The Williamsburg Bridge was built in 1903 for bicycles and horse-drawn buggies, but it has had to carry a much heavier load for most of its 85 years. Now, one of America's busiest commuter spans is closed indefinitely because it's falling apart.

The culprits are decades of rust and the daily rumblings of more than 100,000 cars and 420 subway trains that ferry commuters over the East River between the Williamsburg section of Brooklyn and the Lower East Side of Manhattan.They did, at least, until Tuesday, when the city Transportation Department closed the bridge indefinitely to all traffic. All but four of the Williamsburg's eight traffic lanes had already been closed this week and service suspended on the two subway lines that cross the bridge.

"It was decided it would serve the interests of safety to close the bridge until a further evaluation is made of the material submitted to us," said Transportation Department spokesman Victor Ross after a meeting of city and state engineers and officials.

The city acted after inspectors discovered such extensive corrosion that they feared the bridge might collapse. A team of more than 60 inspectors is being deployed to determine the extent of the problem.

"We're finding that, yes, there is corrosion, yes, the bridge is worse than what we imagined," said Chuck Carlson, deputy commissioner of the state Department of Transportation.

What has happened to the Williamsburg is hardly unique. The Federal Highway Administration keeps track of 271,300 bridges, and of those, 36,000 have some kind of structural deficiency and 41,000 are considered functionally obsolete.

The government estimates it would cost more than $50 billion to bring all the nation's bridges up to standard.

"This is a real crisis in the infrastructure situation in this country that people aren't focusing on," said William D. Toohey Jr., a spokesman for the American Road and Transportation Builders Association. "It's a national crisis, and in New York state it's a lot worse than in other places."

In New York City, decay has forced the closing of two lanes of traffic on each of the two other major bridges across the East River, the Manhattan and the Queensboro.

But the Williamsburg "is certainly one of the worst bridges that we have," said Foster J. Beach III, regional director of the state Department of Transportation. "For 80 years, the bridge really hasn't been maintained."

Problems were built into the bridge. Most suspension bridges at the time were built with galvanized steel cables; the Williamsburg builders chose ungalvanized steel to cut costs.

The cables were coated with oil to protect them from corrosion, but the protection didn't last. And just at the time when the corrosion was beginning to cause serious damage, the city became engulfed in the fiscal crisis of the 1970s. Bridge maintenance and inspections went by the wayside.

Some steel strands within the cables already have snapped.

The Williamsburg has always been the Rodney Dangerfield of New York City bridges. It lacks the elegance of the George Washington Bridge, the romance of the Brooklyn Bridge and the impressive span of the Verrazano-Narrows Bridge.

Stout and practical, the Williamsburg "received a great deal of criticism for its lack of aesthetic consideration" when it was finished, according to engineer Nils D. Olsson, who wrote about the bridge in the Municipal Engineers Journal.

At the time, the Williamsburg, at 1,600 feet, was the longest suspension bridge in the world.

The city now must decide whether to repair the Williamsburg or replace it. Just last month, engineering firms from around the world presented 25 proposals for replacements. This summer, the city is to decide.

The cost of replacing the bridge has been estimated at $200 million to $600 million. The cost of renovating was estimated at $250 million- but that was before the latest damage was found.

In the meantime, said Samuel Schwartz, the city transportation department's chief engineer, there has been yet another unexpected development. "We're finding that thousands of people have returned to walking across the bridge," he said.

Ross said he did not know if pedestrians would be barred from the bridge.