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Building components will collapse and damage and cause casualties if they catch fire or lose stability due to heat.
In order to safely evacuate people, rescue materials and put out fires, buildings are required to have a certain fire resistance capacity.
The fire resistance of buildings depends on the combustion performance and fire resistance limit of building components.
I.
definition of fire resistance limit ― fire resistance limit refers to the time taken from the action of fire to the loss of bearing capacity, integrity or thermal insulation of building components, fittings or structures under standard fire resistance test conditions, expressed in hours (H).
Fire resistance limit is the main index to measure the fire resistance of building components, which needs to be determined by fire resistance test in accordance with national standards.
II.
The determination of fire resistance limit is usually based on whether the building component loses fire resistance stability, integrity and heat insulation to judge whether the component has reached the fire resistance limit.
As long as any of the above phenomena occurs in the building component, it indicates that the building component has reached the fire resistance limit.
1.
Fire resistance stability refers to the ability of load-bearing building components to resist collapse within a certain period of time under standard fire resistance test conditions.
The parameters to determine whether the component can maintain its bearing capacity continuously during the fire resistance test are the deformation amount and deformation rate of the component.
2.
Fire resistance integrity refers to the ability to prevent the penetration of flame and flue gas or the emergence of flame on the back fire surface within a certain period of time when one side of the building partition member is exposed to fire under the standard fire resistance test conditions.
The integrity of the component is considered to be lost in any of the following limiting conditions: (1) the cotton pad is ignited according to the standard fire resistance test.
(2) according to the standard fire resistance test, the gap probe can pass through.
(3) flame appears on the back fire surface and lasts for more than 10s0 # 3 Fire resistance and thermal insulation – fire resistance and thermal insulation refers to the ability to prevent the temperature of the back fire surface from exceeding the specified value within a certain period of time when one side of the building partition member is exposed to fire under the standard fire test conditions.
In case of any of the following limiting conditions for the temperature rise of the back fire surface of the component, it is considered that the thermal insulation is lost: (1) the average temperature rise exceeds the initial average temperature by 140 ℃.
(2) the temperature rise at any position exceeds the initial temperature by 180 ℃.
The initial temperature shall be the initial average temperature of the back fire surface at the beginning of the test.
Load bearing components (such as beams, columns, roof trusses, etc.) do not have the function of blocking flame and heat conduction, so losing stability means reaching its fire resistance limit; Load bearing partition components (such as load-bearing wall, firewall, floor slab, roof slab, etc.) have dual functions of load-bearing and partition.
Therefore, when the component loses stability, integrity or thermal insulation in the test, the component reaches its fire resistance limit; For specially specified building components (such as fire doors, fire rolling shutters, etc.), the thermal insulation fire doors shall meet the fire integrity and heat insulation within the specified time, and the non thermal insulation fire doors shall only meet the fire integrity within the specified time.
III.
factors affecting fire resistance limit in fire, building components with good fire resistance can prevent the spread of fire and prolong the support time.
The fire resistance of components directly determines the instability and collapse time of buildings in fire and the time to control the spread of fire to adjacent fire compartments.
There are many factors affecting the fire resistance of building components, mainly in the following aspects: 1 The combustion performance of the material itself is the main internal factor affecting the fire resistance limit of the component.
If the materials constituting the building components are combustible materials, the components will be ignited and spread the fire, the integrity of the components will be damaged, the thermal insulation capacity will be lost, the bearing capacity will be gradually lost and the stability will be lost, and the fire resistance limit of the components is relatively low.
The combustion performance of the material is good, and the fire resistance limit of the component is low.
The fire resistance of wooden floor slab is lower than that of reinforced concrete floor slab.
2.
High temperature mechanical properties and thermal conductivity of materials # components composed of materials with better mechanical properties and poor thermal conductivity at high temperature have higher fire resistance limit; On the contrary, the fire resistance limit is low.
For example, the fire resistance of reinforced concrete columns under the same stress conditions is much higher than that of steel columns.
Compared with the concrete structure, the steel structure has the advantages of light weight, high strength, good error resistance, easy industrial production and fast construction speed.
However, the fire resistance of steel structure is very poor, which is mainly due to two aspects: first, the heat conductivity of steel is large, and the temperature of steel structure rises rapidly under fire; Second, the strength of steel decreases rapidly with the increase of temperature.
The fire resistance time of steel structure without fire protection is usually only 15 ~ 20min, so it is very easy to be damaged under the action of fire, and often deforms and collapses in the early stage of fire.
3.
The larger the section size of building components is, the higher the fire resistance limit is.
4.
The manufacturing method of components is # the fire resistance limit of components produced by the same material, the same section size and different manufacturing processes is also different.
For example, the fire resistance of prestressed reinforced concrete members is much lower than that of cast-in-situ reinforced concrete members.
5.
Structural mode between members} when other conditions are certain, different connection modes between members affect the fire resistance limit of members, especially the treatment methods of joints, such as welding, screw connection, simple support, cast-in-place and so on.
Under the same conditions, the fire resistance of cast-in-situ reinforced concrete beams and slabs is higher than that of simply supported reinforced concrete beams and slabs.
6.
The greater the thickness of the protective layer is, the higher the fire resistance limit is.
In order to improve the fire resistance of steel members, fire protection is usually carried out by brushing fire retardant coating or coating non combustible materials.
Increasing the thickness of protective layer can improve the fire resistance of members…
