A special inspection of 13 bridges located on NJ 55 between Mileposts 23 and 28 completed by another consultant revealed the presence of a significant number of fatigue cracks – many growing at a rapid rate.Â In addition, the 13 structures, all built prior to 1985, have similar diaphragm details characterized by diaphragm connection plates that are either cut short of the flanges or are flush fit to the flanges, resulting in a short web gap between the end of the connection plate and the flange.Â This detail is known to be sensitive to distortion induced fatigue.
Intelligent Infrastructure Systems, as part of New Jersey Department of Transportationâ€™s (NJDOT) Bridge Resource Program, was engaged to design and implement a testing and monitoring program for one of these bridges – Bridge 0609-163 located at mile marker 27.35, carrying southbound traffic on NJ 55 over SW Blvd. and a CSX rail line.Â It was believed that these bridges were being driven by overloaded (and unpermitted) trucks on a regular basis.Â The objective of the program was to establish the state of stress under controlled load testing and routine truck traffic, track stresses generated by truck traffic (legal and illegal) crossing the bridge, and estimate gross vehicle weights using the load test and monitoring results.
Based on a literature review, field testing, and continuous monitoring, it was concluded that an aggregation of parameters are likely causing the observed fatigue cracking. Specifically, the number and weight of trucks crossing the bridges, both legal and illegal, combined with the diaphragm details on these bridges are contributing to fatigue cracking. Therefore, it is expected that these bridges will exhibit varying degrees of distortion induced fatigue based on bridge geometry, diaphragm details, and loading conditions.
The truck testing and continuous monitoring program implemented for this bridge indicates that there are large numbers of trucks crossing the bridges – ADTT is estimated to be about 900, which is 10% of the recorded ADT.
Intelligent Infrastructure Systems recommended that the monitoring system continue to capture and record truck crossings and that further data analysis and monitoring be performed to provide additional characterization the loading environment of the bridge. Â In addition, it is recommended that finite element modeling of the structure be preformed to calculate differential deflections and estimate web gap stresses for different diaphragm locations to identify any correlations with known cracked locations and differential deflections at diaphragm ends.