On September 11, 2001, The World Trade Center in the United States collapsed after being attacked by a terrorist attack, causing more than 3,000 casualties and $200 billion in economic losses, commonly known as the "911 Incident".
In May 2002, a report investigating the collapse of the World Trade Center was released, but it neither pointed out the flaws of the design and construction nor made any improvement suggestions. This report, which cost $16 million, was surprising!
Wai Lai Berkins is a construction engineer. When he was still in college, he heard from his teacher: "The knowledge learned from collapsed houses is much more than the houses that are still in use.
So, Lai Berkins decided to conduct a re-investigation, summarize experiences and learn from this tragedy.
As high-rise buildings are built higher and higher, people are increasingly worried about the occurrence of fires. This is the collapse of the World Trade Center. The first lesson learned: skyscrapers can withstand the impact of large jet passenger aircraft, but cannot withstand the scorching fire.
In 1966, the World Trade Center Twin Towers were designed in accordance with the requirements of preventing civil aircraft impact. At that time, the 168-ton Boeing 707 was the standard passenger aircraft at that time, so the World Trade Center did not collapse immediately after being hit. Each side of the building was supported by 59 pillars. These columns extending to the 4th-6th floors. Although 30 were destroyed after being hit, Gemini Building still stands.
However, when the 10,000 gallons of gasoline carried by Boeing aircraft caused a fire in the building, the temperature instantly reached 1093℃ and generated 5 gigawatts of energy. Under this high temperature, the strength of the steel used in the building will be reduced by half, causing the steel structure to lose its support capacity and deform the exterior wall. This will directly cause the weight of the floor above the attack point to be pressed on the floor below. As a result, a tragedy of collapse from top to bottom.
The total designed capacity of the World Trade Center is 58,000 people. Since the attack time was 8:46 in the morning, it was working hours, there were about 14,000 people in the building. The death toll total was 2,245, including more than 1,400 in the north building and more than 600 in the south building.
70% of the dead were working on floors higher than the attack point. Most of them were still alive at the beginning of the attack, but they were trapped above the impact point, and it was difficult for them to escape.
In the fire, 5 fire escapes collapsed, 3 in the north building, and 2 in the south building, and only 18 people escaped from the floor above the attack point. This is the second lesson left to us by the building: high-rise buildings should strengthen the use of refractory materials, and gain more time in emergencies to make the people in the building escape from danger.
World Trade Center was built in the mid-1960s, although the building code at that time clearly stipulated that steel structures should be poured into concrete or external building Insulation bricks are used to absorb heat and dissipate heat by dehydration, achieving a heat resistance of 2-3 hours.
However, this insulation layer is too bulky and expensive for the 110-layer World Trade Center World Trade Center with a height of 417 meters. Therefore, sprayed asbestos was used, and later replaced with a 1.9-cm thick inorganic fiber layer, and the thickness gradually increased to 3.8 cm. From the results, the fire resistance of these spray coatings did not achieve the ideal effect and still needed great improvement.
Less than an hour after the World Trade Center was hit by the North Building, an Boeing 757 crashed into the Pentagon . The building area of the Pentagon is 68,000 square meters and can accommodate 25,000 staff. In comparison, the World Trade Center North Building is 47,000 square meters and 20,000 people, but only 140 people died in the Pentagon, but more than 1,400 people died in the North Building, a total of 10 times more.
There is 1 in the Pentagon. /5 area structures are seriously damaged and need to be demolished and rebuilt, the rest are intact as before. The reason for the lightest damage is that it is only 5 stories high, and all structures can be strengthened and thickened, while the 110-story skyscraper cannot be built so thick.
There are also high-rise emergency systems, including fire water pipes, elevators, telephone lines, etc., which cannot be concentrated in a central service area as usual. If damaged, it means that the entire building is paralyzed. If the safety stairs collapse, people will not be able to escape.
After Lai Beginsky restarted the investigation of the World Trade Center, at a construction expert meeting, someone asked Lai Beginsky: "What lesson does the collapse of the World Trade Center provide for future high-rise building design?" His answer was unexpected: "Reduce the height." The audience asked unsatisfiedly: "How low is it?" This question has already been answered in Lai Beginsky's mind, and it is very simple.
is generally calculated based on the highest height that can be achieved by the fire ladder used now. At that time, it was 30 meters, which is the height of 10 floors.
If a building such as this kind of building catches fire, it can be rescued from outside the building. If necessary, both the entry of firefighters and the evacuation of residents can be carried out using the platform outside the building. So his answer was: "The safety height of the building is below 10 floors." (Note: The highest fire ladder in China has reached 101 meters, about 33 floors.)
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