FORGET, for a moment, the Wedge of Light, the 1,776-foot Freedom Tower spire and the birdlike wings of the crystalline train station.
Though the public imagination has been fired by images of a shimmering city in the sky, the new World Trade Center is actually being designed from the ground down, where there is not a cubic inch to spare.
In fact, the complexity of accommodating all the claims to that space - memorial, memorial center, twin-tower footprints, exposed slurry wall, PATH station, subway tunnel, shopping concourses, pedestrian passageways, central air-cooling plant, ramps and roadways, bus and car garages, vehicle screening points, loading docks, storage areas, pipe galleries, ventilation ducts, foundation walls and fan rooms - has proved to be one of the most daunting challenges yet in the reconstruction of ground zero.
"It's a very tight site," said Kenneth J. Ringler Jr., who was appointed executive director of the Port Authority of New York and New Jersey in October. "As a matter of fact, at my first meeting, I was shocked to find that everybody was arguing intensely over inches."
The argument cannot be solved only through innovative engineering, although some remarkable structural legerdemain has been achieved. (Just how the 9/11 memorial will fit into this complex spatial puzzle will become clearer today with the announcement of its working design, expanding on the year-old conceptual renderings.)
At its most basic level, the struggle over the infrastructure is a fight over money.
Depending on what is included, the cost of this subterranean framework may easily exceed $2 billion. It will probably be no less than $1.5 billion. And there is no agreement yet on apportioning the bills for the biggest-ticket items - the roadways, air-cooling plant and foundation walls - that serve all the users on the site, including about 250,000 square feet of underground retail space.
Financing will come largely from the Port Authority, which owns the trade center site; Silverstein Properties, the commercial leaseholder; and the Lower Manhattan Development Corporation, which is overseeing the memorial and cultural programs.
But who, exactly, will pick up the check for what? "We are having on-going, productive discussions," said Kevin M. Rampe, president of the corporation.
Time is closing in. Gov. George E. Pataki announced last month that excavation for the Freedom Tower would begin in February. No simple structure, this skyscraper will rise directly over the outbound PATH tracks and over utility lines that must be relocated around the tower's column footings. Making matters more complex, the tower will share the underground area with the performing arts center, which is only now being designed.
It is difficult to look at a site as vast as the trade center foundation, known as the bathtub, and imagine it starved for space. But fixed elements like the PATH tracks dictate the location of the station, which dictates the location of the concourses, which dictate the location of mechanical space.
In the push and pull over the mechanical systems for the station and concourses, the authority moved some space out of the basement of the Freedom Tower to lower the surrounding street level. But it also added space. Doing its own juggling, Silverstein took the building fitness center out of the basement. More room was needed.
Meeting on the night of Nov. 22, the architects at Skidmore, Owings & Merrill, working for Silverstein with the engineer Augustine A. DiGiacomo and his colleagues from Jaros Baum & Bolles, figured out they could double up some space and create a new sublevel with room for equipment that will serve the television transmission center more than 1,000 feet overhead.
"It's been - and continues to be - a three-dimensional chess game," said Carl Galioto, a partner at Skidmore. "Really four-dimensional, when you bring in the fact that the trains have to keep running."
Freedom Tower is the keystone in another critical part of the common infrastructure. Its basement levels will have to be reinforced by concrete shear walls to resist the tremendous lateral pressure against the north and west sides of the trade center foundation from groundwater, fed by the Hudson River. So if the tower's walls will partly benefit all the other buildings, how much - if anything - should other users pay for those walls?
Another apportioning issue involves the air-cooling plant. When the site is fully developed, the authority said, the five office towers will account for about half the total demand for 40,000 tons of cooling capacity. (A ton of cooling equals 12,000 British thermal units an hour. For comparison's sake, a residential room air-conditioner may have a capacity of less than one ton, while a modest-sized house would require three or more tons.) The Freedom Tower alone will account for almost 10,000 tons.
Given that the plant will probably be constructed in phases, perhaps inside a structural shell designed to handle the full amount of equipment, how much of the cost should Silverstein pay? And when?
The authority estimates that tour buses bound for the memorial will account for roughly 75 percent of the peak-hour underground roadway use in the first years of operation, declining to 40 percent by 2015. The development corporation said traffic can be counted several ways, but in any case said that buses would account for no more than 57 percent of roadway use in 2009 and 12 percent in 2015.
The corporation is adamant about limiting its contribution to elements that benefit the public, rather than commercial and retail users, while authority officials suggest that the entire subterranean roadway system is, in fact, a public benefit and an expression of public policy.
Financing seems settled for the $2 billion PATH terminal and transportation hub. The authority expects to receive $1.7 billion from the Federal Transit Administration and $300 million from insurance proceeds. About 30 percent of the terminal cost involves the construction of 4,350 feet of pedestrian passageways.
Despite the lack of resolution on overall financing, progress has been made in preparing the site for development. "What we've finally pinned down is the street-level plan," said Anthony G. Cracchiolo, the director of priority capital programs at the Port Authority.
Deputy Mayor Daniel L. Doctoroff called it a "collaborative though sometimes contentious process in which everyone was committed to moving forward aggressively."
Perhaps most significantly from the city's standpoint, agreement was reached to move the main vehicle ramp from the north side of Liberty Street, on the edge of the memorial, to the south side, under the future Liberty Park. That will permit traffic on Liberty Street to run two ways and create a much wider buffer between the ramp and the memorial.
To create headroom for the ramp and follow the land contour as it dips down, the west end of Liberty Park may end up 25 to 30 feet above street level. Also, the site of the new St. Nicholas Greek Orthodox Church in Liberty Park will be farther east than its original building, which was destroyed on 9/11.
The main stem of the underground roadway once was to have run along Church Street. It has been shifted to a route closer to Greenwich Street, allowing future utility connections to the office towers. Tenant car parking for the Freedom Tower, and the lifts to serve it, will be under or within the performing arts structure. But the theaters may end up being able to use some of those 300 parking spaces at night for patrons.
The vexatious issue of bus parking, which stirred strong opposition last year when the Port Authority identified an area below the memorial as a possible garage, has been resolved for now by assigning 60 bus spaces to the B3 level under the PATH concourse and 20 spaces to the B3 level under Liberty Park.
As to the security concerns posed by having buses parked below the station, Mr. Ringler of the Port Authority noted that the buses will have already gone through the checkpoint under Liberty Park. "No suspect vehicles can get beyond that point," he said.
The concourse to the Winter Garden in Battery Park City has been lowered from the B2 level to the B3 level, on line with the PATH station mezzanine. This was necessary in part to clear a perpendicular path for a West Street-Route 9A underpass, but it also provides better pedestrian circulation and eliminates the need to relocate a large sewer, authority officials said.
Just above ground, enough right-of-way has been left in the plan to permit the eventual restoration of Dey and Cortlandt Streets, between Church and Greenwich Streets. Beyond that, the city, which strongly favors such a restoration, and the Port Authority, which does not, have agreed to postpone a decision.
An agreement has also been reached on a 60-foot separation between Freedom Tower and the performing arts center to the east. But parts of the center, which is being designed by Gehry Partners, may cantilever into that open space as far as 15 feet, as long as they are opposite unoccupied areas of Freedom Tower. A solution is to be arrived at by the architects.
Given the complex issues, the charting of the trade center's future is in many ways in the engineers' hands now. "At the end of the day, most of these technical problems get solved by professionals," said Janno Lieber, the project director for Silverstein, "not by people whose names are in the papers."
There is at least a greater sense of certainty about what will go where. For instance, before the Signature Theater Company and Joyce Theater Foundation were chosen as the performing arts tenants in June, planners did not know if they would have to allow for loading docks large enough to handle opera sets, said Carla Swickerath of Studio Daniel Libeskind, the master planners of the site.
"When we're fighting in inches," she said, "we've gotten somewhere."