NSTM or Fracture Critical Bridges: Probing the Basics
Back in 1967, the sudden collapse of the Silver Bridge in West Virginia showed that if a single member fails, it could cause the entire bridge to fail, too. This is where the concept of fracture critical bridges (FC) comes into play.
Before heading over there, though, it’s crucial to understand the term “redundancy” and its role in identifying FC bridges.
What is bridge redundancy?
Bridge redundancy is a bridge’s capability to carry loads after one of its structural components reaches its maximum load-carrying capacity, or if one or more of its components fails or gets damaged.
If a bridge can carry loads or traffic despite the failure of any of its members, the structure is considered redundant.
What are the types of redundancy?
There are three different types of redundancy in bridges:
1. Load path redundancy
Load path redundancy relates to the minimum number of primary load-carrying members (like girders or trusses) needed to support the deck.
According to the National Bridge Inspection Standards (NBIS), a bridge must have at least three primary load-carrying members. These bridges are considered redundant because despite a weakness in one girder (member), the other girders (members) can help carry the load.
Put simply, if an alternative and adequate load path exists, you’ll know the structure has load path redundancy.
2. Structural redundancy
The redundancy caused by a continuity within the load path is known as structural redundancy. To be more precise, a structurally redundant bridge is statically indeterminate, that is, a continuous-span structure.
For example, a span in a continuous three-span bridge can redistribute its load or stress to the other spans, making the entire bridge structurally redundant.
In the figure below, the bridge is made up of multiple simple spans. You can notice this fact because of the discontinuity at each pier. So, the bridge is structurally non-redundant.
On the other hand, the second bridge has a continuous superstructure over all the piers. This helps provide structural redundancy for the interior spans.
Additionally, the bridge system performance, involving the participation of the deck, secondary members, and parapets, also yields structural redundancy.
3. Internal redundancy
When there are several parallel elements or separate sections in a member, a fracture or crack in one element isn’t expected to spread out to the adjacent elements within the member. Such a member is said to have internal redundancy.
Built-up members with different plates that are riveted or bolted together are internally redundant.
However, built-up welded members or rolled steel ones don’t have internal redundancy.
What are fracture critical bridges or NSTM bridges?
According to the FHWA’s Bridge Inspector’s Reference Manual (BIRM), a fracture critical member (FCM) is a steel member either partially or fully in tension, or having a tension element, the failure of which can potentially cause the entire bridge, or a portion of it, to collapse.
Expanding on this definition, a fracture critical bridge has one or more FCMs or non-redundant steel tension members, connections, or components.
Some examples of FCMs include suspension cables, link and pin systems in suspended spans, girders having tension components in two-girder bridges, and specific truss members in tension.
However, considering the new Specifications for the National Bridge Inventory (SNBI), 2022, the authorities have suggested referring to fracture critical members as Nonredundant Steel Tension Members (NSTM), moving forward.
An NSTM or simply a nonredundant member is a primary steel member without load path redundancy, system redundancy, or internal redundancy, and partially or fully in tension. Should this member fail, the entire bridge, or a portion of it, may collapse.
Taking an example, let’s suppose a structure has less than three girders, beams, or other load-carrying members. Even if one of these members is too weak to carry loads, it can’t redistribute the peak load to the other members as these won’t have the capacity to carry that load.
So, when one girder in a two-girder bridge fails, the span will probably collapse. This failing girder or member is an NSTM.
What is bridge system redundancy?
In many cases, even after an FCM was damaged, the entire bridge system has remained intact. Such performance of the structure is known as ‘system redundancy’.
The AASHTO Guide Specifications for Analysis and Identification of Fracture Critical Members and System Redundant Members, 2018, list the procedures for analyzing the performance and response of bridge systems during the potential failure of an FCM. These standards apply to steel bridges, including tub girder bridges, truss bridges, and through-girder bridges.
What makes a bridge fracture critical?
As of today, there are 17,468 fracture bridges in the US. Of these, 1,382 FC bridges are in New York alone, followed by Ohio, which has 1,276 FC bridges. The state with the least number of fracture-critical bridges (eight) is Hawaii. It’s preceded by the District of Columbia, containing 24 FC bridges.
That being said, the FHWA helps you single out an FCM bridge using the following criteria:
One or more steel members, connections, or components of the bridge must be in tension. Various loads may also cause some members to experience a stress reversal, ranging from tension to compression. These members are also included in the criteria for identifying fracture critical bridges.
As we’ve mentioned earlier, there must be two or fewer load paths on the bridge. This is so that if one primary load-carrying member fails, no other structural element can carry the load. In short, the bridge must have no load path redundancy.
3. Steel bridges with even one type of redundancy out of internal, system, or load path redundancy are not to be classified as NSTM bridges, according to the SNBI. For example, if a bridge member is internally redundant, but is without load path redundancy, it’s not an NSTM bridge.
You can also take a look at this list of steel FCMs that the FHWA lays out in their reference manual:
Primary suspension cables and steel hangers of suspension bridges
Pin-and-hanger assemblies in bridges with up to two truss lines or girders
Steel cross-girders or floor beams
Two-girder systems (through girder and deck girder)
Eyebars and gusset plates
Arches, trusses, and rigid frames
The manual classifies floating and moveable bridges as fracture critical, too.
Types of fractures in FCMs
Bridge inspectors are primarily concerned about brittle fractures, which are sudden and occur very quickly. Particular members are more likely to fail due to brittle fractures than any other kind of fracture.
Often, you might find brittle fractures in bridges at low temperatures. Then again, heat straightening and impact damage may cause embrittlement, resulting in an elevated risk of brittle fractures.
Other cases that can lead to a high risk of a brittle fracture include welding, galvanizing, and other fabrication steps, and situations where you have identified fatigue cracks on existing bridges.
Ductile fractures are typically slower than brittle fractures, which might give you enough time to help prevent a disaster.
In the case of such a fracture, the net uncracked section of a member shows local plastic deformation. After the material stretches, it necks down in diameter, causing a visible distortion of the member’s shape, and, thus, a ductile fracture.
Factors influencing the type of fracture
The factors we’ve outlined below are at the root of whether the fracture in the member is brittle or ductile:
The degree of constraint or rigidity
Members made up of thick welded plates are more likely to have a brittle fracture than members composed of thinner plates.
That’s because, with thicker plates, the degree of constraint is high, limiting the steel’s ability to experience plastic deformation. The thicker the plates, the higher is the degree of constraint, and the more likely these are to break, rather than bend, under stress.
Loading rate on the bridge
When a steel member experiences rapid loading, as may take place due to a collision or a truck explosion, such a fast-loading rate can create a high energy level. This energy might cause the member to develop a brittle fracture.
Conversely, slow truck-loading rates may result in plastic deformation, and in turn, a ductile fracture.
Scheduling a fracture critical bridge inspection
A hands-on inspection of fracture critical bridge members needs to be done every 24 months. As per the new SNBI regulations, though, the inspections for NSTM bridges would have to be carried out every 12, 24, or 48 months, depending on the fracture criticality or the level of non-redundancy of the member.
For this reason, it’s convenient to plan a schedule according to which the state DOT’s fracture critical bridges are to be inspected.
Some bridge inspection software, like inspectX MultiAsset, come with an in-built inspection schedule planning tool. InspectX MultiAsset offers you the ability to view overdue and upcoming bridge inspections based on inspection types, like FCM inspections, and other data fields like district, county, and bridge owner.
Plus, your DOT’s bridge inspectors can use inspectX MultiAsset to plot these FC bridges on Google Maps. This GIS functionality makes it easier for you to plan and schedule NSTM or fracture critical bridge inspections according to the routes shown on the map.
Once you see the FC bridges on the map, you can hover over each bridge to view its respective data or acquire a street view of the bridge.
Now that you know about fracture critical or NSTM bridges, it’s time to start inspecting bridges the modern way.
Instead of relying on pen and paper and a digital camera every time you’re in the field to collect data, why not switch to tablet-based bridge inspections? Whether you’re inspecting fracture critical bridges or other types of structures in your state, using inspectX MultiAsset on your tablet means you can collect field data offline, without having to worry about connectivity issues.