Two 25-ton steel columns — one bearing signatures of American steelworkers who helped make it — rose at Ground Zero Tuesday, a milestone in prolonged efforts to build the skyscraper that will replace the World Trade Center.

As construction workers, politicians and architects applauded, a massive crane lifted the first, 31-foot-high column, which was painted with an American flag and the words "Freedom Tower," and set it over steel bars on the southern edge of the tower's base.

A second column set a few feet away carried the signatures of steelworkers and politicians from Virginia, where it spent time at a steel company before being shipped to New York.

A third column lay on its side, plastered with signatures of New Yorkers and Sept. 11 victims' relatives as well as pictures of some firefighters killed in the 2001 attack. It will be installed in the next few days.

By next spring, 27 of the jumbo steel columns will anchor the skyscraper and rise to street level — about 70 feet from the bottom of ground zero.

"Today the steel rises, the Freedom Tower rises from the ashes of Sept. 11, and the people of New York and the people of America can be proud," Gov. George Pataki said.

The 1,776-foot tower, set to open in 2011, is to be the tallest of the five skyscrapers planned to replace the trade center.

"Rising from the heart of the World Trade Center site, the Freedom Tower will symbolize the spirit of our city and our nation: inspiring, soaring and undefeated," Mayor Michael Bloomberg said.

Lengthy negotiations over who would build the tower and security concerns have delayed the project.

The tower has had more than one design and groundbreaking; politicians laid a granite cornerstone in July 2004 to begin construction, but had to move the building after city police said it was too close to traffic, making it vulnerable to terrorism.

Construction began again this spring, after the site's owner renegotiated its lease with a private developer and took over construction.

Gov.-elect Eliot Spitzer, who takes office next month, has said he planned to look again at designs for the tower. Federal and state agencies, including the governor's office, have agreed to occupy half of the building's office space.

The columns installed Tuesday — among the largest in the world — were forged in Luxembourg, then shipped to Lynchburg, Va., where workers welded steel plates onto them so they could be properly set in place.

The tower will be built with 45,000 tons of steel, builders say.