Steel is a very stable building material, it will not burn when it encounters a fire source like other materials. Compared with concrete, steel has many advantages, such as earthquake resistance, bending resistance and so on. Therefore, in modern buildings, steel structures are widely used. This can not only improve the carrying capacity of buildings, but also meet the aesthetic and modeling needs of architectural design. For example, a variety of single-storey or multi-storey factories, skyscrapers, warehouses, waiting rooms, departure halls, etc., are generally designed with steel structure.

However, although steel does not burn, it can deform if it encounters high temperatures, which can cause the structure to collapse. In addition, as a building material, steel also has some unavoidable defects in fire prevention. In general, the fire resistance limit of unprotected steel structures is about 15 minutes. When the temperature reaches 450 ~ 650 ° C, the steel structure will lose its bearing capacity, and serious deformation will occur, resulting in bending of steel columns and steel beams and even structural collapse.

In the case of the twin towers, when the aircraft directly struck several floors, severe structural damage, including partial collapse, immediately occurred. But apart from this part, the structure remains intact. However, as the plane hit the building, the jet fuel on the plane was ignited. The impact created a huge fireball that instantly consumed some of the fuel. The remaining fuel flowed down the floor into the elevator shaft and pipe shaft, starting a fire throughout the upper half of the building. As the fire spread, the carrying capacity of the main steel structure was gradually weakened, which eventually led to the overall collapse of the building.

To some extent, the problem of fire prevention is an Achilles heel of steel structures. When the temperature reaches about 600 degrees Celsius, the strength of the steel has dropped to about 50%, and the actual bearing capacity may have been lower than the actual load; When the temperature reaches 1200 to 1400 degrees, the steel will completely lose its structural strength. The weight of the upper part of the building weighs down, like dominoes, causing the entire structure to collapse floor by floor. Fire protection measures were in place for the twin towers, but they were weak in the face of such a disaster.

In order to overcome the insufficient fire prevention of steel structure in practical application, fire prevention treatment must be carried out. The purpose is to increase the fire resistance of the steel structure to the limit range specified in the design code. There are many ways to prevent steel structure from rapidly heating up and deformation collapse in the fire, the key is to take different methods according to different situations. For example, insulation and refractory materials can be used to block the direct burning of the flame on the steel structure, reduce the speed of heat transfer and delay the time of temperature rise and strength weakening of the steel structure. Whatever the approach, the principle is the same. The following introduces the fire protection measures of several different steel structures:

(1) Outsourcing layer: Add an outsourcing layer on the surface of the steel structure, which can be a solid concrete outsourcing layer of cast-in-place molding, or an outsourcing layer formed by spraying method. Cast-in-situ formed solid concrete cladding is usually reinforced with wire mesh or rebar to limit shrinkage cracks and ensure the strength of the shell. The spraying method can be applied to the surface of the steel structure at the construction site to form a protective layer of sand pumps, which can be lime cement or gypsum mortar, or can be mixed with perlite or asbestos. At the same time, the outer layer can also be made of perlite, asbestos, gypsum or asbestos cement, light concrete to make prefabricated panels, using adhesives, nails, bolts fixed on the steel structure.

(2) Filling water (water jacket) : filling water in hollow steel structure is one of the most effective protection measures against fire. This method allows the steel structure to maintain a lower temperature in the fire, the water circulates in the steel structure, and the heat of the material itself is absorbed. After cooling, the heated water can be recirculated or replaced by cool water introduced by pipes.

(3) Shielding: The steel structure is set in the wall or ceiling composed of refractory materials, or the components are hidden in the gap between the two walls, and only a little refractory material or no increase can achieve the purpose of fire prevention. This is the most economical method of fire prevention.

(4) Expansion materials: The use of steel structure fireproof coatings to protect components is an effective fire prevention method. This method has the advantages of good fire and heat insulation performance, and the construction is not limited by the geometry of the steel structure. Generally, there is no need to add auxiliary facilities, and the coating weight is light, and there is a certain aesthetic decorative effect. It belongs to one of the modern advanced fire prevention technical measures.

(5) Spraying fire retardant paint: This is currently one of the widely used steel component fire prevention measures in China. Because of its good fire insulation performance, construction is not limited by structural geometry, easy construction and so on, it has been widely used.

In short, steel structure is more and more widely used in construction. However, due to the characteristics of its low fire resistance, once a fire occurs and the fire prevention treatment is not in place, the building with the steel structure as the main load-bearing component will quickly collapse, causing immeasurable losses to people's lives and property. The current fire prevention treatment method of steel structure can basically meet the requirements of the fire resistance time limit stipulated in the code, but the new fire prevention method still needs to be researched and developed.