Aerial photography captured by a drone showed the crack in the I-40 bridge in 2019. (Credit: Arkansas DOT).
A potentially catastrophic bridge collapse was prevented last week when engineers performing a routine bridge inspection of the Hernando de Soto Bridge that carries I-40 over the Mississippi River between Memphis, TN and West Memphis, AR discovered a significant fracture in one of the primary load-carrying members of the 48-year-old structure. The fracture, near a connection plate in the built-up steel member, had fully severed three sides of the box girder and was about 20 percent through the bottom plate. Had the crack fully propagated through the bottom plate, the entire bridge, which carries upwards of 35,000 vehicles per day, could have collapsed.
The crack through the member was so large that the inspector did not have to look very hard to find it, and he took the immediate action of calling 911 to have all traffic removed from the bridge. This is the type of crack that stops a bridge engineer in their tracks and sends a shiver throughout their entire body. The stability of a steel truss structure such as the I-40 bridge depends entirely on the integrity of the members loaded in tension. Failure of a single one of these members, denoted as fracture-critical, could lead to total collapse in a matter of seconds. When a crack has spread as widely as this one, its behavior can be unpredictable and the final propagation of the crack to reach a fully severed condition could happen in an unknown period. That’s what makes this potential bridge failure scarier – inspections typically a window of several months to be considered completed “on schedule,” so had this year’s inspection been scheduled for later this summer, it’s very possible the bridge would have already collapsed.
The shutdown of the bridge has led to significant delays in shipping and poses a logistical nightmare in Memphis, a crucial city for the country’s shipping networks which are already stretched to their breaking points. Making matters worse, hundreds of barges carrying freight on the Mississippi River were also prevented from passing under the bridge until its stability could be further evaluated. The delays caused by the closed bridge are costing trucking companies millions of dollars per day. The closure will likely drag on for several months while the structural integrity of the entire bridge is evaluated and emergency repair details are designed and installed.
“Memphis is really a nerve center of this country from a supply chain perspective,” said William B. Dunavant III, the chief executive and president of Dunavant Enterprises, a global distribution and logistics company based in the city.
Of course, all of these shipping headaches and lost dollars pale in comparison to the potential loss of life that would have occurred had the crack not been discovered. Further examination of images captured by a drone flying near the bridge in 2019, however, showed that the crack should have been discovered sooner. Two full inspection cycles had passed without the crack being discovered. Failure to identify this crack nearly led to the nation’s first mass-casualty bridge collapse since the I-35 bridge collapse in Minneapolis in 2007.
The shutdown of the I-40 bridge comes at a key moment for infrastructure investment in the country. President Joe Biden has a vision for rebuilding America’s crumbling and rapidly deteriorating bridges, many of which are older and in worse condition than the fractured Hernando de Soto span. If there’s a point that can be made to drive home the fact that we are approaching a critical moment for America’s infrastructure, it is that traffic from the 50-year-old closed bridge is being detoured to a 71-year-old bridge. Thankfully failure of the I-40 bridge was prevented, but next time we might not be so lucky. Until significant headway can be made in reducing the number of structurally deficient bridges, the role of the bridge inspector will remain extremely crucial in protecting the traveling public.
“It’s fortunate that routine inspection averted a potential disaster, but the state of our crumbling infrastructure is deeply troubling,” Representative Steve Cohen, a Democrat from Memphis, said in a statement, adding, “The closure of a major thruway will negatively affect us both here in Memphis and around the country, which depends on us as a national connection hub.”
The development of the crack in this bridge was likely not preventable. Such is the nature of steel truss members being subjected to cyclic loading and carrying thousands of trucks each day. The life of a steel bridge such as this one is finite, and heavier traffic volumes than were originally designed to accelerate the end of its life cycle. Failing to identify the crack, however, before it reached the point of nearly severing the member is inexcusable. These are the type of incidents the nation’s transportation agencies sought to avoid when they implemented rigid standards for inspecting fracture-critical bridges. Every inch of every member whose failure could lead to an imminent collapse of the bridge must be inspected up close and personal. If that hands-on inspection had been done, the crack would have been noted two years ago before it ever reached the point of becoming a ticking time bomb.
“The way we’re supposed to inspect the bridge is you literally go inch by inch along that beam and physically inspect every inch of the beam,” said Lorie Tudor, Arkansas Department of Transportation Director said. “That did not happen.”
In the past, bridge collapses have spurred more attention being paid by the civil engineering community to inspection practices. This incident shows us that even the best-laid plans can go to waste due to sheer incompetence. The FHWA and state DOTs cannot regulate away the potential for human error. Past bridge failures led to re-evaluation in the way we think about bridge safety, like the creation of a national bridge database and implementing regimented inspection schedules and procedures for reporting and monitoring defects.
The number-one role of bridge inspectors across the country is to keep millions of people driving across our bridges out of harm’s way. That is a moral and ethical responsibility that the civil engineer takes on when they sign up for bridge inspection. Failure to take our role seriously leads to situations like this. Luckily, a new team of inspectors stepped in to live up to the high standards all engineers should aspire to and set the ball in motion to prevent the bridge from collapsing.