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Talking about the mechanism of curtain wall building fire spreading upward

  The building curtain wall is more and more accepted by people, and it is also an important outer protective structure of high-rise and super high-rise building structures. The fire prevention of high-rise buildings is the top priority, so the curtain wall fire prevention is an indispensable content in the field of modern engineering. The fire protection design of the building curtain wall must follow the fire prevention work principle of "prevention first, combination of prevention and elimination". Based on the characteristics of fire in high-rise buildings, based on self-prevention and self-rescue, reliable fire prevention measures are adopted to ensure safe application, advanced technology and economical reasonable However, in construction, due to various reasons, the fireproof structure of the curtain wall is not standardized, resulting in hidden safety hazards.

The first way the fire spread upwards.

  When the curtain wall building catches fire, burning produces flame, heat, gas and smoke. At the beginning, the hot air flow rises, forming a temperature difference and a pressure difference, so that the surrounding air source is continuously replenished, the combustion temperature is continuously increased, the nearby combustible substances are ignited, and the fire continues to expand. Will such an air circulation process be terminated due to the depletion of indoor oxygen? In theory, this is the case, but in reality such a situation rarely occurs. The pressure difference across the combustion chamber is different and changes dynamically. The air in the outdoor and the lower floors is sucked into the room through the gaps in the curtain wall building and the floor gaps (such as pipes, stairwells, etc.). Curtain wall construction with good airtightness can delay the expansion of fire at this stage.

  With the continuous increase of indoor temperature, the pressure difference between indoor and outdoor is also increasing. Ordinary glass (non-fireproof glass) will often break within 15 minutes under the constant impact of flame. A large amount of heat and smoke rushed out of the room instantly, causing the temperature on the indoor side of the broken window to drop by several hundred degrees. At the same time, a large amount of air enters the room to participate in the combustion, and the combustion is often led to the outside through the gap, forming an internal and external attack on the glass curtain wall. The non-visible glass between the layers and the upper visible glass are directly exposed to the flame, increasing the possibility of the fire spreading upward. If the fire protection system fails early due to defects in the fireproof material or structure at the wall between windows, it is possible to form the so-called flame effect. According to the statistics of American research on fires in high-rise curtain wall buildings, about 10% of the fire spreads upward through the outdoor side.

The second way the fire spread upwards:

  Concrete floor slabs are generally used to separate fire compartments, which should have a certain fire rating. According to the "Code for Fire Protection Design of High-Rise Civil Buildings" GB 50045 Table 3.0.2, the fire resistance time of building slabs with Class I fire resistance is 1.5 hours, and the fire resistance time of building slabs with Class II fire resistance is 1.0 hour.

  There is a gap between the outside of the concrete floor and the inside of the curtain wall building. The size of the gap is not only related to the design of the building and the size of the aluminum alloy system of the curtain wall, but also to the size error of the concrete structure, the construction of the curtain wall and the error of the production and installation. Most buildings have an actual range of tens of millimeters to 200 millimeters. This gap is also used to compensate for building deformation due to temperature, load, earthquakes, etc.

  This gap should be regarded as an extension of the concrete floor slab. In the fire protection design, it should be reliably filled with fireproof cotton. The surrounding horizontal fire belt with reasonable design must be able to withstand the fire resistance time required by the fire protection code GB 50045.

  Under actual fire conditions, this gap may be further enlarged. Mainly due to the deformation of the aluminum alloy components and the galvanized iron back plate, the looseness of the hardware connectors and load-bearing support members If the fireproof cotton and smokeproof layer cannot effectively compensate for this displacement. Flames and high-temperature airflow will directly enter the upper floors through these gaps and cracks.

  The third way the fire spreads upward is through heat transfer. The methods of heat transfer are conduction, convection and radiation. The aluminum alloy column of the curtain wall building system is a very good heat transfer carrier, and the column often spans two different fire protection zones, and the heat of the fire source layer can be transferred to the upper layer through the column. The form of the curtain wall determines that it is difficult to be defined as a "fire curtain wall" with a hierarchical concept. In a short time, the temperature of the aluminum alloy surface of the upper floor will be higher than the spontaneous ignition point of the paper. Convection is the transfer of heat by air flow. Although opening the window or breaking the glass is good for exhausting smoke, it increases the air flow and oxygen supply. Radiation is a way for objects with higher temperature to transfer heat to objects with lower temperature in the form of energy waves.

  In the fire source layer, when the temperature rises to a certain critical point, the "flashover phenomenon" makes the fire spread from the local to the entire space in a short time. The heat of the fire source layer is transferred to the upper layer through the floor slab, metal curtain wall building and the surrounding horizontal fire belt. If the temperature rise of the upper floor reaches a certain critical point, flashover may also occur, and the fire develops upward in this way.

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