THE 151-foot midriff section of Third Avenue Bridge swings open at a 90-degree angle, allowing oversize barges plying the Harlem River to pass beneath it. Dubbed ``2-40069'' by inspectors, it juts into a New York City skyline cut by the humped double arches of the Triborough Bridge, the steel latticework of the Willis and Madison Avenue bridges, and the blue towers of the Metro North railroad bridge. On the third day of a two-week inspection, Richard Nyman stands in a cherry picker illuminated by three generator-powered lamps and pounds away with a chip hammer at an 18-inch crack scarring the underside of the bridge. Mr. Nyman, leader of a three-man inspection team, is carving out bits of encrusted rust to measure the extent of corrosion in the steel stringers that support part of the superstructure.
Using an ultrasonic testing device, he discovers that the decay warrants a ``flag,'' indicating the imminent failure of a major structural component. After photographing the trouble spot, he adds another detailed entry to a growing 49-page inspection report.
Bridge inspectors work in anonymity, among the shadows cast by abutments and girders. But since cracks in a floor beam forced last month's closing of the Williamsburg Bridge, diverting 240,000 commuters who depend on it for daily passage between Brooklyn and Manhattan, the city's inspectors have been pushed into the public spotlight.
``When a patient dies, they bury him,'' says Salomon Bokman. ``But when a bridge dies, they bury the inspector.''
Mr. Bokman and his bridge inspection teams from the consulting engineering firm Hardesty & Hanover are on the front lines of a battle to chart the condition of New York City's battered bridges.
Transportation officials say the closing of the Williamsburg Bridge illustrates dramatically the ills of New York's bridge system. They warn that years of neglect have left nearly half of the city's 2,098 bridges in need of major reconstruction or rehabilitation.
New York City's bridges span rivers, bays, canals, rail lines, and roadways to form a metropolis. The city's transportation department estimates that more than 160 million trips are made daily across New York's bridges. Yet 371 city-owned bridges are rated structurally deficient, and 32 bridges are completely or partly closed, according to the transportation department's 1987 condition report on bridges and tunnels.
Engineers who specialize in the repair and rehabilitation of bridges generally agree that age, a harsh climate, heavy traffic, and years of substandard maintenance have crippled New York's bridge system.
``Maintenance is the poor stepchild to bridge design,'' says John Alskog, chief of the Bridge Management Branch of the Federal Highway Administration (FHWA). ``It's much more attractive politically to open a brand new bridge than it is to dedicate a new [bridge] paint job.''
Ross Sandler, commissioner of the city transportation department, acknowledges that the city's bridges receive little preventive maintenance, such as scrubbing corrosive salt off roadways and painting steel members on a regular basis. Most maintenance consists of emergency repair work, a kind of knee-jerk reaction that Mr. Sandler describes as a ``formula for constant deterioration.''
Staff reductions from 1938 to the present have sent the city's 160-member bridge maintenance team scrambling to repair the backlog of neglected bridges. The Brooklyn Bridge, for example, once had 200 full-time bridge maintenance workers assigned to it exclusively; now it has the equivalent of fewer than five workers (including part-timers). Sandler is seeking a maintenance budget increase from $8 million annually to $40 million.
``In 1963, when jurisdiction of the city's bridges was transferred to the [city's] Department of Highways, the voice for bridge maintenance was lost in government bureaucracy,'' says Sandler. ``Bridges are an invisible public resource with no natural constituency.''
``People are more concerned with potholes than a defective bridge,'' adds Bokman. ``Unless of course the bridge fails.''
Bokman, a consulting engineer, is supervising a piece-by-piece inspection of the Third Avenue Bridge, one of 13 bridges that join the island of Manhattan with the Bronx. Completed in 1899, it is the sixth-oldest waterway bridge in New York.
Surrounded by piles of broken bridge machinery stored beneath the Third Avenue Bridge, Bokman explains that water has seeped into the bearings and corroded the deck support beams and substructure. He notes that salt - sprayed up from the Harlem River and spread on the roadway to melt snow - has also eaten into structural steel members. Though the bridge was painted recently, the decay was either ignored or undetected.
``All the paint did was hide the defects,'' he says. ``It didn't protect anything.''
Bokman predicts the bridge will be given an overall structural integrity rating of 3 on the scale of 1 to 7, with 1 meaning ``potentially hazardous,'' and 7 meaning ``new condition.'' A 3 means that the bridge, which serves more than 128,000 vehicles daily, has experienced ``serious deterioration.''
Forty-eight people died in the 1967 collapse of the Silver Bridge into the Ohio River. The fatalities alerted the nation to the condition of its bridges, and set in motion the creation of a ``national bridge inspection standard.'' In the mid-1970s the federal government mandated that each municipality use this standard to inspect all its bridges on a two-year cycle.
The inspectors combing the Third Avenue Bridge and the 19,501 other bridges scattered across New York State bring similar backgrounds to their task. All have a minimum of a civil engineering degree and three years of bridge-related experience. Many are professional engineers who worked in bridge design or maintenance before becoming bridge inspectors.
To attain their present jobs, they must complete a one-week training course offered by the New York State Transportation Department. The starting salary for a state-employed bridge inspector is $29,256.
According to John Mladinov, executive deputy commissioner of the state transportation department, 26 employee teams and 35 consulting teams are each responsible for inspecting between 100 to 200 bridges annually. In New York, the state is charged with inspecting city-owned bridges, while the city maintains them.
The Road Information Program (TRIP), a nonprofit organization that evaluates highway transportation issues, says the national bridge inspection program is generally effective, despite its estimates that 150 bridges buckle or collapse each year. TRIP says 4,125 bridges nationwide are closed to traffic.
Some New York City bridges, such as the North Channel Bridge connecting Howard Beach and the Rockaways in Queens, are so badly deteriorated that they are beyond saving. It will cost $70 million to replace the North Channel Bridge, according to the city's transportation department. The Williamsburg Bridge Technical Advisory Committee is due to report by July 1 whether that bridge should be salvaged at a cost of $250 million, or scrapped and replaced at a considerably higher price.
Flaking concrete, holes in the roadway, and heavily corroded steel stringers have forced the closing of two lanes on the Queensboro Bridge. This bridge, which carries the most traffic of the four major East River crossings, has the worst structural problems of any New York bridge, according to Mr. Mladinov of the state's transportation department.
Just as other New York crossings are in worse shape than the Third Avenue Bridge, other states lead New York in the number of deficient bridges more than 20 feet in length, according to the FHWA. Texas tops the nation, with 15,450 deficient bridges, while New York ranks sixth, with 11,726.
Yet according to John Alskog of the FHWA, the eyes of the nation's engineering community remain riveted on New York's engineers as they rush to perform triage on the city's bridges. Mr. Alskog maintains that age has ravaged the bridges. According to TRIP, 38 percent of the bridges have surpassed an average life expectancy of 50 years.
Noting that the 1950s and '60s were years of new highway bridge construction, Alskog concludes that in the coming decades, other states will experience the troubles besetting New York.
``Because the bridge population is not evenly distributed with regard to age, we're concerned that during the next 20 years we will encounter a very serious bridge problem, requiring heavy rehabilitation and reconstruction,'' he says. ``We'll need our best engineering and managerial skills to face this problem.''
Manny Beiglemacher, supervisor of the state's bridge inventory and inspection program for the New York City area, likens inspecting to investigative work. ``We're detectives out here, trying to discover and understand what's going on.''
As a dark rain begins to fall on the Third Avenue Bridge, he adds softly, ``We're well aware that the burden of the public's safety falls on the inspector.''