“Fire protection design of steel structure” must master the knowledge points of fire protection design!

What is a steel structure? In academic language, steel structure is one of the most important types of engineering and construction structures today, with construction steel as the main body of the structure. Construction steel is characterized by high strength, light weight, good overall stiffness, and strong deformation ability, so it is especially suitable for building large-span and very high, lightweight buildings.

In recent years, China’s construction engineering in the field of concrete and masonry structure domination of the situation is changing, steel structure due to its high strength, light weight, construction speed and a series of advantages, steel structure is often as the preferred form of structure is used in large quantities in factories, airports, gymnasiums and other large and medium-sized buildings, and reinforced concrete structure and other forms of structure compared to the steel structure has a fatal shortcoming is Poor fire resistance. With the wide application of steel structure building, the fireproof problem of steel structure is getting more and more attention from researchers and designers.

Compared with concrete materials, steel without fire protection measures degrades greatly in mechanical properties under fire. When the temperature reaches 600°C, the yield strength of steel is only 50% of the normal temperature, when the temperature reaches 700°C, the yield strength of steel is only 20% of the normal temperature, and the mechanical properties of high-strength steel degradation is even more serious, it can be seen that steel at high temperatures, its strength and elastic modulus of the rapid decay of the structural load bearing capacity of steel components and their composition is rapidly reduced. Another characteristic of steel is its high thermal conductivity, which makes it heat up quickly in fire scenarios. Without taking fire protection of steel components, in the fire 15-20 minutes, it may reach a high temperature of 600 ° C or more, it is difficult to meet the “building design fire code” GB50016 fire resistance limit requirements. Based on the steel’s own material and characteristics at high temperature, the fire resistance time of steel structure without fire protection is usually only 15~20 minutes, steel in the fire at high temperature, steel strength and modulus of elasticity rapidly attenuate, and the bearing capacity of steel components and steel structure rapidly reduces, so it can be seen that the fire protection design is an important part of the design of steel structure.

The main factors affecting the fire resistance time of steel components

There are two main factors affecting the fire-resistance limit time of steel components, namely, the load ratio of the component and the cross-section shape coefficient of the component.

(1) The load ratio of the steel member is one of the main factors affecting the fire resistance limit time of the member. Obviously, when the load ratio of the steel member is 0, the steel member will not be damaged under fire, except for high temperature melting; when the load ratio of the steel member is close to 1, the steel member will be damaged when the temperature is low. Therefore, the fire-resistant design of steel structure should consider the actual stress situation of steel members, i.e. the load ratio of steel members.

(2) The cross-section shape factor of steel members is another major factor affecting the ultimate fire resistance time of members. The cross-section shape coefficient of the steel member refers to the ratio of the cross-section perimeter and cross-section area of the steel member, which characterizes the thickness of the steel member. Obviously, the thinner the steel plate of the steel member, the larger the cross-section shape factor of the steel member, the faster the temperature rise under fire, the smaller the fire resistance time.

Precautions for the application of fire protection design method for steel structure

(1)In general, the coating manufacturer does not provide the density ϼi and specific heat ci of the fireproof coating, and only provides the thermal conductivity λ of non-expansion type coating expansion type coating or equivalent thermal resistance R and its corresponding thickness, and it is recommended to use the critical temperature method to calculate. However, the critical temperature method is applicable to the load ratio between 0.3 and 0.9. If the load ratio is less than 0.3 and more than 0.9, the results of the critical temperature method and the structural capacity method differ greatly, and then the structural capacity method shall be applied.

(2)Steel Standard (Section 18.3) applies to the design of steel structure at room temperature and the design of steel structure at long term continuous high temperature, while Technical Code for Fire Protection of Steel Structures in Buildings is the design of steel structure at high temperature for a short period of time (fire-resistant limit time).

(3)When adopting structural bearing capacity method for fire-resistant design, it should be noted that most of the current calculation software is only applicable to the frame structure and the booming combustion environment generated by the standard temperature rise curve, and it is not applicable to some high space structures such as steel trusses and mesh-shell net frames of stadiums, etc. The temperature rise curve of high space buildings should be adopted for stadiums and so on. For the fireproof coating in the calculation of the parameters used in the fireproof design instructions should be specified, and require the design and calculation of the fireproof coating thermal resistance (or equivalent thermal conductivity) and thickness is not less than the test data.

(4)According to Article 4.1.3 of GB51249-2017, it is not suitable to use intumescent fireproof coating for the components with design fire resistance limit greater than 1.5h; the thickness of non-intumescent fireproof coating should not be less than 10mm (fire code is 15mm).

Characteristics of steel structure fireproof coating and selection principles

1.Classification of steel structure fireproof coating and its characteristics

Intumescent fireproof coating is mainly made of polymer materials. With the prolongation of time, these organic materials may be decomposed, explained, dissolved and other irreversible reactions, so that the coating aging failure, chalking, shedding or intumescent performance decline. Therefore, the use of intumescent fireproof coating is limited to a certain extent, but the epoxy class intumescent steel structure fireproof coating has excellent bonding properties, weatherability and fire performance, so the restrictions on the use of the place to be relaxed. The life span of non-intumescent fireproof coating is longer than that of intumescent fireproof coating, and the better the use environment, the longer the life span. However, when the thickness of non-intumescent fireproof coating is too small, the construction is difficult and it is difficult to guarantee the construction quality, so the minimum thickness is not less than 10mm.

2.Principle of selecting fireproof coating

When selecting fireproof coating for steel structure, the structure type, fire resistance limit requirement, working environment should be considered, and the selection principles are as follows:

(1) The design fire resistance limit is more than 1.5h of all steel structure buildings and steel components should choose non-intumescent steel structure fireproof coating (gypsum-based) or epoxy class intumescent steel structure fireproof coating (water-based);

(2) Except for steel pipe concrete columns, steel components with design fire resistance limit greater than 2h shall choose non-intumescent steel structure fireproof coating or epoxy-based intumescent steel structure fireproof coating; steel pipe concrete columns, steel components with design fire resistance limit greater than 2h can choose non-intumescent steel structure fireproof coating or intumescent steel structure fireproof coating;

(3) The indoor concealed steel structure should use non-intumescent steel structure fireproof coating or epoxy type intumescent steel structure fireproof coating;

(4) Don’t use the coatings whose technical performance can only satisfy indoor for outdoor, open air, marine engineering and petrochemical engineering. Outdoor use of the environment is much harsher than indoor, paint in the outdoor to withstand the sun and rain. Wind and freezing, should be selected water resistance, freeze-thaw resistance, aging resistance, high strength fireproofing coatings such as outdoor non-expansion (cement-based) or outdoor epoxy intumescent steel structure fireproofing coatings (oil-based).

(5) Do not easily choose the finish type fireproof coating to protect the steel structure.

Fireproof coating can be divided into steel structure fireproof coating, finish fireproof coating, cable fireproof coating and concrete fireproof coating. Steel structure fireproof coating is divided into indoor fireproof coating and outdoor fireproof coating according to the function place; according to the coating thickness, it is divided into thick-coated fireproof coating, wave-coated fireproof coating and ultra-thin fireproof coating. Thick-coated steel structure fireproof coating Thick-coated steel structure fireproof coating refers to the coating thickness of 8mm to 50mm, and the fire resistance limit of this kind of fireproof coating can be up to 0.5 hours to 3 hours. Thin-coated steel structure fireproof coating Steel structure fireproof coating with a coating thickness of 3 mm to 7 mm is called thin-coated steel structure fireproof coating. Ultra-thin steel structure fireproof coating The thickness of fireproof coating is below 3mm, and it is generally used on the steel structure of the building with fire resistance limit within 2h.