what was the reason for constructing a new building?
The September 11, 2001 terror attacks.
One World Trade Center
At 8:46 a.m. (EDT) on September 11, 2001, five hijackers affiliated with al-Qaeda crashed American Airlines Flight 11 into the northern facade of the North Tower between the 93rd and 99th floors. Seventeen minutes later, at 9:03 a.m. (EDT), a second group of terrorists crashed the hijacked United Airlines Flight 175 into the southern facade of the South Tower, striking between the 77th and 85th floors.

what is the height of the freedom tower 1,776 feet. when was it built? The construction of below-ground utility relocations, footings, and foundations for the new building began on April 27, 2006. and what about the earlier tower, when was that built? 1970s.
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One World Trade Center [SEP] September 11 attacks
The fires burned for hours, compromising the building's structural integrity. Seven World Trade Center collapsed at 5:21 p.m. (EDT). Together with a simultaneous attack on the Pentagon in Arlington, Virginia, and a failed plane hijacking that resulted in a plane crash in Shanksville, Pennsylvania, the attacks resulted in the deaths of 2,996 people (2,507 civilians, 343 firefighters, 72 law enforcement officers, 55 military personnel, and the 19 hijackers). More than 90% of the workers and visitors who died in the towers had been at or above the points of impact. In the North Tower, 1,355 people at or above the point of impact were trapped, and died of smoke inhalation, fell, jumped from the tower to escape the smoke and flames, or were killed when the building eventually collapsed.