Bridge collapses and failures (complete closures for extended periods of time) are often a result of several factors. Whether it is a combination of an unchecked change order mixed with extreme construction loads or cracks missed during inspections mixed with impacts on a structure, generally bridge failures occur when the perfect storm of circumstances come together. Though we cannot often predict when that perfect storm will happen we can delineate bridges which are predisposed to that kind of risk and perhaps do something to avoid taking those structures into the storm.
Recently, a full through crack on I-40 near Memphis, TN gave us a good snapshot of what these “at risk” structures might look like.
Bridges designed and built in the 60s tend to have less redundancy, rendering them fracture critical. Fracture critical means that if one member fails, the whole structure is at risk of failure. An example of these are the tied arch bridges built during that time.
In the 1960s there was a movement to make bridges more elegant, more streamlined, giving them a sleeker look and reducing the factor of safety. Unfortunately, engineers did not yet have a full understanding of key materials properties such as fracture toughness, or the resistance to progressive cracking in metals prior to rapid failure. It wasn’t until 1978 that fracture toughness standards were introduced into bridge steel standards in the AASHTO Guide Specifications for Fracture Critical Non-Redundant Steel Bridge Members.
Why is this important? Good question.
While bridge inspection programs were put into place nationwide in the early 70s due to legislation introduced following the Silver Bridge collapse in 1967, visual inspection is the only inspection mandated on the majority of bridges. As the I-40 bridge incident has shown, these inspections are extremely dependent on the talent, work ethic, and integrity of the inspector, and major cracks or problems may be missed for years. We should all be hailing the 2021 inspector as a hero for finding what others had missed.
So we have bridges with less redundancy, made from steel which does not even meet the standards of the 1980s, and inspection methods which are quite like to finding proverbial needle in a haystack. Definitely a storm, waiting for that one last thing to go wrong…
It is not feasible, economically, to use advanced methods such as conventional NDT (UT, MT, PT, GPR, etc.) or structural health monitoring/condition monitoring on all the bridges in the U.S. And even NDT only gives you a snapshot in time, every two years, granted a “higher resolution” snapshot than the visual inspection. So how do you decide where to use SHM to monitor when there’s so many bridges and so little funding?
There has to be a risk-based screening system that will allow for advanced monitoring to be in place on those structures which are “at risk” for the perfect storm scenario.
Let’s look at a couple of examples of which bridges should be on the “watch list” and be prioritize for monitoring.
Fracture critical steel bridges built from 1960 to 1980 which have low redundancy. Particularly those in colder climates when fracture toughness is reduced even further.
Post-tensioned structures which contain grout with much higher than allowable chloride content due to a grout recall, in environments with consistent exposure to humid environments.
Bridges which are often permitted for heavy loads but which have experience either severe section loss due to deicing salts, evident fatigue cracking, or even fatigue prone connections.
Bridges with a history of cable corrosion in extreme weather environments.
Bridges which are subjected to barge or truck impacts.
These structures all have a reduced factor of safety due to design & materials, environmental conditions, or loading and could be one storm (both literal and figurative) away from a failure. So why not prioritize keeping an eye on them?
We can no longer cross our fingers and hope that an inspector sees a problem in time to avoid fatalities. These structures have known problems and are at higher risks than conventional bridges. A federal mandate with some funding match would be a good place to start to implement a risk-based approach to managing “at risk” structures.
Even then, maybe there’s not enough funding. And SHM can be expensive to get enough coverage. But what if that changed?
What if there were a low power, wireless system which could ping these structure once a week to globally screen for all of these problems? Something that detects structural stiffness changes over time to give an early warning and raise a red flag to get inspectors out to the structure when and where they are needed. A system that can tell them where to look more closely, eliminating the needle in a haystack scenario.
This kind of global monitoring system exists today and should be implemented on these high risk bridges now before the perfect storm comes together. We cannot wait any longer.