【 Weak current College 】 fire conditions cause the collapse of the building structure and response

With economic development and the gradual increase in high-rise buildings, fire load and unsafe. If due to a fire caused by the collapse of a tall building is bound to the rescue and fire fighting more difficult to bear rescue task firefighters put forward higher requirements and challenges. According to the survey, the United States "9.11" the cause of the collapse of the New York World Trade Center is to fire the melting the support bar, instead of the direct impact of aircraft. Therefore, in a building fire fighting, to take full account of the collapse of the building. In the United States "9.11" New York World Trade Center bombing scene-fighting over 300 firefighters were killed as a result of the building collapse. In our country, in recent years due to a fire in the building structure damage, collapse and cause fire casualties of the accident had occurred. Therefore, strengthening of structural damage, characteristics and laws of the collapse of explore, develop practical fire-fighting and rescue operations, no doubt to reduce casualties and property damage with a very great significance.
First, fire conditions cause of the collapse of building structures.
Building components in a variety of buildings, in recent years, with some new material, new technology, new technology in the field of architecture, building elements of performance has become more complex, combustion characteristics of damage also tends to diversity. Different building elements have their own combustion performance and fire resistance. Fire conditions, along with building materials and components of burning and destruction, the whole building structure also affected to a certain extent, until the destruction by partial or whole of collapse. Structure of collapse and destruction, burning, and high temperature conditions, collapse and destruction causes mainly has following several aspects:
(1) prestressed reinforced concrete structure heating, lose pretensioned, reducing structure carrying capacity.
(2) brick masonry heat deformation and cracking. Granite, quartz, feldspar internal, MICA different thermal deformation and fragmentation. Silicate blocks because of internal heat dispersion and loose.
(3) thermal deformation of steel structure. Steel structure after heated plastic deformation occurs quickly, in the burning of about 15-20min, bumper parts into "noodles", with the partial failure, resulting in an overall loss of stability and destruction and damage of steel structure is beyond repair, reuse.
(4) wood surface being ablative, weakened the loading of the section. Wood burning fire, surface charring, if the remaining cross section area will still be able to afford the original full load, the structure is not going to collapse. In particular, the fire brigade arrived at the scene, in fighting fires, Widget, the outer surface of the carbon layer absorbs large amounts of water, on the structure, or a good layer of protection. So the more favourable large section. Experiment in the furnace, you can see, prestressed reinforced concrete components, in the heat for several hours, they appear in large deformation. Obviously, this is because the main results of tensile reinforcement elongation. So prestressed concrete structure in fire-resistant performance, are not as common, reinforced concrete structures must be upsetting the thickness of the reinforced protection.
(5) a high temperature mechanical properties of materials, strength as the temperature rises, as well as fire conditions of thermal stress in the structure, in the current structure of the calculation is not taken into account.
(6) the Interior of the explosion of shock and vibration, often destroying buildings of a main reason. Building structures of explosive, due to shock wave power is large, but with the explosion pressure relief facilities taking into account.
(7) the collapse of superstructure falls to the floor, or fire-fighting water cannot be excluded, or floor on fire extinguishing material absorb water, greatly increasing the load floor, floor collapse due to a large number of overloading.
(8) fire-fighting water fall in the high temperature of brick, concrete or reinforced concrete surfaces, due to the sudden cooling result structure surface due to shrinkage and cracking, peeling skin. Special key is destroyed reinforced concrete structure layer of protection, the fire was extinguished, the fire burned directly to the main la reinforced, reinforced immediately out of strength, and make the entire structure.
Second, the fire destroyed the building structures under the general law.
Under fire conditions and structural collapse, often gives better indoor ventilation, accelerate the combustion chamber and destroys fire compartmentation, so that the fire spread to expand to the correct implementation of fire rescue and self-rescue brings great difficulties. Therefore, the general laws of the collapse of buildings, on fire, it is very necessary. Generally, the laws of the collapse has the following structure:
(1) wood frame roof, the collapse of small, local damage. Steel roof, local fire burned down, the rest is often burned some collapse, also from the wall to the ground.
(2) of the suspended ceiling, roof, wooden floors, white walls, earth wall, inflammable building are all easy to collapse.
(3) Adobe walls of fire is noncombustible, but due to the strong impact of fire Jet will also be damaged.
(4) the order of the collapse, usually after the first ceiling, roof, and finally the walls.
(5) general housing wall is to pour in.
3. fire conditions several common structural damage of concrete conditions and general approach
1. steel structure fire resistance of steel roof. Temperature in the 450-500 ° c, load capacity is greatly diminished. High temperature and then become soft, deformation of steel products, roof, roof with warping will collapse, elapsed time is very short, usually around 10-20min. Since the steel roof space work conditions, between the roof truss connecting many support levers and, therefore, determines that the destruction of the steel roof is overall, or better than most, very few are local. In the steel structure roof building fire, to prevent the fire burn directly to the roof, but also not very hot steel structure for a sudden cooling, because these are accelerating the roof collapse. Use the spray of water, or use the DC gun, roof water to cool roof top chord, in the early stage of fire is valid. You can then save the structure would not be seriously damaged. But fire personnel in fighting,Must not be hidden in the roof collapsed and the wounding of a safe place.
2, reinforced concrete elements and concrete is fire-resistant. On fire, it is more secure and reliable. Reinforced concrete structural fire resistance time, generally more than the 1h. Prestressed reinforced concrete structure, is a new structure, capacity, materials, fire resistance. Fire-fighting, it is necessary to pay attention to its deformation (deflection).
3, thin-shell structure roof shells suitable for large-area roof. It features, mainly because it uses fewer materials. Thin shell of the span, the thickness of the small (reinforced concrete shells at least 25mm, brick shells at least 60mm), shell surrounding support on both sides of the floor, Foundation or just lever. The collapse of thin-shell structure is the whole, it was mainly due to the support of conditions. It is a fire, should promptly cooling just lever. As long as the support of conditions does not destroy, it is perfectly possible to avoid collapse because the shell itself is very high fire resistance performance. When the shell structure of the roof is on fire or fire requires Board shell roofs, special attention should be paid, to the roof of the number cannot be too many people to disperse, do not set, it is best to stand in the center of the shell, or both sides of symmetry in the center position.
4. wood wood members special performances, will soon be lit, the formation of carbon in the surface layer. Wood is burning speed and its density and moisture content of direct exercise of size, wood burning speed, with its density and moisture content of the reduction. According to the test, the wood to inside burning velocity of theoretical average 0.6mm/min, light and dry wood burning velocity of approximation is 0.8mm/min, dense mass of wet wood burning velocity of approximation is 0.4mm/min. In practice, we can use these statistics to estimate the combustion of wood members are.
5, walls and chimney bricks of fire resistance, can stand high temperatures, and built the walls, because the quality and masonry mortar fire resistance, fire resistance of brick wall than brick itself, but the General brick wall subjected to hours of burning is no problem. But when the fire broke out, brick wall or chimney collapsed there occur, mainly due to:
(1) framework, frame fill wall is damaged;
(2) because the floor collapse, or horizontal wall was destroyed, so that the longitudinal walls lost lateral support.
(3) the level of impact by the accident.
(4) is ROWLOCK walls, brick by brick deformation and failure, loss of bearing capacity.