JUST WHEN you thought you were getting over the crick in your neck from looking up at Philly's skyscrapers, along comes a proposal that would be a real test of your flexibility.

There is a proposal afoot to build what would be the tallest building in the Western Hemisphere and the second-tallest building in the world when stacked against existing buildings, at 18th and Arch streets, in Center City. A 2,000-foot-tall condominium building is under construction in Chicago.

The American Commerce Center, at a proposed 1,500 feet, would be 525 feet higher than the Comcast Center, now Philly's tallest building at 975 feet, a block away.

It would surpass the Empire State Building's 1,250 feet.

Phillyskyline.com waxed poetic in its description of what's happening:

"Your Philly skyline is about to change. About to incur a growth spurt. About to shatter any notion of Philadelphian reservedness, about to take A New Day A New Way to a whole other level."

Generating this excitement is the proposal to construct what would be a mix of retail, hotel and office space - and even a movie theater - in an $800 million, 2.2-million-square-foot skyscraper on what is now a parking lot.

Does Philadelphia need another giant office building?

Miller thinks so. He said he believes there is a "void" of newer office stock in Center City that this building would fill and help attract corporate headquarters to the city, according to the Philadelphia Business Journal, which reported the tower story in its Friday print edition.

"I think Philadelphia has a need for a new office building," Miller told the Journal.

"It would be an iconic structure," he said. "This is something that Philadelphia has not seen before."

Nor much of the world. The building's height would be second only to Taipei 101, in Taiwan, at 1,670 feet.

There are broadcast towers that are taller: Canadian National Tower, Toronto, 1,815 feet; Ostankin, Moscow, 1,762; KFVS-TV, near Cape Girardeau, Mo., 1,677; and Oriental Pearl, Shanghai, 1,535. But even with several other taller buildings on the drawing board around the world, the proposed edifice would be part of a small club - and eco-friendly on top of that.

The American Commerce Center would be constructed with a U.S. Green Building Council LEED Gold Certification in mind.

A "green building" is one that increases the efficiency with which it uses resources - energy, water and materials, and which reduces its impact on human health and the environment.

The proposed building will need zoning adjustments, but it apparently has the backing of Mayor Nutter.

Through his press secretary, Doug Oliver, the mayor said he believes the building "would be a spectacular addition to Philadelphia's skyline. Sustainability efforts and building green continue to be hallmarks of this administration and the plans for this particular project are consistent with those goals."

The civic association that covers that neighborhood is watching and waiting.

"Obviously, when you drop something bigger than the World Trade Center into a neighborhood there are bound to be implications," said Rob Stuart, president of the Logan Square Neighborhood Association. "We will identify them and see if they can be mitigated. It is clearly out of scale with a lot of our neighboring buildings."

Stuart said his association was briefed a week ago by the developer and his team and "we are formulating a task force to work on the project."

"There are going to be a lot of discussions. The design might change as a result."

The architect is Kohn Pedersen Fox, which designed the US Airways terminal at Philadelphia International Airport, Huntsman Hall, for the University of Pennsylvania's Wharton School, Mellon Bank Center, Two Logan Square, One Logan Square and the Four Seasons Hotel.

Gene Kohn, one of the partners, is a Philadelphian. He graduated from the University of Pennsylvania in the '50s and once worked for prominent Philadelphia architect Vincent Kling.

The architects, with studios in New York, London and Shanghai, designed buildings - as yet unbuilt - that would be taller than American Commerce Center, in Shanghai and in Hong Kong.

The Shanghai World Financial Center will reach 1,588 feet, and the International Commerce Center in Hong Kong will be 1,608 feet.

Miller, of Walnut Street Capital, said financing is in place for the Philadelphia project but obstacles ahead include the need for zoning adjustments and attracting prospective tenants to occupy the office part of the building.

The Commerce Center will include a 26-story, 473-foot hotel with a garden on top. There would be three to six stories of retail space along Arch Street with a public garden facing the dome of the Arch Street Presbyterian Church.

There would be another garden on the sixth floor, between Arch and Cuthbert streets, overlooking the one below.

The 63-story office tower would be 1,210 feet to the lower portion of the roof, 1,500 feet to the top of a spire.

Miller said some potential tenants of the retail portion of the project include a high-end home store, restaurants, a gourmet food store, a health club and a movie theater with eight screens.

Russell Meddin, Democratic committeeman in the 19th Division of the 8th Ward, said, "It would be a signature building for Philadelphia."

Meddin, who is a board member of the Logan Square Neighborhood Association, said he found the design "fabulous."

While there were mixed feelings in the neighborhood when the Comcast building was proposed, it has been generally accepted, he said.

"This says wonderful things about Philadelphia," Meddin said.

The city has come a long way from the unwritten rule that no building should be higher than the statue of William Penn on City Hall - 548 feet.

There was even a legend that none of the city's sports teams would win a championship if that happened.

The late legendary developer Willard Rouse defied the legend and the old "gentleman's agreement" to keep Philadelphia flat and unexciting when he built the first skyscraper, One Liberty Place, in 1987 at 945 feet. *