The advantages of building steel structure are many. Compared with traditional concrete structure and masonry structure, it has stable performance, light weight and high strength, good seismic performance. It can be processed in the workshop and then assembled on site, not only the integrity of the assembly is good. High precision, and can greatly accelerate the construction schedule and shorten the construction period. At the same time, it is also called "green building", the basic cost is low, the material can be recycled and regenerated, energy saving, land saving and water saving. As a green building, in recent years, steel structure houses have been listed as key promotion projects by the Ministry of Construction. However, such a good form of building structure has a fatal injury, that is, poor fire resistance. Since the steel structure itself has certain combustibles, and the steel structure itself has the characteristics that the heating strength is reduced and the building collapses easily, the occurrence of a fire in the steel structure often causes a mass fire and other serious fire accidents. The most typical example is the "911" incident in the United States. The main character of the incident is the US World Trade Center Building. It is a typical steel structure high-rise building. It is located in the south of downtown Manhattan, New York City, New York, and is surrounded by five buildings. The main building has a double tower shape and uses 78,000 tons of steel. The outer perimeter of the building has a closed steel column. The wall is composed of aluminum plates and glass windows. It is known as the “window of the world”. In the "9.11" terrorist attack, under the influence of the fire, the main building collapsed in just 30 minutes, causing 2,797 deaths and a loss of 36 billion US dollars. Therefore, in order to vigorously promote the steel structure, we must first improve the fire resistance of the steel structure. Only in this way can this energy-saving and environmentally-friendly building structure be popularized and its advantages can be truly realized.

 

1 fire resistance of steel structure

 

The fire resistance of the steel structure depends on the steel. The steel itself does not burn and has good heat resistance. However, the steel is not resistant to high temperatures. As the temperature increases, the strength of the steel decreases, and the deformation increases. Within 200 ° C, the properties of the steel did not change much: the strength dropped sharply between 430 ° C and 540 ° C; the strength was very low at 600 ° C and could not bear the load.

 

Based on the physical properties of steel, in the event of fire, when the temperature reaches 450 ° C ~ 650 ° C, the steel structure will lose its bearing capacity, and a large deformation will occur, causing the steel column and steel beam to bend, resulting in excessive deformation. Can't continue working or even collapse.

 

Once the high temperature is encountered, the deformation or even failure of the steel structural member cannot be avoided. We can only improve its fire resistance limit as much as possible so that the steel structure does not collapse and collapse immediately after the fire. According to the data measured by the laboratory, the fire resistance of commonly used steel structural members is only 15 to 30 minutes. In the national standard "Code for Fire Protection of Building Design" (GBJ16-87), "Code for Fire Protection Design of High-rise Civil Buildings" (GB50045-95), the fire resistance of firewalls, columns and load-bearing walls, stair walls and beams of first-class fire-rated buildings shall be specified. The limit is 3 hours and 2 hours. In comparison, it can be seen that the fire resistance limit of the steel members not covered with the refractory protective layer is very different from the requirements of the fire protection specifications, and the requirements for fire protection of the building under fire conditions cannot be met at all. To overcome the shortcomings of fire-proof performance of steel structural materials in practical applications, it is necessary to improve the fire-resistance performance from various aspects to meet the fire-resistance requirements specified in the specification.

 

2 measures to improve the fire resistance of steel structures

 

2.1 Application of fire-resistant and weather-resistant steel is generally poor in fire resistance, but the material composition of the steel can be changed by relevant technical means, specific components can be added, the structure and metallographic structure of the steel can be changed, and the fire resistance and weather resistance of the steel itself can be improved. . This is fire resistant, weather resistant steel. When the temperature reaches 600 ° C, the strength of the steel is reduced by less than 30%. In addition, because of its better fire resistance, the thickness of the protective layer can be appropriately reduced during construction, and even anti-rust paint can be omitted. A certain cost. The application of refractory and weathering steel is a good cure. It should be promoted. At present, many enterprises in China can produce fire-resistant and weather-resistant steel for construction, which provides a good source of materials for steel structures.

 

2.2 The structural form of the fire-resistant structure and the pure steel structure of the component are relatively poor in fire resistance, but the concrete can be combined with the concrete to form the mixed component, or the reinforced concrete structure can be directly used in some key parts. For example, the column can be made of concrete-filled steel tubular columns, which can effectively improve the fire-resistance limit. Moreover, the larger the diameter (or side length) of the steel pipe, the longer the fire-resisting time, and the concrete-filled steel tubular column with a diameter of 500 mm. The coating can meet the 3h fire resistance time. For members with high fire protection requirements such as slabs and stairs, reinforced concrete structures should be used as much as possible.

 

2.3 Adopting a reasonable steel structure fire protection structure treatment plan Steel structure fire protection construction method should be reasonable, sturdy, economical, easy to construct, and conducive to decoration. When the retaining or separating member of the non-combustion body is on the same axis as the steel member, the non-combustible wall itself can be used to provide fire protection for the steel member without requiring a separate fire protection layer. For steel components that are exposed, reasonable fire protection construction practices should be used.

 

2.4 Implementation of effective fire protection methods Commonly used steel structure fire protection methods include outsourced concrete protection, outsourced steel mesh cement mortar, outsourced fireproof panels and sprayed fireproof coatings. Outer concrete protection is to completely enclose the steel components with concrete. In the concrete, structural steel bars should also be arranged to prevent the concrete from falling off. The fire protection of steel mesh cement mortar is made of metal mesh wipe 50# mortar as a protective layer, which is also a traditional construction method. The outer fireproof board can be wrapped with steel reinforced concrete board (such as TK board, FC board), gypsum board, calcium silicate board, slate board and other non-combustible materials.

 

Spraying fireproof coating on the surface of the steel structure can form a fireproof thermal insulation layer to improve the fire endurance of the steel component. For steel members whose specifications have a fire endurance of less than 1.5 hours, they can be sprayed with a thin fireproof coating, the thickness is generally 1mm to 3mm. The fireproof coating will expand when exposed to fire, forming a porous carbon which is ten to several times larger than the thickness of the original coating. Quality layer. For steel members whose specifications have a fire endurance greater than 1.5 hours, thick-coated fire-retardant coatings should be used. The thickness is generally 7mm~50mm. If it does not swell in case of fire, it has good heat insulation. For high-rise steel structures with high fire endurance requirements, inorganic fiber fireproof cover materials can be sprayed. The inorganic fiber materials have outstanding characteristics of low density, high heat capacity and low thermal conductivity, and have good adhesion. In high temperature fire field, thickness Without obvious changes, the sheath does not melt or fall off, and the protected member can reach the fire endurance of more than 5 hours. Most of China's steel structure construction uses fire-retardant coatings for fire protection.

 

Now the world is calling for energy conservation and emission reduction. The green structure of steel structure is quite popular in developed countries. It has more room for development in China, especially in residential construction. At present, China's steel output has ranked first in the world, blowing the east wind for the promotion of steel structure, do not let fire resistance hinder the pace of steel structure, we should actively take various effective measures to improve the fire resistance of steel structure, accelerate steel The development of the structure.