The metal truss bridge, the 19th Century American engineering marvel that was largely responsible for wiping out the covered wood bridge, is being wiped out by "progress," too.

In Virginia, there are now only 513 of these bridges whose spidery spans played such an important role in the industrial and transportation revolutions that turned the United States into an economic giant.

Many of these remaining bridges are in danger as standards of safety and efficiency force their replacement by modern, concrete and steel structures.

But Virginia, aware of the irreplaceable loss of history and beauty wrought by the careless destruction of covered bridges, has developed a program that it hopes can save many of its best iron truss spans. The spans were constructed on the principle of the rigidity of interconnected triangles.

One truss bridge in Botetourt County is already on the National Register of Historic Places, and the State Highway Department is nominating seven others to that list.

Designation would protect them from the fate of one of the state's finest bridges, which crossed the Rappahannock River west of Warrenton. It was cut into small pieces and sold for scrap last year.

The oldest truss bridge in the state, dating from 1878, has been taken out of service and is to be moved to an interstate highway rest area in Montgomery County, Va., for use as a pedestrian crossover. The state hopes that other bridges also can be saved this way.

A small bridge near Goldvein, in Fauquier County, was sold in June by the Highways Department to Mimsco Steel Corp. for $177.77 and the Lorton firm is now advertising it for resale at a price of $3,700. The Highways Department hopes that this technique might be used to save to save other bridges with private money.

The Virginia Highway and Transportation Research Council, which conducted the nation's first statewide survey of surviving truss bridges, found 12 in Prince William County, including an 1882 bridge over the Southern Railroad tracks near Nokesville still in heavy use.

Fairfax County has four left and Loudoun County has six. Fauquier County has six spans and shares with Culpeper County one of the state's largest, a 433-foot, six-span bridge built in 1898 at Kelly's Ford on the Rappahannock River.

The research council, jointly sponsored by the Highways Department and the University of Virginia, also has developed a rating system based on age, builder, technology, design and environment. It has rated 58 of the state's best truss bridges.

New York State has borrowed the rating system and has begun to rank some of its bridges.

The council issued a series of reports on the bridges - including a history of their development - starting with a 16th Century Italian architect who designed a timber truss bridge.

The concept was not exploited until the 1800s, when it came into widespread use in America. The expanding frontier had abundant timber and an unending number of obstacles to cross. The truss bridges could be precut and assembled, saving scarce labor.

Timber worked well and there was little interest in iron. The wooden bridges were covered to protect the trusses from weather, and these covered bridges were strong enough for the wagon loads of the 19th Century.

Then came the steam locomotive.

The sparks from smokestacks quickly demonstrated the covered wooden bridge's vulnerability to fire; in addition, weight loads increased.

Iron began to replace wood, and the 1840s and 1850s were decades of experimentation with new iron bridge designs. Truss systems were patented and standard types developed.

These were reliable and parts could be mass-produced, sized, cut, drilled and riveted in a shop, broken down and reassembled on the site.

Foundries and forges turned into bridge companies, like the Roanoke Bridge Co., the Keystone Bridge Co. and the King Iron and Bridge Co., which were active in Virginia.

These companies bought components from firms such as Bethlehem; Carnegie, Jones and Laughlin, and steel began to replace iron in the spans.

By 1888, J.L. Ringwalt, a transportation expert, could write: "Spans up to 150 feet can be erected by a gang of 20 men in a single day . . . 250 feet, 3 to 4 days . . .(Bridge builders) have gained extensive reputations for the cheapness and reliability of the structures they erect."

The companies boomed through 1880 to 1910, but most had disappreaded by the 1930s because of the Depression, new techniques and disappearing demand.

One-lane bridges were adequate for wagons and a few cars, but not for the flood of trucks and cars that began to dominate transportation in the United States. The truss bridges began to be replaced by wider, cheaper concrete structures.

Newspaper files filled up with prideful accounts of new bridges, great and small, bottlenecks eliminated and travel speeds increased.

Now, with only about 500 truss bridges left in Virginia - many in picturesque and out of the way places - the state is acting to preserve these important but little noticed pieces of its history.