It is a story of two huge, odd, splendid things inside another: Frank Gehry's soaring, metal-clad constructions squeezed betwixt the columns of Gen. Montgomery C. Meigs' fantastical shed, designed more than a century ago for armed service pensioners and now the National Building Museum.

The primary purpose of the new structures is to celebrate sheet metal and the achievements of those who work with it on the occasion of the centennial this month of the Sheet Metal Workers' International Association. These buildings-within-a-building do accomplish this symbolic aim, triumphantly. They're also useful, sheltering a commendable exhibition, "Sheet Metal Craftsmanship: Progress in Building."

And, not incidentally, even though they aren't quite finished they are demonstrably a thrilling fanfare for the art of architecture, for the sheer excitement of conceiving forms in space and the great pleasure this creative act can elicit.

This can be said even though viewing conditions at present are far from ideal. Volunteer fabricators and union sheet metal workers from across the country (and Canada) made a mighty try to complete the task for the opening last Monday, laboring long shifts and seven-day weeks, but the complexity of the structures and the tight six-week schedule proved too formidable. Access to Gehry's work and the exhibition will remain limited until construction is done in late February.

Plus, the Building Museum itself is a mess. Offices and galleries on the perimeter of the interior court are undergoing a long-overdue but excessively drawn out conversion to contemporary electrical, heating, ventilating and air conditioning systems. As a result the courtyard itself is noisy, dusty and, worst of all, cordoned off. Visitors require special permission to circumnavigate Gehry's work or to get long views of it, which are necessities if it is to be experienced in its full, quirky grandeur.

Gehry was the perfect person for this job. His name automatically comes to mind whenever the subject is ordinary materials. In the course of a decade or so, working in more or less quixotic but quintessentially American landscapes of contemporary Los Angeles, he took possession of happenstance, "nonarchitectural" materials such as chain link fencing and corrugated or galvanized metal sheets, making them integral parts of a sophisticated, ironic, affectionately poetic approach to architecture.

Thus it comes as no surprise that the big, eccentric planes of his structures at the Building Museum, simply clad in four different metals -- copper, polished brass, galvanized iron and a dull gray steel coated with zinc (called terne plate) -- have an effect akin to a prolonged, heartfelt ovation in praise of materials often hidden behind ceilings and walls.

But, of course, it is a surprise -- a big and wonderful one. Great art cannot so easily be predicted. One emerges from the narrow F Street entrance hall into Meigs' stupendous interior space and, off to the west, behind those world record-sized Corinthian columns -- wham! -- there is Gehry's imposing, improbable concoction, two spiraling, muscular tours de force of art and craft, an ensemble of such physical energy that it seems to expand and contract as one attempts to take its measure.

Gehry took on Meigs' space directly, challenged it. His initial sketches, showing soaring towers made of irregular geometric shapes, demonstrate an awareness that significant height would be necessary if his structure was not to seem a little fish awash in an overlarge sea. More surprising, because in an age of steel construction one can comfortably conceive of dramatic, airy, Eiffel Tower verticals, was his decision that weighty, enclosed forms were equally important.

Much of the strength of the (almost) final result is due to the edgy equipoise between height and mass. When seen from the east the taller of the two structures (rising 65 feet) is remindful, in a strong if general way, of a cathedral with a central spire and three apses -- conical forms that house much of the exhibit and that visually push in, with a buttresslike effect, on the tower. But its forces, its internal energies, also push outward. So, too, do those of its companion piece -- at one point the distance separating new and old walls is but a couple of feet. These massive, hybrid buildings seem somehow to have been squeezed into this spacious environment. They elevate the spirit, like a great church would, and they're also very like living creatures conjoined in a whimsical, somber, never-ending dance.

Gehry's structures are all angles, in plan and in three dimensions -- walls don't quite line up with Meigs' rectilinear frame; there are no more than two or three 90-degree corners in the entire complex; all openings are off center and eccentrically shaped, and very few of the surfaces extend for any length at all without colliding with some sharp, protruding plane or a teetering, trapezoidal solid that seems to have been hurled from high above and become stuck, like a meteor. The result is a dynamic clash between the old structure and the new ones, but the friction is creative -- each gains something from the juxtaposition.

The idea of making two buildings was a natural outgrowth of curator David Chase's requirements: He asked for an introductory chamber followed by a sequence of spaces in which to display artifacts tracing the history of sheet metal applications in North America, culminating in an area for the display of tools and for demonstrations of the metalworkers' craft. As he has done before, Gehry divided the functions in a way that allowed him to create separate buildings with an appealing, villagelike space in between. The whole is a work of sculptural architecture, or architectural sculpture, that is almost as interesting to experience from the inside as from the outside.

The exhibition focuses upon materials and a craft that, in recent decades, have been almost totally obscured from public consciousness. By far the largest share of the sheet metal market since World War II has been in duct work for ever more complicated air circulation systems, so we tend to forget the glory days. A few of our noblest historical or architectural treasures -- the Boston State House dome, the Statue of Liberty, the spire of the Chrysler Building, the Gateway Arch in St. Louis -- were fashioned from sheets of metal, as well as the roofs, ceilings, skylights, decorative accouterments and entire facades of hundreds of thousands of apple-pie American buildings.

Economically and lucidly, the exhibition brings this story back into view. It also suggests that the sheet metal industry, by dint of a tremendous nationwide training program established and administered jointly by the union and the contractors (specifically, the Sheet Metal and Air Conditioning Contractors National Association), has proven enormously resourceful in adjusting to new demands, new technologies and new markets. It's a model other building trades would do well to follow. Skilled union sheet metal workers seldom go unemployed.

This is a good show that will be here through August and, one hopes, longer than that. A condition of the museum's occupancy of Meigs' structure is that the great hall be cleared every four years for an inaugural ball. It is perhaps not too much to expect that the inaugural committees of both political parties will be persuaded to give up a third of the space in favor of a crowning work of art.

If not, it'll be a great shame. The Building Museum, established in 1980, open to the public since 1985, has desperately needed an attraction of magnitude to call attention to itself. Even the unforgettable dimension of its interior space has been a curious disadvantage: It's a museum that until now has seemed decisively empty. Gehry's work complements Meigs' and conclusively establishes the new institutional role.

The architect should take a bow, and so should the contractors who fabricated (and donated) the materials and the workers who so skillfully put Gehry's angles together. The show was conceived by founding director Bates Lowry (replaced by Robert W. Duemling last October) together with representatives of the union. The choice of Gehry was curator Chase's inspiration; he also selected the many interesting objects and designed the installation (with consultant Val E. Lewton). The Building Museum is located at Judiciary Square, on F Street between Fourth and Fifth streets NW. Hours are 10 a.m. to 4 p.m. weekdays, noon to 4 p.m. weekends.