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.