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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 material is recyclable. Therefore, both in domestic and foreign steel structure buildings have been widely used.

But the steel structure has a fatal weakness: poor fire resistance. In order to maintain the strength and rigidity of the steel structure in the fire for a relatively long time, and to protect people's lives and property safety, a variety of fire protection measures have been adopted in the actual project. According to different fire prevention principles, fire prevention measures are divided into resistance heating method and water cooling method. Heat resistance method can be divided into spraying method and encapsulation method (hollow encapsulation method and solid encapsulation method). The water cooling method includes the water shower cooling method and the flush water cooling method. Introduce various fire protection measures in detail, and compare their advantages and disadvantages.

1. Fire resistance limit and fire resistance of steel structure

The fire resistance limit of a steel structure refers to the time during which a member loses its stability or integrity and is adiabatic during the standard fire test when it is exposed to fire

Although the steel itself will not ignite or burn, the material properties of the steel are greatly affected by temperature, but the impact toughness of the steel at 250 ℃ decreases, and if it exceeds 300 ℃, the yield point and the limit strength decrease significantly. Under actual fire conditions, the load conditions remain unchanged, and the critical temperature at which the steel structure loses its static equilibrium stability is around 500 ° C, while the general fire site temperature reaches 800-1000 ° C. Therefore, the steel structure will quickly undergo plastic deformation under fire and high temperature, causing local damage and causing the entire steel structure to collapse and fail.

Steel structure buildings must take fire prevention measures to make the buildings have sufficient fire resistance limits. Prevent the steel structure from rapidly heating up to the critical temperature in a fire, prevent excessive deformation and even the collapse of the building, thus winning valuable time for firefighting and personnel evacuation, and avoid or reduce the losses caused by the fire.

2. Fire protection measures for steel structure

The fire protection measures of steel structure are divided into two categories according to the principle: one is heat resistance method, and the other is water cooling method. The purpose of these measures is the same: to make the temperature rise of the component not exceed its critical temperature within a specified time. The difference is that the heat resistance method prevents heat from being transferred to the component, and the water cooling method allows heat to be transferred to the component, and then conducts the heat away to achieve the purpose.

2.1 Heat resistance method

The heat resistance method is divided into spraying method and encapsulation method according to the heat resistance of the fireproof coating and the encapsulation material. The spray method protects the structure by applying or spraying fire retardant paint. Encapsulation method can be divided into hollow encapsulation method and solid encapsulation method.

2.1.1 Spraying method

Generally, fire-resistant paint is applied or sprayed on the surface of steel to form a fire-resistant and heat-insulating protective layer to improve the fire resistance of the steel structure. This method is simple in construction, light in weight, long in fire resistance time, and is not subject to the geometry of steel components. It has good economy and practicality, and is widely used. There are many types of fireproof coatings for steel structures, which can be roughly divided into two categories: one is thin-coated fireproof coatings (Category B), that is, steel structure expansion fireproofing materials; the other is thick-coated coatings (H) category.

Class B fire retardant coating, the thickness of the coating is generally 2-7mm. The base material is * resin, which has a certain decorative effect, and expands and thickens at high temperature. Fire resistance * limit can reach 0.5 ～ 1.5h. Thin coating steel structure fire retardant coating has thin coating, light weight and good vibration resistance. Indoor * exposed steel structure, light roof steel structure, when the fire resistance * is limited to 1.5h and below, thin-coated steel structure fireproof coating should be used. The thickness of Class H fire retardant coating is generally 8-50mm. Granular surface. The main component is an inorganic thermal insulation material with a low density and low thermal conductivity. Fire resistance * limit can reach 0.5 ～ 3.0h. Thick-coated steel structure fire retardant coatings are generally non-combustible, resistant to aging, and more reliable in durability. For indoor concealed steel structure, high-rise all-steel structure and multi-storey factory steel structure, when the fire resistance * is specified to be above 1.5h, thick-coated steel structure fireproof coating should be used.

2.1.2 Encapsulation method

1) Hollow encapsulation method: generally adopt fireproof board or refractory brick to wrap the steel structure along the outer boundary of the steel structure. Most of the domestic petrochemical industrial steel structure workshops use the method of building refractory bricks to wrap the steel members to protect the steel structure. The advantage of this method is high strength and impact resistance, but the disadvantage is that it takes up more space and is more troublesome to construct. Use fire-resistant light-weight board such as fiber-reinforced cement board, gypsum board, vermiculite board, etc. as the fireproof outer cladding. The method of box-wrapping a large steel component has the advantages of smooth and smooth decoration surface, low cost, low loss, no environmental pollution, and aging resistance, and has a good prospect for promotion.

2) Solid encapsulation method: generally by pouring concrete, the steel member is wrapped up to completely close the steel member. For example, the steel column of Shanghai Pudong World Financial Building adopts this method. Its advantages are high strength and impact resistance, but the disadvantage is that the concrete protective layer occupies a large space and the construction is more troublesome, especially on steel beams and diagonal braces.

2.2 Water cooling * method

The water cooling method includes the water shower cooling method and the water filling cooling method.

2.2.1 Water cooling * method

The water spray cooling method is to arrange an automatic or manual spray system on the upper part of the steel structure. In the event of a fire, the sprinkler system is activated to form a continuous water film on the surface of the steel structure. When the flame spreads to the surface of the steel structure, the moisture evaporates to take away the heat and delay the steel structure to reach its limit temperature. The method of water cooling is used in the building of School of Civil Engineering, Tongji University

2.2.2 Water-filled cooling * method

Water-filled cooling * method is to fill the hollow steel member 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 low temperature in the fire, and will not lose the bearing capacity due to excessive temperature rise. In order to prevent corrosion and icing, rust inhibitors and antifreeze should be added to the water. The steel column of the 64-story American Iron and Steel Building in Pittsburgh uses water-filled cooling *.

3. Comparison of fire prevention measures

The heat-blocking method can reduce the speed of heat conduction to the structural member through the heat-blocking material. In general, the thermal insulation method is economical and practical, and is widely used in practical projects. Water protection method is a kind of protective measures against fire, but because this method has special requirements for structural design and high cost, it has not yet been well promoted in the engineering field.

The heat resistance method is widely used in the fire protection of steel structures, so the following focuses on comparing the advantages and disadvantages of the spray method and the encapsulation method in the heat resistance method.

3.1 Fire resistance

In terms of fire resistance, the encapsulation method is superior to the spray method. Encapsulation materials such as concrete and refractory bricks have better fire resistance than general fireproof coatings. In addition, the fire resistance of the new fireproof board is also better than fireproof paint. Its fire resistance limit is obviously higher than that of steel structure fireproof and heat insulation materials with the same thickness, and higher than that of intumescent fireproof coating.

3.2 Durability

Because the durability of encapsulating materials such as concrete is relatively good, it is not easy to cause performance degradation over time; and durability has always been a problem that steel structure fireproof coatings have not solved well. The thin and ultra-thin fire retardant coatings mainly composed of * components, whether used outdoors or indoors, may have problems of *, degradation, aging, etc., which will cause the coating to peel off and powder or lose fire resistance performance.

3.3 Constructability

The fire-resistant spraying method of steel structure is easy to construct and can be constructed without complicated tools. However, the construction quality of spraying fire retardant coating is poor, and it is not easy to control the rust removal of the substrate, the thickness of the fire retardant coating, and the humidity of the construction environment. The controllability is strong and the quality is easy to guarantee. The thickness of the encapsulating material can be changed more accurately to control the refractory limit.

3.4 Environmental protection

The spray method pollutes the environment during construction, especially under the action of high temperature, it can volatilize harmful gases. The encapsulation method has no toxic emissions under construction, normal use environment and fire high temperature, which is conducive to environmental protection and personnel safety during fire.

3.5 Economical

The spraying method is simple in construction, short in construction period, and low in construction cost. However, the price of fireproof coatings is extremely high, and because of the aging and other shortcomings of the coatings, their maintenance costs are relatively high. The construction cost of the encapsulation method is high, but the price of the materials used is cheap, and the maintenance cost is low. On the whole, the encapsulation method has better economic efficiency.

3.6 Applicability

The spraying method is not limited by the geometry of the components *, and is mostly used to protect beams, columns, floors, roofs and other components. It is especially suitable for fire protection of steel structures such as light steel structure, grid structure and special-shaped steel structure. The encapsulation method is complicated in construction, especially steel beams, diagonal braces and other components. The encapsulation method is generally used for columns, and the application range is not as wide as the spray method.

3.7 Taking up space

The fire-retardant coating used in the spraying method has a small volume, and the encapsulation materials used in the encapsulation method, such as concrete and fire-resistant bricks, take up space and reduce the use space. And the quality of the encapsulation material is also relatively large.