What should be paid attention to in the design of the entrance structure of the civil air defense basement?

Aiming at the mouth of the civil air defense basement, this article comprehensively analyzes the problems found in the construction drawing design of the civil air defense basement in our hospital and several civil air defense plan review units in recent years. It analyzes the common problems in the diffusion room and entrances and exits, and analyzes the common problems in the diffusion room and entrance and exit. Suggestions are given to solve these problems in order to improve the quality of civil air defense basement structural design.

1 Entrance and exit

The entrance of the civil air defense basement includes entrances and exits, ventilation openings and other openings (smoke exhaust openings, water supply and drainage openings, electrical openings, etc.). The entrance and exit design is an important part of the oral protection and structural design of civil air defense projects.

1. 1 Problems with stirrup spacing of anti-collapse scaffolding beams 1) Frequently asked questions. The spacing between stirrups of anti-collapse scaffolding beams should be considered to be greater than or equal to 150mm; 2) Principle analysis. From the perspective of protection, the roof of the anti-collapse scaffolding bears two loads. One part is the vertical equivalent static load caused by the collapse of the house, and the second part is the horizontal equivalent static load generated by the air shock wave. Since the effects of these two parts of load must be considered, the structural requirements for anti-collapse scaffold beams should be the same as those for other beams in the anti-collapse basement. According to Article 4.11.10 of GB 50038-2005 "Code for Design of Civil Air Defense Basements", the stirrup spacing in the encrypted area should not be greater than h0/4 (h0 is the effective height of the beam section), and should not be greater than 5 times the diameter of the main bars [3 ]. At the same time, according to Document No. 1 of Beijing Construction Review Expert Committee [2015], anti-collapse scaffolds are also required to be designed in accordance with seismic measures corresponding to the seismic level [4]. In Table 6 of GB 50011-2010 "Code for Seismic Design of Buildings" . 3.3 also has similar requirements [5]. 3) Design suggestions. For the anti-collapse scaffolding at the entrance, try to use a beam height of not less than 400 mm, and pay attention to the diameter of the steel bars not less than 20 mm. To accommodate these adjustments, the column spacing of the anti-collapse scaffolding can be appropriately increased.

1. 2 Problems with the load value and structural requirements of the walls around the stair-type main entrance 1) Frequently Asked Questions. The equivalent static load of civil air defense and related structural requirements are not considered for the walls around the stair-type main entrance; 2) Principle analysis. As the demand for urban underground space becomes higher and higher, the location of civil air defense basements in underground space is also diverse. The diversity of underground space usage has led to the diversity of wall conditions around stairs. Sometimes there are non-civil air defense basements and civil air defense basements. ***When using stairs. For walls that are immediately adjacent to the soil, since the action time and magnitude of internal pressure (air shock wave) and external pressure (compression wave in the soil) are difficult to calculate and determine using simple methods, for safety reasons, the specification stipulates that the internal pressure effect may not be considered. Calculated based on the explosive dynamic load generated by compression waves in the soil [3]. For walls adjacent to ordinary basements, only the effect of air shock waves entering the main entrances and exits is considered; for ordinary basements with openings, ordinary basements and civil air defense basements only use stairwells, although the air shock waves passing through the openings will It has a diffusion effect, but since there is no relevant test basis, the air shock wave load is not reduced in the actual design process. 3) Design suggestions. For the walls surrounding the stair-type main entrance and exit building, when in direct contact with the soil, the full height of the wall shall be considered as civil air defense equivalent static load according to the external wall in the soil; when adjacent to the ordinary basement, regardless of whether there is an opening in the wall, the full height shall be considered as the equivalent static load for air defense. The equivalent static load of the wall is determined; the walls surrounding the corresponding stair-type main entrance must comply with the relevant structural requirements for civil air defense.

1. 3 Problems with load calculation of steel structure protective closed doors 1) Frequently asked questions. For steel structure protective closed doors, the equivalent static load acting on the door frame wall shall be considered in accordance with the relevant provisions of concrete protective closed doors; 2) Principle analysis. When the reinforced concrete protective closed doors commonly used in air defense basements enter a plastic state and reach their ultimate resistance under the action of explosive dynamic loads, their damage develops along the "plastic hinge". Therefore, its calculation diagram can be taken as a simply supported rectangular plate with four sides. The internal structure of the steel structure civil air defense door is determined by the beam-plate system that transmits force in the main force direction.

Therefore, most of the load is transmitted to the door frame wall corresponding to the main force direction, while the door frame wall corresponding to the secondary force direction only bears the equivalent static load transmitted from the local beams and plates.

1. 4. The door frame wall design has the problem of not coordinating peacetime and wartime working conditions. 1) Common problems. The reinforcement and structure of the door frame wall are only considered according to wartime working conditions. 2) Principle analysis. Conventional door frame wall design divides a complete door frame into four retaining walls: upper, lower, left and right. According to the difference in civil air defense equivalent static load of each part of the retaining wall, the reinforcement is calculated. However, because they are positioned as civil air defense door frame walls, and these walls are often found only in civil air defense basements, designers often ignore their stress requirements as ordinary basements. For example, we often forget that the left and right retaining walls should be equipped with edge components from a seismic perspective. The upper retaining wall is also a connecting beam from a functional perspective, and the lower retaining wall is also a floor beam in the basement, etc. 3) Design suggestions. In view of the problem that the force and structure of the door frame wall as a civil air defense component are not coordinated with the force and structure of the earthquake-resistant component, it is recommended that when designing the left and right retaining walls, the range of the door frame wall and edge components, the reinforcement method, the horizontal reinforcement and the vertical Envelope design is carried out based on the amount of reinforcement; the same principle is adopted for the upper retaining wall and connecting beams, and the lower retaining wall and ground beams.

2 Diffusion room

As an important part of the civil air defense basement, the ventilation vent is usually protected by a combination of blocking and diffusion, that is, the explosion-proof valve is mainly used, combined with diffusion A wave elimination system is installed in the chamber to weaken the pressure of the shock wave. The rationality and safety of the diffusion chamber design play a decisive role in the design of the ventilation vent civil air defense structure.

2. 1 Problems with the stress conditions of various parts of the diffusion chamber and unreasonable load values ??1) Frequently asked questions. In the structural design of civil air defense basements, the following situations often occur: except for the wall between the diffusion room and the shaft, other walls are only designed as airtight walls; the equivalent static load of each part of the diffusion room is not considered or the value is inappropriate. 2) Principle analysis. As shown in Figure 3, a diffusion chamber is a room that uses its interior space to attenuate the energy of the incoming shock wave. When the shock wave enters the diffusion chamber with a large cross-section and a certain volume from the entrance with a small cross-section, the high-pressure gas rapidly diffuses and expands, causing its density to decrease and the pressure to decrease, thereby ensuring that the remaining pressure is less than the allowable pressure of subsequent equipment.

The wall between the shaft and the diffusion chamber is the front wall of the diffusion chamber. Its outer side (shaft side) bears the load of the free wall from the shaft, and its inner side (diffusion chamber side) bears the shock wave passing through the suspended wall. The residual pressure load of the plate flap entering the diffusion chamber. The top plate, bottom plate and outer wall of the diffusion chamber that are in contact with the soil are subjected to the residual pressure on the inside, and the civil air defense equivalent static load caused by the compression wave in the soil on the outside. Since the structure in the soil usually only cracks under the action of residual pressure and will not collapse inward, these components only bear the civil air defense equivalent static load caused by the compression wave in the soil.

2. 2 Problems with improper selection of hanging valves 1) Frequently asked questions. The applicable conditions for the residual pressure of the diffusion chamber (for different doors and different sizes of the diffusion chamber) are not considered. As long as the diffusion chamber is used, the equivalent static load value shall be determined according to the design recommendations in Section 2.1 above. A common situation is that buildings in Category A level 6 civil defense basements choose HK series civil defense doors. 2) Principle analysis. A very important parameter of the suspension valve is the valve closing time. The closing time refers to the closing time when the overpressure (ΔP t) acting on the valve is equal to the valve design resistance (ΔP s). When the predetermined overpressure acting on the valve is greater than the design resistance of the valve, the valve will be destroyed; if the former is too small, the valve closing time will increase and the wave elimination rate will decrease. Commonly used hanging panel valves in civil air defense basement projects include HK series and BMH series. The design resistance level of the HK series is only level 5; the design resistance level of the BMH series corresponds to level 5 and level 6.

When the HK series is used in a Class A Class 6 civil air defense basement, there will be a situation where the predetermined overpressure on the door is smaller than the designed resistance of the valve. At this time, the valve closing time becomes longer and the wave elimination rate decreases, resulting in a decrease in the residual pressure in the diffusion chamber. If the diffusion chamber is still designed according to the values ??given in the recommendations in Section 2.1, there will be safety problems, and it may cause the equipment connected to the diffusion chamber to be damaged due to exceeding the allowable residual pressure. 3) Design suggestions. In view of the possible impact of this situation on the air defense basement, it is recommended to communicate with the building in time during the design process and ask it to use the BMH series of suspension valves suitable for level 6 in the level 6 civil air defense basement (when the air volume is less than 14 500 mm2); For some places where only HK series hanging plate valves can be used, the size of the diffusion chamber needs to be adjusted according to GB50038-2005 Article F.0.3 [3] to make the remaining pressure within the allowable range.

2. 3 The problem of insufficient protective thickness of the front wall of the diffusion chamber 1) Frequently asked questions. As shown in Figure 4, the thickness of b in the cross-section of the front wall of the diffusion chamber is 200 mm. 2) Principle analysis. According to Article 4.11.3 of GB 50038-2005, the minimum thickness of the void wall in the civil air defense basement using reinforced concrete is 250 mm. The front wall of the diffusion chamber is the wall between the shaft and the diffusion chamber, which bears the void wall load transmitted from the shaft. Therefore the thinnest part of the wall (marked b in the figure) should meet this requirement.

3) Design suggestions. The thickness of b in the section view of the front wall of the diffusion chamber is greater than or equal to 250 mm.

The structural design of civil air defense basements is very different from ordinary basements. Not only is the civil air defense load it bears a sudden and rapid unloading instantaneous dynamic load, but more importantly, the structural design and construction, ventilation, and protection are closely related to the requirements of other majors. Exploring the protection principles behind structural provisions is particularly important for designing economical, reasonable and safe civil air defense projects.

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