NEW YORK — Adorned with an American flag, the last pieces of a silver spire were hoisted to the top of the World Trade Center on Thursday as construction workers cheered .
The final two segments of the 408-foot spire will rest on a construction platform for several weeks until the entire needle is permanently installed. With the spire as its crown, the trade center will soar to a symbolic 1,776 feet in the air — a reference to the birth of the nation in 1776.
Sunlight glinted off the slender steel structure as it rose slowly. Construction workers inside the building clustered around unfinished floor ledges to get a better glimpse of it.
‘‘It will give a tremendous indication to people around the entire region, and the world, that we’re back and better than ever,’’ said Steven Plate, who is overseeing construction of the World Trade Center.
Composed of 18 parts and weighing 758 tons, the spire floated into Manhattan several months ago on a barge.
With a beacon at its peak to warn aircraft, the spire will provide public transmission services for television and radio broadcast channels. An LED-powered light emanating from it will be seen from miles away.
The building is rising at the northwest corner of the site where the twin towers were destroyed in the Sept. 11, 2001, terror attacks.
With the additional 408 feet, the building would be the tallest in the United States and third-tallest in the world, although experts dispute whether the spire is actually an antenna — a crucial distinction in terms of measuring its height.
The tower is slated to open for business in 2014.