Advantages of steel structure: Steel structures are lightweight, highly seismically resistant, and have a high bearing capacity because of their high strength and ductility. Moreover, the steel structure can be processed on site, construction time is short, and the materials are recyclable, which meets the requirements of environmental protection. Steel structure buildings are therefore widely used at home and abroad.
Fire resistance is a fatal weakness of steel structures. In practical projects, various fire protection measures are adopted to keep the steel structure strong, and rigid in the event of a fire and to protect people and property.
In this article, we will discuss different fire prevention measures in detail according to different fire prevention principles, and compare their advantages and disadvantages.
In terms of fire protection, steel structures can be categorized into two categories: water cooling and heat resistance. The purpose of these measures is the same: the component's temperature is not exceeded for a specified period of time. A heat resistance method prevents the transfer of heat to the component, while a water-cooling method allows the heat to be transferred to the component, after which the heat is conducted away.
Fire resistance refers to the time before it becomes unstable or loses its integrity, and its ability to resist fire in a standard fire resistance test.
It should be noted that although the steel itself will not catch fire, the material properties of the steel are greatly affected by temperature. At temperatures above 250 °C, the impact toughness of the steel decreases; if it exceeds 300 °C, the yield point and ultimate strength will decrease significantly. Under the actual fire, the load condition remains unchanged. The critical temperature for the steel structure to lose its static equilibrium stability is about 500 °C, while the general fire temperature reaches 800 to 1000 °C. Consequently, the steel structure will undergo plastic deformation under the high temperatures of the fire, resulting in local damage, and eventually, the steel structure will collapse.
Steel-structure buildings must take fire protection measures to ensure that they are fire-resistant. Prevent the steel structure from rapidly heating up to the critical temperature in a fire. This will prevent excessive deformation and even the collapse of the building, provide precious time for firefighting and safe evacuation of personnel, and avoid or reduce the loss caused by the fire.
Heat resistance method
According to the heat resistance of fire retardant coatings and the heat resistance of encapsulating materials, the heat resistance method is divided into spraying and encapsulating. In the spray method, fire retardant paint is applied or sprayed on the building. There are two types of encapsulation methods: hollow encapsulation and solid encapsulation.
To improve the fire resistance limit of steel structures, fire retardant paint is painted or sprayed on the steel surface to form a fire-resistant and heat-insulating protective layer. The method is easy to construct, light in weight, has a long fire resistance time, and is not limited by the geometry of steel components. Moreover, it is economically and practically viable. There are several types of fire-retardant coatings for steel structures, which can be roughly divided into two groups: one is intumescent fire-retardant coatings for steel structures (type B); the other is thick-coating type coatings (H).
The coating thickness of Class B fire retardant coatings is generally 2-7mm. It is made from organic resin, which has a decorative effect and expands and thickens at high temperatures. The fire resistance is 0.5 * 1.5 hours. Thin-coated steel structure's fireproof coating is light, thin, and vibration-resistant. Generally, thin-coated steel structure fireproof coatings should be used when the fire resistance limit is specified to be 1.5 h, and below. Fire retardant coatings of the H class are typically 8 to 50 mm thick. The surface is granular. It consists of materials with low density and low thermal conductivity, including inorganic thermal insulation materials. The fire resistance limit can reach 0.5 ~ 3.0h. Steel structures with thick-coated fireproof coatings are generally non-combustible, anti-aging, and more durable. A thick-coated steel structure fireproof coating should be used when the fire resistance limit is specified to be above 1.5h for both indoor concealed steel structures and high-rise steel structures.
1) Hollow encapsulation method: Fireproof boards or refractory bricks are usually used to cover the steel members along their outer edges. In most steel structure workshops, steel components are wrapped in refractory bricks to protect them from corrosion. As an advantage of this method, it is strong and resistant to impact, but as a disadvantage, it occupies a lot of space and is more difficult to construct. Lightweight refractory boards, such as fiber-reinforced cement boards, gypsum boards, and vermiculite boards. are used as the outer layer of a fireproof enclosure. This method of box-wrapping large steel components has the advantage of flat and smooth decoration surfaces, low cost, minimal loss, no environmental pollution, and good promotion prospects.
2) Solid encapsulation method: By pouring concrete over the steel members, they are wrapped and completely enclosed. High strength and impact resistance are advantages, but the disadvantage is that the concrete protective layer takes up a lot of space and construction is complicated, especially when it comes to steel beams and diagonal braces.