Although China achieved remarkable feats in iron structures early on, its scientific development was hampered by over 2000 years of feudal rule, resulting in a prolonged stagnation in iron building technology. It wasn't until the late 19th century that China began adopting modern steel structures. Following the establishment of the People's Republic of China, the application of steel structures saw significant advancements, far surpassing past achievements in both quantity and quality. High levels of proficiency have been reached in design, manufacturing, and installation techniques, including the mastery of complex building design and construction. Across the country, numerous large-scale and structurally complex steel structures have been erected, such as factory buildings, large-span steel civil structures, and railway bridges. Notable examples include the steel roof truss of the Great Hall of the People, the steel reticulated shells of stadiums in Beijing and Shanghai, and the three-hinged steel arch frame of the Qin Shi Huang Terracotta Army Museum in Shaanxi.

　　1. Advantages of Steel Structures

　　1.1 Lightweight: Despite the high specific gravity of steel, its excellent mechanical properties allow it to withstand considerable loads. This results in smaller cross-sectional dimensions for steel structural members. For the same span and load, a steel roof truss weighs only about 1/4 or 1/3 of a reinforced concrete roof truss, facilitating easier transportation.

　　1.2 Homogeneous Material: The consistent internal structure of steel closely aligns with the assumptions used in mechanical calculations. Steel behaves near-isotropically, exhibiting almost perfect elasticity within a certain stress range. Therefore, the actual stress conditions of steel structures closely match the results of structural mechanics calculations.

　　1.3 High Degree of Mechanization in Manufacturing and Installation: Steel structure construction demonstrates a degree of technology-intensive nature. Steel structures utilize a single material and are prefabricated, resulting in simple processing and a high level of mechanization. Construction is swift, quality is easily assured, and mass production is facilitated. From factory fabrication to on-site hoisting, the level of industrialization is far higher than labor-intensive cast-in-place reinforced concrete structures.

　　1.4 Simple Installation and Construction: Steel structure components (beams, trusses, columns, etc.) are produced by specialized metal structure factories. On-site assembly is performed using electric welding or bolts (or high-strength bolts), enabling quick and easy installation. This increases construction speed, a major factor in reducing engineering costs, rendering structural costs secondary.

　　1.5 Flexibility in Layout: Another significant advantage of steel structures lies in their flexibility in terms of layout. In today's information age, demands for interior layout, space, and facilities are stricter. Previous layouts and equipment may soon become less suitable or wholly unusable. Steel structures offer greater possibilities for re-layout, extending the lifespan of buildings.

　　Despite these advantages, steel structures have a fatal flaw: poor fire resistance. Although steel itself is non-combustible, its mechanical properties, such as yield strength and elastic modulus, deteriorate with increasing temperature during a fire. The decline becomes particularly significant around 550℃, resulting in a loss of load-bearing capacity and collapse within approximately 15 minutes. Fire incidents in steel structures worldwide demonstrate that buildings often collapse within 20 minutes of a fire, leaving behind ruins. The Tianjin Sports Stadium fire in May 1973, the Shanghai Cultural Square fire in December 1969, and the Beijing Erqi Locomotive and Rolling Stock Plant fiberboard workshop fire in August 1972 all exposed the critical weakness of steel structures' poor fire resistance.

　　2. Fire Protection Methods for Steel Structures

　　Due to their poor fire resistance and rapid failure under high temperatures, steel structures have a fire resistance rating of only 15 minutes. Protective measures, however, can prevent the temperature increase from exceeding the critical temperature, maintaining stability during a fire. These protective measures can be broadly categorized into two types based on their principles: heat interception and heat dissipation.

　　2.1. Heat Interception Methods

　　Heat interception methods aim to interrupt or impede the heat transfer from the fire to the structural member, ensuring that the temperature rise does not exceed its critical temperature within a specified time. This is achieved by applying a protective layer to the surface of the member, allowing the high temperatures to transfer to the protective material first, before being further transferred to the component. The low thermal conductivity and high heat capacity of the selected materials effectively impede heat transfer, thus providing protection. Heat interception methods include spraying, encapsulation, shielding, and water spraying.

　　2.1.1 Spraying Method

　　The spraying method involves using spray equipment to apply fire-resistant coatings directly to the surface of the members, forming a protective layer. Steel structure fire-resistant coatings are classified into organic and inorganic types based on their adhesives, and into thin and thick coating types based on their thickness. Thin coatings typically have a thickness of 2-7 mm, offering a degree of decorative effect. At high temperatures, the coating expands and thickens, providing fire-resistant and heat-insulating properties with a fire resistance rating of 0.5-1.5 hours. These coatings are also known as intumescent fire-resistant coatings. Thick coatings generally range from 8-20 mm in thickness, featuring a granular surface and low density and thermal conductivity, with a fire resistance rating of 0.5-3.0 hours. These coatings are known as steel structure fire-resistant and heat-insulating coatings.

　　Steel structure fire-resistant coatings operate on three principles: First, the coating acts as a shield, preventing the steel structure from direct exposure to flames and high temperatures. Second, the heat absorption and decomposition of some coating materials release water vapor or other non-combustible gases, consuming heat, reducing flame temperature and combustion speed, and diluting oxygen. Third, the porous, lightweight nature of the coating, and the formation of a carbonized foam layer upon heating, prevents rapid heat transfer to the steel substrate, delaying the reduction in steel strength and increasing the fire resistance rating.

　　Main varieties of thin steel structure fire-resistant coatings include: LB, SG-1, SB-2, and SS-1 intumescent fire-resistant coatings. Main varieties of thick steel structure fire-resistant coatings include: LG fire-resistant and heat-insulating coatings, STI-A, JG276, ST-86, SB-1, and SG-2 fire-resistant coatings.

　　When spraying steel structure fire-resistant coatings, the sprayed thickness must meet the design requirements, with appropriate thickening at the joints. A steel wire mesh connected to the steel structure should be incorporated within the coating under any of the following circumstances to ensure its stability:

　　——Beams subjected to impact or vibration

　　——Design layer thickness exceeding 40 mm

　　——Coatings with adhesive strength less than 0.05 MPa

　　Beams with a web height greater than 1.5m

　　Spraying is the most widely applicable method and can be used for fire protection of any type of steel component.

　　2.1.2. Encapsulation Method

　　The encapsulation method involves creating a fire protection layer on the surface of the steel structure to encapsulate the component. Specific methods include:

　　2.1.2.1. Using cast-in-place concrete as a fire protection layer. Materials used include concrete, lightweight concrete, and aerated concrete. These materials are non-combustible and have a relatively high heat capacity, slowing down the temperature increase of the component when used as a fire protection layer. Because the surface layer of concrete is prone to spalling under high fire temperatures, a wire mesh can be applied to the steel surface to further improve its fire resistance.

　　2.1.2.2. Using mortar or grout as a fire protection layer. Materials used generally include mortar, lightweight magma, perlite mortar or grout, vermiculite mortar or lime grout, etc. The above materials all have good fire resistance, and the construction method is often to apply the above materials to a metal mesh.

　　2.1.2.3. Using mineral fibers. Materials include asbestos, rock wool, and slag wool. The specific construction method is to mix mineral fibers with cement, and then spray them onto the base with a special spray gun and water spray simultaneously to form a sponge-like covering layer, which is then smoothed or left in a concave-convex shape. This method can be sprayed directly onto the steel component or onto a metal mesh on top of it, with the latter being more effective.

　　2.1.2.4. Using lightweight prefabricated panels as a fire protection layer. Materials used include lightweight concrete panels, foam concrete panels, calcium silicate molded panels, and asbestos molded panels, etc. The method is to cover the components with the above-mentioned prefabricated panels. The connection between the panels can be achieved by nailing and bonding. This construction method is simple and has a shorter construction period, and is conducive to industrialization. At the same time, the functions of load-bearing (steel structure) and fire protection (prefabricated panels) are clearly divided, and post-fire repair is simple and does not affect the function of the main structure, thus having good recoverability.

　　2.1.3. Shielding Method

　　The shielding method involves enclosing the steel structure within a wall or ceiling made of fire-resistant materials. Creating a fire-resistant ceiling under steel beams and trusses can significantly slow down the temperature increase of the steel beams and trusses during a fire, greatly improving the fire resistance of the steel structure. This method also improves the aesthetics of the interior, but attention should be paid to ensuring that the seams and holes in the ceiling are sealed to prevent fire spread.

　　2.1.4. Water Spraying Method

　　The water spraying method involves installing a sprinkler water supply network at the top of the structure. During a fire, the system automatically (or manually) starts spraying water, forming a continuous flowing water film on the surface of the components, thus providing protection.

　　As can be seen from the above, the common characteristic of these methods is to reduce the heat flow to the components, therefore they are called interception methods.

　　2.2. Conduction Method

　　Unlike the interception method, the conduction method allows heat to reach the component and then dissipates or consumes the heat, which prevents the component temperature from rising to the critical temperature, thus providing protection.

　　Currently, the conduction method mainly involves water-cooling protection. This method involves filling hollow closed sections (mainly columns) with water. During a fire, the component transfers the heat absorbed from the fire to the water, and the heat is consumed by water evaporation or conducted away through circulation, keeping the component temperature around 100℃. Theoretically, this is the most effective method for steel structure protection. When the system is working, the component acts like a water-filled container being heated, similar to a boiling pot. As long as the water source is replenished and the water level is maintained, and the specific heat and heat of vaporization of water are large, the heat absorbed by the component will be continuously consumed or conducted away.

　　Cooling water can be replenished from an elevated water tank, a water supply network, or a fire truck. Steam is discharged through the exhaust port. When the column height is too large, it can be divided into several circulation systems to prevent excessive water pressure at the bottom of the column. To prevent rust or water freezing, rust inhibitors and antifreeze should be added to the water.

　　Water cooling can be a single-column self-contained system or a multi-column interconnected system. The former relies only on water evaporation to dissipate heat, while the latter can both evaporate and dissipate heat and form a circulation based on the temperature difference of the water, conducting heat to columns with lower temperatures in non-fire areas.