Definition: The FHWA defines a fracture-critical member as a steel member either entirely in tension or with a tension element whose failure would cause either a portion or the entire bridge to collapse. A fracture-critical bridge contains one or more non-load path redundant steel tension members, components, or connections. The FHWA presents two criteria for identifying a fracture-critical bridge:
1. The bridge must have one or more steel members, components, or connections in tension. These loading conditions may include tensile forces and flexure. Load analysis may indicate that some members experience a stress reversal (varies from tension to compression) under various loads. Such members shall be included under these criteria.
2. The bridge must have no load path redundancy, in which no other structural elements can carry the load if a main load-carrying member fails. A bridge must have two or fewer load paths to be defined as nonload paths redundant.
On a typical day in downtown Pittsburgh, as commuters traverse the Rachel Carson Bridge bound for places of business and entertainment, little thought is paid to what is happening among the beams, eyebars, and girders that make up the world-famous river crossing. With every passer-by, the bridge undergoes compression, tension, flexion, and limitless other forces passing through its structure.
As these forces work through the bridge, they follow what is known as a "load path." Invisible forces caused by each car travel like electricity through steel tension members, components, or connections. While each is important, only some receive the designation "Fracture Critical Members" (FCMs). An FCM is a component of the bridge that, should it fail, would cause either a portion or the entire bridge to collapse. Key to the identification of an FCM is a need for more redundancy. Should the main component of a bridge not have redundant load paths to carry the forces of cars traveling over it, this principal component would be considered fracture-critical. The term first came into existence in the late 60s after the collapse of the Silver Bridge in Kanauga, OH. The famous collapse was due to a failure in a suspension chain that supported the bridge's main span, which would today be identified as fracture-critical.
Fracture-critical members are comprised of similarly named fracture-critical materials, which are special metals that have been created with higher strength specifications than the standard. Given the severity of consequences should one of these components fail, the FHWA and AASHTO have strict guidelines governing how fracture-critical materials and members are to be handled.
Over decades, FCMs in high-traffic areas will bear the load of hundreds of thousands of cars, rainstorms, high heat, freezing temperatures, and more. As fatigue sets in, eventually, they will need to be rehabilitated or replaced to maintain their strength. The complex process of rehabbing the most critical components of a bridge structure is not a task that many fabrication shops are willing to take on. It is a niche offering that can be difficult to find certified fabrication shops. Typically, this kind of work is only provided by large-scale fabricators due to the cumbersome nature of adhering to the many regulations and strict quality control measures associated with fracture critical materials and members.
At Advantage Steel and Construction, we are proud to have earned our FCM certifications and are willing to work on these projects in our fabrication shop. Certified by the American Institute of Steel Construction, our Saxonburg, PA shop adheres to the rigid set of quality control measures required of fracture critical materials to ensure that finished members from our shop are stronger than needed for where they will be placed.
Let's revisit the Rachel Carson Bridge in Pittsburgh for an example from the field. Advantage Steel and Construction worked with general contractor Brayman Construction Corporation to rehabilitate the staple Pittsburgh landmark from 2019 to 2020. The project's scope involved rehabbing or replacing components across the entire bridge. The accelerating effects of aging had brought rust into the interior of the main stiffening girders, which are three vertical web plates connected with a bottom flange plate. The leading cause of damage was water ingress from the nearby curb, which, over time, caused the bridge to rust from the inside out.
In addition to sealing this water ingress location, we repaired the stringer connections, floor beam connections, stiffening girders, and sidewalk brackets. Perhaps most important was the work on the bridge's tie-down system and anchors. These components have fracture critical designations, meaning that if one were to show signs of failure, the bridge would be entirely shut down. To restore strength to the aging bridge, we fabricated and installed four new tie-downs and anchors to replace or reinforce the old ones. All fracture critical materials were made of Grade 50 structural steel and manufactured at Advantage's shop. The FCMs used on this project adhered to stringent specifications, meaning the steel was rolled to a particular criterion, making it extremely hard and much stronger than needed. Rigidity and structural integrity were restored by adding this new material to the anchors.
On the way to rehabilitating the bridge, primary consideration was given to historical preservation. Putting a modern spin on old construction practices, we could maintain the bridge's original look without sacrificing strength.
The work done on this bridge by the crews at Advantage Steel and Construction earned the project the 2021 ASHE-Pittsburgh Annual Outstanding Highway Engineering Award.
Today, we continue to work involving fracture-critical materials and members on the Sherman Minton Bridge, a double-decked bridge connecting Louisville, KY, to New Albany, IN. The Sherman Minton Bridge is a critical regional artery, seeing 70,000 cars pass over it daily. We worked with Kokosing, the General Contractor, to fabricate new fracture-critical designated cable brackets. The brackets, which play a crucial role in connecting and anchoring the bridge cables, are an item that many larger shops would pass on due to concerns over cost-effectiveness, and smaller shops typically need more capabilities to work with. We can offer unique value on this project due to its ability to deliver big shop value at the scale demanded by the project.
According to data published by the Department of Transportation, over 18,000 bridges across the nation are designated as having fracture-critical members. With American infrastructure deteriorating, shops that can rehabilitate these structures will be vitally important. Advantage Steel and Construction can meet the growing demand for FCM fabrication and rehabilitation at a unique scale, offering cost-effective and quality work to projects big and small.