Because of its high strength and ductility, steel structure has the characteristics of light weight, good seismic performance and large bearing capacity. At the same time, steel structure can be processed in the field with short construction period, and material can be recycled. Therefore, both domestic and foreign steel structure buildings have been widely used.
The fire resistance limit of steel structure refers to the time when the member loses stability or integrity and insulation during the standard fire resistance test.
Although the steel itself will not ignite and burn, the properties of the steel are greatly affected by temperature, but the impact toughness of the steel at 250 C decreases, and the yield point and ultimate strength decrease significantly when the temperature exceeds 300 C. Under actual fire, the critical temperature of losing static equilibrium stability of steel structure is about 500 C, while the general temperature of fire field is 800 - 1000 C. Therefore, the steel structure will quickly appear plastic deformation and local damage under the high temperature of fire, which will eventually lead to the collapse and failure of the steel structure as a whole.
Fire protection measures must be taken in steel structure building to make the building have enough fire resistance limit. It can prevent steel structure from rapidly rising to the critical temperature in fire and from excessive deformation to collapse of buildings, thus gaining valuable time for fire extinguishing and safe evacuation of personnel, and avoiding or reducing losses caused by fire.
Fire protection measures for steel structures can be divided into two categories according to their principles: one is heat resistance method, the other is water cooling method. The purpose of these measures is the same: to raise the temperature of the component within the specified time and not exceed its critical temperature. The difference is that the heat-resisting method prevents heat from being transferred to the component, while the water-cooling method allows heat to be transferred to the component, and then the heat is transferred to achieve the purpose.
2.1 Thermal Resistance Method
Thermal insulation method is divided into spraying method and encapsulation method according to the heat resistance of fire retardant coating and encapsulation material. Spraying protects the structure by coating or spraying fire-retardant coatings. The encapsulation method can be divided into hollow encapsulation method and solid encapsulation method.
2.1.1 Spraying Method
Fire-proof coating or spraying on the surface of steel is usually used to form a fire-resistant and heat-insulating protective layer to improve the fire-resistant limit of steel structure. This method is simple in construction, light in weight, long in refractory time, and is not limited by the geometric shape of steel members. It has good economy and practicability, and is widely used. There are many kinds of fire-retardant coatings for steel structures, which can be divided into two categories: one is thin-coated fire-retardant coatings (category B), that is, expansive fire-retardants for steel structures; the other is thick-coated coatings (category H).
Class B fire retardant coatings, coating thickness is generally 2-7 mm. The base material is organic resin, which has certain decorative effect and expands and thickens at high temperature. The refractory limit can reach 0.5-1.5 H. Thin coated fire retardant coatings for steel structures are characterized by thin coating, light weight and good vibration resistance. When the fire resistance limit of bare steel structure and light roof steel structure is 1.5h or less, the thin coated fire retardant coating for steel structure should be selected. The thickness of H-type fire retardant coatings is generally 8-50 mm. It is granular. Inorganic thermal insulation material is the main component, with low density and low thermal conductivity. The refractory limit can reach 0.5-3.0 H. Thick-coated fire-retardant coatings for steel structures are generally non-flammable, aging-resistant and durable. When the fire resistance limit of concealed indoor steel structure, high-rise all-steel structure and multi-storey factory building steel structure is over 1.5h, thick-coated fire retardant coatings should be selected.
2.1.2 Encapsulation Method
1) Hollow encapsulation method: Fire-proof board or refractory brick are generally used to encapsulate steel members along the outer boundary of steel members. Most domestic steel structure factories in petrochemical industry adopt the method of building refractory bricks and wrapping steel members to protect the steel structure. The advantages of this method are high strength and impact resistance, but the disadvantages are large space occupied and construction troubles. Fire-resistant lightweight plates such as fiber reinforced cement board, gypsum board and vermiculite board are used as fire-proof coating. The box wrapping method for large steel components has many advantages, such as smooth decoration surface, low cost, low loss, no environmental pollution, aging resistance, etc. It has good prospects for promotion.
2) Solid encapsulation method: steel members are encapsulated and completely enclosed by pouring concrete. For example, the steel column of Pudong World Financial Building in Shanghai adopts this method. Its advantages are high strength and impact resistance, but its disadvantages are that the concrete protective layer occupies a large space and construction is difficult, especially on steel beams and diagonal braces.
2.2 Water Cooling Method
Water cooling method includes water spray cooling method and water filling cooling method.
2.2.1 Water Spray Cooling Method
Water spray cooling method is to arrange automatic or manual spray system on the top of steel structure. In case of fire, the spray system is started to form a continuous water film on the surface of the steel structure. When the flame spreads to the surface of the steel structure, water evaporation takes away heat and delays the steel structure building to reach its critical temperature. Water spray cooling method has been used in the building of Civil Engineering College of Tongji University.
2.2.2 Water-filled Cooling Method
Water-filled cooling method is to fill hollow steel members with water. Through the circulation of water in the steel structure, the heat of the steel itself is absorbed. So that the steel structure can maintain a lower temperature in the fire, and will not lose the bearing capacity because of the high temperature rise. In order to prevent rust and ice formation, antirust and antifreeze should be added to the water. The water-filled cooling method is used for steel columns of the 64-storey U.S. Steel Company Building in Pittsburgh, USA.