A fracture critical bridge is a
bridge
A bridge is a structure built to Span (engineering), span a physical obstacle (such as a body of water, valley, road, or railway) without blocking the path underneath. It is constructed for the purpose of providing passage over the obstacle, whi ...
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.
While members subject to
compressive stress
Compressive stresses are generated in objects when they are subjected to forces that push inward, causing the material to shorten or compress. These stresses occur when an object is squeezed or pressed from opposite directions. In everyday life, ...
may also fail catastrophically, they typically do not fail from crack initiation.
Examples of bridge designs that would typically be considered fracture critical are:
* Most truss bridges with two main load-bearing assemblies
* Two-beam
girder bridge
A girder bridge is a bridge that uses girders as the means of supporting its deck. The two most common types of modern steel girder bridges are plate and box.
The term "girder" is often used interchangeably with "beam" in reference to bridge d ...
s (three-beam bridges in California)
* Two-cell steel
box girder
A box girder or tubular girder (or box beam) is a girder that forms an enclosed tube with multiple walls, as opposed to an i-beam, - or H-beam. Originally constructed of wrought iron joined by riveting, they are now made of rolled steel, rolled ...
bridges (three-beam bridges in California)
* Main suspension cables and hanger cables of
suspension bridge
A suspension bridge is a type of bridge in which the deck (bridge), deck is hung below suspension wire rope, cables on vertical suspenders. The first modern examples of this type of bridge were built in the early 1800s. Simple suspension bridg ...
s
*
Cable-stayed bridge
A cable-stayed bridge has one or more ''towers'' (or ''pylons''), from which wire rope, cables support the bridge deck. A distinctive feature are the cables or wikt:stay#Etymology 3, stays, which run directly from the tower to the deck, norm ...
s
* 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 bridge
A pontoon bridge (or ponton bridge), also known as a floating bridge, is a bridge that uses float (nautical), floats or shallow-draft (hull), draft boats to support a continuous deck for pedestrian and vehicle travel. The buoyancy of the support ...
s
History
The designation and inspection protocols for fracture critical bridges were developed following the failure of an
eyebar
In structural engineering and construction, an eyebar is a straight bar, usually of metal, with a hole ("eye") at each end for fixing to other components. Eyebars are used in structures such as bridges, in settings in which only tension (physic ...
at the
Silver Bridge
The Silver Bridge was an eyebar-chain suspension bridge built in 1928 which carried U.S. Route 35 over the Ohio River, connecting Point Pleasant, West Virginia, and Gallipolis, Ohio. Officially named the Point Pleasant Bridge, it was popul ...
at
Point Pleasant, West Virginia
Point Pleasant is a city in and the county seat of Mason County, West Virginia, United States, at the confluence of the Ohio River, Ohio and Kanawha River, Kanawha Rivers. The population was 4,101 at the 2020 United States Census, 2020 censu ...
, which precipitated the bridge's collapse into the
Ohio River
The Ohio River () is a river in the United States. It is located at the boundary of the Midwestern and Southern United States, flowing in a southwesterly direction from Pittsburgh, Pennsylvania, to its river mouth, mouth on the Mississippi Riv ...
in 1967, resulting in 46 deaths. The disaster resulted in the establishment of the
National Bridge Inventory
The National Bridge Inventory (NBI) is a database, compiled by the Federal Highway Administration, with information on all bridges and tunnels in the United States that have roads passing above or below them. That is similar to the grade-crossing ...
, using the National Bridge Inspection Standards (NBIS) (CFR Title 23, Part 650).
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
See also
*
Fracture mechanics
Fracture mechanics is the field of mechanics concerned with the study of the propagation of cracks in materials. It uses methods of analytical solid mechanics to calculate the driving force on a crack and those of experimental solid mechanics t ...
*
Single point of failure
A single point of failure (SPOF) is a part of a system that would Cascading failure, stop the entire system from working if it were to fail. The term single point of failure implies that there is not a backup or redundant option that would enab ...
*
Jesus nut
References
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Bridge design