Due to the small number of entrances and exits of underground commercial buildings, high tightness and poor ventilation conditions, once a fire breaks out, combustible materials will produce a large amount of smoke, which will spread from the fire site at a faster speed and be in an aggregation state, and the visible distance is generally between 2 and 5 meters. Under normal circumstances, the horizontal evacuation speed of people is 1.0~ 1.2m/s, the horizontal flow speed of flue gas is 1.0~ 1.5m/s, and the fastest speed of people going upstairs is 0.5 ~1.5m/s.
1.2 The buildings are adjacent and connected from top to bottom, with huge space.
In order to attract passengers and facilitate the circulation of goods, there is a phenomenon that several underground shopping malls are not only adjacent, but also connected with commercial buildings on the ground, forming a vast space. Once a fire breaks out, under the action of "stack effect", high-temperature smoke will quickly spread to many underground and above-ground buildings, which will seriously threaten the above-ground buildings and easily lead to vicious consequences.
1.3 with large passenger flow and high fire load density.
Because the city uses underground commercial buildings as crosswalks, the crowds are too crowded during holiday shopping peak hours, and the personnel density index far exceeds the 0.85 person/㎡ on the first floor and 0.80 person/㎡ on the second floor stipulated in Code for Fire Protection Design of Civil Air Defense Engineering.
According to the survey, in several interconnected business centers in Harbin, the weekend passenger flow can reach 200,000 ~ 250,000. Through the calculation of the flow of people in Harbin Nangang Underground Trade City with a total area of 984 1.2㎡, about 40,000 people were stranded in it at the peak of the flow. At the same time, underground shopping malls mainly engage in clothing, shoes, hats and small stones. Most of the goods are flammable and toxic substances such as chemical fiber, leather and rubber, which have high burning speed, large smoke, high temperature and great toxicity.
2. General fire protection design requirements for underground commercial buildings
2. 1 General layout and layout
According to Article 3. 1.4A of Code for Fire Protection Design of Civil Air Defense Works, underground shops shall meet the following requirements:
Goods with fire hazards of Class A and B should not be handled and stored:
The business hall should not be located on the third floor or below:
When the total construction area is more than 20,000m2, the maximum allowable construction area of the fire protection zone should not be more than 500m2, and when the automatic fire extinguishing system is installed, the maximum allowable construction area can be doubled.
2.2.2 When the local underground commercial building is equipped with automatic fire alarm system and automatic sprinkler system, and the interior decoration of the building conforms to the provisions of GB50222, the maximum allowable building area of each fire compartment of its business hall can be increased to 2000㎡. 2. 2. 3 Underground shopping malls that need to be equipped with smoke exhaust facilities should be divided into smoke-proof zones, and the construction area of each smoke-proof zone should not be greater than 500m2, and the smoke-proof zone should not cross the fire-proof zone.
2.2.4 The design of fire prevention and smoke prevention zones should be based on the principles of science, economy, safety and practicality, which should not only limit the diffusion of fire gas during fire, but also be conducive to smoke prevention and control, minimize the harm of fire smoke, and create favorable conditions for safe evacuation and fire fighting and rescue. The area of fire and smoke prevention partition directly affects the effect of smoke prevention and extraction. Too large a fire and smoke prevention area is not conducive to smoke prevention and exhaust, while too small an area is effective, uneconomical, wasteful of investment and even difficult to implement in some places. Compared with the 500 ㎝ specified in the code, through the observation and analysis in the experiment, it is found that the area of fire and smoke prevention area is between 300 ㎝ and 400 ㎝, and the effective sag of smoke prevention vertical wall is between 50 ㎝ and 80 ㎝, and the bigger the better.
2.3 Safe evacuation
2.3. 1 There should be no less than two safety exits in each fire zone, and they should be distributed in different directions. When it is necessary to set outlets in the same direction, the distance between two outlets should not be less than 5M.
2.3.2 The safe evacuation distance shall meet the requirement that the distance between the farthest point in the room and the room door shall not be greater than 15M, and the maximum distance between the room door and the nearest exit or fire door on the firewall shall be 40M, and 20M when it is located in the bag-shaped walkway or the terminal room.
3. Discussion on smoke control and exhaust design of underground commercial buildings
For underground commercial buildings, smoke control is not only the requirement of safe evacuation, but also the need of fire fighting. Therefore, for underground commercial buildings, the setting of smoke control system is particularly important. This paper takes the design of smoke control and exhaust system of underground commercial buildings as an example to discuss.
3. 1 General requirements
Underground commercial buildings, like ordinary large-space buildings, comprehensively use smoke blocking, wall hanging, spraying, smoke exhaust, air supply and other ways to guide the smoke flow reasonably. Among them, the air supply system is mainly used to evacuate stairwells and main evacuation passages. Smoke beggars will go up with the crowd when they enter the evacuation stairwell, which is not conducive to evacuation. Therefore, when it exceeds the first floor, it is recommended to adopt a closed staircase with positive pressure air supply. The installation of smoke-blocking vertical wall is closely related to smoke exhaust system. The design steps of smoke control system are introduced in detail below.
3.2 Design Scheme Selection
There are two ways to set the smoke exhaust system: shopping mall smoke exhaust and aisle smoke exhaust:
A) store smoke exhaust device. Smoke is exhausted from the fire store, and the height of smoke layer is limited, so that smoke can not cross the smoke-blocking vertical wall at the door and enter the aisle, which can ensure the clean air in the evacuation passage.
B) aisle smoke exhaust. Limit the thickness of the smoke layer so that people can pass under it safely.
No matter which smoke exhaust method is adopted, it may be unsuccessful because the building floor is too low. When the floor height is too low, the smoke extraction scheme in the store may cause smoke to overflow and flow into the aisle; However, the use of walkways to exhaust smoke may lead to low smoke interface, which threatens the safety of evacuees. There are two solutions to this problem. One is to set smoke outlets in shops and walkways. The other is to use an open ceiling to increase the height by using the space above the ceiling. However, the influence of equipment and pipelines in the ceiling should be considered in the design, and the detection and spraying system also needs to be adjusted. If the detectors and sprinklers are installed under the ceiling in the conventional way, the start-up of these devices will be delayed and the time to control the fire will be delayed. Smoke screen, vertical wall and other separation equipment should also be extended to the ceiling, unified partition up and down.
3.3 Design calculation steps
There are many calculation methods for flue gas control system. Although the forms are different, the required physical quantities and calculation steps are unified. This paper mainly takes a set of experimental formulas commonly used in Britain as an example, and also partially refers to American standards.
3.3. 1 Determine the fire development process
Determining the scale and development process of fire is the basis of all system parameters. Generally, the heat release rate is expressed as the square of time to describe the development of fire. NFPA92B divides the fire development process into four grades according to the time required for the heat release rate to reach 1000 * * U/s (105 kW), namely, from ignition to 108.
Q= 1000(t/tg)2，…( 1)
Location: Q ... Time after ignition
Time required for Tg… to develop to 1000***u/s
The design is convenient to use and stabilize the fire. The research shows that the influence of automatic sprinkler system on fire development can be considered as follows: when the automatic sprinkler system is started, the fire heat release rate will not increase, so the fire development rate can be selected according to the combustion characteristics of indoor objects, and the heat release rate reached when the automatic sprinkler system is started can be taken as the design value; For different places, a typical fire development process can also be synthesized: a 3m×3m fire with a heat release rate of 5MW (Morgan,1990); The United States has given the density of fire development process per unit area, which is 44***u/ft2vs (about 500 W/m2) for shopping malls and 22 * * U/FT2VS (about 230 w/m2) for houses (NFPA 92b, 199 1). At present, there is no recognized data on the typical fire development process in such areas in China. For the sake of safety, you can choose express fire for design calculation.
3.3.2 Determine the design air layer height according to the safety requirements.
Because there is a transition zone between the smoke layer and the air where the smoke concentration gradually decreases, it is generally considered safe in Europe that the clean air layer is larger than 2.5m~3m ~ 3 m. According to the population height in China, the hanging curtain can be 1.8m off the ground, so the air layer height in shops can be maintained at1.85m ~1.9m. The floors of underground commercial streets in China are all around 3m, so it is obviously unrealistic to keep the air layer height of 2.5m in the corridor rows. It is best to use the smoke exhaust scheme of high-rise shops to prevent smoke from entering the aisle. If the walkway is used for smoke extraction, an open ceiling is used to increase the floor height.
3.3.3 Mass flow into the flue gas layer
It is proved that most of the gas entering the smoke layer is the air inhaled during the rising of the smoke column, and its main influencing factors are the scale of the fire and the height of the smoke layer.
4. Setting of mechanical smoke prevention system
4. 1 Location of mechanical smoke prevention system
According to the Code for Fire Protection Design of Civil Air Defense Engineering, the parts where mechanical smoke prevention facilities should be set are:
First of all. Smoke-proof staircase and its front room or shared front room;
Second. The front room of the refuge aisle.
4.2 Smoke-proof air supply and mechanical pressurization air supply.
4.2. 1 The residual air pressure in the smoke-proof stairwell should not be less than 50Pa, and the residual air pressure in the front room or shared front room should not be less than 25Pa. The mechanical pressurized air supply volume of the smoke-proof stairwell should not be less than 25,000m3/h. When the smoke-proof stairwell is supplied by the front room or the shared front room respectively, the air supply volume of the smoke-proof stairwell should not be less than 16000 m3/h, and the air supply volume of the front room or the shared front room should not be less than 12000m3/h (Note: the door of the stairwell and its front room or the shared front room is pressed by/kl.
4.2.2 The residual pressure of air supply in the front room of the refuge aisle should be the same as that in the front room of the smoke-proof stairwell, and the mechanical pressurized air supply volume should be calculated and determined according to the wind speed of doors and windows at the entrance of the front room not less than 1.2m/s, and the air supply outlet of the front room of the refuge aisle should be opposite to the entrance door of the front room, and the width should be greater than the width of the door opening.
4.2.3 The mechanical booster fan can be used as a centrifugal fan, an axial fan or a diagonal fan. The wind speed of the air supply outlet of the mechanical booster air supply system should not be greater than 7m/s, and the air inlet and exhaust outlet should be the same.
5. Selection of smoke control facilities for underground commercial buildings
The smoke exhaust system in a building often undertakes the smoke exhaust task of one or a central smoke prevention partition. In a smoke prevention zone, multiple smoke outlets can be installed at the same time, but the fire always starts from a part of an area, and the high-temperature fire smoke always enters the smoke exhaust system near the fire site.
5. 1 smoke outlet
Each smoke barrier must have a smoke outlet. Due to thermal expansion, smoke moves upward and sticks to the ceiling, and then flows horizontally. Therefore, it is required that the smoke outlet should be located on the ceiling or the upper part of the wall. It can also be arranged on the upper part of the side wall, but the smoke exhaust effect is not very good, which is slightly worse than that arranged above the aisle. The distance between the farthest point of the fire zone and the smoke outlet should not be greater than 30m；; Separate smoke outlets should be closed at ordinary times and can be opened manually or automatically.
5.2 exhaust fan
The exhaust fan can be a heart-protecting fan, which can work continuously for 30 minutes when the flue gas temperature reaches 280℃. The smoke exhaust fan should be linked with the smoke exhaust port, and the linkage device should also be linked with the automatic fire alarm system. The fan inlet should be equipped with a fire damper that can automatically close when the smoke temperature exceeds 280℃.
6 abstract
To sum up, the most important smoke control and exhaust design problem in the fire safety design of underground commercial buildings, in the design of smoke control and exhaust system of underground commercial buildings, the first consideration should be the determination of system design scheme and the determination of fire development process, which directly affects important issues such as fire prevention and smoke prevention, and needs to be comprehensively considered in combination with the specific conditions of the building.
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