Discussion on fire protection design of large clothing wholesale market

In recent years, with the rapid development of market economy, many large-scale clothing wholesale markets have been set up in various places. Zhuzhou is the first city in Hunan Province to build large-scale clothing wholesale markets. In recent two years, with the transformation of the old city and the development of investment projects, Zhuzhou will build a number of large-scale clothing wholesale markets. However, in the process of inspecting this kind of market in accordance with the current fire protection technical specifications, the imperfection of the specifications has been exposed. According to the current national fire protection technical specifications, there are great differences among the construction units, the design units and the public security fire control organs on the implementation of the specifications. In this paper, combining with practical work experience, according to relevant technical specifications and technical data, the researchers put forward personal opinions on the fire protection design of such markets, hoping to play a role in attracting jade for such market fire protection design.

1. Characteristics of the market

(1) The building area is large. The construction area of this kind of market is more than tens of thousands of M2. At present, the construction area of several markets under construction in Zhuzhou is more than 500 million m2, of which Huiya clothing Plaza has a total construction area of 200000 m2;

(2) The fire load is large. This kind of market is generally divided stalls, each stall area of 20-200m2. The front part of the booth is hung with hanging samples, and the back part is horizontally stacked with clothes to be wholesale. According to the experimental results of Tianjin Fire Research Institute, the peak value of heat release rate of 70kg mixed clothing stacked horizontally on LM2 area is 960.00kw, and there are hundreds of kilograms of clothing in each stall of this kind of market. Especially in the peak sales season, the stalls are full of accumulated clothing, and the burning heat generated when burning will be very amazing;

(3) The area of the openable external window is small, and the natural smoke exhaust capacity is poor. This kind of market creates a commercial atmosphere for the arrangement of stalls, and the external windows are closed or fixed. It is not possible to use external windows for natural smoke extraction;

(4) Personnel concentration, high fire risk, fire fighting is difficult. It is easy to cause mass casualties and heavy economic losses after fire. Such market stalls are generally arranged by external walls, which block the original windows, so it is difficult to enter the market for fighting.

2 fire protection design requirements

2.1 smoke control design of fire compartment

2.1.1 design requirements for fire compartment

According to the requirements of article 5.1.1 of code for fire protection design of buildings: the maximum allowable fire compartment area of each floor in multi-storey market is 2500m2, which can reach 5000m2 when equipped with sprinkler facilities. According to the requirements of article 5.1.2 of code for fire protection design of high rise civil buildings, the maximum allowable fire compartment area of each floor of high-rise market is 1000 ~ 1500m2, and it can reach 4000m2 when equipped with fire alarm system and automatic fire-extinguishing system; the maximum allowable area of each floor of underground market is 500m2, which can reach 2000m2 when equipped with fire alarm system and automatic fire-extinguishing system. Fire compartment is generally separated by fire shutter and firewall. Through the fire risk analysis of the above ground windowless market, it is found that, like the fire risk of underground buildings, the fire risk level of windowless rooms on the ground is classified as underground buildings in the United States. Therefore, for the market without windows, the fire compartment area should be set according to the requirements of underground buildings.

Firewall and fire shutter can be used for partition of fire compartment. Fire shutter is generally divided into two categories: the first type is ordinary fire shutter, because there is no thermal insulation protection measures, the fire resistance limit can be judged only by measuring the radiation heat of a certain distance from the back fire surface and whether the curtain is through the fire, so it can not meet the requirements of fire compartment separation. In use, an independent closed automatic sprinkler system or water curtain protection should be set on both sides of the shutter, and its duration should not be less than the fire resistance limit of the firewall replaced by the fire shutter. In practical work, due to the specification requirements, the automatic spray cooling system or water curtain system is an independent system, and the cooling water consumption is too large, the construction unit is basically difficult to use. In Zhuzhou, neither the built nor the to be built markets adopt this method for fire separation. The second type is super fire shutter. Because of the special material, the temperature rise of the back fire surface is taken as the judging condition of the fire resistance limit, so it can meet the requirements of fire compartment partition. At present, there are many kinds of steel fire-proof rolling shutters in the market, such as double track and double curtain inorganic composite fire-proof rolling shutters, and evaporation type steam fog fire-proof rolling shutters. Because these markets are divided stalls, the partition walls of some stalls can be changed into firewalls in the fire compartment. In this way, we only need to set up special fire shutter on the corridor for separation, which greatly reduces the cost. For the above ground windowless market, the project cost will not increase too much according to the underground building standard.

When the vertical fire compartment is carried out at the openings such as corridors, open stairs, escalators, etc., which connect the upper and lower floors in the market, the standard requirements are the same as the horizontal fire compartment. The vertical fire compartment mainly considers the heat convection spread, heat radiation and heat conduction, which are difficult to expand due to the partition of floor. The horizontal fire compartment should also consider the heat radiation and heat conduction spread of fire. Therefore, the requirements of horizontal fire compartment and vertical fire compartment should not be treated equally. The vertical fire compartment should be relaxed as long as it can prevent the spread of vertical heat convection. However, the side of judging the fire resistance limit should face the business hall. In the vertical fire separation design of the central hall of Huiya Commercial Plaza, the area of the middle hall is 2000m2, the width of each floor corridor is 4m, and the mechanical smoke exhaust facilities are set in the middle hall. Based on the analysis, the design scheme of setting up quick response sprinkler and mechanical smoke exhaust facilities in the middle hall corridor, using fire-proof glass to set up smoke retaining vertical wall at the outer edge of the corridor, and canceling the fire-proof rolling shutter of the facilities. This kind of design scheme has also been adopted in the central hall of Zhengda square in Shanghai. This design can also be used for other large-scale halls.

2.1.2 smoke control design requirements

Because there is no requirement for smoke control design in the aboveground market in the code for fire protection design of buildings, designers do not consider this design in the design. There is no set-up of multi-layer market in Zhuzhou. Although the market fire protection design of high-rise buildings is stipulated in the code for fire protection design of high-rise civil buildings, in the initial stage of construction, designers generally consider the use of external windows for natural smoke exhaust, without mechanical smoke exhaust facilities. However, in the construction or after the completion of the project, the external windows need to be blocked due to the setting of market stalls, so natural smoke exhaust cannot be carried out. Because the clothing in this kind of market is mainly piled up, and the clothing generally uses cotton, hemp, silk and macromolecular compound as raw materials, it will produce a lot of smoke when burning. In addition, due to the poor natural ventilation and smoke exhaust conditions in the market, a large amount of smoke is produced due to insufficient combustion after the fire, which seriously affects personnel evacuation and fire fighting. At the same time, due to the spread of high temperature smoke, the temperature of the fire rises rapidly, which shortens the time of flashover, speeds up the development of fire and increases the difficulty of fire fighting. The purpose of smoke control project is to prevent a large amount of smoke generated by fire, prevent the rapid spread of smoke, ensure the safe evacuation of personnel and improve the conditions of fire fighting. It is required in NFPA that smoke control design should be carried out for all places where people are concentrated. Therefore, Shanghai researchers believe that it is necessary to design mechanical smoke control facilities in such markets.

The mechanical smoke control methods in the market are as follows:

(1) Natural air supply, mechanical smoke exhaust. The smoke is discharged in a short time when smoke is discharged. The visibility in the corridor is ideal, the air distribution is good, and the smoke exhaust time is short, which is very beneficial to the safe evacuation, fire fighting and fire loss reduction. The standard requirements of smoke exhaust volume are 60m3 (M2? H) and 120m3 (M2? H);

(2) Mechanical air supply, mechanical smoke exhaust. In the case of no natural air supply conditions, mechanical air supply and mechanical smoke exhaust can be adopted. This design should be adopted for the above ground windowless and underground market. The smoke exhaust volume of the smoke exhaust system is calculated according to the smoke control volume of the building area of the smoke control zone with the smoke discharge of 60m3 / (M2? H) and 120m3 / (M2? H), and the air supply volume is determined by 50% of the smoke exhaust volume of the smoke control zone. According to the standard requirements, a smoke control zone of 500m2 is designed for smoke exhaust design. It is found that the smoke exhaust volume is large according to the specification requirements. The effective sectional area of the air duct is 1 m2, and the effective sectional area of the smoke outlet in the smoke control zone is 1.67m2. In this way, the space occupied by the air duct is more than 1 m. By improving the height of the building floor, the height of the business hall of the shopping mall will greatly increase the construction cost.

The above calculation does not consider the impact of automatic sprinkler facilities on smoke exhaust. In fact, such markets are equipped with sprinkler facilities. However, the influence on smoke exhaust and ventilation must be considered. The water from the nozzle will reduce the temperature of the fire site and may reduce the smoke emission. In addition, the water will wash the surrounding smoke and the smoke transported by the air to the ground. In the direct data of the interaction between the mechanical smoke exhaust facilities in the buildings with automatic sprinkler facilities, it has always been a research hotspot for researchers. According to the fire simulation model CFAST and evacuation model fegress simulation, under the combined action of automatic sprinkler system and mechanical smoke extraction system, the smoke exhaust volume of smoke exhaust system is 36m37m2? H), which can always keep the smoke interface above 2m, which can ensure that the personnel can have sufficient time to evacuate to a safe place. The purpose of smoke exhaust can be achieved according to the smoke exhaust volume of 72 m ~ 3 for the commercial hall. In the place of automatic sprinkler system, the water ejected from the automatic sprinkler system will reduce the temperature of hot smoke, which may reduce the amount of smoke exhaust, and the water also has the function of smoke elimination. Therefore, under the combined action of automatic sprinkler system and mechanical smoke exhaust system, the smoke exhaust volume of the smoke exhaust system is 36m3 / (M2? H) according to the building area of the smoke control zone it bears, and the air supply volume of the building area is calculated as the air supply volume of 24m3 / (M2? H). In the smoke exhaust design of Huiya Commercial Plaza, the smoke exhaust volume is determined according to 2000x60m3 / (M2? H) because of the high building floor, which has no influence on the diameter of the air duct (the smoke control area is divided into 2000m2).

Since air conditioning systems are installed in these markets, it is also permissible to incorporate ventilation, air conditioning systems and mechanical smoke extraction systems in the code. According to the design calculation, the effective cross-sectional area of smoke exhaust duct is close to that of exhaust duct and supply air duct. Therefore, ventilation, air conditioning system and mechanical smoke exhaust system can be used in the design of mechanical smoke exhaust system. In the design, the air duct of ventilation system or exhaust system is smoke exhaust pipe. However, the air duct shall meet the requirements of smoke exhaust duct. For its own use with the exhaust system, it can be considered to use the two speed axial flow fan with high temperature resistance, high strength and continuous operation for more than 30min when it is higher than or equal to 280 ℃. For self use of the air supply system, the air supply fan and smoke exhaust fan should be set at the end of the air duct, separated by fire damper. At ordinary times, the fire damper opened to the forced draft fan was opened, the fire damper opened to the smoke exhaust fan was closed, and the forced draft fan started to supply air; in case of fire, the forced draft fan and the fire damper opened to the forced draft fan were closed, and the smoke exhaust fan and the fire damper opened to smoke exhaust fan were opened.

The following points should be paid attention to in combination:

(1) The air supply outlet which can be closed by the alarm system shall be adopted;

(2) The smoke outlet is usually closed and the control system is opened in case of fire;

(3) The effective cross-sectional area of air supply outlet and smoke exhaust outlet shall be calculated according to relevant technical requirements, and air supply outlet and smoke exhaust outlet shall be arranged reasonably;

(4) For the fan and smoke exhaust fan shared with the exhaust system, the power supply circuit shall be used for power supply of the exhaust fan. The power supply circuit shall be used for power supply of the exhaust fan, and the power supply circuit shall be switched to the fire power supply circuit in case of fire;

(5) The smoke exhaust outlet is equipped with automatic smoke exhaust fire damper, which can close the smoke exhaust outlet with gas temperature over 280 ℃. The whole system can continue to work to ensure the overall smoke exhaust effect. The area of smoke prevention zone is required to be ≤ 500m2. Through the explanation of this article, we can know that the code is divided by 500m2 mainly considering the influence of too many smoke exhaust shafts on the architectural design. For large shopping malls, it is difficult to set up too many smoke proof vertical walls on the ceiling, so the smoke prevention area can be increased according to the actual situation. In Huiya clothing Plaza, 2000m2 is used as a partition, and each zone adopts an independent smoke exhaust system. In the local technical regulations of Shanghai, 2000m2 is also regarded as the area of a zone. According to the analysis, the best height of the smoke screen is 20% of the height from the ground to the ceiling, and the effective sag of the smoke curtain wall is 60 ~ 80cm when the height of the business hall is 3 ~ 4m. Considering the characteristics of the market, the automatic smoke screen vertical wall is often used. Usually, the vertical plate is in the position of the ceiling, and the vertical wall is matched with the installed fire detector. In case of fire, the fire detector will alarm and drive the automatic smoke screen vertical wall to open and droop for 90 degrees. The smoke curtain wall should not be damaged in the hot smoke flow to ensure that the smoke storage bin will not be damaged. Therefore, high temperature resistant and solid incombustible material should be used as the smoke retaining wall. In order to be beautiful in the market, the smoke curtain wall made of fireproof glass can also be used.

Smoke prevention design is mainly for the evacuation staircase and the front room air supply, according to the requirements of the specification for the evacuation staircase every 2 floors to set up louver air supply outlet. Because the air supply in the front room is only required to be set on each floor in the code, there is no clear how to set it, so it is easy to cause errors in the design. The air supply volume of the front room is determined by opening 2-3 doors to ensure a certain wind speed in the door opening. The evacuation of the fire site belongs to continuous evacuation. All doors may be opened at that time. If the louver air supply outlet is used, the air speed required by the door opening will not be guaranteed if the air volume is insufficient. Therefore, the normally closed air supply outlet with electronic valve should be adopted in the design of the air supply outlet in the front room. In case of fire, the air supply outlet of the fire floor and the upper front room should be opened by the control center, and the air supply outlet of the third floor above the 20th floor should be opened.

The required wind speed of the door opening is 0.7 ~ 1.2m/s. According to the research on the wind speed required to effectively prevent the smoke from affecting the evacuation, it is found that only when the wind speed reaches 4m / s, can the influence of smoke on personnel evacuation be effectively controlled. Therefore, the parameter design of 4m / S is recommended in the design.

In the actual design, attention should be paid to the design of smoke exhaust system and other facilities in buildings. Each fire compartment should be designed as an independent unit, so as to ensure the safety of facilities in fire.

2.2 safety evacuation design

2.2.1 determination of total width of safe evacuation

At present, it is difficult to determine the design area of per capita occupancy reasonably due to the long-time formulation of the national code. In nfpa101, the personnel density in the business hall of the shopping mall is determined as 2.8m2/person. According to the estimation of the existing wholesale market in Zhuzhou City (about one month before the festival), the highest density of personnel is about 1.8m2/person during the peak period of market operation. Based on the analysis of some established markets in China, the researchers suggest that the density index of 2 m2 / person should be used for the above ground part and 1.5 m2 / person for the underground part.

The actual area of the business hall shall be determined according to the construction design drawing of the specific project. In order to reduce the total evacuation width, many designers use the building area of each floor multiplied by 34% (article 3.1.2 of "code for design of shop buildings" (jgj48-88): the building area is more than 15000m2, and the business area should be > X34% of the building area to determine the area of the business hall. Some designers also determine the total number of people according to the number of people in each stall. At the beginning of the design of Huiya Commercial Plaza, the designers calculated the total evacuation width by 2.5 people per stall. The researchers think that the basis of these calculation methods is unreasonable. Because 34% is only the proportion of the construction area occupied by the three parts of operation, storage and auxiliary, and the source of 2.5 per person per stall is not based on. Therefore, these calculation methods are not used in the design audit. When calculating the area, it is determined according to the area available for customers (including shop staff) to stay and activities. According to the analysis of many engineering cases, the calculation area of the large commercial business hall for personnel stay and activities is about 50% of its building area, multiplied by a certain conversion coefficient to calculate the number of evacuation, and then divide the calculated total number of people by the evacuation width index to calculate the total evacuation width.

2.2.2 determination of safe evacuation distance

For multi-layer market, the linear distance from any point in the market to the nearest emergency exit is 40m. When there is sprinkler system, the evacuation distance can be increased to 50m, while the high-rise market is stipulated as 30m in the code for fire protection design of high-rise civil buildings. After the fire broke out in the high-rise market, the automatic sprinkler system opens the sprinkler and mechanical smoke exhaust facilities, and the start of smoke exhaust is bound to prolong the evacuation time. When the evacuation speed is constant, the evacuation time increases. Nfpa101 specifies the maximum distance between two evacuation exits from the maximum diagonal length of the building or the area as a reference. The diagonal length of 4000m2 (rectangle 50x80) can reach 94m. It is mentioned in article 5.3.8 of code for fire protection design of buildings that the evacuation distance can be increased by 25% when the sprinkler system is installed. Under the condition of strengthening the fortification, appropriate adjustment is allowed, so as to give the design some flexibility. Therefore, the evacuation distance of high-rise market can be relaxed. For high-rise buildings equipped with automatic sprinkler system, automatic fire alarm system and mechanical smoke exhaust facilities, the evacuation distance of the market can be extended to 42m, which brings flexibility to the design of large-scale markets. For underground parts and larger shopping malls, safety evacuation corridor can be considered to solve the problem of evacuation distance. A closed evacuation corridor should be set up in the appropriate position to connect with the evacuation stairs, and a positive pressure air supply system should be set up in the corridor to prevent smoke.

2.2.3 design of automatic sprinkler system

At present, the automatic sprinkler systems designed in the markets that have been built or will be built are all ordinary sprinkler systems (ordinary sprinkler heads and ordinary sprinkler systems with hydraulic calculation according to Article 5.0.1 of code for design of sprinkler systems). However, the researchers believe that the design of ordinary sprinkler systems in such markets does not meet the actual requirements, and it is difficult to successfully control the initial fire. This kind of market generally has ceiling, but the ordinary sprinkler head is not recommended in article 6.1.3 of code for design of sprinkler system. The reason is that when the sprinkler is installed under the ceiling, the sprinkler will be seriously blocked, and the water spraying intensity will be reduced by 40%, which will seriously weaken the fire extinguishing capacity of the system. The United States stipulates that all the places with dense people must use the quick response sprinkler head, because the fire mainly depends on the automatic sprinkler system, and the personnel in the fire site must leave the scene of the accident as soon as possible. The rapid response sprinkler can reflect the fire extinguishing in the early stage of the fire, and realize the early automatic fire alarm, so that the fire brigade can receive the alarm in time and arrive at the fire site quickly. At the initial stage of the fire, a large amount of toxic smoke will not be generated, which is conducive to the early evacuation of personnel. After arriving at the fire site, the fire brigade can put out the initial fire as soon as possible, instead of focusing on evacuation and rescue personnel, thus missing the opportunity to put out the initial fire. At the beginning of the fire, the fast acting fire control system can effectively inhibit the spread of the fire. Moreover, the clothing in this market is stacked horizontally, so the water sprayed by the ordinary nozzle is difficult to reach the surface of the combustor, which can not effectively control the fire.

In such markets, the use of ESFR sprinklers in sprinkler systems is effective in suppressing initial fires. If it is difficult to use ESFR nozzle, the nozzle with flow rate of 2.21/s should also be used, and the amount of water should be determined according to the sprinkler action within the action area. In reviewing the preliminary design of Huiya Commercial Plaza, the researchers proposed to design the sprinkler system of the market as a sprinkler with a flow rate of 2.21/s, which was approved by the designers. ESFR (eafiy suppression fast response) sprinkler is called early pressing rapid response sprinkler. It is a kind of quick response sprinkler, that is to say, it can suppress fire when responding. The function of ESFR nozzle is to suppress fire, not to control fire (except for ESFR nozzle, other rapid reaction nozzles cannot be used for fire suppression). According to the definition of nfpa13, fire suppression is to directly and fully spray water through the flame to the surface of the combustor, which can significantly reduce the heat release of the flame and prevent re growth. The reaction speed of ESFR nozzle is the fastest among the current sprinklers. The spray water with large flow rate, large water drop and high speed can penetrate the flame and directly reach the surface of the combustor to extinguish the fire. The design parameters of the sprinkler fire extinguishing system come from the "full scale" or "large scale" fire extinguishing experiments. Only the hydraulic parameters of sprinkler successfully can be used in the design parameters of ESFR sprinkler system.

2.2.4 fire electrical design

(1) Design of automatic fire alarm system. The automatic fire alarm system has intelligent type, full bus type and comprehensive type. At present, photoelectric smoke detector of full bus automatic fire alarm system is mostly used. However, the full bus system has the disadvantages of high false alarm rate and high false alarm rate, which makes it difficult to control a variety of fire facilities at the same time. According to the characteristics of large building area, more detection points and more control facilities, the intelligent automatic fire alarm system is more reasonable according to the performance analysis of various automatic fire alarm systems. The intelligent automatic fire alarm system uses the detector to transmit the analog quantity of smoke, temperature and other generated during the fire to the alarm along with the relevant external environmental parameters. The alarm then judges whether the fire exists according to the obtained data and referring to a large number of internal stored data, which solves the two major problems of the current automatic fire alarm system (false alarm and missing report) 。 At the same time, the automatic fire alarm and linkage fire extinguishing system uses two bus (or three bus) to realize the system information transmission, which brings great convenience to the design, installation and maintenance of the project. The building automation management system is composed of building air conditioning, power supply, water supply, lighting, anti-theft system and other public security and management systems to form a whole, which plays a greater role in providing central monitoring and intelligent decentralized management;

(2) The requirements of fire emergency lighting system and evacuation indication system are 0.5lx. In practice, it is found that this illuminance can not meet the requirements of illumination for evacuation. At the same time, the emergency lighting for fire accidents is mostly set on the ceiling, which is greatly affected by smoke. According to the analysis of the psychological behavior of people in the fire scene when evacuating, it is found that increasing illumination can reduce people's fear. In the review, the researcher asked the designer to increase the illuminance to 15lx. In this kind of market, the emergency lighting of fire accident generally adopts the fire power supply circuit, and the improvement of illumination will not increase the project cost. In case of fire, the steel pipe shall not be covered with fire-proof coating in case of fire, so as to ensure that the power supply system is not damaged in case of fire.

The standard requires that the evacuation indication sign should be set at the place below 1 m from the ground. Due to the particularity of the market, the actual project is usually set under the ceiling, which can not achieve the purpose of evacuation indication. It is suggested that the special evacuation signs should be set at the partition wall between the stalls, and the luminous evacuation signs should be set on the floor of the corridor.

Conclusion

Due to the great difference between large-scale clothing wholesale markets and general stores in terms of fire phenomena, many markets have no clear requirements for the internal design in the initial stage of construction, and the current fire-fighting technical specifications have no clear requirements on the fire-fighting design of such markets, or some regulations bring great difficulties to the design in the actual implementation. Therefore, the characteristics of such markets in case of fire should be scientifically divided Analysis, the implementation of scientific fire protection design, the use of new technology, new equipment, so that this kind of market fire design more reasonable, more scientific.