With the development of building technology and economy, steel structures are often used in buildings, warehouses, stadiums and other buildings. Because of this structure, the structure is simple, the construction is convenient, the span is large, the modern sense is strong, the internal space is large, and the internal freedom is Many advantages, such as large and short construction period, have gradually become the main structural form of enterprise buildings. However, the light steel structure has a low fire resistance rating and poor fireproof performance. Under the action of high temperature, its mechanical properties, including elastic modulus and yield strength, are greatly reduced, which is prone to collapse, resulting in casualties and large fire losses. In recent years, fires in steel structure factories have occurred in many places in China, causing casualties and major property losses, and even causing officers and soldiers to sacrifice for this. Below, the author discusses some unmistakable views on the fire characteristics of steel structure workshops.
First, the fire resistance of steel structure workshop
The steel structure building is mainly supported by steel columns and grids. Therefore, the fire resistance of the steel structure determines the fire resistance of the plant. According to the "Code for Design of Steel Structures" (GB50017-2003), the steel structure workshop mainly uses carbon structural steel and low-alloy high-strength structural steel as the main materials. Generally, the critical temperature for losing static equilibrium stability under full load is about 540 °C, and the overall mechanical properties vary with temperature. When the general structural temperature reaches 350 °C, 500 °C, and 600 °C, the strength decreases by 1 respectively. /3, 1/2, 2/3. In a fire, the temperature of the fire field usually reaches 800~1200 °C. Under such circumstances, the steel structure with high thermal conductivity will generally be around 15 minutes, plastic deformation will occur, local damage will occur, and the bearing capacity will be completely lost. The whole collapsed.
Among them, different wall materials and temperature characteristics have certain influence on the internal temperature rise of the steel structure and the deformation under high temperature. In the existing steel structure workshop, the sandwich panel is usually used as the partition wall and the outer wall, and the interior is made of polystyrene, polyurethane and rock wool with high thermal insulation properties as the filling material. The former two materials have poor fire resistance and are susceptible to heat combustion, which will increase the building fire load, accelerate the temperature rise of the steel structure, and reduce the fire resistance. Rockwool itself is an inorganic silicate fiber that is non-flammable and its combustion properties depend on the type and amount of binder. Under normal circumstances, the fire-resistance limit of the metal sandwich panel is about 10 minutes, that is, it loses its load-bearing function and collapses.
Second, the characteristics of steel structure factory fire
The occurrence and development of fires have the dual characteristics of randomness and certainty. Randomness refers to the cause of fire and the time and place of fire. The factors are uncertain. They are affected by various factors and follow certain statistics. Certainty refers to the law that is determined according to the basic rules. Development spread. Both the combustion process and the flue gas flow process follow physical and chemical laws such as combustion and fluid mechanics. The deterministic law of fire can be studied by quantitative engineering, which is generally divided into four main stages, namely: initial stage, development stage, violent stage and extinction stage.
The fire hazard of steel structure buildings is mainly reflected in the fire source load, the geometry of the plant and the shape of the fire plume. The general steel structure workshop is characterized by large space, sufficient air, no surrounding enclosed buildings, and even large convection inside the large-span large space building, which is the fastest growing axisymmetric fire plume. During the initial growth phase, heat builds up rapidly, creating a higher, rising fire plume above the combustibles. When the plume is blocked by the ceiling, it spreads in the lower direction of the ceiling at an average speed of 0.5 m/s, forming a thin layer of hot smoke flowing parallel along the ceiling surface. Thereafter, the smoke is surrounded by the building. The barrier and cooling of the protection will gradually expand to the internal fire source when it reaches a certain thickness, forming a gradually thickened hot smoke layer, thus shortening the time to enter the violent stage of development. When the fire source develops to many places, a vortex is formed between the fire sources, affecting the air convection, causing the heat to accumulate and accumulate, and reaching a full development stage, the temperature of the hot smoke layer is almost the same as the center temperature.
Third, the steel structure plant fire prevention needs attention
(1) Identification of fire hazard.
Fire hazards should be determined jointly by production, storage, and process. The fire protection requirements of a factory building are different for dangerous production, or for using or storing different dangerous goods. Including dynamic changes in a production phase can also lead to changes in fire risk. Therefore, at the beginning of the design, the process flow, raw materials, finished semi-finished products should be fully understood, and high-risk areas such as clean workshops that may appear in the future should be considered to avoid the problems of subsequent development.
(2) Division of fire zones.
In the absence of special process requirements, the division of fire zones as much as possible is conducive to reducing the spread of fire and providing favorable conditions for evacuation and rescue. The fire protection zone should focus on the parts with large fire load such as combustible and flammable materials that separate the partial pile amplification. According to the fire propagation path, combined with the reinforcement of the structural columns, the firewall, fire curtain, water curtain and other effective separation can be used reasonably.
(3) Sandwich wall material.
Due to the flammability of the filler material, the fire resistance of polyurethane and polystyrene materials is very low. With these two types of buildings, the fire load can be increased by 9% to 30%. In the event of a fire, the temperature rise of the attached steel structure can be accelerated. , causing the rapid collapse of the building. Therefore, low-grade plants should be strictly controlled to use fires with higher risk.
(4) Quality of fireproof coatings.
Fireproof coatings have different prices due to different quality. Some units choose to use poor quality fireproof coatings for investment saving; and the main design life of steel structures is generally 50 years, while the service life of steel structure fireproof coatings is much lower than 50 years. It is generally not possible to repaint after the failure of the fire retardant coating, which results in the fireproof coating applied to the steel structure in the later stage of use.
(5) Ventilation and ventilation of the fire.
Most of the existing plants are naturally ventilated and exhausted, which has many advantages over mechanical exhaust. The domestically-dissolved natural smoke exhaust windows are more suitable for use in steel structure plants, and can meet the lighting needs while being able to discharge smoke from the fire. Under natural weather conditions, the smaller the outdoor wind speed, the faster the smoke is discharged from the top of the fire source. As the wind speed increases, the smoke is blown to the leeward side by the wind, and the natural smoke exhausting effect is reduced. Therefore, a factory building similar to a rooftop power station may consider adding a skylight at an appropriate location or adding a vent to the wall. When the fire is broken, the walls and doors and windows in the upwind direction should be dismantled to quickly discharge the smoke and reduce the flow of fresh air into the fire. Can effectively slow down the development of fire.
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