The advantages of high strength and good ductility determine that the steel structure has the characteristics of light dead 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 can be recycled. But steel structures have an Achilles' heel: poor fire resistance. In order to keep the strength and rigidity of the steel structure in the fire for a long time and ensure the safety of people's life and property, a variety of fire protection measures have been adopted in practical engineering. According to different fire prevention principles, fire prevention measures are divided into heat resistance method and water cooling method. Thermal resistance can be divided into spraying method and encapsulation method (hollow encapsulation method and solid encapsulation method). The water cooling method has a water drenching cooling method and a flushing cooling method.
Fire resistance limit and fire resistance performance of steel structure
The fire resistance limit of steel structure is to point to the component in the standard fire resistance test, from the effect of fire to lose stability or integrity, adiabatic stop this period of time to resist the effect of fire.
Although the steel itself will not catch fire, the material property of steel is greatly affected by the temperature, but at 250℃ steel impact toughness decreased, more than 300℃, the yield point and ultimate strength decreased significantly. In an actual fire, the critical temperature for the steel structure to lose the static equilibrium stability is about 500℃, while the general fire temperature reaches 800 ~ 1000℃. Therefore, the steel structure will quickly appear plastic deformation under high-temperature fire, resulting in local damage, finally, lead to the overall collapse failure of the steel structure. The steel structure building must take fire prevention measures to make the building have enough fire resistance limit to prevent the steel structure from rapidly heating up to the critical temperature in the fire, prevent excessive deformation and building collapse, and avoid or reduce the loss caused by fire.
Fire protection measures of steel structure
The steel structure fire protection measures are divided into two categories according to the principle: one is heat resistance method, 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 at the specified time. The difference is that insulation prevents heat from being transferred to the component, whereas water cooling allows heat to be transferred to the component and then diverted away to achieve the goal.
1.Thermal resistance method
Thermal resistance according to fire-retardant coating thermal resistance and encapsulation material thermal resistance, divided into spraying and encapsulation method. Spraying protects the structure by coating or spraying it with fireproof paint. Encapsulation can be divided into hollow encapsulation and solid encapsulation.
Generally using fire retardant coating or spraying on the steel surface, the formation of fire insulation protection layer, improve the fire resistance limit of steel structure. This method has the advantages of simple construction, lightweight, long fireproof time, and is not restricted by the geometrical shape of steel members. With better economy and practicability, it is widely used.
1) Hollow encapsulation method: fire board or firebrick is generally used to wrap the steel member along the peripheral boundary of the steel member. Most of the steel structure plants in the petrochemical industry in China protect the steel structure by wrapping the steel members with firebrick. The advantages of this method are high strength and impact resistance, but the disadvantage is that the space occupied is larger and the construction is more troublesome. The box-type wrapping method for large steel members has the advantages of smooth decoration surface, low cost, small loss, no environmental pollution, aging resistance and so on, and has a good prospect of popularization.
4.Solid encapsulation method: generally by pouring concrete, the steel members will be wrapped up, completely closed steel members.Its advantages are high strength and impact resistance, but the disadvantage is that the concrete protective layer occupies a large space, the construction is more difficult, especially in the steel beam and inclined brace construction is very difficult.
5.Water cooling method
Water cooling method includes water drenching cooling method and water filling cooling method.
a Drenching cooling method
Water spray cooling method is arranged in the upper part of the steel structure automatic or manual spray system. When a fire occurs, the spray system is started to form a continuous layer of water film on the surface of the steel structure. When the flame spreads to the surface of the steel structure, the water evaporates and takes away the heat, delaying the steel structure to reach its limit temperature.
b Water cooling method
Water cooling method is to fill a 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 in the fire can maintain a low temperature, will not lose the bearing capacity due to too high temperature. In order to prevent corrosion and freezing, rust inhibitor and antifreeze should be added to the water.
Comparison of fire prevention measures
Thermal resistance can be used to slow down the rate of heat transfer to structural members through thermal resistance 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 protection measure against fire, but it has not been well popularized in the engineering field because of its special requirements for structural design and high cost.
Thermal resistance method is widely used in the fire protection of steel structures, so the following focuses on the advantages and disadvantages of spraying method and encapsulation method in thermal resistance measures.
The sealing method is superior to the spraying method in terms of fire resistance. The fire resistance of concrete, firebrick and other sealing materials is better than that of general fire retardant coatings. Additionally, the fireproof performance of a new-type fire prevention board also is superior to fireproof coating. Its fire resistance limit is obviously higher than the same thickness of steel structure fire insulation material, more than the expansion type fire retardant coating.
Because the sealing materials such as concrete have good durability, their performance is not easy to deteriorate over time. And durability has been a steel structure fire retardant coating has not been able to solve the problem. Thin and ultra-thin fire retardant coatings based on organic components, whether for outdoor or indoor use, organic components may produce decomposition, degradation, aging and other problems, so that the coating peeling powder or loss of fire performance.
The spraying method of steel structure fire protection is simple and can be constructed without complex tools. But the spray method of fire retardant coating construction quality control is poor, on the base material rust removal, fire retardant coating thickness and construction environment humidity is not easy to control; Encapsulation method construction is more complex, especially for inclined bracing, steel beam, but the construction of control is strong, easy to ensure the quality. The fire resistance limit can be controlled by changing the thickness of the encapsulating material more accurately.
4 Environmental protection
Spraying method pollutes the environment during construction, especially the harmful gases can be volatilized under the action of high temperature. Encapsulation method has no toxic release in construction, normal use environment and fire high temperature, which is beneficial to environmental protection and personnel safety in fire.
Spraying method construction is simple, short construction period, construction cost is low.
But the price of fire retardant coating is high, and because of the aging of the coating and other shortcomings, its maintenance costs are higher. Encapsulation method construction cost is high, but the materials used are cheap, and the maintenance cost is low.
The spraying method is not restricted by the geometry of the components and is used for the protection of beams, columns, floors, roof and other components. It is especially suitable for fire protection of light steel structure, grid structure and special-shaped steel structure. Encapsulation method construction is complex, especially steel beam, inclined support and other components, encapsulation method is generally more used for columns, the scope of application is not as wide as the spraying method.
7 Occupied Space
The volume of fireproof coating used by the spraying method is small, and the encapsulation material used by the encapsulation method, such as concrete, fireproof brick, will occupy space, reducing the use of space. And the encapsulation material quality is larger.