Cards, patches and mementos of those killed at Ground Zero are are viewed during a preview of the National September 11 Memorial Museum on May 14, 2014 in New York City. (Photo by Spencer Platt/Getty Images)
Lynne B. Sagalyn is a professor emerita of real estate at Columbia Business School and the author of "Power at Ground Zero: Politics, Money, and the Remaking of Lower Manhattan."

In the frequently contentious debate over Ground Zero, the Vesey Street stairs generated an especially heated conflict.

For hundreds of workers who managed to escape the World Trade Center complex on Sept. 11, 2001, the two flights of stairs were a cherished symbol. The stairs had led them from the site’s elevated plaza, away from the collapsing buildings and falling debris, to the relative safety of the streets beyond. In the words of one survivor, “They were the path to freedom.”

For preservationists, the stairs were also important artifacts: the last above-ground remnants of the World Trade Center. “[They] will be the most dramatic original piece of the site that will have meaning to generations to come,” Richard Moe, then president of the National Trust for Historic Preservation, said in 2006, when the group petitioned for the stairs to be kept in place.

Yet for developers and some neighborhood groups, the stairs were an eyesore — damaged not in the attack but in the subsequent cleanup — that were impeding construction of a planned commercial tower and, with it, the revitalization of Lower Manhattan. “To the extent that this is going to delay rebuilding the World Trade Center site, I think New Yorkers have had enough,” John Dellaportas, chairman of the West Street Coalition, a neighborhood group, said five years after the attack .

Only after months of fighting did officials, preservationists and survivors reach an agreement. The 38 steps of the “survivors’ stairs ” were separated from the concrete structure supporting them and moved to the underground National September 11 Memorial Museum. Meanwhile, 2 World Trade Center was allowed to go forward unencumbered by emotion-laden symbolism. Today it is a skyscraper in progress, and the developer is looking for an anchor tenant. (News Corp. and 21st Century Fox had signed a lease but pulled out in January.)

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Again and again during the past 15 years, the same questions have arisen at Ground Zero: How do you resolve the tension between a mandate to remember and a mandate to revive? How do you memorialize, while at the same time creating a space where people want to work and live and visit?

Those twin mandates have compelled continuous accommodation. As a result, the memorial at the Trade Center site is distinctly different from those commemorating the losses at the Pentagon and in Shanksville, Pa., where remembrance is the sole focus. And the revitalization of Lower Manhattan is different than any development project the city, and quite possibly the country, has ever seen.

Gov. George Pataki helped determine the contours of the New York memorial with his unexpected declaration, in June 2002, that “we will never build where the towers stood.”

Pataki also strongly endorsed the commercial mandate to rebuild 10 million square feet of office space and replace the high-grossing retail mall at the original World Trade Center. The Port Authority of New York and New Jersey, as owner of the land, and Larry Silverstein and his investment partnership, as holders of a 99-year lease on the site, were insistent on the full restoration of lost office space. The Lower Manhattan business community wanted the same.

But exactly where commercial development should take place, and what should be considered hallowed ground, proved difficult to settle. Even once the idea took hold that the footprints of the twin towers should be preserved, there was disagreement on how to define the footprints and what preserving them meant.

For example, the first set of plans prepared by the Port Authority proposed putting bus garages (necessary to accommodate the millions of people expected to visit the memorial each year) underground in the vicinity of where the towers used to be. But that proved to be untenable. Although the towers had been built on a concrete slab, many family members objected to any non-memorial construction below ground level, all the way down to bedrock. The distinction between the symbolic and the literal did not hold meaning, especially for families who never received the remains of those they lost on 9/11. (Some 1,115 victims were never found, and a repository containing 7,930 unidentified bone fragments and other remains is maintained by the city’s chief medical examiner within the 9/11 museum.) Ultimately, officials expanded the area for rebuilding by acquiring two blocks south of the Trade Center. This accommodated the Port Authority’s infrastructure and satisfied family members.

A jury selected the memorial design by architect Michael Arad and landscape architect Peter Walker for its “powerful, yet simple articulation of the footprints of the Twin Towers.” Opened to the public on the 10th anniversary of the attacks, the memorial features two voids that borrow their dimensions from the destroyed towers. Each void is framed by walls of water that cascade into pools 30 feet below street level, vanishing into nowhere, seemingly never to fill up. Bronze panels contain the names of those killed on Sept. 11, 2001, and in the first World Trade Center attack, on Feb. 26, 1993. The starkness is softened by a grove of 400 swamp white oak trees. The companion museum, completed in 2014 , is beneath the memorial plaza.

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The commitment to the towers’ footprints presented planners with another sensitive issue: how to balance retail and reverence. Shops could not be too close to the footprints, too close to commemoration. The original master plan separated them by siting the memorial 30 feet below ground and incorporating buffer elements: cultural buildings on the north and east sides. However, when the memorial was elevated to ground level, these elements disappeared. Today, the pavilion entrance to the museum, situated between the two pools, helps in the separation. But at ground level, the visual connection between remembrance and commerce remains. Below ground, these two abut, but none of the commercial spaces have access to or visibility from the museum.

The tension inherent in the dual mandate flared once again with the opening of the museum’s gift shop, which the father of one victim called “crass commercialism on a literally sacred site.”

Another issue that has exposed tensions is culture. With the idea that Ground Zero should be a “living memorial,” planners intended spaces for the arts to help infuse the redevelopment with energy and life. But a small subset of 9/11 family members resisted anything that might interfere with the memorialization of their loved ones. The arts spaces were competition — for public attention, donations, size and pride of place at Ground Zero. And so the families sought to disrupt the planned selection of cultural groups and successfully petitioned to reduce the scale of cultural buildings.

The victims’ families, with their unassailable emotional claim, commanded singular standing whenever they put forth deeply felt desires for specific forms of remembrance or objections to specific plans for Ground Zero. They were a political constituency, and it was very hard for any politician to push against their opposition. But once most issues relating to remembrance were settled, and the focus shifted to commercial arrangements, the power of the activist families dissipated.

Since 2011, a new place has been materializing, a place that balances remembrance of the past with optimism for the future. The many goals officials and development executives set out to achieve have been cohering. One World Trade Center anchors the skyline of Lower Manhattan, and two other commercial towers have been attracting tenants. The World Trade Center transportation hub, with its distinctive, avian-like Oculus structure, is now populated with retail stores. The eight-acre memorial quadrant, with its 400 trees, is becoming more of a park day by day — New Yorkers who are used to small, tight spaces are reveling in its openness. The business community downtown has been cheering the long-sought-after growth that promises to accompany the thousands of new office workers, tourists and residents.

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Rebuilding Ground Zero, “a place made sacred through tragic loss,” as the memorial’s president has said, has been a perilous but essential ambition. Remembrance without rebuilding would have constituted only partial healing of the wound to New York and the nation left by the 9/11 attacks. Only the dual achievement would constitute success in the eyes of New Yorkers, the nation and the world.

That New York succeeded by the 15th anniversary of 9/11, despite a process beset by passionate controversies, political disputes, personal animosities, sensitive preservation issues, broken deadlines, intense debates about developer subsidies and quarrelsome cost overruns, has surprised many who doubted that the years of chaos could yield anything of lasting value.

It is no longer Ground Zero, though the moniker for the Trade Center site is hard to let go.