Quick disclaimer: I’m moving to a new
office, so forgive the mess, and the reverb. 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, France damaging
the Mathilde bridge over the Seine 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. It’s easy to look at a big city like Philadelphia
and be a little bit overwhelmed trying to wrap your head around how it grew into the
place it is today. Cities are complicated; the result of millions of little decisions
by citizens, politicians, engineers, and urban planners. But every once in a while,
all those little decisions add up to something almost serendipitous, like the canals of Venice,
Central Park in New York City, or the incredible Metro System in Shanghai. My friend Dave from the
City Beautiful channel has a whole series about the biggest moments in the greatest cities on
Earth. I was blown away to see the ways Shanghai built out the world’s largest metro system in less
than 30 years in the soft clay soils of the area. Maybe you’ve noticed what I have over the past
few years, which is that all my favorite TV networks are just running reality shows, and
the best video content that I actually enjoy watching is being made my independent creators.
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who have real education and experience to back it up. Dan of the Coding Train is college professor
in computer art. Devin from Legal Eagle is really a lawyer. And Dave of City Beautiful is an urban
planning professor at CalPoly. There’s just something special about videos that are written
and produced by a subject matter expert instead of someone who just knows how to produce a video.
Dave’s series, Great Cities, is only available on Nebula, the streaming platform built by and for
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