Fracture critical bridge
A fracture critical bridge is a bridge or similar span that is vulnerable to collapse of one or more spans as a result of the failure in tension of a single element. While a fracture critical design is not considered unsafe, it is subject to special inspection requirements that focus on the tension elements of its structure.
Definition
For a bridge to be defined as fracture critical:- It must possess structural members that are subject to tensile stresses from bending or axial forces.
- The members must be non-redundant, lacking alternate load paths or means of safely redistributing forces in the event of a tensile failure.
Examples of bridge designs that would typically be considered fracture critical are:
- Most truss bridges with two main load-bearing assemblies
- Two-beam girder bridges
- Two-cell steel box girder bridges
- Main suspension cables and hanger cables of suspension bridges
- Cable-stayed bridges
- Steel ties in tied-arch or tied-truss bridges
- Pin-and-hanger assemblies in two-beam bridges
- Steel floor beams and cross girders
- Steel bent assemblies under tensile stress
- Movable and pontoon bridges
History
In May 2022 new NBIS guidance established additional terminology to describe new forms of redundancy. These are:
- System redundancy, in which the fracture of a primary member will not result in collapse
- Internal redundancy, in which a fracture will not propagate through a member that is not system redundant, the member being itself redundant
- Load path redundancy, where three or more primary load-carrying elements are present