What are the relevant provisions on fire protection, environment and weather in the design code of steel structure workshop?

According to Code for Design of Steel Structures (GB500 17-2003), carbon structural steel and low-alloy high-strength structural steel are mainly used in steel structure workshops. Generally speaking, the critical temperature of losing static equilibrium stability under full load is about 540℃, and the overall mechanical properties change with different temperatures. When the general structural temperature reaches 350℃, 500℃ and 600℃, the strength decreases by 1/3, 1/2 and 2/3 respectively. In case of fire, the fire temperature usually reaches 800~ 1200℃. In this environment, when the steel structure with high thermal conductivity usually reaches 15min, plastic deformation will occur, leading to local damage, complete loss of bearing capacity and overall collapse. Among them, different wall materials and temperature characteristics will have a certain impact on the internal temperature rise and stress deformation of steel structures at high temperature. In the existing steel structure workshop, sandwich panels are usually used as partition walls and external walls, and polystyrene, polyurethane and rock wool with high thermal insulation performance are used as filling materials inside. The first two materials have poor fire resistance and are easy to burn when heated, which will increase the fire load of buildings, accelerate the temperature rise of steel structures and reduce the fire resistance. Rock wool itself belongs to inorganic silicate fiber, which is nonflammable, and its combustion performance depends on the type and dosage of binder. In general, the fire resistance limit of metal sandwich panel is about 10min, that is, it loses its load-bearing function and collapses.

Fire characteristics of steel structure workshop The occurrence and development of fire have the dual characteristics of randomness and certainty.

Randomness means that the cause, time and place of fire are uncertain, influenced by many factors and follow certain statistical laws; Certainty means that the fire will develop and spread according to the basically determined law under certain circumstances. Both combustion process and flue gas flow process follow the physical and chemical laws of 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 fire extinguishing stage. The fire risk of steel structure workshop is mainly reflected in the fire load, the geometric size of the workshop and the shape of Quillfire flow. The general steel structure workshop is characterized by large space, abundant air, no enclosed buildings around, and even air convection in some large-span and large-space workshops, which belongs to the axisymmetric Quillfire flow with the fastest development in the early stage. In the initial growth stage, heat rapidly accumulates, resulting in high temperature and rising Quillfire flow above combustible materials. When the smoke plume is blocked by the ceiling, it spreads in all directions at an average horizontal speed of 0.5m/s under the ceiling, forming a thin hot smoke layer flowing parallel to the ceiling surface. After that, the smoke is blocked by the building envelope, and when it reaches a certain thickness, it will slowly spread to the internal fire source, forming a gradually thickened hot smoke layer, thus shortening the time to enter the stage of violent development. When the fire sources develop in many places, a vortex is formed between the fire sources, which affects the air convection and accelerates the heat accumulation, reaching the stage of full development, and the temperature of the hot flue gas layer is almost the same as the central temperature.

Problems needing attention in fire prevention of steel structure workshop

(1) Identification of fire risk. Fire risk should be determined by production, storage and technological process. When the workshop is used for different dangerous production or different dangerous goods are used or stored, its fire protection requirements are different. Including dynamic changes in three production stages, may also lead to changes in fire risk. Therefore, at the beginning of design, it is necessary to fully understand the technological process, raw materials and finished products, and consider the high-risk areas such as clean workshops that may appear in the future to avoid problems left over from subsequent development.

(2) Fire partition. In the absence of special technological requirements, dividing fire zones as far as possible is conducive to reducing the spread of fire and providing favorable conditions for personnel evacuation and fighting. In the fire zone, the parts with large fire load should be isolated, such as a large number of flammable materials piled up locally. According to the way of fire spreading and the reinforcement of constructional column, firewall, fire shutter and water curtain should be used reasonably for effective separation.

(3) Sandwich panel wall material. Due to the flammability of filling materials, the fire resistance of polyurethane and polystyrene materials is very low. When these two types of buildings are in use, the fire load can increase by 9%~30%, which can accelerate the temperature rise of the attached steel structure and lead to the rapid collapse of the building. Therefore, low-grade factories should be strictly controlled to turn to uses with high fire risk. (4) The quality of fire retardant coatings. The price of fire retardant coatings varies with quality, and some units choose poor quality fire retardant coatings in order to save investment; Moreover, the main design service life of steel structures is generally 50 years, while the service life of fire-retardant coatings for steel structures is much less than 50 years, so it is generally impossible for enterprises to repaint the fire-retardant coatings after they fail, which leads to the failure of fire-retardant coatings applied on steel structures in the later period of use.

(5) Fire prevention, smoke exhaust and ventilation. Most of the existing workshops are natural ventilation and smoke exhaust, which has many advantages compared with mechanical smoke exhaust. Domestic soluble natural smoke exhaust window is more suitable for steel structure workshop, which can not only meet the lighting demand, but also play the role of fire fighting and smoke exhaust. In addition, under natural weather conditions, the smaller the outdoor wind speed, the faster the smoke is discharged from the top directly above the fire source. With the increase of wind speed, the smoke is blown to the downwind side, and the effect of natural smoke extraction is reduced. Therefore, similar factories that build rooftop power stations can consider adding skylights at appropriate locations or adding vents to the walls. When demolishing the fire site, the windward wall, doors and windows should be demolished, and the smoke should be exhausted quickly to reduce the fresh air entering the fire site, which can effectively slow down the development of the fire.