Until now, America's most glamorous tech companies have largely been housed in suburban oases, velvet prisons that offer employees endless supplies of vitamin water and protein bars, but require lengthy commutes in company caravans from San Francisco to the cluttered highway strips of Silicon Valley. There's plenty of interaction inside the bubble, but hardly any with the wider world.

With its new 1,121-foot-tall loft building, designed by Britain's Norman Foster, Comcast fashions a rebuttal to all that. Think of the towering waterfall of glass that was unveiled Wednesday as a skyscraper version of the great, light-filled factory lofts of the early 20th century, but wedged into the unpredictable heart of Center City atop the region's densest transit hub. In the six years since Comcast embedded itself in one of the city's more straight-laced corporate towers, it has done a complete 180: Its second high-rise should be a glorious vertical atelier where employees can make a mess while they invent and build stuff.

In short, this is what the future of the growing Comcast campus at 18th and Arch Streets will look like: Suits to the east, hipster engineers in cutoffs and flip-flops to the west.

Yes, Foster & Partners' glass tower will be the tallest building in Philadelphia when it opens in 2017, the eighth tallest in the United States, the tallest building outside New York and Chicago. But its height, surgically enhanced by the presence of a new Four Seasons hotel on the top 12 floors, is hardly the most interesting thing about the $1.2 billion mixed-use tower.

With this project, Comcast stands to reformulate the architectural imagery of the technology industry. An urban icon for the wired world has been long overdue. Foster's design promises to provide it.

There is a certain irony in Foster's involvement. He's the same guy who is designing Apple's sprawling new headquarters on a 170-acre suburban site in Cupertino, Calif., a low-slung, four-story ring that reinforces the status quo. But Foster also has produced plenty of strong urban buildings, including New York City's Hearst Tower. Comcast, he promises, "is a way to bring a new kind of industry back into the heart of the city."

It's not the shape of the Comcast building that is particularly innovative. A stack of gradually narrowing glass boxes, the tower will rise at mid-block from a podium on Arch Street, separated from the mother ship by the domed Arch Street Presbyterian Church at the corner of 18th Street. In form, Comcast's 59-story tower bears a strong resemblance to Richard Roger's design for 3 World Trade Center, which is still unbuilt.

Aesthetic sensibility

What distinguishes the project, now being called the Comcast Innovation and Technology Center, is the organization and aesthetic sensibility of the interior spaces. The tower's simplicity is as potentially radical as Walter Gropius' Fagus factory was in 1913, because it recognizes that urban skyscrapers are not just for paper pushers, but also for collaboration and creativity.

Foster's office has designed Comcast's tower with an unusual off-center elevator core, located at a 19th Street corner. Ordinarily, offices revolve around a central core that helps support the structure.

By pushing the elevator banks to one side, Foster creates a large, column-free space that can be arranged and rearranged for any purpose. Comcast plans to leave these high-ceilinged floors in a relatively raw state - well, raw for a $60 billion company. The floors will be populated by about 3,000 software engineers, product designers, and other freewheeling types who will adapt the space to their tastes.

All Foster buildings like to wear their structure on their skin, and Comcast is no different. Here, Foster exploits the building's unusual organization for decorative purposes by running the elevator banks up the outside of the building. Glass cabs on the north side will be visible as they travel.

The elevator banks also have been split into two sections, with strips of glass marking the space between. Telescoping as they rise, the towers will form a mast that terminates in a bladelike spire that feels almost art deco. On the west side, Foster expresses the structure in a different way, corseting the facade in a stainless steel ziggurat that will stiffen the building, literally helping it stand up.

Eyeball to eyeball

The tower is placed so that it stares, eyeball to eyeball, at Comcast's headquarters, designed by Robert A.M. Stern Architects. You should be able to see right through the Foster building into the Stern tower. But unlike Stern's slick obelisk, Foster's promises to be minutely detailed.

The north and south facades are divided into three-story sections marked off by a rib of stainless steel. The purpose isn't only aesthetic. Comcast sees each three-story module as a discreet neighborhood, each with its own lounge.

All this is very nice for the people who will work there, but what about the rest of us? Tech companies have become almost pathologically private, and their ground floors are increasingly fortified. After taking over a San Francisco office building recently, Google drove out the lone Starbucks.

Liberty Property Trust, Comcast's partner in developing the Foster tower, as it was with the headquarters, vows to make the ground floor welcoming to the public. On 18th Street, a winter garden will function much like the one in the existing Comcast tower, with a small cafe and a more formal restaurant at mid-block on Arch Street.

The real dazzler, Comcast hopes, will be a 35-foot-high, all-glass penthouse at the top of the building, which will serve both as the Four Seasons' lobby and a swank public lounge. On a clear day, you'll be able to see the hotels in Atlantic City.

But given all the effort Comcast is making to shift the imagery of the building from triumphal corporate trophy to creative loft in the sky, it might be nice to include a more edgy public space. Perhaps an auditorium or small theater. Without the opportunity for people to bring ideas in from the outside, all those cool social spaces are just more bubbles.

Update: Liberty Property Trust Chief William P. Hankowsky is a minority owner of Interstate General Media, the company that operates the Inquirer.