NEW ARCHITECTURE

They are two of the best-looking dinosaurs you'll ever see.The new Hearst Corp. headquarters, by Norman Foster, and the 7 World Trade Center tower, by David Childs of Skidmore, Owings and Merrill, rank among the sleekest, most architecturally accomplished skyscrapers to rise in New York City in decades.

Foster's building for Hearst, at 57th Street and 8th Avenue, is a buoyant, zigzagging composition in glass and steel that sits atop a six-story Art Deco pedestal from 1928. The tower by Childs, built for the developer Larry Silverstein, also performs a complicated dance at street level: It is built above a massive Con Edison substation. But in playing to nearly all of its architect's strengths, it manages to come off as crisp and nearly weightless. Both projects include an unusually wide range of green-design features.Still, as office towers in the purest, most corporate sense, and as symbols of the idea that Manhattan is the center of the world, they are inescapably members of a vanishing breed. All around them — in New York, but also in almost every major city in America — talented architects are recasting the skyscraper as a place to live. Frank Gehry's pair of glass towers on Grand Avenue in Los Angeles, which will contain hotel rooms, apartments and high-end condos, and Santiago Calatrava's tapering, corkscrew-shaped Fordham Spire in Chicago are only the most recent examples of the trend.The condo market that helped propel the change has begun to cool in many American downtowns. But the shift in skyscraper architecture from commercial to residential has been so sharp and widespread that it's difficult not to think of Foster and Childs as an anachronistic pair: as, say, a couple of contemporary composers who have produced dueling string quartets or two television network executives deciding to launch competing half-hour, laugh-track sitcoms that also happen to be very well-made.That in the end is what complicates this discussion: These towers are surpassingly well-made. In Foster's case, that comes as little surprise. The British architect presides over one of the biggest firms in the world that can claim at least a connection to the avant-garde. Its recent projects include the stunningly simple Millau Bridge in southern France, created with the engineer Michel Virlogeux; a new dome for the Reichstag in Berlin; and a gigantic airport in Beijing, which features a dragon motif and is set to open before the 2008 Summer Olympics.There is, certain British critics would tell you, a certain factory sameness to many of Foster and Partners' designs, despite their impeccable detailing and airy futurism. Foster's formal bag of tricks — which mostly involves stretching glass canopies over taut steel skeletons of varying shapes and sizes — is pretty much the same now as it was 15 years ago.But Foster was by all accounts determined to take a particularly active role in the Hearst project, especially after losing out so publicly to Daniel Libeskind in the beauty contest for a new master plan for ground zero. In that competition Foster proposed a pair of broad-shouldered skyscrapers that would recall the forms of the World Trade Center but also lean toward one another at certain points and touch. Foster referred to them as "kissing" towers.Some of the ideas from that proposal are part of the $500-million Hearst design, but the tower is also a singular achievement for an architect who has waited a long time for an American triumph. The 1928 base, designed by Joseph Urban for William Randolph Hearst to prop up an anticipated tower that Hearst never got around to building, contained a rather dense and dark collection of offices and hallways; the great unexpected twist of the Foster design is that it turns the lower levels of the building into a soaring, light-filled atrium. The exterior walls remain in place, propped up by the gigantic trusses of the new building, but the floors that used to fill them have been entirely scooped out.Perhaps the only disappointment is that the new town square that has been dropped into the now-vacant base is, for post 9/11 security reasons, open only to Hearst employees. Even to ride the escalator, which glides at a dramatic diagonal angle through a fountain of glass bricks by the artist James Carpenter, requires that a Hearst staffer vouch for you. It is a shame, because this could have been New York's first great public space (or space that is open to the public, anyway) of the new century.It is in the offices themselves, many of which offer spectacular views of Central Park and the Manhattan skyline, that you get a sense of the structural system that gives the building its architectural personality. The building is made of four-story high triangles, framed in steel, that ripple up and down the facade. This creates offices with floor-to-ceiling glass walls that either cant out toward the city or tilt in toward the center of the building. The system uses about 20% less steel than a conventional skyscraper.Foster and his collaborators on the project realized that in a dense, gridded setting such as Manhattan, pedestrians rarely look at a skyscraper straight on, or capture all of it in a single glance. Instead their sense of a building is made up of a collection of peripheral glimpses. The diagrid is a natural for that sort of architectural setting because it creates a skyscraper with no hard edges, essentially folding four distinct elevations — north, south, east and west — into a continuous exterior.The $700-million 7 World Trade Center tower, which opens officially today, is, on the other hand, pretty much all edges. Because of its odd site, just north of the official ground zero rebuilding parcel, the tower is shaped like a parallelogram in plan. That means its corners are sharper even than the normal office-tower corners that Foster tried to avoid.Childs, who is also the architect of the controversial Freedom Tower just up the street, a job he once shared with and then wrestled away from Libeskind, and of the somnolent Time Warner Center on Columbus Circle, has managed to turn this quirk of urban geography into an asset. Instead of looking like a gangly teenager who is all elbows, the building has transparent corners that give it a light, sharp elegance — and the impression that it is cutting through Lower Manhattan like a ship.That sense of movement is crucial for the ground zero site, which has been stuck in a planning morass. Though 7 World Trade is officially separate from the 16 acres of land governed by the ill-fated Libeskind master plan, it does replace a tower that was destroyed on Sept. 11. And its completion may give an important jolt of optimism to a process that desperately needs it, especially as Michael Arad's memorial for the site sits in fundraising and public-opinion limbo.It is also a sign of how much better the Freedom Tower, another Silverstein-Childs production, could have been. Though that building has been caught up in debates about security, terrorism and American pride, 7 World Trade has gone up almost in stealth, if you can say such a thing about a 52-story structure. In that sense, it is Lower Manhattan's real freedom tower: a skyscraper free from bureaucratic and political wrangling, at least by New York City standards, and all the more successful for it.