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Non-destructive Testing Options for Welded Steel Bridges

Updated: Sep 10, 2021




Welded steel bridges consist of many fracture critical members (FCMs) that are require periodic non-destructive testing (NDT) for fatigue crack detection. Fatigue cracks develop for several reasons including [1]:


- Welded details that are susceptible to fatigue cracks

- distortion-induced fatigue cracking

- Many bridges are over 90 years old

- Corrosion fatigue cracks, and

- Increases in the number of heavy vehicles leading and d allowable leading to significant accumulation of loading cycles.


The article summarizes the most used conventional and advanced non-destructive testing technologies for bridge fracture critical member inspections.


Visual Inspection of Steel Bridges


Visual inspection of steel bridges is by far the most common and cost-effective method to examine bridge steel for visible fatigue cracks. The bridge inspections are carried out mainly in accordance with American Welding Society (AWS) D1.5, Bridge Welding Code (BWC), Chapter 6 Inspection and the National Bridge Inspection Standards (NBIS) require that bridge FCM be inspected every 24 months. NBIS Bridge Safety Inspectors are required to attend and pass two and one-half week bridge safety inspection training courses prior to leading any inspections.




Many state DOTs publish Bridge Safety Inspection Manuals that detail the minimum recommended inspection reporting requirements and supporting non-destructive testing techniques. PennDOT’s bridge inspection manual can be reviewed here:



An example fatigue crack indication on a steel girder is shown in Figure 1. Wet visible magnetic particle testing (WVMT) was used on an area over which paint cracking was observed along the weld toe line. Upon removal of the paint and proper surface preparation, a light coating of white contrast paint was applied to the welded area. Black wet bath was applied while magnetizing the area and a reproduceable indication from the crack was reproduced.


Wet visual magnetic particle indication