Twenty years is not a long time in the life of a building, but it is long enough to tell.

Dulles Airport, celebrating its 20th birthday this weekend, remains what it seemed to be when it opened, a triumphant monument of our time, a masterpiece that will be looked to and talked about long after our time is up.

Eero Saarinen, who designed the building and who died at 51 the year before it was completed, preferred to describe the achievement matter-of-factly. "It was a hard-boiled problem and we wanted to solve it in a hard-boiled way," he wrote.

And this is exactly what Saarinen and his superb staff did. Much of the beauty of the place is due to its crystalline rationality. The first commercial airport to be designed with jet aircraft in mind, it still sets a model in most respects for efficiency and convenience.

Of course Saarinen knew it was more than that or at the very least, he guessed. Although he didn't live to actually see it, he described the thrilling effect of the building as it first comes into view round the final bend in the long access road: "a hovering form between earth and sky" on a "beautiful flat plain" in Virginia.

And as construction progressed he ventured to say, "I think this airport is the best thing I have done. I think it is going to be really good. Maybe it will even explain what I believe about architecture."

Much of what Saarinen believed about architecture was imbibed from his father, Eliel Saarinen, himself a significant architect whose greatest work, the Cranbrook Academy in Bloomfield Hills, Mich., also is celebrating an important anniversary this year (its 50th).

"Always think of the next larger thing" is the famous aphorism handed down from father to son: wholeness in architecture depends upon the interdependence of the parts, the widening ripple from little to big. How deeply the lesson took can be gauged in Eero Saarinen's mature achievement at Dulles, where one thing leads to another with a seamlessness that belies the intense concentration involved.

The success of the Dulles design can be measured in the ways it combines imaginative, lucid solutions to functional problems, studied attentiveness to a great site, superb engineering, sculptural clarity, expressive exaggeration and conscientious attention to detail. These are qualities not often found in a single piece of work, and they exert a lasting, almost subliminal impression.

The search began with a thorough reexamination of the helter-skelter ways people, baggage, cargo and planes meet in conventional airports. From this analysis emerged the brilliant innovation of the mobile lounges, conceived of as "a part of the terminal which detaches itself from the building" to bring "the passenger to the plane rather than the plane to the passenger."

Selling this novel idea to the Federal Aviation Adminstration, and then getting the lounges built by Chrysler, was in itself no mean feat--Saarinen called in his old friend Charles Eames to help the pitch along--but in the end its logic proved unassailable. Lumps come easily to throats of anyone watching the slow gait of the big buses carrying loved people to the remote runways, but the system remains the most civilized of them all.

By eliminating the need for attaching interminable pedestrian "fingers" to the terminal building the concept allowed Saarinen to accomplish his great vision of a majestic, free-standing, horizontal temple hovering in a vast field. In lesser hands it could have been a bad idea but unlike Edward Durrell Stone at the Kennedy Center, for instance, Saarinen did not simply dress his colonnaded temple in vaguely 20th-century clothes.

Instead, he remade the temple with 20th-century technology and modernist esthetics. The structural system is in itself still a marvel--a roof of precast concrete slabs attached to steel suspension cables hung from those dramatically angled reinforced concrete piers. And, as it should be in a genuinely modernist enterprise, everything does precisely what it seems to be doing, and then some. The roof obviously droops, and the pier-columns clearly pull against the roof's weight and force, and thus support it.

This engineering feat, expressed with such clarity, doesn't explain the building's magic, but it made the magic possible. As always, the art in the architecture cannot be entirely explained, but one can look for it in the details large and small: the glass walls on all sides that make the building seem to float, the dramatic mass of the piers at ground level (more mass by far than function demanded), their impressive shape and slope, the awesome tilt of the building itself, higher in front than in back, the clean unencumbered sweep of the interior space, the crisp horizontality of the service core . . . and so on.

In fact, on its 20th birthday Dulles is looking better than it has in a long time. Saarinen was nothing if not forward-looking, but he didn't foresee jumbo jet crowds and bomb-toting terrorists, which explains the new 50-foot extension on the rear of the building, sensitively conceived and executed by the Washington office of Hellmuth, Obata and Kassabaum.

Unfortunately the FAA insisted that the new security checkpoints, four in all, be placed between pairs of the concrete piers, rather than behind them. Although this interrupts the strong rhythm of the rear wall, the building is big enough, and good enough, to withstand the unnecessary (and correctable) intrusion. The main misfortune of Dulles is that it remains hostage to a misguided, unbalanced federal policy favoring National Airport.

But then we hear about that constantly. A 20th birthday seems an apposite time to reflect upon the irreplaceable grandness of the achievement. Saarinen's airport is a great place to be, and it stays in the mind.