The National Transportation Safety Board revealed that Baltimore’s Francis Scott Key Bridge was “fracture critical”

meaning the entire structure could crumple if one vital piece failed. 

 Data shows that 17,468 bridges out of 615,000 nationwide are "fracture critical" or lacking built-in redundancies and other protective measures. 

To be "fracture critical," a bridge has to lack built-in redundancies, meaning if a single vital piece of the structure were to fail, either a part or the entire bridge could crumple.

https://www.yahoo.com/news/over-17-000-us-bridges-170051259.html?.tsrc=1013