The compressed air shock wave described in this paper refers to the shock wave formed when the building (structure) is demolished and blasted, due to the disintegration of the building and the collapse of the towering structure, compressing the air inside and adjacent to the ground. People have already realized the damage it brings to the surrounding protective materials in the blasting. However, there is no specific discussion on how to quantitatively calculate the shock wave. This paper discusses the method and makes a method to protect the air shock wave in the demolition blasting. Summary.
2 Compressed air shock wave generation and damage In the demolition blasting, the air existing in the interior of the building (the structure) and its ground is strongly compressed due to the blasting disintegration and collapse of the building (structure). A very strong air shock wave is formed.
It and the high-speed airflow that follows it will not only arouse the dust on the ground to pollute the surrounding environment, but also cause unexpected damage to nearby buildings and facilities.
For example, during the blasting of the Guangzhou Gymnasium, due to the collapse of the roof of the pavilion, the air in the hall was compressed and the air shock wave and high-speed airflow formed from the inside. The water pipe racks that were ready to spray the water flow were all pushed down, so that the water spray device did not play. Desirable dust.
In 1996, the reinforced concrete chimney of Maoming, Guangdong Province, was demolition blasting. When the chimney fell to the ground, the air in the compression cylinder produced strong shock waves and high-speed moving airflow, and a nearby 7m metal distribution cabinet was impacted by a distance of 3m. On the second floor balcony of the far building, some angle steel and straight 8m steel pipes and concrete fragments flew up to the 160m high reinforced concrete chimney of the Sanhexi Thermal Power Co., Ltd., and the central cylinder was pressed before the chimney collapsed. The frame structure of the factory burst (about 5m high), the air inside the chimney is subjected to intense compression, and the generated air shock wave is ejected from the top of the chimney, and the brick wall of the factory area with a height of about 2m and a thickness of 24cm is pushed down at 30m in front to form a long An inverted trapezoidal notch of about 25m. The situation in which the compressed air shock wave generated by the collapse of the chimney blasting collapses pushes the wall as shown.
On May 9, 2008, in the blasting of 105m high reinforced concrete chimney of Taiyuan No. 2 Thermal Power Plant, the author once again witnessed the damage caused by compressed air shock waves. The 105m chimney of this blasting was originally a high chimney with a height of 210m. Due to the limited space, more than 105m was manually removed. Therefore, the blasting demolition is a special chimney: the bottom diameter is 20.68m, which is about twice the diameter of the normal 105m high chimney, while the chimney has a height-to-diameter ratio of only 5:1, which is 1 times smaller than the normal chimney; the chimney weighs about 7000t. It also greatly exceeds the weight of the normal 105m high chimney. In order to prevent flying stones, the construction unit will use the original wall of the chimney blasting incision at a distance of 1.50m from the chimney blasting section to be a concrete wall with a height of about 1m from the bottom of the chimney and a height of about 6m. Curved wall. When the chimney collapses from south to north, the lower cylinder is acted upon by the lever of the front high wall, so that the root of the chimney is increased, the reinforcement is reserved, and the cylinder is broken at the bottom of the chimney to form a To the rear (south) large bell mouth, the chimney hit the ground and is rapidly compressed to form an air shock wave, which is ejected south along the "horn". This powerful air shock wave and high-speed airflow caused the glass windows on the north side of the chemical treatment workshop close to the south side of the chimney center to be completely destroyed. Many of the window frames were impacted and landed; the brick wall outside the workshop appeared near the window. There are many cracks of about 12mm; the main building of the east side is about 30m away from the chemical treatment workshop. A light aluminum alloy wall with a length of 15m and a width of about 20cm is about 1520m high at the corner of the wall. Stretching deformation. The compressed air shock wave destruction path and the damage condition of the workshop are as shown.
The damage of the compressed air shock wave to the chemical treatment workshop can be clearly seen from these examples. When the blasting collapse of the building (structure), the damage caused by the air shock wave formed by the compressed air and the high-speed moving air flow behind it cannot be ignored. Prevent and control.
3 Calculation of Compressed Air Shock Wave Wavefront Parameters According to the shock wave theory, there are the following relationship between different characteristic parameters of the air shock wave wave surface:: household two households one p) two 2P (D2―c2)/g (1+ wave overpressure, Pa; p is the pressure of undisturbed air, Pa; o' is the envelope speed of undisturbed air, c, = 331 (1 + t, / 546), m / s; t is not Disturbing air temperature, G, P is the compressed air density, kg/m3; P is the undisturbed air density, taking P=125kg/m3; Y is the air adiabatic index, Y=14. For ideal gas, at its pressure p, temperature t, volume V has the following relationship: for compressed air shock wave, because the building is in the process of blasting collapse, the internal air is compressed for a short time, so it can be assumed that the temperature of the compressed air is constant, then formula (4) Can be written as: pressure, volume and temperature.
Applying the above theory, the examples in the previous section are checked.
Demolition of a reinforced concrete chimney with a height of 160m in the Hexi Thermal Power Plant in Linyi, Shanxi Province, caused by air shock wave damage. It is estimated from the strength and damage condition of the on-site fence that the air shock wave of the action exceeds 15Pa. If the air is at a normal temperature, c々331m/s and P=1.25kg/m3, the wave velocity of the air shock wave can be obtained by using formula (1). 916Om/s; the flow rate of the compressed air calculated by the formula (3) is u=551 1~6637m/s; if the chimney collapses, the internal air pressure is the standard atmospheric pressure p=1. 15Pa, the pressure after air compression p= Ap+p=(1 15Pa, the air volume inside the chimney is V/V=064~Q57 by formula (5)). 6, that is, the air volume is compressed by about 40%. The damage caused by the compressed air shock wave in the blasting of the 105m reinforced concrete chimney of the second thermal power plant in Taiyuan. Judging from the cracks in the window and brick wall, the overpressure of the air shock wave is Ap=(.
15Pa also applies the above formula to obtain the wave velocity D 5~5029m/s of the air shock wave. As in the above example, it can be calculated that the air volume inside the chimney is compressed by about 30%. The reinforced concrete chimney of Guangdong Maoming blasting height is 120m, and the chimney is poured. When the ground is broken, the air in the compression cylinder generates a strong shock wave and high-speed moving airflow, and a nearby 2 1OmXQ7m metal distribution cabinet is impacted and flies on the second floor balcony of the building which is 3m away. From the analysis of the minimum conditions for the formation of air shock waves, the shock wave velocity at the power distribution cabinet is at least greater than the local air sound velocity, according to the current temperature (January 1996) = 15 °Ci ten air density P = 3401 m / s. According to blasting safety The description of the degree of damage of the air shock wave in the regulation, when the overpressure reaches the level of Ap=(25~4)X15Pa, the tile roof of the building moves to a large amount of all the turbulence, and the airflow velocity corresponding to the air shock wave is about 335. ~452m/s. If the air velocity is u=335m/s, Ap=025X15Pa, the air shock wave velocity D=597m/s is calculated. The calculated air density is P=1.27kg/m3, and the kinetic energy density of compressed air is e1. =fti2/2=:127X(335)/2=71269N/m2; assuming the quality of the metal distribution cabinet is M=320kg (assuming the iron box is made of iron plate with a thickness of 5mm), the volume is Ve= 4m3, then pick up It flies 3m away, 3m high (on the second floor balcony), the energy density required to overcome gravity work e=M(gh+gs)/Ve=137143N/m2, e1 is much larger than £2, which shows that compressed air The shock wave is completely possible to pick up the tin distribution cabinet and fly out to the second floor, 3m away from the chimney, 3m high.
4 Prevention and control of compressed air shock waves For the prevention and control of compressed air shock waves, according to past experience, the main method is to use “pressure relief†and “blocking wave†measures.
41 “Relief†method For building blasting, first use the existing door and window window of the building. It is possible to open some "relief" windows on the outer wall of the non-cutting part of the floor where the blasting cut is located, and also to cut some 'window' on the floor, so that the compressed air in the floor can be discharged in all directions when the building is dumped. Avoid forming a strong shock wave in a certain direction. Secondly, the building can be fully disassembled after blasting. In addition to the consideration of blasting incision and blasting parameter design, the load-bearing beam and stair beam of the floor can be broken or clothed in advance. The hole is blasted and cut off simultaneously with the main blast, which can reduce the generation of compressed air shock waves and reduce the intensity of the air shock wave.
For high-rise structure blasting (such as reinforced concrete high chimney), only when it is blasted and collapsed, all parts of the barrel can be fully broken, so that the compressed air in the cylinder can be shunted from all directions to reduce the compression air shock wave. strength. In particular, when the chimney is to be lowered, the first part of the chimney is subjected to impact compression, resulting in a note:
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