Collapse law and Countermeasures of the hottest bu

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Collapse law and Countermeasures under the action of building fire

in recent years, with the rapid development of China's economic construction, the proportion of building fire is on the rise. The number of deaths and injuries caused by such fires is shocking, and the direct property losses caused are also on the rise. This kind of fire accident not only brings great danger to the fire fighting force's fire extinguishing pressure experimental machine strain curve, which can be roughly divided into five situations, but also poses a greater challenge to the fire prevention supervision of public security fire protection institutions. The "9.11" World Trade Center in New York, the United States, collapsed due to the terrorist attack, resulting in 2797 deaths and a loss of $36billion; A fire broke out in the workshop of Qingdao Jimo Zhengda food company, which led to the collapse of the steel structure roof truss, causing more than 20 employees to be buried in the workshop because they failed to evacuate in time; The building collapse accident caused by the "11.3" fire in Hengyang, Hunan Province, led to the sacrifice of 20 firefighters, creating the most sacrifice of firefighters in a fire accident in New China. Frequent building fire collapses pose a severe challenge to the specific analysis of the test machine fixture and the fire protection work in the new era. It is of great practical significance to actively study and discuss the characteristics and laws of the destruction and collapse of various structural buildings under the action of fire, strictly review and accept the fire protection design of buildings, and formulate practical fire-fighting and rescue plans to reduce casualties and property losses in such fire accidents

first, the general law of building collapse of several common structures under the action of fire

there are many kinds of building component materials. The traditional building materials include brick, wood, cement, sand and stone. In recent years, with the massive application of new materials such as steel and plastic, the forms of steel structure, thin shell structure, frame structure and other building structures are increasing, and the physical and chemical properties of building components are becoming more and more complex, The characteristics of combustion damage also show diversity and complexity. Different building components and materials have their own combustion performance and fire resistance limit. Under different fire conditions, they will also show different forms of deformation and collapse. Some are local damage, some are local collapse, resulting in full collapse, and some are the rapid and full collapse of the whole building.

(I) brick (soil) wood structure buildings:

brick (soil) structure buildings have been built for a long time and are common in rural areas. Their general walls are generally built with clay bricks or adobe, and their roofs are built with wood and other building materials. When wood is on fire, its surface will be charred and ablated, thus weakening the cross-sectional area, causing the bearing capacity to decline and collapse. If the area of the remaining section can still bear all the original weight, the structure will not collapse. When the fire brigade arrived at the fire scene to put out the fire, because the carbonized layer on the outer surface of the wood component absorbed a lot of water, it could form a good protective layer, which was generally not easy to collapse. Therefore, the roof of wood structure buildings has less overall collapse and more local damage.

bricks have good fire resistance and can withstand high temperature. After being built into walls, due to poor masonry quality and fire resistance of mortar, the fire resistance of brick walls is not as good as the bricks themselves, but generally the fire resistance limit of brick walls is relatively high. For example, the fire resistance limit of 24 double-sided plastered non load-bearing ordinary clay brick solid walls is 8 hours. Generally, it is no problem for brick walls to withstand a few hours under fire, and generally they will not collapse.

the adobe wall is fire-resistant and incombustible, but the adobe will absorb water when soaked in water, resulting in softening, and will be damaged by the strong impact of the water gun jet, losing its bearing capacity and collapsing.

(II) mixed structure building:

mixed structure building is mainly a building whose walls are made of clay bricks, blocks, stones and other building materials, built with mortar, and whose floors are made of floor slabs or cast-in-place concrete. The building materials used in such buildings are clay bricks, blocks, stones, concrete and other non combustible materials, and the fire endurance of each building component is relatively high. For example, the fire resistance limit of 24 ordinary clay brick solid bearing wall is 5.5 hours, that is, under the standard fire resistance test conditions, this kind of wall will not lose its bearing capacity or damage within 5.5 hours. However, in case of a fire in a building with a large fire load, the temperature can exceed 1000 ° C. the temperature difference between the fire facing surface and the fire facing surface of the masonry is very large, which will generate great internal stress. At the same time, carbonates and silicates will decompose under high temperature, which will destroy the masonry. Therefore, under the action of fire, the construction of mixed structure buildings is controlled by buttons with different colors on the equipment. Generally, the building components will not be damaged within their fire resistance limit, but under the action of high temperature for a long time, the building may collapse locally, and generally there will be no large-scale collapse accident.

(third, SEBS is made of rubber like material) reinforced concrete structure buildings:

reinforced concrete structures are common in some large-span workshops, workshops, warehouses and high-rise buildings, including frame structures, bent structures, shear wall structures, etc. The main load-bearing members of this kind of building structure are reinforced concrete members, and the wall composed of clay bricks and blocks only plays the role of separation and function division.

the physical and chemical properties of reinforced concrete building materials such as steel bars, cement, sand and gravel change under high temperature, and the strength changes differently with the increase of temperature. At present, no one in this field has conducted a detailed study on the stress changes of these components under the action of fire. However, the tensile strength of the reinforcement in the concrete will be reduced under the influence of temperature rise, and carbonate and silicate will decompose under high temperature, which will destroy the masonry. Therefore, the thickness of the protective layer of the reinforcement has a great impact on the fire resistance of tensile reinforced concrete structural members such as beams and floors. For example, the fire resistance limit of simply supported non prestressed reinforced concrete beams is 1.2 hours when the protective layer thickness is 1cm and 2.9 hours at 4cm. Therefore, increasing the protective layer thickness of tensile reinforced concrete structural members can effectively improve the fire resistance limit of members.

when the prestressed reinforced concrete structure encounters heat, it will cause the prestressed reinforcement to lose prestress, thus reducing the bearing capacity of the structure. The performance of prestressed reinforced concrete structure in terms of fire resistance is not as good as that of ordinary reinforced concrete structure. In order to extend its fire resistance limit and ensure the safety of buildings, personnel and materials, the thickness of reinforcement protective layer must be thickened.

reinforced concrete structures are statically indeterminate structures. Under the action of fire, the destruction of a member may cause local collapse, but generally there will be no large-scale collapse.  

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