The high strength and good ductility of the steel determine 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 domestic and foreign steel structures have been widely used.
But the steel structure has an Achilles heel: poor fire resistance. In order to maintain the strength and rigidity of the steel structure in the fire more often, 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 heat method and water cooling method. The thermal resistance method can be further divided into a spraying method and an encapsulation method (hollow encapsulation method and solid encapsulation method). The water cooling method includes a water shower cooling method and a flush cooling method. This article will detail the various fire prevention measures and compare their advantages and disadvantages.
1.Fire resistance endurance and performance of steel structures
The fire endurance of a steel structure refers to the time during which the component resists fire from the action of the fire, from the action of the fire to the loss of stability or integrity, and the heat insulation.
Although the steel itself does not ignite, the material properties of the steel are greatly affected by temperature, but the impact toughness of the steel at 250 °C decreases, exceeding 300 °C, the yield point and ultimate strength are significantly reduced. Under actual fire, the load condition is unchanged, the critical temperature at which the steel structure loses static equilibrium stability is about 500 °C, and the general fire field temperature reaches 800-1000 °C. Therefore, the steel structure will quickly undergo plastic deformation under high temperature and cause local damage, which will eventually cause the overall collapse of the steel structure to fail.
Steel structures must be fireproofed to provide adequate fire endurance. Prevent the steel structure from rapidly rising to a critical temperature in a fire, prevent excessive deformation and collapse of the building, thus gaining valuable time for fire fighting and safe evacuation of personnel, avoiding or reducing the damage caused by fire.
2. Steel structure fire protection measures
Steel structure fire protection measures are divided into two categories according to the principle: one is the heat resistance method, and the other is the water cooling method. The purpose of these measures is the same: to keep the component from rising above its critical temperature for a specified period of time. The difference is that the heat-blocking method prevents heat from being transmitted to the member, while the water-cooling method allows heat to be transferred to the member and then conducts the heat away for the purpose.
2.1 Thermal resistance
The thermal resistance method is divided into a spraying method and an encapsulation method according to the heat resistance of the fireproof coating and the heat resistance of the encapsulating material. The spray method protects the build by applying or spraying a fire retardant coating. The encapsulation method can be further divided into a hollow encapsulation method and a solid encapsulation method.
2.1.1 Spraying method
Generally, fireproof coating is applied or sprayed on the surface of the steel to form a fireproof thermal insulation layer to improve the fire resistance of the steel structure. This method is simple in construction, light in weight, long in fire resistance, and is not limited by the geometry of steel members. It has good economy and practicability and is widely used. There are many varieties of fire-retardant coatings for steel structures, which are generally divided into two categories: one is thin-coated fire-retardant coating (Class B), that is, steel-structured fire-retardant material; the other is thick-coated paint (H).
Class B fire retardant coatings, coating thickness is generally 2-7mm. The base material is an organic resin, which has a certain decorative effect and expands and thickens at high temperatures. The fire resistance can reach 0.5~1.5h. The thin coated steel structure fireproof coating is thin, light in weight and good in vibration resistance. Indoor bare steel structure, light roof steel structure, when the fire limit is specified to be 1.5h or less, thin-coated steel structure fireproof coating should be used. The coating thickness of the H-type fireproof coating is generally 8 to 50 mm. It has a granular surface. The main component is an inorganic heat insulating material with a low density and a low thermal conductivity. The fire resistance can reach 0.5~3.0h. Thick coated steel structure fireproof coatings are generally non-combustible, resistant to aging and durable. Indoor concealed steel structure, high-rise all-steel structure and multi-storey steel structure. When the fire resistance limit is specified above 1.5h, thick-coated steel structure fireproof coating should be used.
2.1.2 Encapsulation method
1) Hollow encapsulation method: Generally, a fireproof board or a refractory brick is used to wrap the steel structure along the outer boundary of the steel member. Most of the steel structure plants in the domestic petrochemical industry use the method of laying refractory bricks to wrap steel components to protect the steel structure. The method has the advantages of high strength and impact resistance, but the disadvantage is that the occupied space is large and the construction is troublesome. Refractory lightweight panels such as fiber reinforced cement board, gypsum board, slate, etc. are used as fireproof outer cladding. The method of box-wrapping large steel components has the advantages of smooth and smooth decoration surface, low cost, low loss, no environmental pollution, and aging resistance, and the promotion prospect is good.
2) Solid encapsulation method: Generally, the steel members are wrapped by pouring concrete to completely close the steel members. This method is adopted for the steel column of the Shanghai Pudong World Financial Building. The advantage is high strength and impact resistance, but the disadvantage is that the concrete protective layer occupies a large space and the construction is troublesome, especially on steel beams and diagonal braces.
2.2 Water cooling method
Water cooling method includes water shower cooling method and water filling cooling method
2.2.1 Water shower cooling
The water shower cooling method is to arrange an automatic or manual sprinkler system on the upper part of the steel structure. In the event of a fire, the sprinkler system is activated to form a continuous film of water on the surface of the steel structure. When the flame spreads to the surface of the steel structure, the evaporation of water removes heat and delays the construction of the steel structure to its limit temperature. The water shower cooling method was used in the Civil Engineering College building of Tongji University.
2.2.2 Water-filled cooling method
The 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. Therefore, the steel structure can maintain a lower temperature in a fire, and the bearing capacity is not lost due to excessive temperature rise. To prevent rust and ice, add rust inhibitor and antifreeze to the water. The steel column of the 64-story US Steel Company building in Pittsburgh, USA, uses a water-filled cooling method.
3. Comparison of fire prevention measures
The thermal resistance method can slow the conduction of heat to the structural members through the heat-resistant material. In general, the heat insulation method is economical and practical, and is widely used in practical engineering. The water cooling method is an effective protection measure against fire. However, this method has special requirements for structural design and its cost is high, so it has not been well promoted in the engineering field. The heat-resistance method is widely used in the fire protection of steel structures. Therefore, the following are the advantages and disadvantages of the spraying method and the encapsulation method in the thermal 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 are better in fire resistance than general fireproof coatings. In addition, the fire resistance of the new fireproof board is also superior to fireproof coatings. The fire resistance limit is significantly higher than the steel structure fireproof and heat insulating material of the same thickness, which is higher than the intumescent fireproof coating.
Since the encapsulation material such as concrete has better durability, it is not easy to deteriorate performance over time; and durability has always been a problem that steel structure fireproof coatings fail to solve. Thin and ultra-thin fire-retardant coatings based on organic components, whether used outdoors or indoors, may cause decomposition, degradation, aging, etc., which may cause the coating to flake off or lose fire resistance.
The steel structure fireproof spraying method is simple to construct and can be constructed without complicated tools. However, the quality of the fireproof coating construction method is poor, and it is difficult to control the derusting of the substrate, the coating thickness of the fireproof coating and the humidity of the construction environment. The construction of the encapsulation method is more complicated, especially for the bracing and steel beams, but the construction is particularly obvious. The controllability is stronger and the quality is easy to guarantee. The thickness of the encapsulating material can be changed more accurately to control the fire endurance.
3.4 Environmental protection
The spraying 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 during construction, normal use environment and high temperature of fire, which is beneficial to environmental protection and personnel safety during fire
3.5 Economic performance
The spraying method is simple in construction, short in construction period and low in construction cost. However, the price of fireproof coatings is high, and the maintenance cost is high due to defects such as aging of the paint. The enveloping method has a high construction cost, but the materials used are cheap and the maintenance cost is low. In general, the encapsulation method is economically effective.
The spraying method is not limited by the geometrical constraints of the components, and is mostly used for the protection of beams, columns, floors, roofs and the like. It is especially suitable for fire protection of steel structures in space structure systems such as light steel structures, grid structures and profiled steel structures. The encapsulation method is complicated to construct, especially for steel beams and slanting braces. The encapsulation method is generally used for columns. The application range is not widely applied.
3.7 Space occupation
The fireproof coating used in the spraying method is small in volume, and the encapsulating materials such as concrete and fireproof brick used in the encapsulation method take up space and reduce the use space. Moreover, the quality of the encapsulating material is also large.
Through the discussion, the following conclusions can be drawn:
1) The adoption of steel structure fire protection measures should consider the influence of various factors, such as component type, construction difficulty, construction quality requirements, durability requirements and economic benefits;
2) By comparing the spraying method with the encapsulation method, the main advantage of the spraying method is that the construction process is simple, and the shape of the component does not change greatly after spraying. The main advantages of the encapsulation method are lower cost, good fire performance and durability;
3) Various fire prevention measures have their own advantages and disadvantages. In the application of the project, it is possible to make up for the shortcomings and combine various fire prevention measures. And you can set up multiple fire prevention lines by taking different measures.