NSTM or Fracture Critical Bridge Inspection: A Guide
The 1967 Silver Bridge collapse and the 1983 Mianus River Bridge failure caused much concern about fatigue and fracture critical bridges. As a result, mandated fracture critical bridge inspection made it to the priority list of the federal and the state governments in the US.
Let’s start exploring.
What is a fracture critical bridge?
According to the FHWA’s Bridge Inspectors’ Reference Manual (BIRM), a fracture critical member (FCM) is a steel member in tension that, in the case of failure, may cause the entire bridge, or a part of it, to collapse. A bridge containing one or more FCMs is called a fracture critical bridge.
However, with the recent release of the Specifications for the National Bridge Inventory (SNBI), it’s been proposed to refer to fracture critical bridges as Non-redundant Steel Tension Members (NSTM).
As per the new definition, an NSTM is a primary steel member partially or fully in tension, that’s load path non-redundant, or is without any other redundancy (system or internal redundancy) that inspectors have found through an FHWA-approved process. The failure of this member may cause the entire bridge, or a portion of it, to collapse.
What is a fracture critical inspection?
An NSTM or fracture critical bridge inspection is a hands-on inspection of non-redundant steel members that are subjected to tension.
In this inspection procedure, the fracture critical (FC) bridge inspector needs to give close visual attention to identify and measure fatigue cracks or fractures on steel members. While doing so, they should maintain a distance no further than an arm’s length from the FCM or NSTM.
During a hands-on or visual safety inspection, the FC bridge inspector looks for any:
Occurrence, location, and direction of visual cracks
Weld terminations in tension areas
Physical damage, such as arc strikes
Rust, possibly forming around a working crack
Who can inspect FC or NSTM bridges?
To perform an inspection of fracture critical bridge members, inspection team leaders need to possess special qualifications. They must:
Achieve a 70 percent minimum passing score on the end-of-course assessment
Have sufficient knowledge and can understand the functioning of a fracture critical bridge and the possible location of defects
Be able to recommend or perform advanced testing procedures at areas most susceptible to fractures and fatigue
Be able to perform hands-on inspections on all fracture critical members, components, or connections on the bridge
Also, only a registered Professional Engineer (PE) can perform load ratings on an NSTM bridge. This is because performing a load rating would require precise engineering calculations and judgment, keeping the safety of the traveling public on a pedestal.
Any other person performing the load rating would have to do so only under the direct supervision of the PE.
Fracture critical inspection techniques for steel bridges
Several types of FC inspection techniques are available to determine the condition of a member. These include:
Radiographic inspection: Used to inspect possible cracks at the ends of welded cover plates
Magnetic particle inspection: Magnetic field irregularities help in finding inclusions and other subsurface defects
Ultrasonic inspection methods: Used to inspect potential cracks in pins
Dye-penetrant inspection: Used to inspect cracks next to fillet welds at tee-joints
Please note that one single inspection technique may not be suitable for all situations. Instead, FC bridge inspectors might have to combine more than one inspection technique to adequately assess the damage.
How to perform a fracture critical bridge inspection
Identifying NSTM inspection necessity
According to the SNBI, inspectors need to use the following codes to report whether or not a certain bridge requires an NSTM inspection:
Planning an NSTM or FCM inspection
To ensure a thorough and complete FCM inspection in the field, bridge inspection team leaders need to develop, review, and update a fracture critical inspection plan.
Such a plan typically includes:
A description of the steel bridge, covering the bridge’s maintenance history, past load ratings, previous inspection reports, and any rehabilitation or bridge repair plans. The inspectors should keep themselves up to speed with the condition states, the types of structural elements, and recommendations from previous bridge inspection reports.
Identification of each NSTM or fracture critical element of the bridge on plan sheets, sketches, or drawings
The location of each NSTM or FCM on the bridge, encompassing fatigue cracks and other fatigue-prone details
The inspection method or technique to be used
The access equipment required to perform the fracture critical inspection (such as man lifts, ladders, unmanned aerial systems, or non-destructive testing equipment like ultrasound devices)
A plan for managing traffic on the bridge (like flagging railroads and closing lanes)
Vehicles necessary for accessing the NSTM or fracture critical member (such as snooper trucks and bucket trucks)
The best time of the year for carrying out the FC bridge inspection (considering bird nesting seasons, snow, ice, or any other situation)
Anything else the inventory manager feels the inspection team leader should familiarize himself with, before executing the field inspection of the FC bridge
Along with the points mentioned above, the inspection plan should also encompass the fracture critical bridge inspection frequency.
Existing FC bridges are inspected at an interval of 24 months. However, as per the SNBI, the NSTM inspection interval may be 12, 24, or 48 months, depending on the condition of the bridge.
The inspectors would need to document the criteria by which they are to inspect NSTM bridges in less than a 24-month interval.
Scheduling a fracture critical inspection
After the inspection plan is made, it’s time to schedule the inspection according to the NSTM inspection interval guidelines laid down in the SNBI.
Carrying out the fracture critical bridge inspection in the field
On the day for which the inspection has been scheduled, the FC bridge inspectors execute the techniques laid down in the inspection plan.
The inspection starts with an overall evaluation of the entire bridge and NSTMs or fracture critical members. After the general evaluation, they need to identify and closely investigate all the current fatigue cracks and other deficiencies recorded in the previous bridge inspection reports.
For example, on bolted or riveted bridges, fatigue cracks may develop at end connection angles, tack welds, or truss hangers. Meanwhile, the primary members in welded bridges may have fatigue cracks on back-up bar splices, intersecting groove welds, or other stress risers.
In such cases, the FC bridge inspectors should take either or all of the following courses of action:
If they find a new crack, they are to document the type of the crack, its location, length, and other details in their inspection report. Pictures of the crack are to be attached to this documentation. The inspectors must also note the crack details on the bridge component where they’ve found the crack.
If there’s an existing crack that’s grown since the previous inspection, the inspectors should punch the crack’s new tip. This helps them monitor any new propagation during the next fracture critical bridge inspection.
You can use the inspectX tablet app available for iPad and Android devices to efficiently perform your inspection in the field in offline mode. Please visit the inspectX help center to learn more.
inspectX allows you to record your findings, take photos, and draw sketches of FC bridges without any internet connection. So, there’s no hassle of carrying a pen, paper, or a digital camera in the field.
Reporting the NSTM inspection condition
The SNBI requires inspectors to report the condition rating of the NSTMs using one of the following codes:
Incorporate the condition rating of an NSTM into the substructure or superstructure condition rating. In case you identify one or more NSTMs in both the superstructure and the substructure of a bridge, only the lower of the two condition values for the NSTM condition are to be reported.
If an NSTM inspection is not required, you don’t need to report the NSTM condition rating at all.
In addition, inspectors must now report the bridge inspection type for an NSTM inspection using the code ‘4’.
Generating an FCM bridge inspection report
If an NSTM bridge fails, the consequences could potentially be catastrophic. Therefore, it’s crucial to document the entire inspection accurately, without any omissions.
This brings us to the final step in the process, which is, to submit the inspection to the inspection supervisor or program manager for review and approval, and then generate a report documenting all the NSTM or FC bridge inspection findings.
In inspectX, once you upload your fracture critical inspection data from your tablet to the server, you can review the inspection to ensure it is complete and send it to the reviewer for approval.
You can easily generate a PDF report in inspectX. Please refer to this article to learn how.
The report formats generally differ from one agency or state DOT to another. Nevertheless, every fracture critical bridge inspection report must follow a set of minimum requirements.
For instance, inspectX helps bridge inspectors generate PDF reports of their FC bridge inspections. Such reports primarily contain:
The bridge number and inspection type (fracture critical)
Location of the bridge on the map
The inspection team lead and the date of the inspection
FC bridge identification, structure type, condition, and load rating
The description and the condition state (CS) rating of elements
Description of deficiencies identified
Bridge maintenance requirements
Recommendations regarding critical findings
Supplementary images and sketches of the FC bridge taken during the inspection
Wrapping it up
Now that you know the NSTM or fracture critical inspection procedures and processes, it’s time to get to work.
As you can see, with inspectX, you can not only schedule fracture critical inspections in batches, but collect inspection data in the field in offline mode using the inspectX App and download detailed PDF inspection reports, too.
Along with its schedule planning tool, you get access to past inspection data about the FC bridge. This data is valuable in assessing the extent of propagation of fatigue cracks and the condition of the structure.
Ultimately, the safety of the traveling public is the highest priority. Keeping this in mind, a fracture critical bridge inspection needs to be executed thoroughly and with the utmost precision. And evidently, a commercial bridge inspection software solution like inspectX can help you do just that, and more.
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