What Really Happened at the Hernando de Soto Bridge?

Summary

In May 2021, a critical crack was discovered in the Hernando de Soto Bridge's arch tie, a key structure of the I-40 bridge over the Mississippi River. This fracture in a non-redundant member threatened the bridge's integrity, prompting an immediate shutdown. The incident raised questions about the bridge's maintenance and inspection procedures, especially given the bridge's importance in a high-traffic freight corridor. The cause of the crack is speculated to be fatigue from the dynamic traffic loads, and the response highlights the necessity for rigorous inspection processes to ensure public safety.

Takeaways

• 🚧 In May 2021, a critical crack was discovered in the Hernando de Soto Bridge's structural member, prompting an immediate shutdown to prevent potential failure.
• 🛣️ The bridge, part of US Interstate 40, is a vital east-west link between West Memphis, Arkansas, and Memphis, Tennessee, carrying about 50,000 vehicles daily.
• 🔨 The bridge features a distinctive double arch design and has undergone seismic retrofits to withstand earthquakes in the New Madrid Seismic Zone.
• 🔍 The crack was found during a detailed inspection by an external engineering firm, highlighting the importance of regular maintenance and inspection.
• 🚨 The decision to close the bridge was crucial for safety, causing significant traffic disruptions and demonstrating the bridge's critical role in regional transportation.
• 📚 Arch bridges, like the Hernando de Soto, use compressive forces rather than bending forces, relying on arch ties that are susceptible to fatigue and fracture.
• ⚒️ The bridge's tied arch design allows it to sit on piers primarily designed for vertical loads, as the arch ties handle the horizontal thrust forces.
• 🔧 The crack in the bridge's tie was a significant concern because these ties are considered fracture-critical, meaning their failure could lead to bridge collapse.
• 🔎 The cause of the crack is speculated to be fatigue due to repeated loading cycles, which is common in welded steel members like the bridge's arch ties.
• 🔍 Inspections are crucial for detecting such issues, but the bridge's crack was missed in previous inspections, raising questions about the effectiveness of the inspection process.
• ⚠️ The incident has prompted a reevaluation of inspection procedures and a deeper inspection for similar issues, emphasizing the need for rigorous quality management in engineering.

Q & A

What was the significant event that occurred on the Hernando de Soto Bridge in May 2021?

In May 2021, inspectors discovered a major crack in one of the steel members of the Hernando de Soto Bridge, which led to the immediate closure of the bridge to both vehicle and maritime traffic.

What is the importance of the Hernando de Soto Bridge?

The Hernando de Soto Bridge is a crucial east-west link carrying US Interstate 40 across the Mississippi River, with an average of 50,000 vehicles per day, making it a vital part of the freight corridor between West Memphis, Arkansas, and Memphis, Tennessee.

What is the distinctive design feature of the Hernando de Soto Bridge?

The bridge has a distinctive double arch design, giving it the appearance of a bird gliding low above the river, which is both aesthetically pleasing and structurally sound.

Why was the bridge retrofitted with seismic improvements?

The bridge is located in the New Madrid Seismic Zone, an earthquake-prone region, so seismic retrofits were added from 2000 to 2015 to help the bridge withstand a major earthquake and serve as a post-earthquake lifeline.

What is the role of ARDOT and TDOT in maintaining the Hernando de Soto Bridge?

ARDOT and TDOT share maintenance responsibilities for the bridge, with ARDOT being in charge of inspections, while TDOT assists in other maintenance tasks.

How did the crack in the bridge's structure go unnoticed for a significant period?

Despite national bridge standards requiring inspections every two years, and the bridge having fracture-critical members that necessitate more frequent inspections, the crack was missed in previous inspections, possibly due to human error or inspection oversight.

What is the significance of the crack being in a tied arch bridge's tension member?

The crack in the tension member, specifically one of the arch ties, is significant because tied arch bridges rely on these ties to balance the arch's thrust forces. Losing a tie can lead to the collapse of the entire bridge structure.

What could be the potential cause of the crack in the steel tension member?

The primary reason for a steel tension member to crack is fatigue, which occurs due to repeated cycles of loading. Microscopic flaws can grow into cracks over time, especially in welded sections where residual stress concentrations can initiate fatigue cracks.

What are the consequences of the bridge closure on traffic and transportation?

The closure of the bridge caused significant traffic disruptions, forcing vehicles to detour to the older I-55 bridge and creating a backup of barges and ships on the Mississippi River below the bridge.

What steps are being taken to address the issue with the Hernando de Soto Bridge?

The bridge is closed indefinitely for repairs, which include a temporary reattachment of the tie using steel plates for contractor safety. A detailed structural analysis and forensic investigation will determine the design for the second phase of repairs.

What broader implications does this incident have for bridge inspection procedures?

This incident highlights the need for a thorough review and potential overhaul of bridge inspection procedures, emphasizing the importance of quality management, including quality assurance and control, to prevent such critical oversights in the future.

Outlines

00:00

🌉 The Memphis I-40 Bridge Crack Incident

In May 2021, inspectors discovered a critical crack in the Hernando de Soto Bridge, a vital I-40 crossing over the Mississippi River between Arkansas and Tennessee. This led to an immediate shutdown to prevent potential collapse. The bridge, opened in 1973, is a key freight corridor with 50,000 vehicles daily. Despite recent seismic retrofits, the crack's discovery raised questions about its age and maintenance. The bridge's distinctive tied-arch design, which uses compressive forces and horizontal ties to balance thrust, was compromised by the crack in one of its arch ties. This fracture-critical member's failure could lead to bridge collapse, emphasizing the urgency of the situation.

05:05

🔍 Investigating the Hernando de Soto Bridge Crack

The script delves into the potential causes of the steel tension member's crack, suggesting fatigue as the likely culprit due to repeated traffic load cycles. Despite the bridge's significant traffic, fatigue was possibly exacerbated by poor welding practices introducing flaws. The discussion highlights the challenges of bridge inspections, the importance of quality management in engineering, and the consequences of human error. It also addresses the failure of inspection processes, as evidence suggests the crack was present in a drone inspection two years prior but was overlooked.

10:10

🛠️ Addressing the Bridge Crisis and Future Implications

The script concludes with a discussion on the necessary steps for bridge repair and the broader implications for bridge maintenance. It emphasizes the need for thorough inspections, including non-destructive testing, and a detailed structural analysis to assess potential overloading of other elements. The incident underscores the importance of robust quality management processes to prevent such issues. The bridge remains closed, and while the immediate focus is on safety and repair, the incident serves as a stark reminder of the fragility of infrastructure and the critical need for diligent maintenance and inspection.

Keywords

💡Hernando de Soto Bridge

The Hernando de Soto Bridge is a major bridge that carries US Interstate 40 across the Mississippi River between West Memphis, Arkansas, and Memphis, Tennessee. It is an iconic structure with a distinctive double arch design, resembling a bird gliding above the river. The bridge is crucial for the transportation of an average of 50,000 vehicles daily, making it a vital east-west link. The video discusses a significant structural issue with the bridge, emphasizing its importance in regional infrastructure.

💡Structural Crack

A structural crack refers to a fracture or break in a structural member, which can compromise the integrity and safety of a bridge or building. In the context of the video, inspectors discovered a crack in a major steel member of the Hernando de Soto Bridge, which raised serious concerns about the bridge's stability. The crack's discovery led to the immediate closure of the bridge to prevent potential collapse.

💡Seismic Retrofit

Seismic retrofitting is the process of upgrading and reinforcing structures to withstand seismic events, such as earthquakes. The video mentions that the Hernando de Soto Bridge underwent seismic retrofits from 2000 to 2015 to improve its resistance to earthquakes, highlighting the bridge's location in the earthquake-prone New Madrid Seismic Zone.

💡Fracture-Critical Members

Fracture-critical members are structural elements whose failure would likely result in the collapse of the entire structure. The video discusses that the cracked tie member of the Hernando de Soto Bridge is considered fracture-critical, emphasizing the severity of the crack and the potential risk it posed to the bridge's stability.

💡Fatigue

Fatigue in materials science refers to the weakening of a material due to repeated loading and unloading, which can lead to cracks and eventual failure. The video speculates that fatigue could be the cause of the crack in the bridge's steel member, due to the millions of load cycles from heavy traffic the bridge experiences annually.

💡Tied Arch Bridge

A tied arch bridge is a type of arch bridge where the thrust of the arch is balanced by a tension member, or tie, rather than being transferred to abutments. The video explains that the Hernando de Soto Bridge uses tied arches, which allows the arches to sit atop piers designed for vertical loads, making the bridge's design both structurally efficient and visually distinctive.

💡Inspection and Maintenance

Inspection and maintenance are routine activities performed to ensure the safety and longevity of infrastructure like bridges. The video discusses the importance of these activities in the context of the Hernando de Soto Bridge, where a missed inspection could have led to a catastrophic failure. It also touches on the need for improved inspection processes following the incident.

💡Quality Management

Quality management encompasses the organizational and administrative activities related to ensuring that work is performed correctly and to a high standard. The video emphasizes the importance of quality management in engineering, particularly in bridge inspections, to prevent human error and ensure public safety.

💡Forensics in Engineering

Forensic engineering is the investigation of materials, products, structures, or components that fail or do not operate as intended. The video mentions that a detailed forensic review would likely be conducted to understand the cause of the crack in the Hernando de Soto Bridge, which would inform future maintenance and repair strategies.

💡Repair and Reinforcement

Repair and reinforcement are processes used to restore the structural integrity of damaged infrastructure. The video outlines the initial steps taken to temporarily reattach the cracked tie member with steel plates, indicating the start of a broader repair and reinforcement plan for the bridge.

Highlights

In May 2021, a critical crack was discovered in the Hernando de Soto Bridge's structure, prompting an immediate shutdown.

The bridge, a vital link between Arkansas and Memphis, Tennessee, carries an average of 50,000 vehicles daily.

The bridge's double arch design, resembling a bird gliding, is both aesthetic and functional.

Seismic retrofits were added from 2000 to 2015 to enhance the bridge's resilience in the earthquake-prone New Madrid Seismic Zone.

The crack was found in a steel member below the bridge deck during a detailed inspection by an outside engineering firm.

The decision to close the bridge caused significant traffic disruptions and highlighted the importance of the structure.

Arch bridges, like the Hernando de Soto, use compressive forces to transfer weight, making them efficient for spanning large gaps.

The bridge's tied arch design allows it to sit on piers without transferring horizontal thrust forces.

The crack in the bridge's arch tie is a serious concern as these ties are considered fracture-critical members.

Fatigue, caused by repeated loading cycles, is speculated as the primary reason for the steel member's crack.

Welding, a common fabrication method, can introduce flaws that lead to fatigue cracks in steel bridges.

The bridge's inspection history revealed missed opportunities to detect the crack earlier, raising questions about inspection procedures.

The Arkansas Department of Transportation fired an inspector over the incident, indicating a potential flaw in quality management.

The bridge remains closed indefinitely, with repairs and a detailed structural analysis underway to ensure safety.

The incident underscores the importance of thorough and regular inspections to prevent potential bridge failures.

The bridge's closure has caused significant inconvenience, but the focus remains on public safety and structural integrity.

Transcripts

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.