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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.
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.
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:
To perform an inspection of fracture critical bridge members, inspection team leaders need to possess special qualifications. They must:
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.
Several types of FC inspection techniques are available to determine the condition of a member. These include:
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.
According to the SNBI, inspectors need to use the following codes to report whether or not a certain bridge requires an NSTM 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:
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.
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.
You can easily schedule a fracture critical inspection in inspectX by following the instructions in this article.
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:
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.
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’.
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:
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.