On the afternoon of 6:00pn 4 August, 2020 local time in Beirut Lebanon, two explosions occurred at the port of the city of Beirut, the capital of Lebanon. The extremely powerful blasts has been linked to about 2750 tonnes of ammonium nitrate, equivalent to around 1155 tonnes of TNT. The blast was detected by the United States Geological Survey as a seismic event of magnitude 3.3. Houses that are closed to the blast are demolished, glasses are shattered, mushroom red cloud arise in the sky, broken buildings and scattered glass shards everywhere. Even the Lebanese Presidential Palace, which is more than 10 kilometers away from the explosion site, had glass shattered. But the 48 sets reinforced concrete silos in three rows just next the explosion site, survives after this extreme blast with half of the concrete silos still standards there, and the last row stands magically well, Why?
To answer this question, we need to analyze the material composition and structure type of concrete silo, the chemical principle of the ammonium nitrate explosion, the stress state and failure mode of concrete silo when it suffers shock wave of the ammonium nitrate explosion.
1.Material composition and structure type of concrete silo
As a container for storing bulk materials, the silo has the advantages of convenient for use, small land occupation, large capacity, smooth discharge, and easy mechanized operation. It is very popular in various industries. The reinforced concrete silo is made of steel bar, cement, sand, and gravel, which is strong and durable and has a strong resistance to wind load and other impacts. Reinforced concrete silo has features of good air tightness, less affection by external temperature change, good performance for moisture and rain resistance, thus it is very suitable for grain storage. The silo that survived the Lebanon explosion is reinforced concrete silo.
Reinforced concrete silo is mainly composed of silo roof, silo wall, hopper and supporting base. Grain enters from the inlet and discharges from the outlet. The mechanical properties of reinforced concrete are very good. The walls of the concrete silo are generally made of C35 concrete. The standard value of compressive strength reaches 35 Mpa. The tensile strength of steel bars is generally above 200 MPa. During the process of bearing load, the concrete bears the compressive stress, the steel bars bear the tensile stress part.
2.The chemical principle and explosive power of ammonium nitrate explosion
Pure ammonium nitrate is stable at atmosphere temperature and is not sensitive to impact, collision, or friction. It is a blunt materials that only explode at a high temperature of more than 400 degrees Celsius. Therefore, there is no need to worry about the explosion of ordinary civilian fertilizers. Because of this, many countries take this chemical lightly. Ammonium nitrate will decompose after being heated and can release reddish-brown nitrogen oxide gas and oxygen. This decomposition is very easy to cause an explosion. The chemical reaction formula when an explosion occurs is 4NH4NO3─→3N2+2NO2+8H2O+123kJ. Ammonium nitrate is quite powerful when it explodes. The Hiroshima nuclear explosion released 13,000 tons of TNT equivalent energy, and the TNT equivalent produced by the explosion in Lebanon may exceed 1,500 tons also, reaching 1/13 of the Hiroshima nuclear explosion.
3.Force state of the reinforced concrete silo when it suffers the shock wave of ammonium nitrate explosion
After the ammonium nitrate explodes, high-temperature and high-pressure air is generated, which squeezes the adjacent air to form a dense zone, pushes outwards layer by layer, forming an explosion shock wave. When the shock wave overpressure reaches 0.1 atmospheres pressure, it will damage doors, windows and break the glasses. An overpressure of 0.1 atmospheres is equivalent to a force of 0.1 kg per square centimeter. 0.1 atmospheres pressure looks very small, but when it acts on an object of 1 meter in length and 1 meter in width, the force is as high as 1000 kg. Although the action time is very short, this large impact can produce great destructive effects. According to a research in explosives thesis, a shock wave generated by a 6-ton equivalent TNT explosion can form a punch of 10 atmospheres pressure to an object 16 meters away. The TNT equivalent of the explosion in Lebanon may exceed 1,500 tons, the punch is very big. The impact force on the reinforced concrete silo next to the explosion point is terrible, but there are still 16 reinforced concrete silos in 48 stands well, showing the robustness of reinforced concrete silo. Next, we will analyze the stress state of the concrete silo at the center of the explosion when it is impacted by the explosion.
As shown in the figure above, when the explosion impacts, the impact force F acts on the first row of the reinforced concrete silo. Since the silo is round, when the impact force acts on the silo, the impact force is dispersed on the arc surface of the silo, which enlarges the force bearing area and reduces the unit force on the arc surface. The silo is a reinforced concrete structure, which can resist a pressure of 35 Mpa per square meter. The silo is filled with wheat during the explosion, it can also resist a part of the pressure. Due to the huge impact pressure of the explosion, the first row of silo is destroyed later, but it consumed a part of the impact energy. The impact on the second row of silos was weakened, and only destroy the facing semicircle of the second layer of silos. The third row of the silos standards well as “building strong and brave”. This shows the sturdiness of reinforced concrete. From the satellite photos comparation before and after the explosion, we can see that, except for this strong and brave silo building, the other nearby buildings have disappeared and are in a mess.
In summary, the reinforced concrete silo has unique structural safety, impact resistance, good air tightness, little influence from external temperature differences, good moisture and rain resistance, and is suitable for grain, cement, fly ash, slag powder, and clinker, powder and granular materials. As a modern storage and transportation system design, supply, and construction company, our company can provide you with safe and reliable, modern, automated reinforced concrete silo, steel silo, and other storage and transportation systems.