Let's set aside, for a second, the hardheaded question of whether Spanish architect and engineer Santiago Calatrava's twisting 2,000-foot Chicago tower will ever get built. In a way, it doesn't matter. Last Wednesday's unveiling of the dazzling, but still-evolving 115-story hotel and condo tower marked a major milestone in an ongoing revolution: The skyscraper and the tall office building no longer are synonymous.

For more than a century, they were.

When the great Chicago architect Louis Sullivan articulated the famous principle "form ever follows function" in 1896, he was writing an influential essay, "The tall office building artistically considered." It aimed to impart grace upon a then-new and cantankerous building type, the skyscraper, which was blocking views, darkening streets and making buckets of money for ruthless capitalists.

"This sterile pile, this crude, harsh, brutal agglomeration," Sullivan called it.

More recently, all the holders of the world's tallest building title have been office buildings, from the Empire State Building to Sears Tower to the latest to wear the crown, the pagoda-inspired Taipei 101 in Taiwan. To think about building tall, in other words, has been to think about tall office buildings. And it was to assume that downtowns would be places of work -- noisy, dirty, perhaps industrial -- and that people would live outside them in quieter city neighborhoods or leafy suburbs.

But as Calatrava's design reveals, life and cities have changed and the skyscraper is free to adapt to those changes in stunning new ways. Though far from faultless, it is one of the freshest and most captivating skyscraper designs Chicago has seen in decades, fully taking advantage of the possibilities offered by the fact that it would be a place to live rather than work.

Tall residential buildings are apt to be thinner than tall office buildings so residents can be closer to the views for which they paid so dearly. They do not have to project the businesslike image of a corporation, whether it be Sears, Roebuck & Co. or the Seagram liquor empire. They also reflect the fact that cities, at least Chicago, no longer are the brutal industrial beasts of Sullivan's day, having been softened and suburbanized by the likes of Mayor Richard M. Daley. This is the age of the post-industrial city, which is as much a place to play as a place to work. Calatrava's tower reflects that, flaunting a Baroque dynamism that could not be more different from the gridded, hard-boiled Chicago architecture of old.

The problem of the tall residential tower is hardly new, of course. There have been tall residential towers before, as well as tall mixed-use buildings that included homes. Architects in Chicago have designed them in a variety of ways: the mansion in the sky, a skyscraper version of a traditional manse (Park Tower); the skeletal tower, which vigorously expresses its structural framework (the John Hancock Center); the slab plopped on an enormous base (Water Tower Place); and the organic high-rise, whose sensuous curves take their visual cues from nature (Marina City, Lake Point Tower). Calatrava's tower, which the architect compares to the twisting trunk of a tree, clearly belongs to this tradition.

Too much product?

What's different today is that there are so many tall residential buildings either planned or under construction in Chicago. And three of them are not just any tall buildings, but supertall buildings, which aim to crack the barrier of 1,000 feet: Calatrava's tower, which would be called the Fordham Spire and would rise on a riverfront site just west of Lake Shore Drive; the underconstruction Trump International Hotel & Tower, which, at 1,360 feet, will be just 90 feet shorter than Sears Tower; and the planned Waterview Tower at 111 W. Wacker Drive, a thin setback tower that would rise to 1,050 feet. The plans alone are a stunning rebuke to the post-Sept. 11 prophets who predicted the death of the skyscraper.

The profusion of luxury condos has some real estate experts predicting that the Fordham Spire, where typical units are expected to carry the lofty price tag of $1 million to $2 million, will never materialize because there is too much product out there. But it would be foolhardy to dismiss the $500 million project, even if the odds against it being built appear to be as long as the tower itself.

If Trump, the star developer, had the "X" factor of his reality-TV show, "The Apprentice," working in his favor, the developer of the Fordham Spire, Chicago's Christopher Carley, chairman of the Fordham Co., has his own "X" factor: a star architect, Calatrava, who rivals Frank Gehry in his ability to create attention-getting icons.

As anyone who has been to Calatrava's lyrical, birdlike Milwaukee Art Museum addition knows, his startling, almost surreal buildings have the capacity to alter the rules of everyday life, or at least make people behave in seemingly non-rational ways. The Milwaukee museum's budget grew astronomically as the $125 million project progressed, and the museum only recently obtained pledges to retire the $25 million debt the project caused.

Will people plunk down their millions to live in a high-rise that could, by virtue of its height, be a terrorist target? Trump, who scaled down his tower after buyers told him they didn't want to live in a record-setting building, says no. Carley and Calatrava disagree, saying their residential project won't be an economic symbol like the twin towers of the World Trade Center. Ultimately, the market will decide whether the nation has gotten over its post-Sept. 11 fear of heights.

Calatrava, it turns out, had been at work on the problem of the tall residential building long before Wednesday's unveiling at the Museum of Contemporary Art, a setting presumably chosen to communicate the idea that buying owning a unit in the tower would be like purchasing a work of art. His answers have been aesthetically radical, albeit hyperexpensive, as in the planned 80 South Street Tower, a lower Manhattan design to be built near the Brooklyn Bridge. Announced last year, 80 South Street will consist of 12 glassed-in 45-foot-square cubes, each four stories high and designed to house just one or two families each. Arranged like a ladder, the cubes will cantilever off the tower's exposed core. Rising more than 1,000 feet, the tower would offer a dazzling new take on verticality.

The living concept behind 80 South Street is just as intriguing as the form. The cubes, in effect, are townhouses in the sky, offering all the advantages of high-rise living, such as panoramic views, plus the variety of interior spaces that townhouses provide. True, there is talk in the real estate community that the cubes may be cut into multiple units. Even so, this tower represents a wonderfully creative rethinking of the ziggurat-like pile of cubes in Moshe Safdie's Habitat housing development at Expo '67 in Montreal. Construction could start as early as this fall, a Calatrava spokeswoman says.

Calatrava's planned Chicago tower is less radical than its New York counterpart, yet no less striking. It would consist of a tiered four-story podium that would contain shops and restaurants. The podium would serve as a pedestal for a sculptural, glass-faced tower in which each successive floor would rotate slightly from the one below it, making it appear as if the tower were twisting into the sky. The building's roof would stretch to the height of 1,458 feet, 8 feet taller than Sears Tower. Its steel spire would soar to roughly 2,000 feet, surpassing Sears and the planned 1,776-foot Freedom Tower in New York as the nation's tallest. Superlatives like this do not come cheap. Despite the tower's straightforward internal structure, Carley acknowledged that the tower will cost at least 35 percent more per square foot than a typical residential high-rise. "The Calatrava factor," he calls it, with a smile.

While other architects have proposed twisting towers, few have been done with such panache. Already being compared to a drill bit, the tower would whir without interruption into the sky, meeting Sullivan's standard that a skyscraper should be "every inch a proud and soaring thing, rising in sheer exultation . . . without a single dissenting line." In a sense, it shows that skyscrapers can return to the slender, romantic forms they had in the 1920s, before air conditioning and the desire for huge office floors made office buildings bulge with ungainly girth.

In harmony

Yet Calatrava's song of the sky would not be a showoff solo; his twisting forms would engage in a memorable skyline dialogue with Lake Point Tower's undulating curves. Nor would this skyscraper be a flashy eye-grabber whose beauty is only skin-deep. The design integrates sculpture, structure and space in a way that recalls the tree-inspired high-rise plans of Frank Lloyd Wright. Its floors would cantilever beyond its trunklike concrete core and circle of columns, opening unobstructed views. Still, Calatrava must figure out how to incorporate the balconies Carley wants for some condo units without compromising the tower's sculptural form.

There are other issues to confront, including the awkward way the tower emerges from its podium, a move that recalls a stripper popping out of a cake. In an interview after the unveiling, Calatrava said he wants to eliminate the podium and have the tower come directly to the ground, where it would be surrounded by crescent-shaped gardens. The base of the tower would still contain restaurants and shops, he said. Yet ditching the podium raises the question of how the tower can transform itself to a pedestrian-friendly scale at ground level, an issue that takes on added weight because it will rise near the adjoining riverwalk.

Traffic congestion is another concern since this enormous building would be reached by a narrow, out-of-the-way road called North Water Street. Three-flats on Damen Avenue are easier to get to. Calatrava said he is exploring an exit off Lake Shore Drive, presumably off the lower level, that would provide direct access. Whether his solution works or not, it shows that Daley ought to be taking neighbors' concerns about congestion seriously rather than dismissing them as not-in-my-back yard complaints.

The same goes for the threat of terrorism, which the mayor brushed off Wednesday by saying, "We have to move on with life." Of course we do, but we cannot build exactly as we did in the past. We have to intelligently confront new realities, something Calatrava begins to do by encasing separated stairwells in protective concrete. Yet more security measures may be necessary because the tower's proximity to Lake Shore Drive could make it vulnerable to a vehicle-delivered bomb. Even the new Hyatt Center office building, a much shorter tower, incorporates security features designed to thwart such an attack.

Evolving points of reference

There is, finally, the question of whether a tower this tall belongs along the lakefront and whether it would be wise to change the way the skyline now tiers down gracefully toward the shoreline. Calatrava makes the persuasive case that the tower's site, close to the meeting of the lakefront and the Chicago River, deserves to be marked like "a symphony finale." In addition, he accurately points out, our skyline reference points are forever changing. The Hancock seemed shocking when it was built in the late 1960s. Now people revere it. Perhaps the same would happen with the Fordham Spire. But I, for one, want to see more drawings of what this tower would look like from the pedestrian's point of view.

Despite those concerns, the design has an extraordinary sense of possibility. So much of what has been built in the current residential building boom has been visual junk food -- hulking concrete condos and faux pieces of history. Carley deserves credit for breaking out of that box and asking Calatrava to vault to a higher standard. This could be a great tower, one that meaningfully extends Chicago's innovative skyscraper design traditioninto the 21st Century and explores new possibilities of the residential skyscraper, artistically considered.

Now comes the hard part: Getting it built.

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bkamin@tribune.com