After two rejuvenations going back nearly 40 years, this great, once-grimy river city has a building, indeed, a cluster of buildings designed by the same architect at the same time, that somehow encapsulates its promise and its achievement, its raw energy and its enormous wealth.

This is PPG Place, the brightest star on the new Pittsburgh horizon, the icy flower of the city's self-proclaimed Renaissance II: a work of the architect's art that can take your breath away even as you smile. The designer is Philip Johnson, of Johnson/Burgee Architects of New York.

PPG Place is not the only massive new downtown development, but in esthetic and urban terms it is by far the best. Its centerpiece is PPG tower, a silver cube of mirror glass facets rising 42 stories (not all that high, as local buildings go) to a theatrical pinnacled top. The tower is the exclamation point for a large, complex development in the heart of the downtown district. When completed in mid-1984 it will consist of six buildings of differing sizes and heights, plus a glass-enclosed "winter garden" and a large, wholly new public square. (The tower and two other buildings will open this fall.)

It is too early to tell in certain key details exactly how the completed project will work and look. One wonders about glare, about formal issues such as how the buildings meet the ground, about the cumulative effect of all those pinnacles surrounding the new square. But the overall significance and excellence of the achievement can hardly be overstated. PPG Place is imaginative urban design on a grand scale, and it is architecture of exhilarating quality.

Not that the first Pittsburgh renaissance lacked significant accomplishments. For one thing, it transformed an industrial juggernaut that had nearly buried itself in atmospheric grit into a city with clean (well, relatively clean) air. For another, by dint of an unusual partnership between corporate, Republican Pittsburgh (notably the awesomely rich Richard King Mellon) and a powerful Democratic mayor (the almost legendary David Lawrence), it began the dynamic process of rebuilding downtown Pittsburgh, as Lawrence fondly stated, with "the sleek new forms of the future."

The thing is, none of this vast effort really redeemed Pittsburgh's reputation as the national capital of grime. Neither, yet, has Renaissance II, so named by Richard Caliguiri, another popular Democratic mayor, just now entering his second full term.

One reason is that life is by no means easy in the Pittsburgh region--department stores just reported their worst Christmastime in recent years, steel mills are shutting down, the unemployment rate is 14 percent. The news is bad, but the building goes on as the corporations do a repeat performance, pouring money into the still-golden Golden Triangle downtown. As is said in proclamations on the sides of trash receptacles, Pittsburgh is "still growing strong."

Cooperation between private capital and the city government for downtown renewal is nothing new in Pittsburgh, but at PPG Place the effort reached a new level of efficacy. The initiative was taken by PPG Industries Inc., a huge diversified corporation ($3.4 billion in sales in 1981) that wanted to consolidate and expand its headquarters in Pittsburgh. (PPG Industries developed from the Pittsburgh Plate Glass Co.) The company asked the city to use its urban renewal powers to help assemble the land downtown. The city agreed, setting certain conditions concerning placement, bulk, height and use of the buildings and surrounding spaces.

There were problems, of course, such as a rental consultant's notion of floor plans for the office tower that would have negated the city's desire for sun to fall on open spaces, but the problems tended to vanish once the company had chosen its architect. As Jonathan Barnett, an urban design consultant to the city, observed, "Philip Johnson has the authority, and the temperament, to put the rental consultant's optimal floor plan straight into the wastebasket, the same place that he says he put the city's urban design and planning criteria."

Johnson, now 76, is one of the century's preeminent architects and among its more preemptive personalities. Whether he did it with or without unwanted hints from the outside, his design met or exceeded most of the city's criteria. In urbanistic terms PPG Place rivals, and may supersede, his own IDS Center in Minneapolis, one of the more successful center-city mixed-use projects of the last two decades, and the new center has been aptly compared to Rockefeller Center.

PPG Place has a tremendous reach. It encompasses the equivalent of six city blocks. It faces upon three important public open spaces: Gateway Center, the modernist mix of buildings and formal parkland that was the pride of Renaissance I; Market Square, an intimate place bordered on three sides by low commercial buildings recently clustered into a historic district and now mostly converted into fern bars; and PPG Plaza, Johnson's wide-open paved square surrounded by his arcaded glass buildings with their picturesque pinnacles.

The mix of uses (ground floor retail with offices above), sizes (buildings vary from four to 42 stories) and spaces (a variety of low and high interior areas including the glass-enclosed winter garden with its towering gables) is contrasted with the uniformity of material (PPG's own Solarban reflective units) and style (a movieland sort of Gothic in gleaming contemporary dress). Pittsburghers already are arguing over the complex. To some it looks funny-ha-ha, to others funny-strange, and to others it is the last word in shining elegance.

It could be that all are right. The combination of wit and utter seriousness, of ornamental silhouette and International Style sleekness, of local relevance and world-class panache, of ice-palace corn and contemplative rigor, is perhaps the secret of PPG's beauty. (In this unlikely combination of qualities it is not unlike Robert Venturi's original design for Western Plaza in Washington.)

Johnson clearly grasped the importance of the commission; he created an instant image for a downtown becoming too crowded with vertical boxes (better than K Street or Rosslyn but, like them, pretty much look-alike despite individual differences), and a striking symbol for the changes the city is going through. This is the highest service an architect and a client can provide for a place: Pittsburgh can consider itself lucky for this, and luckier still that PPG is one of many signs that the city is learning how best to profit--in the widest possible sense of the word--from its plentiful advantages.

The city's most impressive natural asset is its magnificent setting in a low plateau between the Allegheny and the Monongahela rivers as they come together to form the mighty Ohio. The site prompted an 18th-century lawyer, perhaps history's first true-blue Pittsburgh booster, to proclaim, "There is no more delightful spot under the heaven . . ." What man has done to this setting in 200 years has not always been delightful, of course.

Recently, for instance, the riverfronts, their commercial usefulness over, were given over to the automobile. Intrusive, high apartment buildings have been allowed to mar the great tree-lined ridges, especially the stupendous long run of Mount Washington (Coal Hill in the old days), south of the city. Old buildings, particularly low commercial and residential structures, have been gobbled up by Progress at an alarming rate.

But the first renaissance did bring many important improvements, such as the splendid (though isolated) Point Park at the intersection of the rivers, with Three Rivers Stadium not much more than a stone's throw (and a bridge) away. Renaissance II has brought several more subtle, positive changes. Planners have laid out a parade of public open spaces between the two riversides of the downtown area. This linear sequence of spaces (including PPG Place) clearly will make the downtown area a more inviting place to work, walk, shop, eat and live in, and it also shows an increased awareness of the city's splendid vistas. Station Square, a cluster of trendy restaurants, shops and offices in a delightfully reconditioned railroad terminal and yard, demonstrates how the city can expand across the rivers (despite the architecturally lamentable addition of an ugly new Sheraton hotel).

And there are numerous studies--to recapture the waterfronts for pedestrians, to "downzone" in the few remaining low-rise historic areas, to improve the city's streets once subway construction is complete. The city government simply needs the courage, some added power, and the support of the corporations to push ahead with its best-laid plans.

To a visitor PPG Place--a transformative gift of art--is the most extraordinary signal that Pittsburgh is serious about putting to rest its undeserved repute of dismal, nondescript backwater. Pittsburghers have another idea. Like many natives, Fred Swiss, senior urban designer for the city's planning department, believes that the 1979 triumphs of the Steelers and the Pirates (in the Super Bowl and World Series, respectively) were the crucial, consciousness-raising events.

"We're no longer a national joke," he says, seriously. It is without question a combination to be reckoned with: athletics and architecture, raw power and street corner smarts linked to urbane wit and urban sophistication.