The main calculation process is (1) to determine the fire resistance rating, fire resistance time limit of components, and fire retardant coating type of components.
② Temperature rise calculation: this is determined according to Article 6.2.1-2 of steel fire protection.
The design fire resistance limit of supports between columns shall be the same as that of columns, the design fire resistance limit of floor supports shall be the same as that of beams, and the design fire resistance limit of roof supports and tie bars shall be the same as that of roof load-bearing members.
(5) Perform internal force calculation and internal force combination [relevant description] This article considers the possible simultaneous load (action) on the structure during fire according to Article 3.2.2 of Steel Fire Protection, and shall be determined according to the most unfavorable value of the following combination values.
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The section shape factor Fi/V of steel members with fire protection is not only related to the section characteristics of steel members, but also related to the practice of fire protection layer.
With the increase of component temperature under fire, the strength of materials will decrease and the bearing capacity of components will also decrease; When the bearing capacity of the member is reduced to the most unfavorable combination effect, the member reaches the limit state of fire resistance bearing capacity.
Such purlin damage may lead to the loss of the overall stability of the main structure and the overall overturning.
Main design methods (1) The fire resistance calculation and fire protection design of steel structure members can adopt the fire resistance limit method, bearing capacity method or critical temperature method.
In addition, for places where combustible materials are mainly hydrocarbon materials, such as petrochemical buildings and workshops where hydrocarbon materials and products are produced and stored, the hydrocarbon (HC) temperature rise curve should be adopted.
2.
1、 Step 1 of steel structure fire protection design.
(2) Calculate the section fireproof shape parameter Fi/v [relevant description] This article is based on the description of Article 6.2.2 of steel fire protection and the article description Fi – the fire surface area of the unit length of steel members with fire protection (m2); For the outer edge type fire protection, take the internal surface area of the fire protection material of the unit length steel member; For non-outer edge type fire protection, take the minimum inner surface area of the possible rectangular packaging measured along the unit length of steel members; V – volume of steel member per unit length (m3).
(4) Section stiffness reduction (actually elastic modulus reduction) and strength reduction caused by temperature rise [relevant description] According to Article 5.1.2-5.1.3 of Steel Fire Protection, the strength and elastic modulus reduction curves are as follows.
Therefore, such purlins should be considered as an integral part of the main structural system of the roof, and their design fire resistance limit should be taken according to the requirements of Table 1 for “roof support and tie bar”.
[Related notes] According to Article 3.1.1 of Steel Fire Protection, the design fire resistance limit of steel structural members shall be determined according to the fire resistance rating of the building and the current national standard Code for Fire Protection Design of Buildings (GB50016).
For combustible materials, such as wood, paper, cotton, cloth, clothing, etc., which can be mixed with a small amount of plastic or synthetic materials, the temperature rise curve based on fiber fire should be adopted.
Even if it is damaged in the fire, it will not cause the overall failure of the structure.
(1) The first type of purlins only support the roof slab.
According to these two formulas, the author compiled the relevant iterative solution table, which was verified to be accurate and consistent with yjk and other software.
(2) The second type of purlins, in addition to supporting the roof panel, also serve as longitudinal tie bars, which play a role of lateral support for the main structure (such as roof truss); Or as the web member of the transverse horizontal support bay.
(3) Determine the indoor fire temperature rise curve of the environment where the component is located, and conduct the temperature rise calculation of the protected component (or the temperature rise calculation of the unprotected component, which is used for the critical temperature method) [relevant description] ① Fire curve: this article is determined according to Article 6.1.1 of the steel fire protection, and pay attention to distinguish the ambient temperature rise curve of the general indoor fire and the large space fire, which are two typical building fires.
[Related notes] This article is in accordance with Article 3.2.6 of the Technical Code for Fire Protection of Building Steel Structures (GB51249-2017).
The bearing capacity method or critical temperature method is used to directly check whether the members meet the requirements of fire resistance bearing capacity limit state within the design fire resistance limit time.
The fire resistance limit method is to determine whether the fire resistance of the component meets the requirements and determine its fire protection by comparing the actual fire resistance limit and the design fire resistance limit of the component.
The fire protection layer commonly used in the project can be divided into two types: (1) outer edge protection, that is, the fire protection layer is protected along the outer surface of the steel member; (2) Non-external edge protection, that is, all or part of the fire protection layer is not protected along the outer surface of the steel member.
For such purlins, the fire resistance limit may not be required.
The time from the fire to the ultimate state of fire resistance bearing capacity is the fire resistance limit of the component; The critical temperature of the member is the temperature when the member reaches its ultimate state of fire resistance bearing capacity.
(2) The fire protection design of steel structure shall be based on the fire resistance check calculation of the whole structure or the fire resistance check calculation of the components according to the importance of the structure, the structure type and the load characteristics, and shall comply with the following provisions: 1.
Such purlin damage only affects the local roof panel and has little impact on the overall mechanical performance of the roof structure.
The fire protection design method based on the fire resistance check calculation of the whole structure should be adopted for the large-span steel structure with a span of not less than 60m; 2 Prestressed steel structures and steel structures in large-span buildings with a span of not less than 120m shall adopt the fire protection design method based on the fire resistance calculation of the whole structure.
Table 11 shows the calculation example of section shape coefficient of common steel members with fire protection.
Therefore, it should not be considered as an integral part of the main structural system of the roof.
