NEW YORK – Officials hoisted a 70-foot piece of World Trade Center steel at ground zero Tuesday and vowed to open the Sept. 11 memorial by next year, although they acknowledged that the ongoing construction at the site would limit where and how the public could visit.

The memorial, with reflecting, waterfall-filled pools set above the footprints of the fallen towers, its wall of victims' names, its trees and green spaces, is expected to open by the 10th anniversary of the 2001 attacks. Officials have said it would be open to the general public after that.

But the public will only be able to enter the memorial from the western edge of ground zero, while fenced boundaries that surround the site on three other sides of the 8-acre plaza will still be there, Mayor Michael Bloomberg said Tuesday.

Every once in a while they're going to have to move a beam or something and they'll close off a part of it," the mayor said, but said visitors would be able to still walk through the cobblestoned plaza and pause by the memorial pools, which have been built up to street level.

You'll be able to come, walk the plaza, sit, contemplate, the fountains will be working, look at the names, you can reflect," he said.

Thousands of visitors come to peer inside the fenced construction site or visit two adjoining museum sites a day. Once the official memorial is open, officials estimate 7.1 million people will visit it in the first year.

Bloomberg said a northeast section of the plaza would be closed at some point while a transit hub is being built underneath it, and said other parts of the plaza could close as needs warrant. Several of the 400 trees that are planned at the plaza — including 16 that were planted this summer — will still need to be installed after the 10th anniversary passes.

Bloomberg and other officials described progress at the memorial and at half a dozen other projects underway at the 16-acre site, including the signature, 1,776-foot skyscraper meant to replace the fallen trade center towers. It was followed by the hoisting of a salvaged 50-ton steel column that was once part of the north tower's facade to mark what will be the entrance of the memorial museum.

I think there had been doubts, there had been concerns but we are here today to truly acknowledge what has happened recently," said Chris Ward, executive director of the Port Authority of New York and New Jersey, which owns the site.

Thirty-six stories of 1 World Trade Center, formerly known as the Freedom Tower, are rising from the northwest corner of the site. It is scheduled to open in 2013, along with a second office tower.

The column put in place Tuesday was part of that. The 70-foot structure, with its three-pronged trident top, was salvaged from the rubble after the Sept. 11 attacks. A second column is expected to be raised Wednesday. The columns will be at the entrance of the museum, which will occupy space underneath the site and is scheduled to open in 2012.

Officials cited an agreement reached with developer Larry Silverstein in recent months that they said would speed up the financing and construction timetables of up to three towers the private developer hopes to build. The Port Authority agreed to put up to $1.6 billion in public financing towards two of Silverstein's towers, including one where it plans to rent office space.

The Port Authority is also building a transit hub expected to be close to the size of Grand Central Terminal for commuter rail lines to New Jersey and connections to a dozen subway lines. The hub is expected to be finished in 2014.

Plans for two other towers are uncertain and dependent on market conditions. A performing arts center is also planned for the site, but its design, financing and construction schedule are not complete.

But once the towers, memorial and hub open, Silverstein said, the area known as ground zero would transform into something else.

It will be an extraordinary dimension to this neighborhood in terms of what it will have created," he said.