September 11, 2001 morning, two Boeing 767 crashed into two buildings, respectively north and south of the World Trade Center burning 56 minutes after the south tower collapsed. North Building (Building 1) and a half hours later collapsed. Later that day, Building 7 also collapsed. While other building did not collapse but due to heavy create disability, can not be repaired, after being removed.
North Building knocked over 1344 people at the building level no way to flee. After a plane crash than eccentric Flight 11, so there is a staircase can be used, but before the south tower collapsed only a few people able to escape from there. While the floor is lower than the south tower hit the north tower, so the layers affected than the north tower, but the number was blocked or hit the north tower of death less than a total of fewer than 700 people.
But in fact, after the twin towers were hit persisted long enough for a lot of people provided valuable time for evacuation. World Trade twin towers designed by LERA company. Later LERA designer of the company, explains: If a Boeing 707 in the fog Trek, accidentally hit the Twin Towers, our design can withstand such an impact. Even 767 terrorists were captured, although larger size, flying faster and still can not rely on hit to destroy the twin towers. However, the fire protection system is not able to control the Twin Towers fire caused thousands of gallons of jet fuel, in fact, it goes beyond the capabilities of fire protection systems.
Twin towers were flown directly hit several floors, immediately there was a serious structural damage, including the partial collapse. But in addition to other parts of the local outside structure remains intact. However, since the plane hit the building, causing the plane aviation fuel is ignited. Huge fireball from the impact of the moment part of the fuel consumed. The remaining fuel flows along the floor of the elevator shaft and pipe wells, in the upper part of the building caused the fire. With the spread of the carrying capacity of the main steel structure fire was gradually weakened and eventually led to the collapse of the whole building.
Structural steel at around 600 degrees Celsius, the intensity has fallen to about 50%, in fact, may have been lower than the actual bearing capacity of the when the temperature reaches 1200 to 1400 degrees, the loss of all the structural strength of steel. The weight of the upper part of the building come down, like dominoes, resulting in collapse of the whole structure of the building layer by layer. While the Twin Towers took fire protection measures, but in the face of such a disaster, it is a drop in the bucket.
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