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.