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

On Jun 11, 2023, a fuel tanker truck caught fire on an exit underneath Interstate 95 in Northeast Philadelphia. The fire severely damaged the northbound bridge, eventually causing it to collapse. Sadly, the driver of the truck was killed in the crash, but fortunately there were no other injuries or deaths. Although it didn’t collapse, PennDOT officials said that the southbound bridge was also compromised in the fire and had to be demolished. All of I-95 through a major part of Philly was shut down for a couple of weeks, and (as of this writing) the off-ramp underneath it will likely will be closed for the near future as the bridges are rebuilt. Fires at bridges haven’t really been a major concern for transportation engineers in the past, but increasingly, they’re becoming a more serious problem. The cost to rebuild I-95 may pale in comparison to the indirects costs of having the highway shut down for so long. Or maybe not - it’s hard to say. Let’s talk about what happened and how engineers think about fire hazards at bridges. I’m Grady and this is Practical Engineering. In today’s episode, we’re talking about the I-95 bridge collapse.

The details in the intro are really all the details we know at the moment. A tanker truck crashed below the bridge, eventually leading it to collapse. There are some wild videos taken by motorists on I-95 during the fire, probably only minutes before the bridge fell, with the road deck sagging significantly. Fortunately, emergency crews were able to shut down the highway before anyone was seriously injured. The National Transportation Safety Board has had a crew on site to begin their investigation, but knowing the meticuluous pace at which they work, it will likely be a year or more before we get their report. But, the basics are pretty clear already. And in fact, even though we don’t know all the details of this particular event, we’ve seen similar collapses on several occasions. And the sequence of events is almost always the same.

In 2002, fire caused the main span of the I-20 interchange in Birmingham, Alabama to sag by 3 meters or 10 feet, necessitating replacement of the bridge. Cause of the fire? A crashed fuel tanker. In 2006, a temporary part of the Brooklyn Queens Expressway in New York collapsed during a fire. Again, the cause of the fire was crashed tanker truck under the bridge. 2007: The MacArthur Maze Interchange in Oakland, California collapsed during a fire from a crashed fuel tanker. 2009: A bridge over I-75 in Detroit collapsed after a tanker truck crashed into the overpass. 2013: A diesel tanker crash damaged a bridge in Harrisburg, Pennsylvania that had to be demolished. 2014: A gasoline tanker exploded on I-65 in Tennessee, destroying two overpass bridges. Of course, this isn’t just a US phenomenon. In 2012, a tanker overturned in Rouen (roo-AHN), France damaging the Mathilde bridge over the Seine (sehn) River and requiring part of it to be replaced. And of course, bridge fires don’t only come from tanker truck crashes. In 2017, a massive fire under I-85 in Atlanta, Georgia that resulted in collapse happened because someone set fire to construction materials stored below the bridge.

Incredibly PennDot was able to reopen this bridge a mere two weeks after it collapsed with a pretty clever solution. Rather than wait until the original bridges could be rebuilt to get I-95 back open, they decided to simply build a temporary embankment instead. After the demolition of the fire-damaged bridges was complete, the less-critical off-ramp below the bridges remained closed so that crews from PennDOT’s emergency contractor, Buckley & Company, could fill the area in and simply pave over the top. My friend Rob, built a little model of this on his channel you should check out after this.

This temporary highway wasn’t built using soil or crushed rock, the typical backfill material used in roadway embankments (at least not mostly). That stuff is heavy, so most roadway embankments have to be built slowly to allow time for the foundation to settle as each layer is added to the top, a process that can take months or even years. (Not an option in this case.) Plus there are sewer lines below the existing road that could have been overloaded by a mountain of backfill on top. Instead, the design called for lightweight backfill called foamed glass aggregate. I have a whole video we produced earlier this year about different types of lightweight backfills and how they work, so check that out if you want to learn more. This foamed glass aggregate is not cheap, many times the cost of standard backfill. But, it’s strong enough in compression to support the overlying roadway without overloading the foundation below which would lead to settlement over time or damage to underground pipes. I actually have some of it here in the studio. It feels kind of like floral foam, just a lot stronger.

The other innovative design aspect of the temporary embankment is that it leaves room on either side for the permanent repairs to the bridge. Eventually the City needs this off-ramp back open for travel, after all. The emergency embankment is sited in the center of the right-of-way to give as much space as possible for the next phase of the repairs that will replace the bridges. That also required that both sides of the embankment have a retaining wall, in this case mechanically stabilized earth walls that use reinforcing elements between each layer of backfill to keep the tall structure from collapsing. I’ve also done a few videos explaining MSE retaining walls if you want to learn more about them. The basics are easy to see in this drone footage. The reinforcement turns the backfill itself into a stable wall, making it able to both withstand vertical loads and hold back the rest of the embankment backfill. I built a little MSE cube many years back and put one of my car tires on top to show how strong it really is. Looks like the cube built by PennDOT will hold up even more cars than mine!

To their credit, PennDOT kept a live feed of construction going for most of the project. You can see the flurry of activity as workers and equipment build the embankment up to the level of the highway on either side. Traffic was rerouted onto the temporary embankment starting June 24th. But, why did a fire cause so much damage in the first place?

We, collectively, put a tremendous amount of research and engineering into the fire resistance of buildings and tunnels, but when it comes to fires at bridges, we know a lot less. In fact, most bridges in the world are designed with little, if any, consideration for fire resistance. Neither the Eurocode or the US bridge design criteria address fires or have any guidelines or requirements for how or when to engineer against them. Of course, we think about thermo-mechanical behavior of bridges all the time. I have a video all about thermal expansion and contraction of large structures. But, when you get above a few hundred degrees, there just hasn’t been much consideration. And the reasons for that are kind of obvious, at least at first glance. Less then 3% of US bridge failures between 1980 and 2012 resulted from fire. Compare that to hydraulic damage from scour and flooding that makes up nearly 50% of all failures. That alone isn’t enough reason to ignore fires in the design codes. After all, earthquakes make up only 2% of those failures, and we spend considerable resources and engineering to design bridges against seismic loads. But, you also have to consider safety. Even when bridges collapse due to fire, people are rarely injured because most places have robust emergency response capabilities. Roads are closed well before a fire is able to significantly weaken a structure. The relative infrequency of serious fires at bridges and their unlikelihood of causing a public safety issue mean that we just don’t devote a lot of resources to the problem right now… at least not proactive resources.

The National Fire Protection Association does have some guidance for fires at bridges, but it’s nebulous. They don’t recommend what fire loads should be considered, how to protect a bridge against fire, or how to analyze a structure after a fire. And, the guidelines only apply to bridges longer than 1000 feet or 300 meters. When you think about bridges, you often think about these long-span structures over major valleys or waterbodies. They’re iconic, but they’re also just the tip of the iceberg when it comes to bridges. In the US alone, there are over half a million bridges in service today, and nearly all of them are short-span bridges used mainly for grade separation (to let streams of traffic cross each other without interruption). They’re overpasses, structures you traverse every day without even noticing. But you definitely notice when one of these bridges is taken out of service. Bridges used for grade separation are more vulnerable to fires because, unlike the ones over waterways, a tanker truck can crash underneath one where the fire is most likely to cause damage. But protecting them is not as easy as it might seem.

A robust engineering guideline for design of bridges against fires would actually be pretty complicated. There are so many different variables and scenarios, and we really don’t have any collective agreement about what level of protection is appropriate. What would be the fuel source, footprint, flame height, intensity, and duration of the fire? With that information, we can try to predict the response. How does the heat transfer from the fire to the structural elements through radiation and convection, and how much do the structural elements increase in temperature as a result? These are tough questions to answer on their own, but they still don’t get to the heart of the matter, because what we really care about is how those structural elements respond. What happens to the material properties of steel and concrete when they increase in temperature way beyond what they were designed to handle? And more importantly, how does the overall structure behave? You have thermal expansion, weakening of materials, loss of stiffness, load redistribution, and a lot more. This is an extremely complicated scenario just to characterize through engineering, let alone to design protections against.

And the biggest question right now seems to be “Should we?” Bridge fires are primarily economic problems. As I mentioned, they rarely result in injuries or life safety concerns because the roadway is closed ahead of failure. But that doesn’t mean there aren’t impacts, and if you regularly drive on I-95 in Philadelphia (or any of the other roadways I mentioned before), you definitely know what I’m talking about. Replacing a bridge is an expensive endeavor, but the indirect costs that come with having a major highway closed are often higher. When the MacArthur Maze in Oakland collapsed from a tanker fire, the indirect costs of having the bridge out was estimated in the millions of dollars per day, way more than the cost of reconstruction. In fact, the rebuilding job was bid with a bonus to the contractor for each day ahead of schedule they were able to finish the job. SFGate has a great story about how they got that bridge reopened in just 26 days that I’ll link below.

Road construction often seems slow, and part of the reason is to keep the costs down. It’s not very efficient to dedicate expensive resources like equipment, engineers, and specialty construction crews to a single project. Instead, resources get spread across many jobs so that people, crews, vendors, and equipment can stay busy. Even if seemingly slow progress is often frustrating to see, it’s usually less a result of incompetence or corruption and more just government agencies trying to be good stewards of limited public resources. But a major bridge failure changes that math. Fabricators, equipment suppliers, painters, truckers, operators, and laborers are all willing to set aside their other obligations for the right price. And government agencies will happily devote their engineers and inspectors to sit and wait for questions or problems to arise on a single job if the politicians can deliver the funds for it. In the industry, they call it “accelerated construction.” It comes at a steep price, but sometimes that price is worth it.

Like the MacArthur Maze, I-95 is a busy stretch of roadway, carrying roughly 150,000 vehicle trips per day. Some of that traffic was redistributed to other routes, but some of the capacity was simply lost while the roadway was out. That means deliveries were cancelled, workers had trouble reaching their jobs, emergency response times went up, and more. The gridlock was not as apocalyptic as predicted, but there were still some major slowdowns. In most large American cities, unexpectedly closing a major highway has real economic consequences through lost commercial shipping, lost productivity, lost retail sales, more wear and tear on roadways not meant to accommodate the detour traffic, and a lot more. And those indirect costs play into the consideration in whether or not its worth it to include fire protection in the design of highway bridges.

But what’s on the other side of that equation? Of course it would have been worth the cost to protect this one bridge in Philly from a tanker fire if we knew it was going to happen, but would it have been worth the cost of protecting all the bridges just in case? Or is that gold-plating our infrastructure where it’s not really needed? We know adding highway capacity induces traffic demand, but we also know the corollary. Reducing capacity decreases traffic demand as people find alternatives to making trips in cars, and maybe a highway bridge outage isn’t quite as big a deal as the politicians and news coverage suggest. And maybe investing in some diversity in our transportation infrastructure and giving people better alternatives to driving can do more good than putting that money toward protecting bridges against the unlikely event of a fire.

Like a lot of things in engineering, the costs and risks and alternatives aren’t that easy to weigh out. Your answer might depend on how many fuel tanker trucks you see on your everyday commute. The International Associaiton for Bridge and Structural Engineering has a group working on guidelines for designing bridges against fire hazards. That’s a long way from incorporating fire protection in the design codes, but it will at least give engineers some tools to include fire resistance in designs where the situation calls for it. That group is scheduled to finish their work later in 2023, but hopefully PennDot is able to get I-95 fully repaired before then.