The spire brings the iconic building to a height of 1,776 feet -- an allusion to the year the United States declared its independence.
The Port Authority of New York & New Jersey said in a tweet Thursday that weather was promising for the installation.
The pieces installed Friday morning were hoisted to a temporary platform atop the building last week.
The spire contains 18 steel sections and three communication rings. The first -- and heaviest -- steel section was installed in January. It weighs more than 67 tons, according to a statement from the Port Authority.
It will serve as an antenna for a television broadcast facility housed in the building, which rises from the site near the original World Trade Center towers, which fell in the 2001 attacks.
Last week, construction director Steven Plate told CNN affiliate WABC that the spire is a "beacon that'll be seen for miles around and give a tremendous indication to people around the entire region, and the world, that we're back and we're better than ever."
Construction on the building began in April 2006.