Steel structures offer numerous advantages over traditional concrete and masonry structures. They are stable, lightweight, high-strength, and have good seismic performance. Fabrication can be done off-site, allowing for high-precision assembly and significantly faster construction. They are also considered "green buildings," with low initial costs and recyclable materials, contributing to energy, land, and water conservation. As an eco-friendly building material, steel-structured housing has been identified as a key promotion project by the Ministry of Construction. However, a significant drawback is their poor fire resistance. Steel structures themselves contain combustible materials, and the strength of steel decreases significantly at high temperatures, making buildings prone to collapse in the event of a fire. This often leads to serious casualties. The collapse of the World Trade Center towers in the "9/11" terrorist attacks serves as a prime example. These twin towers, a prime example of high-rise steel structures, were located in lower Manhattan, New York City, and comprised five buildings. The main towers were twin skyscrapers made of 78,000 tons of steel, with closely-spaced steel columns and walls of aluminum panels and glass windows. Known as the "Window to the World," they collapsed within 30 minutes of the fire, resulting in 2,797 deaths and $36 billion in losses. Therefore, to promote steel construction, fire resistance must be improved; only then can this energy-efficient and environmentally friendly building material gain widespread use and truly showcase its advantages.

　　

　　1. Fire Resistance of Steel Structures

　　

　　The fire resistance of steel structures depends on the steel itself. Steel is non-combustible and has good heat resistance. However, it is not high-temperature resistant; its strength decreases, and deformation increases with rising temperature. Below 200℃, steel properties remain largely unchanged; between 430℃ and 540℃, strength drops sharply; and at 600℃, the strength is too low to support loads.

　　

　　Based on these physical properties, when temperatures reach 450℃～650℃ during a fire, steel structures lose their load-bearing capacity, undergoing significant deformation, leading to the bending of columns and beams, resulting in functional failure or collapse.

　　

　　Deformation and failure of steel components at high temperatures are inevitable; we can only strive to improve their fire resistance to prevent immediate failure and collapse. Laboratory data indicate that the fire resistance of common steel components is only 15-30 minutes. National standards such as the "Building Design Fire Prevention Code" (GBJ16-87) and the "High-Rise Civil Building Design Fire Prevention Code" (GB50045-95) stipulate fire resistance ratings of 3 hours and 2 hours for firewalls, columns, load-bearing walls, stairwell walls, and beams in Class A fire-rated buildings. The fire resistance of unprotected steel components is significantly lower than these requirements and cannot meet fire safety standards. To overcome the shortcomings of steel in fire safety, improvements must be made from multiple aspects to achieve the required fire ratings.

　　

　　2. Measures to Improve the Fire Resistance of Steel Structures

　　

　　2.1 Application of Fire-Resistant and Weather-Resistant Steel: Ordinary steel has poor fire resistance, but this can be improved by altering the material composition, adding specific components, and modifying the structure and microstructure of the steel to enhance its fire and weather resistance. This is fire-resistant and weather-resistant steel. At 600℃, the strength of this steel decreases by less than 30%. Its good fire resistance allows for reduced protective layer thickness during construction, potentially eliminating the need for anti-rust paint, saving costs. This is a fundamental solution that should be widely promoted. Several Chinese companies now produce fire-resistant and weather-resistant steel for construction, providing a good source of material for steel structures.

　　

　　2.2 Using Structural Forms and Components with High Fire Resistance: Pure steel structures have poor fire resistance but can be combined with concrete to form composite components, or reinforced concrete can be used in critical areas. For example, steel pipe concrete columns can be used to significantly improve fire resistance. Larger pipe diameters (or side lengths) result in longer fire resistance times; a 500mm diameter steel pipe concrete column with a 15mm thick heat-insulating fire-resistant coating can meet the 3-hour fire resistance requirement. Components with high fire resistance requirements, such as floors and stairs, should use reinforced concrete.

　　

　　2.3 Implementing Rational Fire Protection Design Schemes for Steel Components: Fire protection design for steel structures should be rational, robust, economical, easy to construct, and facilitate decoration. When non-combustible walls or partitions are in line with steel components, the non-combustible materials can provide fire protection without requiring additional layers. For exposed steel components, appropriate fire protection designs must be used.

　　

　　2.4 Implementing Effective Fire Protection Methods: Common fire protection methods for steel structures include external concrete encasement, external wire mesh cement mortar, external fire-resistant panels, and spray-applied fire-resistant coatings. External concrete encasement completely encloses steel components with concrete, reinforced with structural steel to prevent concrete detachment. Wire mesh cement mortar fire protection uses a metal mesh and 50# mortar as a protective layer, a traditional method. External fire-resistant panels can include fiber-reinforced cement boards (such as TK boards, FC boards), gypsum boards, calcium silicate boards, vermiculite boards, and other non-combustible materials.

　　

　　Spray-applied fire-resistant coatings create a fire-resistant and heat-insulating protective layer, improving the fire resistance of steel components. Thin fire-resistant coatings, typically 1-3mm thick, can be used for components with fire resistance requirements less than 1.5 hours; these coatings expand when exposed to fire, forming a porous carbon layer many times thicker than the original. For components requiring more than 1.5 hours of fire resistance, thick fire-resistant coatings (7-50mm) are used; these do not expand and have good heat insulation. For high-rise steel structures with high fire resistance requirements, inorganic fiber fire protection materials can be sprayed; these have low density, high heat capacity, and low thermal conductivity, with good adhesion; at high temperatures, the thickness does not change significantly, and the protective layer does not melt or detach, achieving fire resistance of more than 5 hours. Most steel structures in China use spray-applied fire-resistant coatings for fire protection.

　　

　　The world is now calling for energy conservation and emission reduction. Steel structures, a green and environmentally friendly form of building construction, are already quite popular in developed countries and have vast development potential in China, especially in residential buildings where it should be widely promoted. Currently, China's steel production ranks first in the world, providing a tailwind for the promotion of steel structures. We shouldn't let fire resistance hinder the progress of steel structures; we should actively take various effective measures to improve the fire resistance of steel structures and accelerate their development.