How do forensic engineers investigate bridge collapses, like the one in Miami?

The collapsed pedestrian bridge in Miami. Image: Getty.

On 15 March, a 950-ton partially assembled pedestrian bridge at Florida International University in Miami suddenly collapsed onto the busy highway below, killing six people and seriously injuring nine. Forensic engineers are taking centre stage in the ongoing investigation to find out what happened and why – and, crucially, to learn how to prevent similar tragedies in the future.

I’m not actively involved in this investigation, but I’ve been a forensic engineer for nearly 20 years and am the 2018 president of the National Academy of Forensic Engineers. Similar to forensic scientists, we visit scenes of disasters and crimes to determine what role engineering practices played in what happened.

The first step in any forensic investigation, collecting evidence, often can’t begin until survivors are rescued and victims are recovered. Those operations displace material and can damage evidence, which means forensic engineers must study the emergency response as well, to be able to tell whether, for instance, a support column collapsed during the event or was destroyed to reach a victim in need of help. During the FIU recovery efforts rescuers used large equipment to break up massive blocks of concrete so that victims’ bodies could be recovered.


In Miami at the moment, forensic engineers and technicians from the National Transportation Safety Board are on the scene. Right now they’re collecting samples of materials from the bridge to test for their physical properties. They’re reviewing drawings and plans, and examining both industry standards and site engineers’ calculations to understand what was supposed to be built – to compare with what was actually constructed. They’ll look at photographs and videos of the collapse to identify the sequence of events and locations of key problems. Of course, they’ll also talk to witnesses to find out what workers and passersby saw and heard around the time of its collapse.

Then they’ll combine and analyse all that data and information to identify as clearly as possible what went wrong, in what order. Often there are many factors, each leading to or amplifying the next, that ultimately caused the disaster. Putting that puzzle together is a key part of the forensic engineer’s role.

Weakness in partial structure

The FIU bridge was being built using a method called “accelerated bridge construction,” with separate sections that needed to be put together: the footings were installed beside the road and the span was built nearby and lifted into place just days before the collapse. In a plan like that, each piece must be able to withstand the forces acting on it as they’re all being put together. A weakness in one place can cause problems elsewhere, ultimately leading to catastrophe.

Two key elements of the bridge design, the tall centre pylon and pipe supports, were not yet in place when the structure collapsed. They hadn’t been scheduled to be added until later in the process – and the bridge wasn’t slated to open until next year, so it’s likely that the project’s designers and engineers expected the bridge segment to hold while construction continued.

An artist’s rendering of what the final bridge was supposed to look like. Image: City of Sweetwater.

Part of a forensic engineering evaluation will investigate whether that was a reasonable expectation, and whether those missing elements reduced the strength of what was there enough for it to collapse.

Searching for clues

There are some other publicly available clues, too, that shed light on avenues likely under investigation already. Dashcam video of the bridge collapse seems to indicate that the initial failure was very close to the north end of the structure. It has been reported that a couple of days before the collapse, a crack had been discovered near the bridge’s north end.

Additionally, the bridge span might have been either undergoing stress testing or other adjustments when it collapsed. It’s too early to say now – but the inquiry will certainly reveal – whether the crack and the stress testing put too much load at the north end of the bridge.

There will be other questions too, like “Why didn’t they use temporary supports to shore up the bridge?” There may be a perfectly sensible explanation: Perhaps the bridge was supposed to be strong enough to support itself, for example. Or maybe temporary supports would have created a traffic hazard on the road below.

Some of those questions will not be entirely engineering-related. For example, many are asking “Why wasn’t the road closed?” The Tamiami Trail was shut down for a few hours while the bridge span was put in place. But then it was reopened to cars – a decision that would have been informed by engineering, of course, but could also have been influenced by concerns about public safety or traffic congestion.

The ConversationAt the moment, many of the questions the public has are also being investigated by forensic engineers. Their goal is to ensure that eventually those questions are all answered, and many more as well, about designs, materials, processes, procedures and safety precautions. Those lessons will inform not just any replacement for this particular bridge in Miami but future bridge construction projects elsewhere in the country and around the world, as the rest of the engineering community takes lessons from whatever the investigation uncovers, so builders can avoid similar mistakes – and tragedies. In a sense, it is fortunate that one of the leading centres for accelerated bridge construction is right on the FIU campus.

Martin Gordon, Professor of Manufacturing and Mechanical Engineering Technology, Rochester Institute of Technology.

This article was originally published on The Conversation. Read the original article.

 
 
 
 

How can cities become more bike friendly? The Netherlands offers useful lessons

(Aurore Belot/AFP via Getty Images)

It might seem like cycling is in the DNA of the Netherlands, a country where even the prime minister takes his bicycle to work. But the Dutch haven’t always lived as one with their bikes. In the Amsterdam of the early 1970s, cars were considered the wave of the future. They can be seen filling up squares and streets in historical photographs, and killed an average of over two Amsterdammers per week, including many children.

It is nothing more than an “accident of history” that the Netherlands embraced cycling, says Marco te Brömmelstoet, the director of the Urban Cycling Institute in Amsterdam and a man better known as the city’s cycling professor. Today’s bike rider’s paradise was created after parents and activists took to the streets to protest “child murder” by car. A Saudi oil embargo, rising gas prices, concerns about pollution and anger about the destruction of entire neighbourhoods to build motorways did the rest. 


Amsterdam, 1958. Not a cyclist's paradise. (Keystone/Getty Images)

What’s important about this history is that it can be replicated in other cities, too. Of course, the Netherlands has certain advantages – it’s flat as a pancake, for example. But in the eyes of traffic reformers, the rise of e-bikes (and even cargo bikes) means there’s no excuse for prioritising cars everywhere. 

So how can cities, flat or not, follow Amsterdam’s path to creating places where cycling is a pleasant, safe and common way to get around? The Dutch have some tips. 

Separate bikes from car traffic

Any city could start painting dedicated bike lanes on the streets. But in the Netherlands, those white marks indicating space for cyclists are considered just a minor first step. 

“A line on the road is not enough. Motorists will ignore it,” says Frans Jan van Rossem, a civil servant specialising in cycling policy in Utrecht. If other cities want their residents to choose bikes instead of cars when dodging pandemic-era public transport, protecting them from fast-moving car traffic must be the priority, Van Rossem says. 

The Dutch research institute CROW developed a widely praised design manual for bicycle infrastructure, full of tips for creating these protected lanes: A row of vertical white posts or a curb can serve as a physical separator, for example. Still, cyclists tend to feel safest in a "solitary" path, separated from the road by grass, trees, or an elevated concrete island. 

“The main bottleneck, the main reason why people don’t cycle, is that they don’t feel safe,” Van Rossem notes. “To start, construct separate paths.”

Turn those bike paths into a network

Many cities may have some bike lanes on some streets, but leave cyclists to roll the dice everywhere else. Will conditions still be safe when they turn left or right? Often they have to continue their way without any protected facilities for cyclists. 

“In many cases, cities take fast action, without thinking it through very well,” says Lucas Harms. He leads the Dutch Cycling Embassy, a partnership between the Dutch government and several companies, which promotes Dutch bike knowhow globally. “Don’t build small pieces of bike lane from nothing to nowhere. Think about a network of cycling infrastructure.” 

Utrecht aims to have cyclists within 200 to 300 metres of a connected path anywhere in the city, Van Rossem says. Avoid constructing those paths in sketchy industrial areas, he warns. “A connection through an unattractive area may be fast, but won’t be used a lot.”

Embrace the ‘fietsstraat’, a street where bikes come first


On some streets, drivers have to give up their privileges. (Rick Nederstigt/AFP via Getty Images)

A peculiar Dutch invention called "fietsstraat" (cycling street) holds strong potential for the rest of the world, Kevin Krizek says. He’s a transportation professor from Colorado who spent three years at Radboud University in Nijmegen. 

On cycling streets, cars are “guests”, restricted by a speed limit of 30 kilometres per hour. Drivers are not allowed to pass, so cyclists comfortably dominate the road. In the Netherlands the fietsstraat is usually paved with red asphalt, to resemble a bike path and notify drivers of their secondary status. But creating a cycling street can be easy. “All you need to do is put signs at intersections,” Krizek says. The effect is revolutionary in his view. Drivers have to give up their privileges, and cyclists can take the lead. 

Some Dutch traffic experts worry the cycling street won’t work if a city doesn’t also have a robust cycling culture. In the Netherlands, drivers are aware of the perils of urban cycling because they too use bicycles. Moreover, Dutch cities use sophisticated “circulation plans” to direct cars away from city centres and residential areas, onto a few main routes. 

Without “calming” traffic this way, the cycling street could be a step too far, Harms says. “In a city like New York, where all roads are equally accessible and full, it’s better to separate bicycles and cars,” he says.

Redesign intersections for cyclists' safety

If cyclists have to cross intersections “at the mercy of the Gods”, you’re not there yet, says Harms. When he travels abroad, he often finds clumsily designed crossings. As soon as cars turn, cyclists may fear for their lives. 

Harms recommends placing physical barriers between cars and bikes in places where they must cross. The Dutch build elevated islands to direct traffic into separate sections. The golden rule: cars wait behind bicycles. That way, drivers can see cyclists clearly at all times. Barriers also force Dutch cyclists to turn left in the safest way possible. They cross the street first and wait for their turn again before making their way left.

“You can create that with simple temporary measures,” Harms says. Planters work fine, for example. “They must be forgiving, though. When someone makes a mistake, you don’t want them to get seriously injured by a flower box’s sharp edge.”

Professor Krizek points out how the Dutch integrated cycling routes into roundabouts. Some are small; some are big and glorious, like the Hovenring between Eindhoven and Veldhoven, where cyclists take a futuristic-looking roundabout lifted above the highway. Most of those traffic circles move high volumes of cars and cyclists through intersections efficiently and safely. For a simpler solution, the Dutch manual suggests guiding cyclists to quieter streets – crossing a block up or down may be safer. “Nobody knows how to do intersections better than the Dutch,” says Krizek. 

Ban cars, or at least discourage them


A man rides down from a three-level bicycle parking garage near Amsterdam's main train station. (Timothy Clary/AFP via Getty Images)

The quickest, most affordable way to make a city more bikeable is to ban cars, says Ria Hilhorst, cycling policy advisor for the City of Amsterdam. It will make streets remarkably safe – and will most likely enrage a significant amount of people. 

Amsterdam doesn’t outlaw cars, but it does deliberately make their owners feel unwelcome in the historic city’s cramped streets. Paid parking is hugely effective, for example. Many car owners decide to avoid paying and use bicycles or public transportation for trips into the city. Utrecht, meanwhile, boasts the world’s largest bicycle parking garage, which provides a dizzying 12,500 parking spots.

To further discourage drivers from entering the city’s heart, Amsterdam will soon remove more than 10,000 car-parking spaces. Strategically placed barriers already make it impossible to cross Amsterdam efficiently by car. “In Amsterdam, it is faster to cross the city on a bike than by car,” Harms says. “That is the result of very conscious policy decisions.”

Communicate the benefits clearly

Shopkeepers always fear they will lose clients when their businesses won’t be directly accessible by car, but that’s a myth, says Harms. “A lot of research concludes that better access for pedestrians and cyclists, making a street more attractive, is an economic boost.”

Try replacing one parking space with a small park, he recommends, and residents will see how it improves their community. Home values will eventually rise in calmer, bike-friendlier neighbourhoods without through traffic, Van Rossem says. Fewer cars mean more room for green spaces, for example.

“I often miss the notion that cycling and walking can contribute a lot to the city. One of the greatest threats to public health is lack of exercise. A more walkable and bikeable city can be part of the solution,” says Ria Hilhorst. “But in many countries, cycling is seen as something for losers. I made it, so I have a car and I’m going to use it, is the idea. 

“Changing this requires political courage. Keep your back straight, and present a vision. What do you gain? Tranquility, fewer emissions, health benefits, traffic safety, less space occupied by vehicles.” 

Again, she points to Amsterdam’s history. “It is possible; we were a car city too.”

Karlijn van Houwelingen is a journalist based in New York City.