The advantages of high strength and good ductility of steel determine that steel structures have the characteristics of light weight, good seismic performance, and large bearing capacity. At the same time, steel structures can be processed on site, with a short construction period, and materials can be recycled. Therefore, steel structure buildings have been widely used both domestically and internationally.
But steel structures have a fatal weakness: poor fire resistance. In order to maintain the strength and stiffness of steel structures for a longer period of time during fires, and to ensure the safety of people’s lives and property, various fire protection measures have been adopted in practical engineering. According to different fire prevention principles, fire prevention measures are divided into resistance heat method and water cooling method. The heat resistance method can also be divided into spray coating method and encapsulation method (hollow encapsulation method and solid encapsulation method). Water cooling method includes water spray cooling method and flushing cooling method. This article will specifically introduce various fire prevention measures and compare their advantages and disadvantages.
- The fire resistance limit and performance of steel structures refer to the time during which a component loses stability or integrity, insulation, and resistance to fire during a standard fire resistance test.
Although steel itself does not ignite or burn, its properties are greatly affected by temperature. However, the impact toughness of steel decreases at 250 ℃, and over 300 ℃, the yield point and ultimate strength significantly decrease. Under the actual fire, the critical temperature for the steel structure to lose its static equilibrium stability is about 500 ℃, while the general fire site temperature reaches 800~1000 ℃. Therefore, under high fire temperatures, the steel structure will quickly undergo plastic deformation, resulting in local damage and ultimately leading to the overall collapse and failure of the steel structure. Steel structure buildings must adopt fire prevention measures to ensure that the building has sufficient fire resistance limit. To prevent steel structures from rapidly heating up to critical temperatures in a fire, prevent excessive deformation and even building collapse, thereby gaining valuable time for firefighting and safe evacuation of personnel, and avoiding or reducing losses caused by fires.
- Fire protection measures for steel structures
The fire protection measures for steel structures can be divided into two categories based on their principles: one is heat resistance method, and the other is water cooling method. The purpose of these measures is consistent: to ensure that the temperature of the component does not exceed its critical temperature within the specified time. The difference is that the heat blocking method prevents the transfer of heat to the component, while the water cooling method allows heat to be transferred to the component and then dissipated to achieve the purpose.
2.1 Heat resistance method
The heat resistance method is divided into spray coating method and encapsulation method based on the heat resistance of fireproof coatings and encapsulation materials. The spraying method protects the construction by applying or spraying fire-resistant coatings. The encapsulation method can also be divided into hollow encapsulation method and solid encapsulation method.
2.1.1 Spraying method
Generally, fire-resistant coatings are applied or sprayed on the surface of steel to form a fire-resistant and thermal insulation protective layer, which improves the fire resistance limit of steel structures. This method is easy to construct, lightweight, and has a longer fire resistance time, and is not limited by the geometric shape of steel components. It has good economy and practicality, and is widely used. There are many types of fire resistant coatings for steel structures, which can be roughly divided into two categories: one is thin coated fire resistant coatings (Class B), which is also known as expansion fire resistant coatings for steel structures; The other type is thick coating (H). Class B fireproof coating, with a coating thickness of generally 2-7mm. The base material is organic resin, which has a certain decorative effect and expands and thickens at high temperatures. The fire resistance limit can reach 0.5-1.5 hours. Thin coated fireproof coating for steel structures has the advantages of thin coating, light weight, and good vibration resistance. When the fire resistance limit of indoor exposed steel structures and light roof steel structures is specified to be 1.5 hours or less, thin coated steel structure fire retardant coatings should be selected. The coating thickness of Class H fireproof coating is generally 8-50mm. It appears as a granular surface. The main component is inorganic insulation material, with low density and low thermal conductivity. The fire resistance limit can reach 0.5-3.0 hours. Thick coated fireproof coatings for steel structures are generally non combustible, aging resistant, and have reliable durability. When the fire resistance limit of indoor concealed steel structures, high-rise all steel structures, and multi-layer factory steel structures is specified to be above 1.5 hours, thick coated steel structure fire retardant coatings should be used.
2.1.2 Encapsulation method
1) Hollow enveloping method: generally, fireproof boards or Fire brick are used to wrap the steel members along the peripheral boundary of the steel members. The steel structure of domestic petrochemical industrial steel structure plants is mostly protected by building Fire brick wrapped steel members. The advantages of this method are high strength and impact resistance, but the disadvantages are that it occupies a large space and is difficult to construct. Using fire-resistant lightweight panels such as fiber-reinforced cement board, gypsum board, vermiculite board, etc. as fire-resistant outer layers. The method of box wrapping large steel components has the advantages of flat and smooth decoration surface, low cost, low loss, no environmental pollution, and aging resistance, and has good prospects for promotion. 2) Solid encapsulation method: Generally, by pouring concrete, the steel components are wrapped and completely enclosed. For example, the steel column of the World Finance Tower in Pudong, Shanghai, adopts this method. Its advantages are high strength and impact resistance, but its disadvantages are that the space occupied by the concrete protective layer is large and the construction is relatively troublesome, especially on the steel beams and slant support.
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
The water spray cooling method is to arrange an automatic or manual spraying system on the upper part of the steel structure. When a fire occurs, 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 water evaporates and carries away heat, delaying the steel structure building from reaching its limit temperature. The water spray cooling method has been used in the building of the School of Civil Engineering, Tongji University.
2.2.2 Water filled cooling method
The water filling cooling method involves filling water into hollow steel components. By circulating water within the steel structure, it absorbs the heat generated by the steel itself. Thus, the steel structure can maintain a lower temperature in a fire and will not lose its load-bearing capacity due to excessive heating. To prevent corrosion and icing, rust inhibitor and Antifreeze shall be added to the water. The steel columns of U.S. Steel Building on the 64th floor in Pittsburgh, USA, are cooled by filling water.
- The comparison of fire prevention measures and thermal insulation method can slow down the speed of heat transfer to structural components through thermal insulation materials. Overall, the insulation method has good economy and practicality, and is widely used in practical engineering. Water cooling method is an effective protective measure against fires, but due to its special requirements for structural design and high cost, it has not been well promoted in the engineering field at present.
The thermal insulation method is widely used in the fire protection of steel structures, so the following focuses on comparing the advantages and disadvantages of the spray coating method and the encapsulation method in the thermal insulation measures.
3.1 Fire resistance
In terms of fire resistance, the encapsulation method is superior to the spraying method. The fire resistance of enveloping materials such as concrete and Fire brick is better than that of ordinary fireproof coating. In addition, the fire resistance performance of the new type of fireproof board is also better than that of fireproof coatings. Its fire resistance limit is significantly higher than that of steel structure fire insulation materials of the same thickness, and even higher than that of expansion type fire retardant coatings.
3.2 Durability
Due to the good durability of sealing materials such as concrete, it is not easy to experience performance degradation over time; Durability has always been a problem that steel structure fireproof coatings have not been able to solve. Thin and ultra-thin fire-resistant coatings mainly composed of organic components, whether used outdoors or indoors, may cause decomposition, degradation, aging, and other problems, resulting in coating peeling and pulverization or loss of fire resistance.
3.3 Constructability
The spray coating method for fire prevention of steel structures is simple and can be applied without the need for complex tools. However, the controllability of spray coating construction quality is poor, making it difficult to control the rust removal of the substrate, the coating thickness of the fireproof coating, and the humidity of the construction environment; The construction of enveloping method is relatively complicated, especially for slant support and steel beams, but the construction is highly controllable and the quality is easy to guarantee. The thickness of the sealing material can be accurately changed to control the fire resistance limit.
3.4 Environmental friendliness
Spraying method pollutes the environment during construction, especially when it can emit harmful gases under high temperatures. The encapsulation method has no toxic releases in construction, normal use environment, and high fire temperatures, which is beneficial for environmental protection and personnel safety during fires.
3.5 Economy
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 due to the drawbacks of aging, their maintenance costs are relatively high. The construction cost of the encapsulation method is high, but the materials used are cheap and the maintenance cost is low. Overall, the encapsulation method has good economic efficiency.
3.6 Applicability
The spraying method is not limited by the geometry of components and is often used for the protection of various components such as beams, columns, floors, roofs, etc. Especially suitable for fire protection of steel structures in spatial structural systems such as light steel structures, grid structures, and irregular steel structures. The construction of enveloping method is complex, especially for steel beams, slant support and other components. The enveloping method is generally used for columns, and its scope of application is not as wide as the spraying method.
3.7 Occupying Space
The fireproof coating used in the spraying method has a smaller volume, while the packaging materials used in the packaging method, such as concrete and fireproof bricks, will occupy space and reduce usage space. And the quality of the packaging material is also relatively high.
- Summary
Through discussion, the following conclusions can be drawn:
1) The adoption of fire protection measures for steel structures should consider 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 appearance of the components does not change significantly after spraying. The main advantages of encapsulation method are low cost, good fire resistance and durability;
3) Various fire prevention measures have their own advantages and disadvantages. In engineering applications, they can complement each other’s strengths and weaknesses, and multiple fire prevention measures can be combined. And multiple fire prevention lines can be set up by taking different measures.
