Fracture-Critical Bridges Have No Redundancy and Cannot Fail Gracefully

Approximately 18,000 bridges in the United States are classified as fracture-critical, meaning they contain steel members whose failure would cause the collapse of the bridge. These bridges have no structural redundancy: if one key member cracks or fails, there is no alternate load path, and the bridge comes down. The I-35W bridge that collapsed in Minneapolis in 2007 was a fracture-critical design. This matters because fracture-critical bridges require a fundamentally different safety approach than redundant bridges. On a redundant bridge, a cracked member redistributes load to other members, giving inspectors time to find and repair the damage. On a fracture-critical bridge, there is no second chance. The inspection must catch every crack before it propagates to failure, which is an unrealistic standard given the limitations of visual inspection and the difficulty of detecting subsurface fatigue cracks. The practical consequence is that these bridges require more frequent and more sophisticated inspection, including hands-on inspection of every fracture-critical member, often requiring special access equipment and traffic closures. This makes them disproportionately expensive to maintain. Yet many are also high-traffic bridges carrying interstate highways, so closures for inspection cause significant disruption. This problem persists because replacing a fracture-critical bridge is enormously expensive, often $50 million to $500 million for major crossings, and the bridge is typically still functional. It is politically impossible to justify closing and replacing a bridge that is carrying traffic today because of a theoretical risk of sudden failure. The calculus only changes after a failure, by which point it is too late. The structural root cause is that fracture-critical designs were standard practice in the 1950s through 1970s because they used less steel and were cheaper to build. The engineering profession understood the theoretical risk but did not appreciate how difficult it would be to maintain zero-tolerance inspection standards over a 75-year bridge life. Modern bridge design avoids fracture-critical configurations, but the legacy fleet of 18,000 bridges will be in service for decades to come.