In its new role as the National Building Museum, the Pension Building this week opened a show that takes us back to when carpenters built airplanes, houses were moved by horses, and the eight-hour workday was an issue. Scattered out of sequence in the atrium, "Building America" is an array of photo blow-ups and posters marking the centennial of the United Brotherhood of Carpenters and Joiners of America. While long on text and union patriotism, the exhibit will fascinate anyone who can't pass up a construction site.

A 16th-century drawing of a shipyard from Diderot's encyclopedia matches a 1917 photo of the WWI Emergency Fleet under construction. Another shipyard view reveals that when grease was in short supply workers made do with halved bannanas. The first trade-union seal bears a bridge under construction; a shot of Washington's Metro in the works shows nothing's changed.

From a history of the Brotherhood the exhibit moves on to pay tribute to the dams, skyscrapers, brides, matches and bowling pins for which the trade deserves credit. Most affecting are the photos of those who handled the wood: joingers, millwrights, piledrivers, railway crews, a wicker chairmaker, planers and timbermen. The shift to large-scale and nuclear-age construction is noted, as is the appearance of blacks and women in the ranks. (Today the union is 30 percent female. In 1919 an item on the Brotherhood's program called for "the principle and fact of equal pay for equal work, irrespective of sex.")

What's missing are simple explanations. We see a Massachusetts wheelwright's shop in an 1885 picture. But how they made the wheels remains a trade secret.

The Pension Building is worth a trip anyway: the tiered red-brick Victorian monstrosity at 4th and G Streets NW was built in 1882 to hold the 1,500 civil servants who processed pensions for Civil War vets. A huge water fountain is lost in the vast empty interior; arches upon arches raise the roof some 15 stories high and its martial terracotta frieze runs around the block for 1,200 feet. "BUILDING AMERICA" -At the National Building Museum, 440 G Street NW (Judiciary Square Metro stop), through July 5.