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 For Trade Center Builders, the Future Starts With a Very Old Number

October 15, 2005 By DAVID W. DUNLAP
With the public captivated in recent weeks by talk of freedom, patriotism, memory, honor and dishonor at ground zero, the engineers who are actually shaping its future have been working in another dimension.

Inches.

And they have now set what is perhaps the most important dimension for the future World Trade Center: the height of the memorial plaza that will be its spatial and spiritual centerpiece.

What makes that height so important is that it determines the relationship of the plaza, which must be tabletop flat, to everything around it: the Freedom Tower, the PATH terminal and the surrounding streets and sidewalks. But setting the dimension is not easy on a site that slopes down more than 20 feet as it approaches the Hudson River.

The plaza itself needs to be as close to the sidewalk as possible, no more than three and a half feet higher or lower, so that it can be seen and approached easily by all visitors - including the disabled - from all directions, without walls or long flights of stairs. "The appearance of the memorial as well as the accessibility of the memorial were paramount," said John P. Cahill, chief of staff to Gov. George E. Pataki and the top downtown redevelopment official.

After months of pushing and pulling, the plaza elevation was finally set last month at 313 feet above a subterranean benchmark known as the World Trade Center Downtown Restoration Program Datum. "Giving up inches got a lot done," said Steven Plate, deputy PATH Trains Hudson Tubes Downtown WTC Station director for priority capital programs of the Port Authority of New York and New Jersey, which owns the trade center site. "As a result of this effort, we've been able to move forward." 

A datum is an imaginary horizontal plane against which a construction project is measured. Typically, it is set far below ground to ensure that all elevations - even those in the deepest subbasement - are above that plane, to avoid construction mistakes.

The trade center datum, related to one originally used for the Hudson & Manhattan Railroad, the predecessor to PATH, is 297.347 feet below mean sea level. A higher plaza would have meant more room for utility lines, ductwork, mechanical areas and public spaces below ground. Too much height, however, would have set the memorial plaza apart from its environs, recalling the Austin J. Tobin Plaza of the original World Trade Center, which met West Street as a formidable wall.

In recent weeks, the calibration of the plaza level - which must be set before final construction drawings are prepared - turned largely on reconciling the difference between the Freedom Tower lobby and the roadbed of Greenwich Street. Negotiations involved the Port Authority, the Lower Manhattan Development Corporation, New York City Transit, the State Department of Transportation and Silverstein Properties, the developers of the Freedom Tower. "Everybody had to make compromises," Mr. Cahill said.

Greenwich Street, between Dey and Cortlandt Streets, started at elevation 316 feet, allowing for the No. 1 subway line underneath and the structural support, ductwork, fans and utility lines between the tunnel and the street surface. At this location, there could be only a one-foot difference between street and plaza, to permit emergency vehicles to get onto the memorial plaza from Greenwich Street. That tentatively set the plaza level elevation at 315 feet.

But across Fulton Street from the memorial is the Freedom Tower. Its lobby runs through to Vesey Street, at the lowest end of the site. Silverstein Properties wanted the Fulton Street end of the lobby to align with the memorial but did not want too great a change in levels between the Fulton and Vesey Street sides.

During several weeks of intense re-engineering, a way was found to lower Greenwich Street to elevation 314 feet by reconfiguring the underground ducts and reducing their size, changing the thickness of structural slabs, moving utilities and altering the slope of the street. That allowed the plaza level to be set at elevation 313 feet. Meanwhile, Silverstein Properties raised the Fulton Street side of the Freedom Tower lobby to elevation 311.2 feet. The 1.8-foot grade change between the tower lobby and the memorial across the street can be rendered almost imperceptible.

The benefit is not only to the memorial design, said Stefan Pryor, president of the Lower Manhattan Development Corporation, but also to the goal of knitting together Battery Park City and the World Financial Center with the financial district to the east. Mr. Plate of the Port Authority said that elevation 313 was still subject to minor modification, but that the recent problem-solving exercise would serve as a "model for many other issues to come." He explained the secret of its success: "Locking the doors and saying, 'We don't leave the room till we've worked it out.' "

http://www.nytimes.com/2005/10/15/nyregion/15rebuild.html

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