With the development of architectural technology and economic, workshop, warehouse, stadium and other buildings in the more use of steel structure, because this kind of architectural form has simple structure, convenient construction, large span, contemporary feeling strong, interior space is large, large internal degrees of freedom and short construction period and many other advantages, gradually become the main structure form of enterprises plant. However, light steel structure building fire resistance rating is low, poor fireproof performance, its mechanical performance under high temperature including elastic modulus, yield strength will reduce, prone to collapse, causing casualties and large fire loss. In recent years, many domestic steel structure factory building fire, causing casualties and major property damage, even causing officers and soldiers injured sacrifice for this purpose. Here the author talk about the fire characteristics of steel structure factory building some not mature opinion.
A fire resistance, steel structure factory building
Steel structure workshop is mainly made of steel column, rack support way, thus the fire resistance of steel structure determines the fire resistance of the plant.According to the specification for design of steel structures (GB50017-2003) of steel structure workshop mainly adopts carbon structural steel and high strength low alloy structural steel as the main material. Usually, in full load under the condition of losing stability of static balance critical temperature about 540 ℃, with the type of overall mechanical properties with temperature changes, the general structure temperature of 350 ℃, 500 ℃, 600 ℃, the intensity decreased 1/3, 1/2, 2/3 respectively. In the fire, the fire temperature often up to 800 ~ 1200 ℃, in such an environment, tend to have higher thermal conductivity of steel structure in about 15 min, plastic deformation will occur and produce local damage, and to lose bearing capacity cause the overall collapse.
Among them, the different wall materials and the temperature characteristic, the internal temperature of steel structure and the forced deformation of the high temperature is going to have an impact. Existing steel structure workshop, usually using sandwich board as interval wall and exterior walls, interior to have high insulation performance of polystyrene, polyurethane and as a filler material such as rock wool. Before both the refractory material performance is poor, easy to heat burning, will be building fire load increase, accelerate the rise of temperature of the steel structure, reduce the ability to resist fire. And rock wool itself belongs to the inorganic silicate fibre, nonflammable, combustion performance depends on the type and amount of adhesive. In general, the fire resistance of metal sandwich board for about 10 min or lose bearing function and collapse.
Second, the steel structure factory building fire characteristics
The occurrence and development of fire has dual characteristics of randomness and uncertainty. Randomness is refers to the cause of the fire and is uncertain factors such as time, place, influenced by various factors, follow certain statistics;Certainty that occurred in a particular situation of the fire will spread according to basic certain rule. Combustion process and gas flow process follows the combustion, fluid mechanics and other physical and chemical laws. Fire the certainty of law can be used quantitative engineering research, generally divided into four main stages, namely: at the beginning stage, developing stage, hard phase and phase out.
Steel structure factory building fire risk is mainly manifested in the fire load and fire plume and the factory building geometry size of shape, etc. Ordinary steel structure workshop is the feature of the space is larger, the air is enough, not affiliated to surround close around buildings, some large span large space buildings and even form the air convection, belongs to the early of the fastest growing axisymmetric fire plume. In the early growth stage, the heat accumulating rapidly, higher formation temperature, rising above the fuel of the fire plume. When plume after exposure to the roof of block, and under the ceiling direction at average level of 0.5 m/s speed spread in all directions, forming along the roof surface parallel to the flow of hot smoke layer with thin, after that, the flue gas block and cooling by building palisade, reached a certain thickness and will gradually expand to the internal fire place, form gradually thickening of hot smoke layer, so as to shorten the time of the stepped into the stage of development of violent. Development up to place in front of the fire, the fire between the vortex formation, affect air convection, cause heat accumulation, to achieve full development stage, the temperature of the hot smoke layer and the core temperature.
Three problems that should pay attention to fire prevention, steel structure plant
(a) fire hazard.
Fire hazard should be made by the production, storage and process to determine. A factory building as the production of different risk, or use, different storage of dangerous goods, the fire control requirements are different. Including the dynamic changes of the production stage, change could lead to a fire hazard. Therefore, at the beginning of the design should fully understand the process flow, raw materials, semi-finished products, think of the future possible high risk grade clean workshop area, avoid legacy problems in further development.
(2) the fire zoning.
In the absence of process under the condition of special requirements, as far as possible the division of fire partition, is beneficial to reduce the spread of fire, providing favorable conditions for the evacuation and fighting. Fire partition to separate local combustible piled up a large number of flammable materials such as fire load large area as the key point, according to the ways of the spread of fire, combined with the reinforcement of structure column, rational utilization of firewall effective separation, fire shutter, water curtain, etc.
(3) the sandwich board wall materials.
Due to the flammability of filling materials, the fire resistance of polyurethane and polystyrene material is very low, the use of the two buildings, the fire load can increase 9% ~ 30%, there is a fire, can accelerate the attachment of the rise of temperature of the steel structure, leading to the rapid collapse of building. Therefore, low levels of plant should strictly control switch to fire risk higher purpose.
(4) fireproof coating quality.
Fire retardant coatings with different quality with high and low prices, poor quality of some units to save investment choice of fireproof paint;And use fixed number of year is usually the body of the steel structure design for 50 years, and the service life of steel structure fire retardant coatings is far less than 50 years, the enterprise generally unlikely to besmear to brush after fire-retardant coating failure, thus causing late used in steel structure, no fire retardant coating coated on the fire.
(5) the fire smoke exhaust ventilation.
Most of the existing factory building are natural ventilation and smoke exhaust, many advantage compared with mechanical smoke exhaust, domestic soluble natural smoke exhaust window using suitable for steel structure plant, at the same time in the meet the need of daylighting can rise the fire smoke. The other natural weather conditions, the smaller the outdoor wind speed, the faster the top smoke above is on fire, with the increase of wind speed, smoke blown to the leeward side, the natural smoke exhaust effect is reduced.Similar building roof power station factory building, therefore, can consider to add skylights in appropriate locations or additional vents on the wall in the fire rescue smoke, should break down the upper hand in the direction of wall and doors and Windows, smoke quickly, reduce fresh air into the fire, can effectively slow down the fire development.