In an era of ephemeral architecture, Gen. Montgomery C. Meigs' Pension Building remains the grandest space in Washington. The Italian Renaissance-revisited barn teaches everybody what makes a structure into a landmark, and what makes a landmark work. There could be no more appropriate place to open the National Building Museum.

In its vast space you understand what an insignificant creature is man and how mighty was Meigs -- which was just what the architect, engineer, innovator and egotist intended. You come in off the street, enter through a dim hall, then step cautiously into the magnificent reaches of the Great Hall, a cortile, 316 feet long, 116 feet wide.

Meigs wanted you to feel like a fly under the sky. He built the eight tallest Corinthian columns in the world (75 feet high, 25 feet around); he set the ridgepole 159 feet up; he created the firmament of arched windows (each one a little sun) and the tiers of arcades (their columns marching as to war), embellished with 244 urns with lion-head handles.

The edifice (anything on that scale deserves the grandiose word) should serve its new role as well as it did as the city's major memorial of the Civil War. When he designed it in 1881, Meigs, retired quartermaster general of the Union Army, must have intended the building as an arch of triumph, not a statement but a shout, in the capital of the hard-won re-United States of America.

But Meigs was not only a form giver. He also thought of himself as a practical man, a functionalist.

With such a framework, almost nothing anyone could do to it would minimize its majesty -- short of Jules de Sibour's 1934 plan to robe it in a Greek Revival sheet, and flatten its roof.

But even well-meant "improvements" have not always been helpful. To make the interior look good enough for an inaugural ball last January, the General Services Administration performed what could best be described as quick, superficial face makeup -- decidedly less than a face lift. Some committee -- no single person could have made such a mistake -- tarted up the walls and ceilings with green and blue eye shadow and pinky-beige face-powder colors, then drew "marbleizing" squiggles on the eight great columns, bronzed the arcade columns and the flower urns with what looks like radiator paint and covered up the glorious but badly broken tile floor with a rug the color of cigarette burns. Sculptor Gretta Bader's recent hard work to cast busts for the very high upper niches seems hardly worth the trouble. The fountain is a nice grace note, but the planter around it, while keeping people from falling in the pool, seems obtrusive.

The government's restoration money has come in dribbles, rather like a leak in the roof. So ugly doors and solid transoms still close off the offices Meigs intended to be open, blocking the light that he intended would shine through the office windows into the cortile.

The building is supposed to be fully restored in 1988, Congress and the administration permitting. David Condon, of the restoration architects Keyes, Condon and Florance, says work will begin next spring to let in the sun, while closing the lower roof to rain. Also scheduled is replacement of the existing stupid double-hung windows with the T-shaped windows that God and Meigs intended.

Window air conditioners and garlands of electrical wires will come down then, if all goes well. Condon has some problems -- air-conditioning the cortile is like cooling the whole outdoors. Too much humidity, on the other hand, would make the court a rain forest.

On the first floor a small auditorium and eight galleries are complete, but more than 20 others are not touched. On the second floor only the presidential suite (originally the commissioner's office) was restored quickly for the inaugural ball. It's still empty, though its splendid fireplace is enough furniture if you don't need to sit down or write. Nothing has been done on the other two floors.

Encircling the outside of the building, Casper Buberl's Union Army veterans march eternally in the magnificent frieze, an original and astonishing achievement of architectural decoration.

The building's unstuccoed dark red brick was an advancement for its day, an effort at honesty about basic material and structure. It remains a ruby in an undistinguished setting, adding just the right relief to the eye from the chaste, pale restraint of its neighbors, the General Accounting Office, the Judiciary Square buildings and the Washington Metropolitan Transit Authority.

Functionally, the Building Museum will add a grand bit of fun and information to the neighborhood between Fourth and Fifth and F and G streets. The location is fine -- the Metro entrance is directly across from the F Street museum entrance, and hot and sour soup is but a block or two away in Chinatown. Museum Director Bates Lowry, who like his mentor, the late Joshua Taylor, never trusts a connoisseur who doesn't like to eat, hopes for a restaurant, overlooking the cortile.

The unfortunate necessity remains of making the building pay -- as though its majesty were not reason enough for being. Inevitably, in the manner of impoverished southern gentlewomen after the War of the Northern Aggression, the museum will have to rent out rooms. Why not congenial roomers from allied professions and crafts: architects, contractors, terra-cotta manufacturers, bricklayers and such?

The four opening exhibits provide a taste of what's to come.

"An Architectural Wonder: The U.S. Pension Building" was organized by Isabel Barrett Lowry, head of the documentation center, from more than 13,000 letters, bills, bids and memos collected by Meigs. The photographs record how to build a landmark. If I had my way, Lowry would have four times as much space and money to expand this show. It's a crime that no thick, fully illustrated catalogue on the Pension Building, its amazing construction and its 10 hilarious inaugural balls, has been published on this auspicious occasion.

"Building a National Image: Architectural Drawings for the American Democracy, 1789-1912" is a handsome show (and book) of rare drawings. Its draftsmen (too often anonymous) now can be seen as important artists. The Benjamin Latrobe and William Thornton designs for the Capitol and the White House are familiar, thanks to the Octagon show of a few years back. But who has seen the Montgomery Meigs design for a fan room in the House of Representatives that looks like a Charles Rennie Mackintosh drawing? Or Smithmeyer & Pelz's proposals for the Library of Congress, not to mention Henry Bacon and John Russell Pope's designs to honor Abraham Lincoln?

Other shows in this opening phase are: "The Anatomy of a Bridge: Seven Steps in Constructing the Brooklyn Bridge" and "America's Master Metalsmith: Samuel B. Yellin, 1885-1940."

All, except for the bridge models, are shoehorned into far too pinched a space. The exhibit boards and cases appear big and bulky. False walls obscure the wonderful vault and columns of the rooms, sometimes covering up Meigs' precious natural light. The obligatory audio-visual presentations make it hard to see the real objects. Museums think no one can understand anything except secondhand via television.

The museum shop, a great drawing card for architecture addicts, has been cluttered by the restoration designers with a Post-Modern shrine, an aedicula (nicknamed the ridicular by the staff), with no discernible purpose. Post-Modern posts stand like guards to discourage entrance.

But carping about cosmetic pimples and the need for the building to earn a living doesn't minimize the achievements: The restoration of the Pension Building has begun. Washington has gained a new museum. Shoot off fireworks! Commend Chloethiel Woodard Smith, first to document why the building should be saved; Wolf Von Eckardt, whose "The Building Building" was the major manifesto; Cynthia Field, who mounted the congressional onslaught; and present company: the Lowrys, Richard O. Haase, president of the museum's board of trustees, and congressional supporters. For all these, a loud 19th-century hurrah -- this building won't come tumbling down.