Steel - structured buildings are widely used due to their advantages such as high strength and rapid construction. However, the fire - sensitive nature of steel poses a severe challenge in case of a fire. Therefore, it is of great importance to understand the basic knowledge of fire protection for steel - structured buildings.
I. Fire - resistant Performance of Steel Structures
Although steel has high strength, its mechanical properties will decline sharply at high temperatures. When the temperature reaches 300℃ - 400℃, the strength of steel begins to decrease significantly. When the temperature is around 600℃, its strength is almost completely lost. In a fire, if effective fire - protection measures are not taken for a steel - structured building, components such as steel beams and columns may deform rapidly due to the increase in temperature, leading to the instability and collapse of the building structure, endangering lives and property. For example, in an unprotected steel - structured factory building, the steel structure may be severely deformed within ten - odd minutes after a fire breaks out.
II. Classification of Fire - resistance Ratings
The fire - resistance ratings of steel - structured buildings are classified according to factors such as building types, functions, and importance. Generally, they are divided into four levels: Level I, Level II, Level III, and Level IV, with Level I being the highest. Different fire - resistance ratings have clear regulations on the combustion performance and fire - resistance limits of building components. For a building with a fire - resistance rating of Level I, the fire - resistance limit of columns is required to reach 3.00 hours, and that of beams is 2.00 hours. For a building with a fire - resistance rating of Level II, the fire - resistance limit of columns is 2.50 hours, and that of beams is 1.50 hours. The fire - resistance limit refers to the period during which a building component undergoes a fire - resistance test according to the time - temperature standard curve. It starts from the moment the component is exposed to fire and ends when it loses its load - bearing capacity, its integrity is damaged, or it loses its fire - separation function.
III. Fire - protection Measures
(I) Fire - resistant Coatings
Types:
Thin - film Fire - resistant Coatings: The coating thickness is generally 3 - 7mm. When exposed to fire, the coating expands and foams to form a heat - insulating and fire - resistant foam layer, preventing heat from being transferred to the steel. It has advantages such as a thin coating, light weight, and good decorative properties, and is often used in steel - structured buildings with high aesthetic requirements, such as commercial buildings and public buildings.
Thick - film Fire - resistant Coatings: The coating thickness is usually 8 - 50mm. It mainly delays the heating of steel through its own heat - insulating properties. It has good fire - resistant performance and a relatively low cost, but its appearance is relatively rough. It is mostly used in buildings with low aesthetic requirements, such as industrial plants and warehouses.
Construction Key Points: Before construction, the steel surface should be treated for rust removal and degreasing to ensure the adhesion of the coating. Construction methods such as spraying and brushing are used, and the coating thickness and construction interval should be strictly controlled according to the product instructions. The coating should be uniform without defects such as missed coating and hollowing.
(II) Fire - resistant Board Cladding
Materials: Commonly used fire - resistant boards include rock - wool boards, glass - wool boards, calcium - silicate boards, etc. These boards have good heat - insulating properties and can effectively block the heat transfer to the steel. For example, rock - wool boards, with their non - combustibility, heat - insulation, and sound - absorption properties, have become a commonly used material for the fire - resistant cladding of steel structures.
Installation: The boards are fixed to the surface of the steel structure through bonding, nailing, etc. During installation, attention should be paid to the tight splicing between the boards to avoid gaps that may affect the fire - protection effect. For steel columns, a full - cladding method is usually adopted. For steel beams, methods such as cladding the bottom and both sides can be selected according to the actual situation.
(III) Structural Fire - resistant Design
Reasonable Layout: During the building design stage, the layout of the steel structure should be reasonably planned. Important steel - structure components should be arranged in relatively safe areas to reduce the probability of being exposed to fire. For example, the steel structures of key parts such as evacuation stairs and fire - fighting elevators can be set in the center of the fire - compartment, and fire - walls or fire - shutters can be set around for protection.
Increasing the Cross - sectional Dimensions of Components: Appropriately increasing the cross - sectional dimensions of steel - structure components can improve their fire - resistant performance. Because a larger cross - section can absorb more heat and slow down the rate of temperature rise of the steel. However, this method will increase the amount of steel used and costs, which need to be considered comprehensively.
IV. Daily Maintenance and Inspection
Regular Inspection: Regularly inspect the fire - protection facilities of steel - structured buildings, such as fire - resistant coatings and fire - resistant boards, to check whether the coatings are peeling or cracking, and whether the boards are damaged or displaced. Conduct a comprehensive inspection at least once a year to promptly discover and repair damaged fire - protection facilities.
Maintenance and Upkeep: For slightly damaged fire - resistant coatings, repair them in a timely manner. For severely damaged areas, re - apply the coating. For damaged fire - resistant boards, replace them in a timely manner. At the same time, keep the surface of the steel structure clean to avoid the accumulation of debris affecting the fire - protection effect.
