its position is above the middle of the test furnace 15, so that it is evenly exposed to fire during the test.

[0062] Step 8: Hoist and install the cable test model 1 on the steel-concrete fire-resistant support 11, and use aluminum silicate wool to cover the ends of the cable test model 1, the steel-concrete fire-resistant support 11, and the surface of the thermocouple 6 for fireproof and heat-insulating protection. This can prevent the thermocouple wire from being burned and heat from entering from the end, affecting the accuracy of the test results.

[0063] A fireproof expansion end 16 is used to fireproof both ends of the cable test model 1. The fireproof expansion end 16 is a sealed shell and its bottom is also adapted to be equipped with a steel-concrete bracket fireproof protection body 17 that can wrap the steel-concrete fire-resistant bracket 11. A steel gasket is installed on the surface of the thermocouple 6 exposed outside the cable test model 1 to isolate the flame. The thermocouple wire of the thermocouple 6 is led out of the furnace from the fireproof expansion end 16 and the steel-concrete bracket fireproof protection body 17 to prevent the thermocouple wire from being damaged by high temperature.

[0064] Step 9: After completing the fire protection of the cable test model 1 end and the thermocouple wire, close the furnace body and connect the thermocouple wire to the data acquisition instrument to collect temperature data. The test is ready and the test is completed according to the specified fire environment and target fire exposure time.

[0065] Different fire environments and target fire times can be set during the test to meet different fire conditions. The test termination conditions include the steel surface temperature reaching 300°C and the test duration reaching the target fire time.

[0066] When a cable fire resistance test is required, a detachable fireproof and anti-corrosion layer 18 is applied to the outer surface of the steel wire 5 , and the detachable fireproof and anti-corrosion layer 18 is made of high-temperature resistant aerogel felt.

[0067] Obviously, those skilled in the art may make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if such changes and modifications fall within the scope of the claims and their equivalents, the present invention is intended to include such changes and modifications.

Claims

1. A closed space fire resistance test method for a large diameter cable test model, characterized in that: The large diameter cable test model includes: A cable test model (1) of a cable strand assembly type has a cross-sectional diameter that is proportionally reduced based on a full-scale cable. The cable test model (1) is composed of a plurality of parallel cable strands (3) of the same length and aligned at both ends, which are locked and fixed by a plurality of annular steel clamps (4). The reduction ratio of the cable strands (3) is consistent with the reduction ratio of the cable test model (1), and the cable strands (3) are formed by extruding a plurality of bundles of fine steel wires into round shapes and are staggered and fixed at both ends and in the middle by steel tie ties. Thermocouples (6) are welded to the measuring point surfaces of a plurality of strands (3) inside the cable test model (1) and are led out through the gaps between the strands (3). The strands (3) welded with thermocouples (6) are positioned in sequence within the cable test model (1) according to the target measuring point positions. Steel gaskets are installed on the surfaces of the thermocouples (6) exposed outside the cable test model (1) to isolate the flame. A steel wire (5) is wound around the surface of a cable test model (1); wherein the cable prototype diameter corresponding to the large-diameter cable test model is R, 80 mm ≤ R ≤ 1500 mm; The method for making a cable test model (1) of a strand-assembled type comprises the following steps: Step 1: Determine the diameter of the cable test model (1), use thin steel wire to make an appropriate number of cable strands (3), and tie each cable strand (3) several times with steel tie bands at both ends and in the middle to squeeze and fix the thin steel wire. The steel tie bands are arranged in a staggered manner, and the tying positions of different cable strands (3) are staggered; Step 2: Place a number of annular auxiliary devices (2) in the same straight line at both ends and the middle of the cable test model (1) to be made, respectively. The annular auxiliary devices (2) at the two ends are positioned outside, and the annular auxiliary devices (2) in the middle ensure that the cable test model (1) is straight as a whole. The annular auxiliary device (2) includes a base steel plate (8), the base steel plate (8) is welded and fixed to a vertical ring-cut steel plate (9), the middle portion of the vertical ring-cut steel plate (9) is ring-cut into a hole according to the size of the cable test model (1), and the upper portion of the vertical ring-cut steel plate (9) is opened from the center of the side length, and the opening size is larger than the outer diameter of the cable strand (3); Step three, counting the number of cables (3) that need to be welded with thermocouples (6) according to the preset target measuring point positions, and then welding the thermocouples (6) to the surfaces of the cable strands (3) at the measuring points; Step 4: Using the annular auxiliary device (2) as a mold, the cable strands (3) are placed in layers, and at the same time, the cable strands (3) welded with thermocouples (6) are placed and positioned in a preset order according to the target measurement point position, ensuring that the cable strands (3) are parallel and the cable strands (3) at both ends are flush, and the thermocouple wires of the thermocouples (6) are led out of the cable test model (1) through the gaps between the cable strands (3); Step 5: After the cable strand (3) is installed, the two ends are fixed with annular steel hoops (4), and the completed cable test model (1) is taken out from the annular auxiliary device (2) to complete the production of the cable test model (1); the annular steel hoop (4) is composed of two semicircular steel plates fixedly connected, and the cable test model (1) is symmetrically provided with a plurality of annular steel hoops (4) at both ends thereof, and two adjacent annular steel hoops (4) on the same side are arranged at a preset distance; Step 6: Wrap the steel wire (5) around the surface of the cable test model (1), and install a steel gasket on the surface of the thermocouple (6) exposed outside the cable test model (1) to isolate the flame; The fire resistance test method of the cable test model (1) comprises the following steps: Step 7: Place the two steel-concrete refractory supports (11) in a closed test furnace (15), and adjust the placement direction and the distance between the two steel-concrete refractory supports (11); Step eight, hoisting and installing the cable test model (1) on the steel-concrete fire-resistant support (11), and using aluminum silicate wool to cover the ends of the cable test model (1), the steel-concrete fire-resistant support (11) and the surface of the thermocouple (6) for fireproof and heat-insulating protection; Step 9: After completing the fire protection of the cable test model (1) end and the thermocouple wire, close the furnace body, connect the thermocouple wire to the data acquisition instrument, and the test preparation is completed. Complete the test according to the specified fire environment and target fire exposure time.

2. The closed space fire resistance test method for a large diameter cable test model according to claim 1, characterized in that: In step seven, the steel-concrete refractory support (11) is made of square steel pipes and concrete, and includes an I-shaped bottom support (12). Two columns (13) are provided on the bottom support (12), and a crossbeam (14) is fixedly connected between the two columns (13).

3. The closed space fire resistance test method for a large diameter cable test model according to claim 1, characterized in that: In step eight, a fireproof expansion end (16) is used to fireproof both ends of the cable test model (1). The fireproof expansion end (16) is a sealed shell and its bottom is also adapted to be equipped with a steel-concrete support fireproof protection body (17) that wraps the steel-concrete fire-resistant support (11). A steel gasket is installed on the surface of the thermocouple (6) exposed outside the cable test model (1) to isolate the flame. The thermocouple wire of the thermocouple (6) is led out of the furnace from the fireproof expansion end (16) and the steel-concrete support fireproof protection body (17).

4. The closed space fire resistance test method for a large diameter cable test model according to claim 1, characterized in that: In step 2, a plurality of stiffening ribs (7) are provided between the base steel plate (8) and the vertical ring-cut steel plate (9); the length of the base steel plate (8) is greater than the outer diameter of the cable test model (1), and the width of the base steel plate (8) is half of its length; the vertical ring-cut steel plate (9) is a square steel plate, and the side length thereof is equal to the length of the base steel plate (8).

5. The closed space fire resistance test method for a large diameter cable test model according to claim 1, characterized in that: In step six, when a cable fire resistance test is required, a removable fireproof and anti-corrosion layer (18) is applied to the outer surface of the steel wire (5), and the removable fireproof and anti-corrosion layer (18) is made of high-temperature resistant aerogel felt; different fire environments and different target fire exposure times are set in the test to meet different fire conditions, and the test termination conditions include the steel surface temperature reaching the preset temperature and the test duration reaching the target fire exposure time.

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