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. In today’s episode 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. Watch what happens if I simulate a crack in
my homemade bridge model. 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 for watching, and let me know what
you think.
It was great to get a decent explanation of what’s wrong with this bridge. It seems like they’re incredibly lucky that it didn’t collapse.
How is it possible that this wasn't discovered from the drone inspection footage that shows it?
Grady does a great job with his videos - there are toms of them on his YT channel that explain a lot of fundamental (and many beyond fundamental) concepts in plain language than can be understood by most.
Great video!
I really appreciate that this video pointed out that if your process relies on a single person to catch major damage like this, then there's something wrong with your process.
It is interesting that there's evidence (some even from earlier inspections) that this crack took years to propagate to the point it was at. Hopefully that means that there isn't an impending catastrophic failure, or even another failure like this one elsewhere on the bridge.
Seems like this was a failure in the inspection process for sure and maybe from the studies we'll learn there were other mistakes in the design of the original structure or the retrofits that were performed more recently.
Wow. So that crack was there likely since at least 2016, possibly longer? That's nuts.
The guy who inspected it was terminated, also they are inspecting the other bridges he was responsible for. There is a bridge on 55 there is 3 miles south of that bridge that is as old as that bridge. They are looking to Inspect it as soon as possible
Besides the points everyone else has made, I really want to point out how Grady was super responsible about assigning blame and root cause - namely, that you cannot say for sure at this time. Too many armchair engineers are happy to point the finger even before initial collection of evidence is complete.
The redundancy of inspectors and quality control folks is re-assuring. I suspect the crack was missed for the same reasons 100% inspection NEVER works. Humans, it seems, remain human.
Still lots of questions in my mind. Was there a latent crack in the beam? Did corrosion have anything to do with causing/perpetuating the crack? Why didn't the bridge catastrophically fail? Is the original design faulty?