SPANNING PITTSBURGH

THESE are no spectacular miles of ribbon such as cross San Francisco Bay, or mammoth structures such as join the boroughs of New York. Pittsburgh bridges are local wonders, built with Pittsburgh steel, by home-grown heroes such as Andrew Carnegie and steelmaker Andrew Kloman. Most bridges here are less than 1,000 feet long, clearing the rivers by a scant 60 feet. Many, built into the roll of the land, are secrets from all but their makers. Yet year after year the international bridge conference bypasses Paris and Venice, San Francisco and New York, to meet (as it did in June) in Pittsburgh, whose 1,765 bridges in rocky, river-cut Allegheny County include virtually every type of bridge built.

``Every bridge is a unique structure, kind of like a person,'' says Ken Seibert, a Pittsburgh civil engineer with the Pennsylvania Department of Transportation. ``Every one's an individual.''

Some 200 bridges bind together the 55 square miles of the city, crossing its rivers, valleys, railroads, and canals. Delicate, supple spans of steel are the warp on which the city is woven. To go almost anywhere at all in Pittsburgh, you must cross a bridge.

The city's dense business district is on the point where the Monongahela and Allegheny Rivers meet to form the Ohio. From 16 blocks bristling with skyscrapers, 12 river bridges fan out from the heart of the city to suburbs and Interstates.

At Point State Park, downtown workers lunch by the riversides, and festivals are held in the summer. Here the twin bridges of Fort Pitt and Fort Duquesne arch to opposite shores. These are double-decker, arch-truss bridges, their roadways suspended by cables from pale yellow arches. Fort Pitt, completed in 1958, is named after the 18th-century outpost built between the two rivers. This bridge is now the main gateway into the city, connecting Fort Pitt Tunnels and Interstate 376, and handling about 100,000 vehicles a day.

Behind the twin arches at the Point are three blond suspension bridges slung between their pillars, a black-steel railway bridge cluttered with webs of struts and braces, a sleek new concrete bridge still under construction, and a steel truss bridge that carries the rapid-transit trains.

In the midst of the arches and cables is the extraordinary tangle of steel that is the Smithfield Street Bridge, probably the city's oldest, and certainly its most striking.

It is called a lenticular truss, the rare flexible frame made of 2-inch steel bars bolted together in a shape like a figure 8 on its side. Swooping like a roller coaster across the deck of the Smithfield Street Bridge, the peeling, silver-painted truss has held the bridge since 1883.

When it was built, this was the only industrial-strength bridge to link Pittsburgh with the railroad terminal and the glass and steel factories on the Monongahela's southern shore. Now the historic landmark is the main route to work for thousands of pedestrians who live or park in the lofty Mount Washington suburb and ride the century-old Monongahela Incline down the riverbank. Out over the water, pedestrians can feel the old bridge tremble beneath the wheels of every passing bus, and can glimpse the yellow-brown rushing river below.

Younger bridges also bear the stamp of history. Liberty Bridge, connecting the Liberty Tunnel with Boulevard of the Allies, was named in the triumphant days following World War I. The 1920s were a time of city expansion, when several river bridges were built with local taxpayers' money. During this time the city's art commission gained some say over bridge design. It influenced selection of the graceful suspension bridges at the Sixth, Seventh, and Ninth Street crossings. With nothing to anchor them to on either side of the Allegheny, they became the nation's first self-anchored suspension bridges.

With its 27 miles of waterfront, Pittsburgh was once a major inland port, shipping in raw materials to feed its voracious steel industry. But most steel factories have closed, and commercial traffic has declined. Tugboats still push barges of sand and gravel to concrete factories on the Monongahela. But most boats traveling under the bridges today are recreational. Small boats - 25,000 are registered in the area - skim like waterbugs down the rivers, passing through the light and shadow of bridge after bridge after bridge.

As the steel and coal industries have declined, the rivers have become cleaner. Local fishermen now cast for the bass and walleye that hide in the shade beneath the Glenwood and Birmingham Bridges. They say that at the spillway beneath the Highland Park Bridge there are even trout.

And in Schenley Park, where most roads are closed to traffic, city dwellers wander over footbridges through acres of leafy hills, picnicking beyond the city's intensity.

For most in Pittsburgh, bridges are such an integral part of life that they are barely noticed, unless they are the scene of a traffic jam or are closed for major repairs. Yet few cities in the world depend on them more. Here bridges connect what nature sliced asunder, and man's engineering marvels are as various as the rivers and the hills.