THE NATIONAL BUILDING MUSEUM hosts its grand opening this Saturday, and four inaugural exhibits are open there as well -- five, if you count the building itself.

The museum is better known as the Pension Building, with its Great Hall the size of a football field and eight faux marble columns for goalposts. The first floor has been painted, marbleized and gilded to look much the way it did when the building was completed in 1887. A single-spray fountain erupts in the center of the hall, and 244 heads peer down from niches high above it -- architects, engineers, hardhats, cast from eight busts by sculptor Gretta Bader.

The National Building Museum is a monument to the building arts in America, and apparently overdue. This sentiment was expressed by two retired electrical contractors, who wrote "for our showplace" on their contribution check for $20. (The government makes the Pension Building available free, maintenance included, but the museum raises private money for its doings.)

At the museum this Saturday from noon to 4, experts will demonstrate bricklaying, marbleizing, bust sculpting, metal crafting and, for ambitious do-it-yourselfers, suspension-bridge building. These "crafts" all relate in some way to the building's reconstruction or to the exhibits.

In the Brooklyn Bridge exhibit, you can wander among eight wooden models that represent stages in bridge-building. These 13- foot mockups were part of Brooklyn's 1983 exhibit celebrating the bridge's centennial.

Another permanent exhibit details the history and construction of the Pension Building, designed by General Montgomery C. Meigs, who had to deal with everything from too- shallow foundations to pigeons in the Great Hall.

"America's Master Metalsmith: Samuel B. Yellin" is a third exhibit, one that suggests the possibilities when artisans work with architects and builders. A Philadelphian, Yellin gave us some fine ornate ironwork gates at the National Cathedral, as well as iron stair railings at the Federal Reserve Board. His genius was in transferring organic forms to metal: Rather than a clanging gate of iron bars, a gate to a crypt would have an iron tree growing through it.

In a darkened gallery can be found the most delightful exhibit here. Muted lighting keeps the "Architectural Drawings for the American Democracy" from fading. Here we see the Capitol building inside and out, down to the design for the pedestal for "Freedom," the statue on the dome.

These drawings show federal buildings in macrocosm -- impressive closeups that enlarge the mind about things taken entirely too much for granted, such as skylights, stairs, plasterwork and chandeliers.

And what might have been! An entry for the competition to design the Library of Congress made it look like a cathedral, in a medieval style appropriate to Oxford. Our own Lincoln Memorial could have been a pyramid, or, worse, a Mayan temple complete with smoldering funeral pyre, had not reason prevailed.

If this exhibit is any indication, the National Building Museum will certainly meet its specifications.

NATIONAL BUILDING MUSEUM -- On F Street NW between Fourth and Fifth, across from the Judiciary Square Metro stop (Red line). Hours: Monday through Friday, 10 to 4; Saturday and Sunday, noon to 4.