Greetings from Bridging Solutions!

Did you know that the catastrophic failure of the Silver Bridge (Point Pleasant) spanning the Ohio River in 1967 was the result of the fatigue failure of a steel eye bar on this non-redundant suspension bridge? Its failure prompted the US Congress to pass legislation that led to the establishment of the National Bridge Inspection Standard (NBIS) and the Special Bridge Replacement Program.

Silver Bridge over Ohio River, Dec. 1967 (Excerpted from the Herald-Dispatch)

This installment of “A Bridge Moment” provides a quick introduction into the topic of fatigue cracking of steel bridges. Fatigue cracks may form in steel bridge members as a result of repetitive cycles of stress due to truck loadings (live loads) in the portions of the bridge members that resist tensile stresses. Over time these fatigue cracks can propagate into larger cracks that can eventually cause brittle fracture of a bridge member. If a primary bridge member is non-redundant, its failure can cause the entire bridge to collapse. Examples of bridge types with non-redundant members are truss, suspension and 2 girder bridges. 

To illustrate the concept of fatigue, take a metal bobby pin and bend it back and forth a couple of times and all seems OK – but if you continue to bend it back and forth multiple times – eventually it will crack and break. This is a simple example of fatigue cracking and brittle fracture. Our next installment will dig a little deeper into this structural phenomenon.