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There are approximately 2,100 bridges in the Metropolitan area of New York City; 842 of the bridges, counting over 4,500 spans, are the responsibility of the City administration. According to New York State inspection records, 56% of these bridges are rated less than 4.5 on a 7 grade system. This is regarded by the New York City Department of Transportation (NYC DOT) Bridge Bureau as an indication of the need for structural repair of some magnitude. In the situation involving 42% of the City's bridges, a ten-year, $3.5 billion program envisions complete rehabilitation. A recent NYC DOT study examined the remaining 58% of the bridges and concluded that considerable structural repairs are necessary in order to avoid the need for premature rebuilding of that portion as well.
A further step is the identification of structural components requiring repair or replacement, and the optimal frequency of such measures. A program for component rehabilitation is expected as a result of this study. The elimination of certain problem-prone structural components from future design can be considered as well. Ultimately, schedules for timely repairs can be generated in order to extend the useful life of a bridge. This is viewed as a supplement to the New York City Preventive Maintenance Program which is already applied to bridges in good condition. The following structural components emerge as the primary sources of problems on most of the bridges:
Joints—Approximately 90% of the structural deterioration is concentrated in the vicinity of joints and is precipitated by joint failure. Experience suggests that “the only good joint is no joint.” Yet joints are inevitable and their maintenance while still in serviceable condition is vital to the structure as well as to traffic.
Drainage—More than 60% of the bridge drainage systems are rated “not functioning as designed.” This results in saturating bridge decks and the entire superstructures with water and de-icing salt, causing rapid corrosion.
Wearing Surface—Deck—The deck and wearing surface, frequently monolithic in recent years, are the first to suffer from the malfunctioning of the above components. Further, they are subjected to the increasingly severe dynamic loadings due to the growing volume and weight of traffic.
The consequences to the bridge structure are grave. Concrete piers crack, spall and fail due to the dynamic loadings. Steel members crack under the combined effect of fatigue and corrosion. In all of these cases, problems originate at the roadway surface and rapidly propagate throughout the structure. The progress of this path of rapid deterioration is traced for several bridge types frequently encountered in New York. Measures used with limited success in mitigating the surface and structural deficiencies are described.
bridge surface, decks, drainage, dynamic load, failure, fatigue, grating, impact, joints, superstructure
Assistant Commisioner, AC Bridge Inspection/R&D Unit, Bureau of Bridges, New York, NY