The last pieces of the 104-floor Lower Manhattan skyscraper that replaces the twin towers destroyed in the Sept. 11 attacks are set to be put in place this week. One World Trade Center is already New York’s tallest building and will become the tallest in the Western Hemisphere when the final pieces of its spire are attached to the building’s roof, according to the Port Authority of New York and New Jersey. The structure will top out at 1,776 feet in a reference to the year the Declaration of Independence was adopted by Congress, NBC News reported. But the "tallest" designation won't be official until the Council on Tall Buildings and Urban Habitat signs off. If the council doesn't count the building's spire as an "architectural top," One World Trade Center would rank third in the Western Hemisphere behind Willis Tower and the Trump International Hotel & Tower, both in Chicago.