In May of 2021, inspectors on the Hernando de Soto Bridge between Arkansas and Memphis, Tennessee discovered a crack in a major structural member. They immediately contacted emergency managers to shut down this key crossing over the Mississippi River to vehicle traffic above and maritime traffic below. How long had the crack been there and how close did this iconic bridge come to failing? I’m Grady and this is Practical Engineering. Today, we’re discussing the Memphis I-40 bridge incident.

The Hernando de Soto Bridge carries US Interstate 40 across the Mississippi River between West Memphis, Arkansas and Memphis, Tennessee. Opened for traffic in 1973, the bridge’s distinctive double arch design gives it the appearance of a bird gliding low above the muddy river. I-40 through Tennessee and Arkansas is one of the busiest freight corridors in the United States, so the Mississippi River bridge is a vital east-west link, carrying an average of 50,000 vehicles per day. Although it was built in the 70s, the bridge has had some major recent improvements. It’s located in a particularly earthquake-prone region called the New Madrid Seismic Zone. Starting in 2000 and continuing all the way through 2015, seismic retrofits were added to the bridge to help it withstand a major earthquake and serve as a post-earthquake lifeline link for emergency vehicles and the public. ARDOT and TDOT share the maintenance responsibilities for the structure, with ARDOT in charge of inspections.

On May 11, 2021, a climbing team from an outside engineering firm was performing a detailed inspection of the bridge's superstructure. During the inspection, they noted a major defect in one of the steel members below the bridge deck. The crack went through nearly the entire box beam with a significant offset between the two sides. Recognizing the severity of the finding, several of the engineers called 911 to alert local law enforcement agencies and shut the bridge down to travel above and below the structure. This decision to close the bridge snarled traffic, forcing cars and trucks to detour over the older and smaller I-55 bridge nearby. It also created a backup of hundreds of barges and ships needing to pass north and south on the Mississippi River below the bridge. Knowing how significant an impact closing the bridge would be on such a vital corridor, how did engineers know to act so quickly and decisively? In other words, how important is this structure member? To explain that, we need to do a quick lesson on arch bridges. There are so many ways to span a gap, all singular in function but remarkably different in form. One type of bridge takes advantage of a structural feature that’s been around for millennia: the arch.

Most materials are stronger against forces along their axis than those applied at right angles (called bending forces). That’s partly because bending forces introduce tension in structural members. Instead of beams that are loaded perpendicularly, arch bridges use a curved element to transfer the weight of the bridge to the substructure using almost entirely compressive forces. Many of the oldest bridges used arches because it was the only way to span a gap with materials available at the time (stone and mortar). The Caravan Bridge in Turkey was built nearly 3,000 years ago but is still in use today. Even now, with the convenience of modern steel and concrete, arches are a popular choice for bridges. When the arch is below the roadway, we call it a deck arch bridge. Vertical supports transfer the load of the deck onto the arch. If part or all the arch extends above the roadway with the deck suspended below, it’s a through-arch bridge like the Hernando de Soto.

Arches can be formed from many different materials, including steel beams, reinforced concrete, or even stone or brick masonry. The I-40 Mississippi River bridge has two arches made from a lattice of steel trusses. One result of compressing an arch is that it creates horizontal forces called thrusts. So, arch bridges normally need strong abutments at either side to push against that can withstand the extra horizontal loads. So why do the arches of this bridge sit on top of spindly piers? Just from looking at it, you can tell that this support was not designed for horizontal loading. That’s okay, because the Hernando de Soto uses tied arches. Instead of transferring the arch thrusts into an abutment, you can tie the two ends together with a horizontal chord. This tie works exactly like a bowstring, balancing the arch’s thrust forces with its resistance to tension. Tied arch bridges don’t transfer thrust forces to their supports, meaning they can sit atop piers designed primarily for vertical loads.

This tension member is the subject of our concern. The crack in the Hernando de Soto bridge went right through one of the two arch ties on the eastern span. It’s hard to understate the severity of the situation. These ties are considered fracture-critical members - those non-redundant structural elements subject to tension whose fracture would be expected to result in a collapse of the entire bridge. Obviously, this member did fracture without a collapse, so there may be a dispute about whether it truly qualifies as fracture-critical, but suffice it to say that losing the tie on a tied-arch bridge is not a minor issue. So why would a tension member like this crack?

Let me throw in a caveat here before continuing. Structural engineering is not an armchair activity. Forensic analysis of a failure requires a tremendous amount of information before arriving at a conclusion, including structural analysis, material testing, and review of historical information. Without such an investigation, the best we can do is speculate. A detailed forensic review will almost certainly be performed, and then we’ll know for sure. With all that said, there’s really only one reason that a steel member would crack like what’s shown in the photos of the I-40 bridge.

When steel fails, it is usually a ductile event. In other words, the material bends, deforms, and stretches. But, steel can experience brittle failures too, called fractures, where little deformation occurs. And the primary reason that a crack would initiate in a steel tension member of a bridge is fatigue. Fatigue in steel happens because of repeated cycles of loading. Over time, microscopic flaws in the material can grow into cracks that open a small amount with each loading cycle, even if those loading cycles are well below the metal’s yield strength. If not caught, a fatigue crack will eventually reach a critical size where it can propagate rapidly, leading to a fracture. Bridges are particularly susceptible to fatigue because traffic loads are so dynamic. This bridge sees an average of 50,000 vehicles per day. That is tens of millions of load cycles every year.

Fatigue is common on steel members that have been welded because welding has a tendency to introduce flaws in the material. When weld metal cools, it shrinks generating residual stress in the steel. These stress concentrations are where most fatigue cracks occur. And the box tie member at the I-40 bridge is a built-up section. That means it was fabricated by welding steel plates together. It’s a common way to get structural steel members in whatever shape the design requires. But, if not carefully performed, the welds have the potential to introduce flaws from which a fatigue crack can propagate.

Of course, these ties aren’t purely tension members holding the two sides of the arch together. If they were, the load cycles would probably be a lot less dynamic. The ties don’t support these lateral beams below the road deck - that’s done by the suspender cables hanging from the arch above - but they do have a rigid connection. That means when the deck moves, the tension ties move with it, potentially introducing stresses that could exacerbate the formation of a crack. Again, without a detailed structural model, it’s impossible to say how the dynamic cycles of traffic forces are distributed through each member. We can’t say whether the original design or the seismic retrofits had a flaw that could have been prevented. Fatigue and fractures are difficult to characterize, and in some cases inevitable given the construction materials and methods, even with a good design. That’s why inspections are so important. One of the biggest questions everyone is asking, and rightly so given the severity of the situation, is “how long has this structural member been cracked?”

National bridge standards require inspections for highway bridges every two years. Bridges with fracture-critical members, like this one, are usually inspected more frequently than that, and inspection of those members has to be hands-on. That means no drones or observations from a distance - a human person has to check every surface of the steel from, at minimum, an arm’s length away. Given those requirements, you would think that this crack, discovered in May of 2021 did not exist the year before. Unfortunately, ARDOT provided a drone inspection video from 2 years earlier, clearly showing the crack on the tie beam. Although it hadn’t yet grown to its eventual size, the crack is nearly impossible to miss. And it could have been there well before that video was shot. One amateur photographer who took a canoe trip below the bridge in 2016 shared a photo of the same spot, and it sure looks like there’s a crack.

Bridge inspections are not easy. Even on simple structures they often require special equipment - like snooper trucks - and closing down lanes of traffic. Complicated structures like the I-40 bridge require teams of structural engineers trained in rope access climbing to put eyes on every inch of steel. And even then, cracks are hard to identify visually and can be missed. Inspectors are humans, after all. But, none of that justifies this incident, especially given how large and obvious the fracture was. ARDOT announced that they fired an unnamed inspector who was presumably responsible for the annual inspections on this bridge. We don’t know many details of that situation, but I just want to clarify that it’s not a solution to the problem. If your ability to identify a major defect in a fracture-critical member of a bridge hinges on a single person, there’s something very wrong with your inspection process. Quality management is an absolutely essential part of all engineering activities. We know we’re human and capable of mistakes, so we build processes that reduce their probability and consequences.

That includes quality assurance which are the administrative activities of verifying that work is being performed correctly such as making sure that bridges are inspected by teams and that inspectors are properly trained. It also includes quality control, the checks and double-checks of work products like inspection reports. And, quality management should be commensurate with the level of risk. In other words, if an error would threaten public safety, you can’t just leave it up to a single person. Put simply and clearly, there is absolutely no excuse for this crack to have sat open on the bridge’s tie member for as long as it did.

This story is ongoing. As of this video’s writing the bridge is closed to traffic indefinitely. But, that doesn’t mean the incident is over. There’s a chance that, as the forces in the bridge redistributed with the damage to this vital member, other structural elements became overloaded. The second tension tie may have taken up much of its partner's stress and the pier supporting the arch may have been subject to a lot more horizontal force than it was designed to withstand. In addition, bridges are full of repetitive details. If this crack could happen in one place, there’s a good chance similar cracks may exist elsewhere. The Federal Highway Administration recommends that, when a fatigue crack is found, a special, in-depth inspection be performed to look for more. That will involve hands-on checking of practically every square inch of steel on the bridge, and probably non-destructive tests that can identify defects like using x-rays, magnetic particles or dyes that make cracks more apparent.

The repair plan for the bridge is already in progress. Phase 1 was to temporarily reattach the tie using steel plates to make the bridge safe for contractors. The design for Phase 2 will depend entirely on the findings of detailed structural analysis and forensic investigation. In the meantime, it’s clear that ARDOT and TDOT have some work ahead of them. Most importantly, they need to do some reckoning with their bridge inspection procedures, and thank their lucky stars that this fracture didn’t end in catastrophe. There’s no clear end in sight for the inconvenienced motorists needing to cross the Mississippi River, but I’m thankful that they’re all still around to be inconvenienced. Thank you, and let me know what you think.