Steel is valued for tensile strength far superior to other construction materials, and for ductility, which is the ability to stretch before fracturing, possibly shifting force to unimpaired members and causing deformation that may be recognized as a warning of imminent failure. However, both strength and ductility can be compromised, as evidenced by the broken hulls of World War II liberty ships continuously battered by frigid North Atlantic waves. In addition to low temperature and fatigue due to repetitive loading, other factors that reduce resistance to fracture include material with inadequate notch toughness (a combination of strength and ductility measured by capacity to absorb strain energy) and stress risers (e.g., cracks, nicks, dents, pits and section loss due to corrosion, re-entrant corners, holes, welds, and abrupt changes in cross-section.)

Ship builders improved their product, but bridge designers gave minimal attention to the possibility of brittle (i.e., non-ductile) fracture until after December, 1967, when the Silver Bridge at Point Pleasant, West Virginia, suddenly collapsed into the Ohio River, killing 46 people. Investigators concluded that the forty-year-old bridge complied with design standards when it was built and was not overloaded when it failed. Examination of the wreckage disclosed that collapse was precipitated by brittle fracture of a single steel eye bar. Bridge engineers promptly fixated on the problem of fracture-critical members.

A fracture-critical member (FCM) is a tension member or component the fracture of which would cause the bridge to collapse. Typical FCMs are lower chords of trusses, diagonal truss members that slope downward toward mid-span, and bottom flanges of bridge girders if there are only two. Eye bars are usually fracture-critical, but not all tension members are.

  

A redundant tension member is not fracture-critical because other members can accept its force if it fails. The cables of a cable-stayed bridge exemplify redundancy. Since the Silver Bridge disaster, bridge designers incorporate redundancy to minimize the use of FCMs, which are now subject to stricter requirements and consequently are more expensive than redundant tension members.

  

FCMs abound in trusses, and FCMs in older bridges may not comply with current standards. Typically, material notch toughness is not known, and material strength may be uncertain. There is no guarantee that tested samples represent the entire bridge. Visual inspection for stress risers is necessary but unreliable because flaws may be tiny, hidden, or internal to members. More thorough inspection methods are costly and require disassembly.    

Bringing an older bridge into compliance with current standards may require reducing forces in members. The reduction for FCMs will be greater than that for redundant tension members. Member forces are caused by load, and both can be reduced by installing lighter decking, eliminating a traffic lane, or narrowing the deck of a pedestrian bridge. Deficient components can be replaced with new ones. Strengthening existing members by attaching new material is an option, but riveting or bolting is inadvisable unless existing holes can be reused. Like holes, welds are stress risers. Nevertheless, welding can be used to repair flaws or attach new material, even on FCMs, if the welds are properly designed and appropriate procedures are followed. It may be possible to remediate a deficient member by adding a new member parallel to it. A system of cables paralleling existing truss members can provide redundancy, removing tension members from the FCM category and its concomitant requirements.

Customized arc welding courses are being offered at Lansing Community College, West Campus, Lansing, Michigan. These courses can be designed for individuals or groups for either introductory or advanced training in the Flux Cored Arc Welding (FCAW), Metal Cored Arc Welding (MCAW), and Shielded Metal Arc Welding (SMAW) processes.

  

Depending on the client's needs, customized courses can include lectures, demonstrations, and hands-on activities with instructions on the essential variables (voltage, amperage, and other operating parameters) of the arc welding processes. 

Contact Vern Mesler at meslerv@gmil.com for more infomation