CHICAGO.-By Michael Webb The first steel-frame skyscraper was erected in Chicago 120 years ago, and no city has a greater concentration of elevated architectural landmarks. Harry Cobb, a founding partner in the New York firm of Pei Cobb Freed & Partners, has been designing high rises for 50 years, but few have given him greater satisfaction than the Hyatt Center. "I've waited all my life to build in Chicago and add my piece to that incomparable urban fabric," he says.

Cobb's elliptical tower stands out from neighboring blocks in its iconic form, clarity of plan, nobility of proportion, refinement of detail, and complex layering of spaces. It is the latest contribution to a legacy of distinguished commercial buildings by such architects as Adler & Sullivan, Burnham & Root, and Mies van der Rohe, as well as the Marina Towers of Bertrand Goldberg, and the John Hancock Tower of Skidmore, Owings & Merrill. What makes this achievement even more remarkable is that Cobb was commissioned to design a speculative office building--not a prestigious headquarters--on a tight budget and a tighter schedule, while meeting the city's tough new energy code.

In downtown Chicago, most buildings shoulder up to the sidewalk, creating high walls along the street grid, in contrast to the set-backs and plazas that are mandated in Manhattan. Developers everywhere favor rectilinear blocks that provide the greatest permitted amount of leasable floor space. Like the grid plan of most American cities it's a formula everyone can live with. Cobb perfected that model in Boston's John Hancock Building, then broke the mold for the multi-faceted Library Tower in Los Angeles, and the elliptical Tour EDF in Paris, which directly inspired the Hyatt Center.

In plan, the 49-story tower resembles a football with sharply notched ends presenting a slender, elegant profile to South Wacker Drive and Franklin Street. The long façade on Monroe Street curves gracefully back to accommodate a public garden that complements the sleek expanse of glass and stainless steel spandrels, softening the sharp lines and hard surfaces. A mighty beech shades benches that encourage commuters to take a refreshing pause on a busy street. The seating does double duty as security barriers, replacing the customary bollards, and transforming the defensive perimeter into an amenity. A six-story "bustle," as the architect calls it, juts from the rear façade, enlarging the floor plates over ground-level parking. At either end, lofty entry vestibules are set into the junction of the bustle and the tower, and vibrant paintings by British artist Keith Tyson beckon visitors from afar. Like the garden, they are gifts to the city, enhancing the public presence of this private venture, which can be glimpsed from Grant Park at the east end of Monroe.

Architecture adds value to every kind of building, from a modest row house to a mile-long airport terminal, and good architects turn every problem to advantage. When the Pritzker family decided to develop the site they envisioned a headquarters for the Global Hyatt that would be shared with other tenants. Following 9/11, Harry Cobb was invited to design a no-frills building. The immediate need was to attract an additional anchor tenant, which would be the law firm of Mayer, Brown, Rowe & Maw, and to provide space for Hyatt when the lease on their existing offices expired in December 2004. Pritzker Realty Group and their co-developer, Higgins Developer Partners, gave the architect just four weeks to develop a schematic design for a building that would maximize the site and provide column-free, 33,000-square-foot floors, spanning 45 feet from core to exterior wall.

Cobb likes to quote a comment by one of his favorite writers, the philosopher Isaiah Berlin, who declared "my brain is like a taxi-it responds when hailed." As the architect explains, "I don't have projects waiting to go; as a modernist I respond in a fresh way to each program and site. I had no interest in doing another cereal box, and it took about 20 minutes to realize the advantages of curved faces on a narrow site with these monsters on every side." The curves would play off the sharp angles, open up oblique views of the city from every floor, and provide a distinctive signature. However, Cobb knew that his clients would need to be convinced of the superior merits of an elliptical building. So he and his team raced to sketch models of that and a conventional scheme, and they presented the contrasting designs side by side in their New York offices.

"For Mayer, Brown it was love at first sight-but we would never have got this building without the box," says the architect. Goldman Sachs, the financial services company, were willing to relocate, provided they could have even larger floors and higher security than the law firm required. "This is where architecture can be fun," says Cobb, who seized the opportunity to add the bustle and six extra floors to the tower, dramatically improving its proportions, and pushing up the total floor area to about two 1.7 million square feet. Higgins explains that the garden and grassy roofs brought bonuses from the planning department, allowing them to build higher without having to seek waivers. He describes the project as "three build-to-suits, and a speculative building in one package." IBM became the fourth anchor in a building that is now almost fully leased.

The Hyatt Center was conceived in the months following 9/11, and that catastrophe shaped its design in obvious and subtle ways. As Cobb notes, "security demands allow us to make a case for more elaborate entries-- which no developer in his right mind would have agreed to before. The first modernists rejected the grand staircases and deep entries of classical buildings. You just walked through an opening in a glass wall, and we would have put ours at the center of the long façade. Returning to a more elaborate, processional route allows you to integrate barriers with the architecture and achieve a more satisfying sense of arrival." To provide convenient access to a building that is a block long and approached from all directions, there had to be entries at either end. These created a formal separation between public and private, and preserved the integrity of the long façade. Tapered steel canopies with sharp edges shelter and define the entries, balancing the horizontal and vertical thrusts.

The glazed reception vestibules are 50 feet high-the tallest spaces in the building. Glass ceilings pull in natural light and allow you to look up one side of the tower. Cobb put tiny reproductions of a Wassily Kandinsky painting into his model as markers for two boldly colored artworks--something he regarded as an indispensable complement to the architecture. Penny Pritzker brought in art consultant Thea Westreich to work with the architect and developers. They considered 25 artists before selecting a proposal from Keith Tyson, who was then little known in the United States, but has since won the prestigious the Turner Prize and is now represented by the Pace-Wildenstein Gallery.

The two 40 x 10-foot painted aluminum panels, located on the inner walls of the vestibules, were conceived as an abstract expression of the dynamic spirit of the building. Tyson picked colors-such as the yellow background-that would stand out, and would emerge or recede as the light changed. Black frames give the panels a sense of transparency. The composition of overlapping spheres-large at the top and finely detailed at the bottom-is an artistic abstraction of a molecule. The design was generated by Maya, a 3D computer modeling program that is used in the film industry, and was hand-painted by 15 people over eight months.