[Note that this article is a transcript of the video embedded above.]

On January 28, 2022, about an hour before dawn, the four-lane Fern Hollow Bridge in Pittsburgh, Pennsylvania, collapsed without warning. Five vehicles, including an articulating bus, fell with the bridge, and another car drove off the abutment after the collapse, not realizing the bridge was gone. Although there were no fatalities, several people in the vehicles were seriously injured. And this bridge had been listed as being in ‘poor condition’ for over a decade. Anyone who walked by the supports in the park below would have had reason to question its safety, as seen in this sadly prophetic tweet from 2018, four years before the collapse. So, why was it left open in this state?

While some initial findings were released earlier, the official NTSB report was delivered to the public this year in February, more than two years after the collapse and over a year after the replacement bridge was built and open. The report included the use of some really cool technology for the forensic study of structures and revealed systemic flaws in how we inspect, analyze, and prioritize repairs for bridges. In fact, the NTSB issued recommendations to basically every organization involved in this bridge from the bottom to the top, and they referenced that tweet that got so much attention. This is a crazy case study of how common sense can fall through the cracks of strained budgets and rigid oversight from federal, state, and city staff. And the lessons that came out of it aren’t just relevant to people who work on bridges. It's a story of how numerous small mistakes by individuals can collectively lead to a tragedy. I’m Grady, and this is Practical Engineering.

The Fern Hollow Bridge was opened in 1973, replacing an aging arch bridge built at the beginning of the 20th century, crossing Frick Park and Fern Hollow Creek. The 1973 bridge used a K-Frame design with continuous steel girders supporting the deck, each with two angled steel legs supported on concrete thrust blocks. At the time, the bridge’s design was celebrated because it blended well into its settings. It was featured on the cover of the 1974 edition of Prize Bridges by the American Institute of Steel Construction. And a big part of why it looked so harmonious with the park below was the type of steel used for the design.

The Fern Hollow bridge was fabricated from weathering steel, sometimes referred to by its genericized trademark, COR-TEN. And it was developed commercially right there in Pittsburgh by US Steel in the 1930s. Unlike most types of steel, whose rust can continually flake away, exposing more material to corrosion, the oxide layer on weathering steel (called its patina) is more stable and protective, shielding the underlying material against exposure to the elements. In that way, weathering steel acts kind of like aluminum, which protects itself from corrosion in a similar way. And, architecturally, it’s a nice material. You get this rustic look that can give structures a more comfortable and less obtrusive appearance. But weathering steel has a limitation: For a stable patina to form, the material has to stay mostly dry. If water pools or the steel is kept damp for extended periods, that patina of rust isn’t enough to protect the underlying steel, and it will continue to corrode. Corrosion of structural steel is called section loss by engineers, and it's easy to see why in these photos of the bridge.

What’s more alarming than what’s in those photos is where they came from: inspection reports of the bridge. It’s not that this deterioration somehow went unnoticed. The bridge supports were clearly visible from a popular walking trail. Between 2005 and 2021, this bridge was inspected a total of 14 times! In those reports, you get a clear and vivid story of its decline. First were the drainage problems. You can see in these images from multiple previous bridge inspections there were drainage grates on the roadway that were 100 percent clogged. Rainwater, and even worse, salty meltwater from the frequent snow that Pittsburgh sees each winter couldn’t follow the prescribed drainage paths off the deck and into the creek below. Instead, that water would leak through the bridge deck, dribbling over the structural steel, and pooling in portions of the legs where webbing and tie-plates could catch puddles of water, leaves, and debris.

The City was aware of the section loss due to these drainage problems for many years before the collapse. Nearly every inspection report noted problems with the drains and the accelerating corrosion that was resulting. In fact, in 2009, the cross-braces connecting each pair of legs were found to be failing, and steel cables were installed as a temporary retrofit until the framing could be replaced. These cables were lightly tensioned to add structural integrity to the bridge but were never meant to be a permanent solution.

You can see the ends of two of these cables in this now-infamous tweet from 2018. Of course, the more notable feature of this image is the fully separated steel cross-brace! That photo was taken about nine years after the temporary cables were installed. And they remained in place for the rest of the bridge's life, which ended up being only four more years. But the cross-bracing between the legs wasn’t the only place where corrosion was an issue. The legs themselves were also fabricated from weathering steel, and that steel was suffering, too. Since 2005, inspection reports marked them in fair to poor condition with areas where the steel had completely rusted through. By 2019, all four legs were given the worst assessment possible for an individual bridge element. According to the code, that should trigger a structural review to check whether the integrity is affected by the poor condition of a structural element, but it was never done. And that’s not all.

An important part of inspecting steel bridges is identifying members that are “fracture critical.” That’s engineering jargon, but the idea is actually pretty straightforward. It’s any piece of steel under tension that lacks redundancy. If it breaks, the bridge collapses. And these types of members get special attention because of their importance, so inspection teams identify them ahead of time to make sure they get a proper look. This drawing shows in green which elements of the bridge were considered to be fracture-critical. Notice that while the girders crossing the span are identified, the legs are not. And, at first glance, that might match your intuitions. Bridge piers, columns, and vertical supports usually don’t experience tension forces, right? They’re in compression. So if there’s a crack or break, the forces just squeeze it together, generally not that big a deal. But K-frame bridges are different. By splaying out the legs, there are loading conditions that can apply bending forces, putting part of each beam in tension. And, this particular bridge had another feature that was absolutely essential to its performance.

Each leg of the bridge was essentially an I-beam: a central web with a top and bottom flange. To simplify the foundation design, each leg had a “shoe”: a tapered end that would connect to the concrete block. It’s clear that the narrower section would have less strength, so larger stiffeners were added to each shoe to strengthen that portion of the leg. These are just steel plates welded to the web and flanges to increase the leg’s rigidity. And I built a little cardboard model to clarify this point. This particular stiffener, called the transverse tie plate, bridges the two flanges right where they taper down. And if I apply a compressive force on the leg, it’s easy to see what kind of force that tie plate experiences as a result. It’s tension. These tie plates were fracture-critical members of the bridge, but never identified as such, and so, even though it was clear they were deteriorating quickly over time, the inspectors never elevated the concerns to a priority level that might have spurred a more immediate response. But there was another opportunity to catch the problem.

In 2013, an inspector was concerned enough about the bridge's safety to recommend that it be reviewed for a load rating. Most bridges are designed to allow any legal load to pass over, but sometimes, either because of an old design or poor structural conditions, it's necessary to limit the weight of vehicles allowed. Engineers analyzed the bridge in 2014 and decided it could only handle 26 tons per vehicle, just over half of its previous rating. When NTSB reviewed that decision in hindsight, they found some pretty serious errors.

For one, the load rating for the bridge was based on a layer of about 3 inches of asphalt paving on top of the concrete road deck. In reality, the bridge had about double that amount. The City’s records of the removal and addition of pavement were poorly kept, so the engineering firm doing the load rating had no idea there was so much extra weight. For two, the engineers didn’t fully account for all the corrosion on the legs. This was partly because inspectors hadn’t cleaned off the rust to measure and report the actual thickness of the remaining steel. Even so, the engineers used a method that distributed the section loss from corrosion evenly along the entire leg, instead of applying it where it actually was, at the shoes and tie plates. That’s a pretty commonly-used simplification that usually generates conservative results (since the worst corrosion is rarely located at the most critical part of a structure), but again, that wasn’t the case for the Fern Hollow Bridge, and no one had recognized how important those tie plates really were.

And for three, those engineers made an incorrect assumption about how the bridge’s legs would buckle. A structural member under compression will buckle at different loads depending on how much restraint it has at the ends. This is something you learn in year one of engineering. If you keep a column from rotating at its ends, you substantially increase the amount of force it can withstand, and with the original cross-bracing between the legs, that would have been the case. But I’m sure I don’t have to tell you that steel cables don’t provide the same support as rigid members. Again, this is engineering 101: “You can’t push a rope.” The cables provided some restraint, but not in the same way that the original bracing could, so the load rating applied to the bridge ended up significantly overestimating its actual capacity. In fact, when NTSB updated the load rating with these errors fixed, they found that the bridge should have been limited to 3 tons, basically nothing for a bridge. In effect, this load rating exercise should have closed the bridge to traffic entirely. These structural issues were exactly what the process was meant to identify. But instead, the bridge stayed open to everyone except the largest of trucks,and here’s what happened, courtesy of NTSB’s animation team.

The transverse tie plate on the southwest leg, weakened by corrosion, failed first under tension, separating the flanges on the leg, and ultimately causing it to buckle. With no redundancy in the supports, the loads had nowhere to go, and so the rest of the bridge fell into the valley below. The articulated bus had both rear-facing and forward-facing cameras on it, which captured some truly harrowing footage of the event. Looking at the rear-facing camera, you can see the western portion of the bridge begin to fail. Keep an eye on the railing, and you can see the exact moment it starts. Once it started, there was no stopping it. Within two seconds, the front-facing camera shows that the collapse had propagated all the way to the eastern abutment, and the bridge fully failed.

Thankfully, the collapse happened during particularly inclement weather. School delays and generally poor conditions meant that traffic was lighter than normal, and the weather also likely kept folks away from the trail underneath. On a fair weather day during rush hour, it wouldn’t be uncommon for the eastbound lanes of the bridge to be fully occupied by heavy traffic, and the trail underneath to be populated with dog walkers, families, or even classes on field trips from nearby schools. The bridge also carried a large gas line, which was severed during the collapse, leading to a major leak and some evacuations, but they got it shut off in time. It really is remarkable that nobody was killed in a failure of this magnitude. But there were still multiple victims needlessly affected for the rest of their lives by the collapse, not to mention the overall erosion of trust in the organizations and engineering systems meant to keep the public safe.

By pure happenstance, President Biden was set to arrive in Pittsburgh on the very day of the collapse to speak in support of the Infrastructure Investment and Jobs Act, so he rearranged his trip to make some remarks at the site. One of the entities supported by the act is the National Highway Performance Program, which ultimately funded the replacement of the collapsed bridge. But at that time, no one understood the full scope of neglect and bad assumptions that led to the gradual, and then sudden, demise of the bridge. In those two years after Fern Hollow Bridge fell, the NTSB conducted numerous interviews with those involved, from the paving contractors to the inspectors to the bridge rating engineers. They performed 3D laser scanning of the bridge components to compare them to as-built conditions. They tested sections of steel for strength and durability. They reviewed all the previous records of the design, construction, and repairs. And they built a detailed finite-element model of the bridge to confirm that the gradual corrosion of one small structural element, the transverse tie plate on the southwest leg, initiated the collapse. And then they documented why it got to that point: the City of Pittsburgh just didn’t fix it.

This figure in the NTSB report tells the story as clearly as I think is possible. From 2005 onward, recommendations from inspection reports to repair parts of the bridge didn’t fall off the list. They just kept being repeated by each new inspection, year after year. Since 2007, every single inspection report that included recommendations said to repair the stiffener plates in the legs that were heavily corroded. These were Priority 2 recommendations, which means the timeframe to complete them is before the next inspection. But it was never done. They didn’t fix the drainage problems that were accelerating the corrosion, they didn’t apply the protective coatings that might have slowed it down, and they never analyzed the capacity of the legs after they were rated the worst possible condition a structural element can have. And, apparently, there was no mechanism to follow up on those recommendations by the state charged with overseeing the bridge inspection program. When there was finally a chance to recognize how deficient the structure really was through a new load rating, the engineers made a few bad assumptions, missed it by a mile, and left the bridge open, a ticking time bomb, for years. (Years, by the way, in which the City still didn’t address the recommendations from inspection reports.)

Due to the nature of the emergency, the site was cleaned up quickly, with a huge crane brought in to remove the bus, and building of the replacement bridge happened on a fast-track schedule. The new bridge uses a more conventional design: pre-stressed concrete girders on vertical piers. The formed stone texture on the columns certainly doesn’t blend into the park as well as the graceful and patinaed K-frame once did, but I doubt anyone involved in the project could stomach another structure built from weathering steel, given the circumstances. The new bridge might not win any awards for beauty, but it could win some for speed. After a colossal effort from the design and construction teams, it opened to limited traffic less than a year after the collapse in December of 2022, and by July the next year, it was fully operational. It would be almost a year later before the NTSB concluded why the previous bridge collapsed, not for the purpose of blame, but to issue recommendations to prevent something like this from recurring in the future. And unlike the recommendations from those inspection reports, many of the NTSB recommendations have already been put into practice.

They published a special report on weathering steel bridges to highlight the specific challenges of keeping them in good condition, and they identified several similar bridges that needed a closer look. The City of Pittsburgh quadrupled their spending on inspection, maintenance, and repairs. And they redid the load ratings on all the bridges they owned, resulting in one bridge being closed until it can be rehabilitated and two more having lane restrictions imposed. PennDOT released a technical bulletin to shore up their bridge inspection program. And even the federal government has implemented a process to identify, prioritize, and follow up on recommendations related to weathering steel bridges.

But as I read through those recommendations from the NTSB, one thing struck me: They all add up to more paperwork. And this is just my own personal opinion as someone who did this kind of work for nearly a decade (not on bridges, but other large infrastructure projects). We have these national inspection standards and procedures - huge documents that you could spend an entire career understanding. We have the Federal Highway Administration overseeing the program, state DOTs charged with carrying it out, individual bridge owners, like Pittsburgh, responsible for inspecting their own bridges, and then the private contractors who do most of the actual work. We have this huge machine with thousands of people, federal, state, and local involvement, and millions of dollars meant to keep the traveling public safe. And what did it do for us when a photo like this is all it would take any reasonable person to say: “This bridge needs to be fixed”?

This big machine, in a lot of cases, has all the work sectionalized out. The inspectors see the bridge up close, but they have no autonomy to do anything but document and give recommendations. It’s not their bridge. But the owners who are charged with the safety of their bridges just see a piece of paper. Each recommendation is just another one on the list of sometimes hundreds of action items, to sort and prioritize and try to find the budget to cover. All the NTSB recommendations feel a little bit like bandaids if the real source of the problem was that no one person in this whole machine had both a full appreciation of the bridge's condition and the authority to do something about it. And if that’s the case, I’m not sure any of those recommendations really fixes that problem. I don’t know what the answer is, and I’m still wrestling with trying to understand how something like this can fall through the cracks of the enormous system we’ve built for the sole purpose of trying to prevent it.

If you take a walk on the Tranquil Trail through Frick Park beside Fern Hollow Creek and look carefully, you can still see remnants of the old bridge. And I’m glad the City left them, because they’re a good reminder that design and construction are two parts of a three-part system for keeping people safe. Maintaining infrastructure is thankless work. Don’t get me wrong, it can be a really rewarding career. Inspections involve a lot of time out in the field seeing cool structures up close. And repair projects are often interesting challenges for contractors. But they’re not rewarding in the same way that designing and building new stuff can be. No one holds a press conference and cuts a big ribbon at the end of a bridge inspection or structural retrofit. Building a new structure is not just an achievement in its own right; it’s a commitment to take good care of it for its entire design life, and then to rehabilitate, or replace, or even close it when it’s no longer safe for the public. And I think this is the perfect case study to show that there’s more we could do to encourage and celebrate that kind of work as well.