NEW YORK — A final steel beam was lifted today atop a new World Trade Center skyscraper — the first expected to open at the site next year since the twin towers were decimated on Sept. 11, 2001.

With BeBe Winans singing “God Bless America,” workers raised their hardhats in tribute as the mammoth beam rose slowly into the Manhattan sky, swaying from a steel rope hoisted by a crane.

A U.S. flag attached to the bottom of the beam fluttered above several hundred spectators at the topping-off ceremony.

“Ten years later, it’s pretty remarkable,” said a teary-eyed Sally Rexach, a nurse who aids workers constructing 4 World Trade Center.

She was at ground zero just after the attack, supporting workers who combed through the smoking debris in search of remains.

“This is a very proud moment; it’s full circle,” she said, glancing across the 16-acre site where the uncompleted One World Trade Center in the northwest corner is already New York’s tallest structure.

In the southeast corner facing the 9-11 memorial, the 72-story tower that was topped off Monday is to open for business in the fall of 2013 — the first occupied high-rise at the new trade center site since the Sept. 11 attacks.