Once intended as a shimmering, soaring declaration of political resolve and architectural ambition, the Freedom Tower at the site of the World Trade Center is being reimagined as something more impregnable. So the challenge now faced by its designers is to keep it from looking like a high-rise bunker.
The Freedom Tower may end up with a structurally massive base that is distinctly different from the upper office floors. The designers may use stainless-steel cladding, reinforced glass or even translucent concrete, a new material embedded with strands of glass that transmits light and shadow through seemingly impervious walls.
The redesign, ordered by Gov. George E. Pataki last month in response to security concerns raised by the New York Police Department, will add tens of millions of dollars to the building's estimated $1.5 billion cost. Less than four weeks before the governor's deadline of the end of June, details of the redesign are still being worked out, said Silverstein Properties, developers of the Freedom Tower and 7 World Trade Center, and Skidmore, Owings & Merrill, the architects of both buildings.
The urgent push to fortify what had already been called ''probably the safest building in the world'' by its chief architect, David M. Childs of Skidmore, has raised broader questions about the site planning and architectural criteria for the trade center redevelopment project.
Today, two City Council committees will consider security concerns at the Freedom Tower and the larger issue of Police Department involvement in the design and construction of big buildings and development projects.
If antiterrorist security had been the one and only consideration at the trade center site, the first designs for the Freedom Tower and other projects would seem to flout some basic axioms put forth by the Defense Department, the Federal Emergency Management Agency and the American Institute of Architects.
In publicly available reports, they advise that attention-getting architectural symbols are prime targets and should be located far from potential vehicle-borne bombs; glass facades can be lethal in a blast; train stations and underground garages are especially vulnerable to attack; and spacious, column-free interiors under other structures may be liable to collapse.
At the trade center site, planners have envisioned a defiantly tall skyscraper laden with symbolism on a site bounded by a heavily trafficked state highway; a memorial that is expected to draw millions of visitors; and a transparent, glimmering museum directly above the broad, column-free mezzanine of a busy commuter rail and subway station. City streets will lace the site. Below them will be a network of ramps, roadways, loading docks and parking spaces.
Officials still intend to follow this broad development outline, but the design of the Freedom Tower is being revised to provide a greater shield from vehicle-borne explosives. This is generally described as ''the most important consideration'' in antiterror structural design by the emergency management agency, even though the last attack on the trade center was an aerial assault. Cars and trucks have proven to be an effective way of delivering large explosive charges.
The first version of the Freedom Tower was unveiled in 2003 and publicly embraced by city and state leaders, including officials of the Port Authority of New York and New Jersey, which owns the site. The design placed the building as close as 25 feet from West Street-Route 9A. From a parallelogram base ringed by an arcade of exposed columns, the tower was to twist as it rose, culminating after 70 floors of occupied space in an open-air superstructure of cables, power-generating windmills and an off-center spire meant to echo the Statue of Liberty.
People who have been informed about the redesign discussions say that the architects will probably set the building back at least twice as far from the street to limit blast damage from a car or truck bomb, and untwist it. The new building is likely to have a square floor plan, with a different summit and several more occupied floors than were in the original design. Windows below the 150-foot level will almost certainly be fewer and smaller, and views will be limited. The arcade around the base may be eliminated.
Managers of the redevelopment insist that the trade center site can be made safe through a series of protective zones, with checkpoints, bollards and other barriers.
And they say the Freedom Tower and other sensitive buildings can be hardened with blast-resistant materials. Some, like Daniel Libeskind, the site's master planner, say the new World Trade Center ''will be as safe as any site in the world today, if not safer.''
''We are responding to the need for security, but we are also responding to life,'' Mr. Libeskind added. ''We cannot make the Freedom Tower a tower of fear.''
There is no denying, however, that this site has been attacked twice. In this context, the assessment of security is something of a gamble against unknowable, and possibly deadly, odds.
Much about security preparedness has been deliberately shrouded in secrecy. Some experts and public officials spoke anonymously for this article because they are forbidden contractually, or by the agencies they work for, to publicly discuss questions of safety. They also say that specifying security details is an invitation to terrorists to exceed them.
Some with knowledge of the Police Department's evaluation of risk on the site said that its experts favor caution in security tradeoffs, but do not want to be unrealistic in demanding assurances of absolute safety. These experts acknowledge that an urban site cannot be built that is totally immune from attack, but insist that its buildings must not be so vulnerable that they invite an attack.
The Freedom Tower's original ''threat and risk assessment'' report, these experts say, specified that the building achieve the security standards for a high-level government structure in Washington, capable of resisting a 5,000-pound truck bomb -- more than the size of the 4,000-pound Oklahoma City explosion in 1995.
Though such criteria were unusually high for an office building, the police rejected them as inadequate, given the Freedom Tower's height, its role as a political statement, its planned glass exterior and its proximity to West Street-Route 9A.
The police did suggest some redesign options and submitted a report detailing the history of truck-bomb attacks, according to some with knowledge of the ensuing dialogue. Finally, in early May, the governor announced that the Freedom Tower would be moved away from West Street, and that the tower exterior would be strengthened.
Untwisting the torqued design of the original tower would reduce the size of the building's base, and help meet the Police Department's requirement that it be set back farther from the street.
The original distance of 25 feet from West Street-Route 9A will be doubled, at a minimum; as much as 65 feet is conceivable. Since the distance from Vesey and Fulton Streets will be considerably less, traffic on those streets will most likely have to be limited.
Such seemingly minor differences matter ''because explosive pressures decay so rapidly with distance. Literally, every foot counts,'' said Eve Hinman, the president of Hinman Consulting Engineers and a principal author of ''Primer for Design of Commercial Buildings to Mitigate Terror Attack,'' issued in 2003 by the emergency management agency.
The Freedom Tower architects have said that the building's stairwells, like those of the new 7 World Trade Center but unlike those of the twin towers, will be protected by reinforced concrete instead of wallboard, and will be pressurized to keep stair passages from filling with smoke.
Architects are planning to include separate emergency-responder staircases and elevators that could be used in a crisis. The designers hope to incorporate redundant sets of pipes feeding the fire sprinklers. And signal-boosting radio repeaters are planned to enhance radio communication.
The security consultant on the Freedom Tower project is Weidlinger Associates, an engineering firm with decades of experience in blast resistance.
Over all, Mayor Michael R. Bloomberg has said, the redesigned Freedom Tower will be built to achieve United States embassy security standards. The base, up to 150 feet, will have what consultants call ''a hard bottom,'' built to Department of Defense standards of blast resistance. Although the size of this threat is classified, an emergency management agency report recommends protection against at least 10,000 pounds of explosives. (The largest truck bombing against an American target, the Marine barracks in Beirut in 1983, involved some 12,000 pounds of explosives.)
Among the sheathing materials under consideration for the Freedom Tower are stainless steel, laminated glass and translucent concrete, an expensive material, some of which is sold under the brand name LiTraCon, for light transmitting concrete.
The redesign is likely to eliminate arcade columns, which the emergency management agency recommends avoiding in its design primer since they are ''primary structural elements that hold up part of the building,'' said Dr. Hinman, the author of the report, in an interview.
To compensate for the reduction in floor space caused by the redesign, the occupied portion of the building, originally to be 70 stories, will probably increase by another few floors.
Publicly trumpeted flourishes at the top of the building -- a wind farm, an observation deck, restaurants and exterior cabling at the spire -- are likely to change. But none of the original elements has thus far been written off. One possibility is that a single antenna structure will allow the tower to reach 1,776 feet.
That height remains the governor's goal, even though some security experts say it substantially raises the security ante. ''In my opinion the height is a negative because of the risk of another aerial attack,'' Dr. Hinman said.
Mr. Libeskind, however, said that ''a four-story building on that site wouldn't be less attractive to those who want to destroy this country.''
Carl Galioto, a partner at Skidmore, said, ''Our client and the governor have asked us to design something great.''
Greatness aside, the security goal is to make the Freedom Tower structurally reusable after an explosion. Such engineering standards do not assume that the lives of all the people within the building could be saved in an attack, though hardening the structure could prevent many deaths.
The original plaza of the World Trade Center, which acted as a sometimes wind-swept barrier to the surrounding neighborhood, might have been preferable purely from a security point of view because it eliminated streets running through the site.
But Alexander Garvin, an urban-planning professor at Yale University, championed the re-creation of the old street grid when he headed planning at the Lower Manhattan Development Corporation in 2002 and 2003.
''Either we live normal lives with normal streets and normal sidewalks, or we go back to living in fortresses as they did in the Middle Ages,'' he said in an interview.
Many prominent buildings are quite close to streets. These include Grand Central Terminal and the Empire State Building and structures under construction like the Bank of America Tower at 1 Bryant Park and the future glass-clad headquarters of The New York Times across Eighth Avenue from the Port Authority Bus Terminal. This transportation hub has for months been undergoing a security upgrade. Bare steel support columns in upstairs parking areas have been encased in reinforced concrete to enhance their fire and blast resistance.
Several experts question the location of the Freedom Tower near a state highway, three through streets and the headquarters of at least two high-profile companies, American Express and Verizon.
''The best thing from a security standpoint would be to move the Freedom Tower to Church Street,'' said Guy J. Nordenson, a structural engineer and a professor of architecture at Princeton University who is currently involved in several projects at the trade center site. ''You could control truck traffic access better on Church Street than on West Street, and you would be able to achieve setbacks more easily on that site.''
Trucks are not the only threat. ''Large passenger vehicles like S.U.V.'s are able to carry more explosives unobtrusively and are a major hazard,'' Dr. Hinman said. She said the current estimate of how much a big S.U.V. can carry is the equivalent of the charge used in the Oklahoma City bombing.
To make underground parking safe, all vehicles entering the subterranean roadways would be subject to inspection at the entrances off Liberty and Vesey Streets, said Stefan Pryor, president of the Lower Manhattan Development Corporation.
Security experts underscore that terrorists' intentions cannot be predicted, and they recommend the installation of specially designed air intakes and ventilation systems in buildings at the site to counter radiological, chemical and biological threats.
Also undergoing scrutiny is the $2 billion PATH station and transportation hub by the architect Santiago Calatrava, which poses many engineering challenges, including the routing of the PATH tracks directly under the Freedom Tower site, meaning that the tower's supporting columns must be threaded among the rails.
Mr. Calatrava said in an interview that he could deliver ''a beautiful building that accomplishes all the needs of security,'' adding that ''95 percent of people in the station will be in lower levels,'' and therefore more shielded from the possibility of explosion.