The most notable fact about a new proposal to build a record-breaking 1,500-foot skyscraper at 18th and Arch Streets isn't the building's stunning height. It's that its height isn't an issue for Philadelphians.

Two decades after this city underwent a wrenching public soul-searching over the prospect that the 945-foot-tall Liberty Place would overshadow the City Hall tower, the first question many are asking about the proposed American Commerce Center is: "What are the odds of its getting built?"

Slim but better than zero, city real estate handicappers say. Yet even if this statuesque tower never takes its place alongside the skyline's current pinnacle, the 975-foot Comcast Tower, it's a good bet Center City will see skyscrapers of the same immense scale piercing the clouds in the not-distant future.

One Liberty Place held the title of Philadelphia's tallest building for 20 years, until it was overtaken last year. Its successor is unlikely to wear the mantle for long, predicted John Gattuso, the Liberty Property Trust executive who built Comcast's glass obelisk.

"There's a high probability we'll see super-tall towers in the next round of building," he said. "When will we get to the height of the one that's proposed? Who knows? But it's wonderful that Philadelphians are thinking in those terms."

In general, it seems that tall buildings are getting taller. During the recent condo boom, 400-foot towers became commonplace in many cities, including Philadelphia. Now, super-tall towers, as their champions call them, are going through the stratosphere.

New York is building the 1,776-foot Freedom Tower at Ground Zero to replace the lost World Trade Center (1,368 feet). It soon will be bested by Santiago Calatrava's swirling 2,000-foot Chicago Spire. Both buildings, however, will have to look up to Burj Dubai, which will soar a half-mile above the earth at 2,300 feet after its completion in the United Arab Emirates next year.

Heavyweights of the super-tall club are mainly rising in go-go financial centers such as Dubai, Shanghai and Hong Kong, eager to present a visible statement of their ambitions. But the architect A. Eugene Kohn, who has designed two of the world's highest buildings and is overseeing the American Commerce Center plans, contends that any city that expects to remain competitive in the energy-conscious 21st century will have to embrace super-tall buildings.

"When these towers are well planned and served by transit, they're incredibly energy-efficient," said Carol Willis, director of New York's Skyscraper Museum. They make cities more livable by providing the critical mass to justify amenities, she believes.

Walnut Street Capital, the investment group behind the American Commerce Center, clearly thought the lot at 18th Street met the criteria for a super-tall building when it paid $30 million for the half-block property last year. Though the site does not connect to the Suburban Station Concourse, architects are looking at ways to extend that underground passage, which now terminates under the Comcast building. The site is accessible to SEPTA, Amtrak and the airport.

Like most super-tall towers, American Commerce would offer more than one use. The complex would include a hollowed trapezoid on the 18th Street side that would rise 477 feet and include a 33-story hotel, three levels of shops, movie theaters, and a conference center, as well as landscaped plazas served by cafes and restaurants. Another retail level is designated for a big-box store such as Crate & Barrel, and there is room in the underground level for a supermarket.

Yet despite the center's boast of soaring 1,500 feet, its office tower would top out at just 1,200 feet. A 300-foot spire would poke out from its asymmetrically sliced crown - like a toothpick in a martini glass. That crown, incidentally, is a virtual clone of the original design that Daniel Libeskind submitted for Freedom Tower in 2003.

The complex probably would be faced in glass, but the architects are a long way from completing the details. While northwest corners would be angled, the Center City side appears dully squared off.

One odd thing about the 1,200-foot tower, which would be on the 19th Street side of the site, is that it would contain just 62 office floors above the eight-story base of amenities. That would give American Commerce about 1.3 million square feet of office space, roughly the same as the Comcast building, which has 57 floors and a similar footprint. American Commerce's total would be 2.2 million square feet.

Most of today's super-tall towers, Willis noted, are residential buildings. They need fewer elevators than offices do, and they don't have to waste precious floor space on bulky service cores.

For many Philadelphians, it's hard to imagine a big project of either type getting off the ground with a recession looming. Yet this market doesn't follow a typical pattern.

Other than the Comcast building and the petite Cira Centre, there has been virtually no office construction here since the 1991 real estate crash. The city's office buildings are as technologically out of date as a five-year-old laptop and have trouble satisfying companies that want fast Internet and high energy efficiency.

So, although the real estate market has stalled, American Commerce's investors believe there is a pent-up demand here for new high-tech offices. The strategy, said attorney Peter Kelsen, is to take advantage of low office rents to lure out-of-town companies that are eager to cut costs. Philadelphia's rents are half of New York's.

If American Commerce can find a starter tenant to take 30 percent of its floor space, the project could get a green light. Because it is backed by a pension fund, Walnut Street Capital won't need to scramble for bank financing.

But it will be competing with Liberty Property Trust and Brandywine Realty, which also plan new office buildings. Unlike Walnut Capital, those companies have proved they can build a tower.

Contact architecture critic Inga Saffron at 215-854-2213 or isaffron@phillynews.com.