Caveat: Anon could provide a definitive analysis if multiple hi-res images/views were available. Just a professional opinion based on what has been posted.
All /pb
No…
ME Anon here. I’d like to put this to bed.
This is a Stress-Strain fracture that has occurred over time.
Occurs more than one may think on structures. Especially structures like bridges which see intermittent varying loading, vibration, temperatures, etc.
Usually the fault lies with the following:
Poor design
Faulty materials
Maintenance (corrosion issues)
Some fuck cut corner(s)
What appears to you as “grind” marks are fretting of the “broken” joint due to motion from vibration & changing temps. There is a better image in past breads where you can see the rust and flaking of paint. This would not be left by a slim grinding (whizz) wheel. One would not be able to duplicate the “jagged” break shown with a wheel. The “chatter” indicated by Anon could come from anything that has rubbed the painted surface.