Fire Protection of Large Span Steel Structure Buildings

In recent years, with the rapid development of china’s social economy, more and more types of large-span steel structures are used in buildings, such as workshops, bridges, warehouses, gymnasiums, exhibition halls, supermarkets, airport waiting halls, etc. Large-span steel structures are widely recognized and used by society for their advantages of high strength, self-weight, light weight, large span, convenient construction, and short construction time. Buildings, Shenzhen SEG Building with a height of 291m, Dalian World Trade Center, Wuhu Yangtze River Bridge with a span of 216m, Shanghai Baosteel Large Steel Rolling Plant, etc.

1 Types of large-span steel structures

(1) space frame structure. It is widely used in the roof structure of gymnasiums, exhibition halls, clubs, theaters, canteens, conference rooms, waiting rooms, hangars, workshops, etc. It has the characteristics of a high degree of industrialization, lightweight, good stability, and a beautiful appearance.

(2) Suspension cable and cable truss structure. A series of cables are used as the main load-bearing components. These cables are composed of various forms according to certain rules and are suspended on the corresponding supporting structures so that the strength of the material can be fully exerted under tension. It is suitable for large-span roofs and has the characteristics of saving steel, beautiful appearance, and complicated design and construction.

(3) Reticulated shell structure. Like the space frame structure, the reticulated shell is also composed of many rods arranged according to a certain rule, connected by nodes to form a space rod structure, but the shape of the space frame is flat, and the shape of the space frame is curved. Generally single-layer or double-layer, according to the shape of single-curved or hyperboloid, it forms various forms such as reticulated dome, reticulated cylindrical shell and hyperbolic parabolic reticulated shell. The reticulated shell structure has the characteristics of beautiful appearance, good transparency, large building space, less material, and more complicated design and construction.

2 Basic characteristics of large-span steel structure buildings

(1) The structural span is large and the structural space is large. The building uses various steel structures as load-bearing components, such as roof trusses, beams, slabs, etc., which have the characteristics of large span, large space, and strong load-bearing capacity.

(2) The building structure and function are complex. Most of the large-span steel structure buildings are industrial buildings, where machinery, equipment, raw materials and finished products are piled up. In addition, in order to meet the needs of production and technology, the upper part of modern large-span steel structure buildings is equipped with various pipes such as ventilation, water, central air conditioning, and steam pipes. There are also flammable gas and oil pipelines installed, and the outer walls and partition walls are mostly made of iron sheets and polyurethane. In addition, some non-load-bearing components, such as roofs, ceilings, floors, etc., and other decoration materials use polymer organic materials. These have greatly increased the sources of danger in the building.

(3) The fire resistance of large-span steel structure buildings is poor, and the strength drops sharply in high-temperature environments. In the case of heating, the mechanical properties of steel change with increasing temperature. Generally, the elastic modulus, yield strength, and ultimate strength decrease with the increase of temperature, and the plastic deformation increases with the increase in temperature. The so-called “blue brittleness” phenomenon occurs in hot-rolled steel at 200~350℃. At this time, the ultimate strength of the steel increases and the plasticity decreases, and it becomes “brittle” compared with other temperature ranges. At 500 °C, the ultimate strength and yield limit of the steel are greatly reduced, and the plasticity increases. At 450~600℃, carbides tend to be graphitized and spheroidized. If the heating temperature is higher and the time is longer, the carbon content of the steel is higher, and the spheroidization of carbides will be more severe. The presence of graphitization and spheroidization indicates that the steel is weakened at high temperatures and the mechanical properties are reduced. The addition of alloying materials generally increases the temperature required for the above-mentioned changes in the steel. The test results show that the change of strength is not obvious within 200℃, the yield strength decreases slightly, and the ultimate strength basically does not change. After 200°C, the rate of yield strength decreasing with increasing temperature begins to accelerate. The ultimate strength at 200~300℃ is slightly higher than that at room temperature due to the appearance of “blue brittleness”, and after 300℃, the ultimate strength decreases obviously with the increase of temperature. At 600 °C, the yield strength and ultimate strength of low carbon steel are only 35% to 40% of those at room temperature, while the strength of carbon steel wire is lower. As the temperature further increases, the strength of the steel basically disappears at 800 °C. At the same time, the elongation rate and section shrinkage rate of the steel increase with the increase of temperature, indicating that the plasticity of the steel increases at high temperatures and is easy to deform.

3 Fire characteristics of large-span steel structure buildings

(1)Large-span steel structure buildings have poor fire resistance and are prone to collapse. The steel structure material itself will not catch fire without fire prevention treatment, but its strength will drop rapidly in the event of a fire. According to the test, under full load, the critical temperature for the steel structure to lose its static equilibrium stability is about 500℃, while the general fire temperature can reach 800~1000℃. At such a high temperature, the exposed steel structure will quickly deform, resulting in local damage, resulting in the overall collapse of the steel structure. The collapse time of steel structure buildings is very short, generally after continuous burning for 15 minutes, and in the fire, when the steel structure temperature reaches 350, 500, and 600 ℃, the strength decreases by 1/3, 1/2, and 2/3 respectively.

(2)Large-span steel structure building fire: fierce fire, densely populated, large load, fast heating, and difficult to put out. The large-span steel structure building has a large space and sufficient air. Once a fire occurs, the fire can spread in all directions, burn violently, and it is easy to generate strong high-temperature smoke to form and spread, resulting in large-scale burning. Large-span steel structure buildings are densely populated and have a large fire load. When fighting fires, there are many obstacles, making it difficult to evacuate people, and it is easy to cause mass deaths and injuries. On April 5, 2003, a fire broke out in the steel-structured cooked food processing workshop of Qingdao Zhengda Co., Ltd. Due to improper handling and poor evacuation at the beginning of the fire, the employees on duty blindly escaped, resulting in 21 people being burned to death on the spot.

4. Fire protection of large-span steel structure buildings

The purpose of fire protection for large-span steel structure buildings is to increase the fire resistance limit of the steel structure to the limit range specified in the design specification, and the measures are various. Spraying fire retardant coatings has the characteristics of convenient construction, lightweight, low cost, not limited by the geometry of components, the widest application range, and high efficiency. It is a relatively common and relatively mature practice.

4.1 The fire prevention principle of large-span steel structure fireproof coating The fireproof principle of large-span steel structure fireproof coating is to use adiabatic or heat-absorbing materials to block the flame to directly burn the steel structure, reduce the speed of heat transfer to the steel, and delay the temperature rise and strength of the steel structure. weakening time. According to “Fireproof Coatings for Steel Structures” (GB14907-2002), fireproof coatings for steel structures are defined as coatings that are applied to the surface of buildings and steel structures to form a fire-resistant and heat-insulating protective layer to improve the fire resistance of steel structures.

4.2 Types of fire retardant coatings for large-span steel structures

According to the thickness of the fireproof coating, it can be divided into thin coating type, thick coating type, and ultra-thin type.

Thin-coated steel structure fireproof coating, the coating thickness is generally 2~7mm, which has a certain decorative effect. At high temperatures, the coating expands and thickens, which has the effect of fire resistance and heat insulation. The fire resistance limit can reach 0.5~2h, so it is also called a steel structure. Intumescent fire retardant coating. Thick coating steel structure fireproof coating, the coating thickness is generally 8~50mm, granular surface, low density, low thermal conductivity, fire resistance limit can reach 0.5~3h, also known as steel structure fireproof and heat insulation coating. Thick-coat fire retardant coatings are generally non-flammable, non-toxic, resistant to aging, and have good permanent resistance, and are suitable for permanent buildings. Ultra-thin fireproof coating for steel structure, the coating thickness is generally 1~3mm. Compared with thick-coated and thin-coated steel structure fireproof coatings, ultra-thin steel structure fireproof coatings are characterized by smaller and finer particle size, good decoration and coating. Thinner, the fire resistance limit can reach 05~10h.

4.3 Hidden dangers of fire retardant coatings for large-span steel structure buildings

Although long-span steel structure fireproof coating plays an important role and fully reflects its value in construction engineering applications, in addition to its own advantages, it also has defects. In terms of production and construction, most of the domestic steel structure fireproof coating manufacturers are small in scale, the production process automation level is not high, the research on raw materials dedicated to fireproof coatings is not enough, the detection and control of raw materials is not enough, and the detection methods in the production process are not enough. Incomplete, the construction equipment needs to be improved, and the compatibility with anti-rust paint can not be strictly tested; in terms of testing standards, “Fire-retardant coatings for steel structures” (GB14907-2002) only stipulates one coating thickness for the same kind of fire-resistant coatings However, in the actual project, the fire resistance limit required by the steel beam, steel column, and steel floor specifications is different. For example, the test report of the ultra-thin fireproof coating with an indoor thickness of 2mm is issued with a fire resistance limit of 1.5h. The actual project requires The fire resistance limits of steel beams, steel plates and steel columns are 1.5, 1.0 and 2.0h respectively. What thickness of fire retardant coatings should be used to protect steel plates and steel columns is currently inconclusive in theory or in practical projects. In addition, there are still some problems, such as the test methods, the differences between the standard components and the actual engineering components, the low quality of the construction team, the poor construction quality, the improper selection, the uneven product quality, and the lack of strict implementation standards of the testing agencies.