It's been 20 years since 9/11, but much talk about that terrorist attack remains a source of attention. As the largest terrorist attack on the United States since the beginning of the new century, this incident has indeed caused many casualties.
<h1 class="pgc-h-arrow-right" data-track="2" Why did > the 9/11 twin towers collapse? </h1>
The two 110-story World Trade Center towers, iconic buildings in Manhattan, New York, collapsed after two airliner crashes. There are many opinions about the reasons for the collapse of the two buildings, because the impact of the passenger plane only caused a part of the upper floor of the building to break, but did not affect the overall structure of the building.
So the collapse of the building was not mainly caused by the impact of the airliner, other claims include high temperatures melting steel, and some people say that the incident was caused by directional blasting. Because the collapse of the building raised a lot of dust, the scene is very similar to a directional blast. To understand the reasons for the real collapse of the building, we have to start with the structure of the building.
In 1961, the Port Authorities of New York and New Jersey decided to build the World Trade Center, which was built by Miro Yamazaki and Ross. In view of the cost of construction, Yamazaki proposed a design plan, and the interior of the building was a huge elevator cylinder, built with steel columns and reinforced concrete. The outer wall is framed by 236 steel columns, and a lightweight steel truss is used as a fixing part between the two. The floor is paved with 10 cm thick cement slabs.
<h1 class="pgc-h-arrow-right" data-track="16" > twin tower design scheme was the main reason for the collapse of the building</h1>
This design is not only faster to build, but also lightweight enough to ensure the basic strength of the building and the absence of shaking in strong wind conditions.
However, before the design of the twin buildings, it did not take into account the event of an aircraft impact, and the design to ensure the basic strength led to the collapse of the building. The 9/11 passenger plane crashed into the Twin Towers, breaking only a few steel columns on the outer wall. Due to the support of other steel columns, the building did not collapse immediately after the impact.
Both airliners carried 90,000 litres of fuel that burned violently inside the building after the impact. Some people speculate that the collapse of the twin towers was caused by the high temperature generated by the combustion of these fuels, which caused the steel to be baked. In fact, the strength of steel in high temperature environments is indeed greatly affected, but the roasting is not enough. The fuel is burned at a high temperature, but not to the point of melting steel. Smoke billowed from the impact site after the impact, and it was clear that the fuel was not fully burned.
The most likely to cause the twin towers to collapse may be the floor slab design with limited load-bearing capacity. The steel floor truss connecting the floor slab expands in the high temperature environment and becomes no longer strong, and the degree of expansion of the steel column of the outer wall is different from the expansion degree of the steel floor truss, resulting in the truss falling off. Smashing on the floor slab on the next floor, the floor at the impact site has such a problem.
These floor slabs were smashed on one floor after they fell. When designed, a floor slab can carry up to 1300 tons, so because it cannot bear such a large weight, the floor slab continues to fall. There were no firefighting facilities for the entire building, and the steel columns on the outer wall could not support the collapsed building, and there was a scene of the entire building collapsing rapidly in a short period of time.
<h1 class="pgc-h-arrow-right" data-track="17" > the collapse of the twin towers in the 9/11 incident gave designers more inspiration</h1>
The collapse of the World Trade Center also inspired the construction of other skyscrapers later, and before designing such a tall building, designers would think more about the scene after the building was attacked by the air. By adjusting the steel structure of the building, the building is made stronger, so that it will not suffer a single impact, which will cause the entire building to collapse completely.
If the World Trade Center had not completely collapsed, it would not have caused so many casualties. This example appears to be an ordinary terrorist attack, but may actually be a precise plan by terrorists. If they don't understand the structure of the building, they may not hit the upper floors of the building, but the lower layers of the building.
After that, the United States launched a crackdown on al-Qaida, and after eliminating al-Qaida, the possibility of such a terrorist attack again is very small. 9/11 was also a disaster for Americans.