The plane hit the roof of the building in the 911 incident. Why did the whole building collapse?

Because the high temperature softens the supporting steel bars in the concrete, it is difficult to support the huge gravity, forming a chain reaction and collapsing from top to bottom.

In the 911 terrorist attacks, after the Twin Towers of the World Trade Center in New York were hit by hijacked planes, the protective insulation panels on the surface of their steel structures fell off. The steel structure of the twin towers was therefore completely exposed to the fire, which at that time reached a temperature close to the softening point of steel of 500 degrees Celsius.

The two buildings withstood the impact of the plane for a period of time after the collision. The North Tower of the World Trade Center held up for 122 minutes before collapsing after the attack, and the South Tower held up for 57 minutes. During this time, despite the destruction of supporting elements on the building's exterior, the main structure successfully distributed its forces to other supporting points.

After the plane hit the building, the huge fireball immediately consumed 1/3 of the 10,000 gallons of fuel per plane, but this did not cause much damage to the building itself. However, the remaining fuel continued to burn and ignited the furniture, electrical appliances, paper and cargo on the plane within minutes, ultimately leading to the catastrophic consequences of the overall collapse of the twin towers.

Extended information

The incident and handling results of the 911 incident:

On the morning of September 11, 2001 (US Eastern Time), two aircraft were attacked by terrorists The hijacked civil airliner crashed into Building 1 and Building 2 of the World Trade Center in New York, USA. The two buildings collapsed one after another after being attacked. The remaining five buildings of the World Trade Center were also affected by the earthquake and collapsed and were damaged.

At about 9 o'clock, another hijacked passenger plane crashed into the Pentagon of the US Department of Defense in Washington, USA. Part of the Pentagon's structure was damaged and collapsed. After the incident, military forces across the United States were placed on maximum alert. Although the Taliban issued a statement saying that the terrorist incident had nothing to do with bin Laden, the U.S. government still identified bin Laden as the number one suspect in the terrorist attack.

In response to the attack, the United States launched the "War on Terror," invaded Afghanistan to eliminate the Taliban harboring al-Qaida terrorists, and passed the USA Patriot Act. On October 7, 2001, U.S. President George W. Bush announced the launch of a military offensive in Afghanistan.

The "911" incident was the most serious terrorist attack in the United States, with a total of 2,996 victims. There are different statistics on the property losses caused by this incident. The United Nations issued a report stating that the economic losses caused by this terrorist attack to the United States amounted to 200 billion US dollars, equivalent to 2% of the GDP of that year. The damage caused by this incident to the global economy even reached about 1 trillion US dollars.

This incident has had a profound psychological impact on the American people, and their sense of economic and political security has been severely weakened.

Baidu Encyclopedia--9·11 incident