Seeking to capitalize on Millennium Park's soaring popularity, the Art Institute of Chicago on Tuesday will unveil the final design for its soon-to-be-built new wing, including a superlong footbridge that would shoot like a glistening knife over the park's south end and deposit thousands of parkgoers on the building's rooftop.

The stainless steel span, still in conceptual stages but already endorsed by Mayor Richard Daley, is a dramatic new element of the $258 million expansion, which is expected to open in spring 2009 and will provide a concentrated showcase for the museum's scattered collection of modern and contemporary art.

Known as the north wing, the sparkling temple of steel, glass and limestone will be built at the southwest corner of Monroe and Columbus Drives, on a blighted, sunken site that contains the historic but vacant Goodman Theatre. It will be the Art Institute's largest structure since its familiar Beaux-Arts edifice along Michigan Avenue rose in 1893 atop rubble from the Great Chicago Fire.

"What we really have here is a new museum. It's a museum of 20th Century art. It will look like Chicago has this temple of modern art that the world didn't know we have," said John Bryan, the chairman of the museum's board and its chief fundraiser. He still needs to raise more than $100 million for the project, which was delayed by the economic downturn that followed the Sept. 11, 2001, terrorist attacks.

Workers will begin erecting construction barriers on Wednesday, and the old Goodman Theatre building will be demolished in the fall, said James Cuno, the museum's president.

Museum officials view the proposed bridge, designed by the expansion's celebrated architect, Renzo Piano of Italy, as a way to lure crowds from neighboring Millennium Park, which will mark its first anniversary on July 16. According to a study done for the City of Chicago, the 24.5-acre park is expected to draw 3 million visitors this year.

"The whole impetus to break ground now and to develop this bridge was the palpable success of Millennium Park," Cuno said.

Straight as a knife when seen from above, with an estimated length of 800 to 900 feet, the gently sloping bridge is sure to invite comparisons with Millennium Park's other destination span, the snaking, 960-foot-long BP Bridge by Los Angeles architect Frank Gehry. That bridge is as much a place as a passageway, inviting parkgoers to stop and take in views of the park, Lake Michigan and the skyline.

"It's like a lazy river moving around," Piano said by telephone from his office in Genoa, Italy. "In some ways, this is my response to that one."

A winner of the Pritzker Architecture Prize, his field's highest honor, Piano has a credit list of finished museums that includes the high-tech Pompidou Center in Paris (for which he was co-designer), the serene Menil Collection in Houston and the graceful Nasher Sculpture Center in Dallas. His recent success at garnering U.S. museum commissions has spawned a backlash, with critics complaining that he represents a default "safe choice" for risk-averse museum directors.

Long-delayed plans

Yet his bold bridge plan is anything but timid, marking the biggest change in the Art Institute's expansion blueprint, which was first made public in the spring before the Sept. 11 attacks.

The subsequent economic downtown slowed fundraising, forcing the museum to postpone a groundbreaking originally envisioned for early 2003. In addition, the expansion's overall size has shrunk to 264,000 square feet from a planned 290,000.

Still, the north wing will increase the museum's gallery space by roughly one third--adding 65,000 square feet of display space--and the wing retains enough heft to qualify as a stand-alone museum. When completed, it will be more than 100,000 square feet larger than Chicago's Museum of Contemporary Art. The budget will be princely, nearly $1,000 a square foot.

And while other museums have been forced to cancel or suspend ambitious expansions--last week, the Corcoran Gallery of Art in Washington, D.C. put on hold its plans for a flashy, $110 million Gehry-designed addition--the Art Institute has cobbled together enough support from its inner circle of donors to get the privately funded north wing off the ground.

The museum has gifts and pledges of $170 million for the wing. However, it still must raise another $115 million from foundations, corporations and the public to meet its overall $285 million goal, which includes funds for an endowment and to pay for the bridge. Museum officials estimate that the span will cost either the same or less than Gehry's bridge, which cost $14.5 million. And the museum continues to troll for a lead gift of at least $50 million. Whoever gives that much gets their name on the building.

"We'll start at $50 [million]," Cuno joked. "We'll take $75 [million]."

The timing of the museum's fundraising campaign is hardly coincidental. Civic pride should be running high Tuesday night when the billionaire Pritzker family awards this year's Pritzker Architecture Prize at Millennium Park's Gehry-designed Pritzker Pavilion.

The ceremony and the park's approaching first anniversary will enable Bryan, the fundraising wizard behind the $475 million urban pleasure ground, to make the case that Chicagoans should open their wallets to support the city's next grand project--which happens to be right next door.