Steel has the advantages of high strength and good ductility, which determines that the steel structure has the characteristics of light weight, good seismic performance and large bearing capacity. At the same time, the steel structure can be processed in the field, the construction period is short, and the materials can be recycled.Therefore, steel structure building has been widely used both at home and abroad. 

But steel structure has a fatal weakness: poor fire resistance.In order to keep the strength and stiffness of steel structure in fire for a long time and ensure the safety of people’s life and property, a variety of fire protection measures are adopted in practical engineering.According to different fire prevention principles, fire prevention measures are divided into heat resistance method and water cooling method.Heat resistance method can be divided into spraying method and encapsulation method (hollow encapsulation method and solid encapsulation method).Water cooling methods include water spray cooling method and flushing cooling method.This paper will introduce various fire prevention measures and compare their advantages and disadvantages.

1Fire resistance limit and fire resistance performance of steel structure

The fire resistance limit of steel structure refers to the time from the moment when the member is subjected to fire to losing stability or integrity and thermal insulation in the standard fire resistance test. 

Although the steel itself will not ignite and burn, the properties of the steel are greatly affected by temperature. However, the impact toughness of the steel decreases at 250 ℃, and the yield point and ultimate strength decrease significantly when the temperature exceeds 300 ℃.In actual fire, the critical temperature of steel structure losing static equilibrium stability is about 500 ℃, while the temperature of general fire field reaches 800-1000 ℃.Therefore, the plastic deformation of steel structure will appear quickly under high temperature of fire, resulting in local damage, and eventually the overall collapse of steel structure failure. 

Fire prevention measures must be taken in steel structure building to make the building have enough fire resistance.In order to prevent the steel structure from rapidly rising to the critical temperature in the fire, to prevent excessive deformation and even building collapse, so as to win valuable time for fire fighting and personnel evacuation, and to avoid or reduce the loss caused by fire. 

2Fire protection measures for steel structure

According to the principle, the fire protection measures of steel structure can be divided into two categories: one is heat resistance method, the other is water cooling method.The purpose of these measures is consistent: to make the temperature rise of the component within the specified time not exceeding its critical temperature.The difference is that the heat resistance method is to prevent the heat from transferring to the component, while the water cooling method allows the heat to be transferred to the component, and then the heat is guided away to achieve the purpose. 

2.1 Heat resistance method 

According to the heat resistance of fire retardant coatings and the heat resistance of encapsulation materials, heat resistance method can be divided into spraying method and encapsulation method.Spraying protects the structure by coating or spraying a fire retardant coating.Encapsulation method can be divided into hollow encapsulation method and solid encapsulation method. 

2.1.1 Spraying method 

Generally, fireproof coating is applied or sprayed on the steel surface to form a fire-resistant and heat-insulating protective layer to improve the fire-resistant limit of steel structure.This method has the advantages of simple construction, light weight, long fire resistance time and is not limited by the geometry 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 roughly divided into two categories: one is thin coating fire retardant coating (class B), that is, steel structure intumescent fireproof material; the other is thick coating coating (H). 

Class B fire retardant coating, the coating thickness is generally 2-7mm.The base material is organic resin, which has a certain decorative effect, and it expands and thickens at high temperature.The fire resistance can reach 0.5 ~ 1.5h.Thin coating steel structure fire retardant coating is thin, light weight and good vibration resistance.When the fire resistance limit of indoor exposed steel structure and light roof steel structure is 1.5h or less, thin coating steel structure fire retardant coating should be selected.The thickness of class H fire retardant coating is generally 8-50 mm.It is granular.The main component is inorganic thermal insulation material with low density and low thermal conductivity.The fire resistance can reach 0.5 ~ 3.0 H.Generally, the fire retardant coating for thick steel structure is non combustible, aging resistant and reliable in durability.When the fire resistance limit of indoor concealed steel structure, high-rise all steel structure and multi-storey workshop steel structure is more than 1.5h, thick coating steel structure fireproof coating should be selected. 

2.1.2Encapsulation method

1) Hollow encapsulation method: generally, fireproof board or firebrick is used to wrap steel members along the peripheral boundary of steel components.Most of the steel structure workshops in domestic petrochemical industry use the method of building firebrick to wrap the 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 troublesome construction.Fire resistant light board such as fiber reinforced cement board, gypsum board, vermiculite board and so on are used as the fireproof outer layer.The box wrapping method for large steel members has the advantages of smooth and smooth decoration surface, low cost, small loss, no environmental pollution, aging resistance and so on. 

2) Solid encasement method: generally, steel members are wrapped by pouring concrete to completely seal steel members.For example, the steel column of Shanghai Pudong world financial building is adopted.Its advantages are high strength and impact resistance, but the disadvantages are that the concrete cover occupies a large space and the construction is troublesome, especially on the steel beam and diagonal brace. 

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 upper part of steel structure.In case of fire, the sprinkler system is started to form a continuous water film on the surface of steel structure. When the flame spreads to the surface of steel structure, water evaporation takes away heat and delays steel structure building to reach its limit temperature.Water spray cooling method is used in the building of Civil Engineering College of Tongji University. 

2.2.2 Water filled cooling method 

Water filling cooling method is to fill the hollow steel members with water.Through the circulation of water in the steel structure, the heat of the steel itself is absorbed.The temperature of the steel structure will not be too high to keep the temperature of the structure low due to the fire.In order to prevent corrosion and freezing, rust inhibitor and antifreeze should be added in water.In Pittsburgh, the 64 story steel column of the American steel company building is filled with water. 

3Comparison of fire prevention measures

The heat resistance method can slow down the speed of heat conduction to structural components through heat insulation materials.Generally speaking, the heat insulation method is economical and practical, and it is widely used in practical engineering.Water cooling method is an effective protective measure against fire. However, due to its special requirements for structural design and high cost, it has not been widely used in engineering field. 

Thermal insulation method is widely used in fire protection of steel structure, so the advantages and disadvantages of spraying method and enveloping method in heat resistance method are compared. 

3.1 Fire resistance

In terms of fire resistance, encapsulation method is better than spray method.The fire resistance of concrete, firebrick and other sealing materials is better than that of common fire retardant coatings.In addition, the fire resistance of the new fireproof board is better than that of the fireproof coating.Its fire resistance is obviously higher than that of the same thickness of steel structure fireproof and thermal insulation material, and even higher than that of intumescent fire retardant coating. 

3.2Durability 

Due to the good durability of enveloping materials such as concrete, it is not easy to degrade the performance over time; however, the durability has always been a problem that the fireproof coating for steel structure has not been solved.Thin and ultra-thin fire retardant coatings mainly composed of organic components may cause decomposition, degradation, aging and other problems whether they are used outdoors or indoors, so that the coatings will flake and pulverize or lose their fire resistance performance. 

3.3Constructability

The spraying method for fire prevention of steel structure is simple and can be applied without complicated tools.However, the construction quality of spraying fire retardant coating is poor, and it is difficult to control the rust removal of base material, the coating thickness of fire retardant coating and the humidity of construction environment; the construction of enveloping method is more complex, especially for diagonal bracing and steel beam, but the construction controllability is strong and the quality is easy to be guaranteed.The thickness of sealing material can be changed to control the fire resistance limit. 

3.4 Environmental protection 

Spraying method pollutes the environment during construction, especially harmful gas can be volatilized under high temperature.There is no toxic release in the construction, normal use environment and high temperature of fire, which is conducive to environmental protection and personnel safety in case of fire. 

3.5 Economy 

Spraying method has the advantages of simple construction, short construction period and low construction cost.However, the price of fire retardant coating is high, and the maintenance cost is high due to the aging of the coating.The construction cost of encapsulation method is high, but the cost of materials is cheap and the maintenance cost is low.Generally speaking, the economic efficiency of encapsulation method is better. 

3.6 Applicability 

The spray method is not limited by the geometry of the components, and is mostly used for the protection of beams, columns, floors, roofs and other components.It is especially suitable for fire protection of steel structures of space structure systems such as light steel structures, grid structures and special-shaped steel structures.This method is widely used in the construction of complex steel ladle beam. 

3.7 Occupied space 

The volume of fire retardant coating used by spraying method is small, while the enveloping materials such as concrete and fireproof brick used in enveloping method will occupy space and reduce the use space.And the quality of the encapsulation material is also large. 

4Conclusion

The following conclusions can be drawn from the discussion

1）The fire protection measures of steel structure should consider the influence of many factors, such as component type, construction difficulty, construction quality requirements, durability requirements and economic benefits;

2）Through the comparison of spraying method and enveloping method, the main advantage of spraying method is that the construction technology is simple, and the shape of components will not change greatly after spraying.The main advantages of encapsulation method are low cost, good fire resistance and durability;

3）All kinds of fire prevention measures have their own advantages and disadvantages. In the engineering application, we can learn from each other’s strong points to make up for each other’s shortcomings.Different measures can be taken to set up multiple fire defense lines.