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.

 
 
 
 

Green roofs improve cities – so why don’t all buildings have them?

The green roof at the Kennedy Centre, Washington DC. Image: Getty.

Rooftops covered with grass, vegetable gardens and lush foliage are now a common sight in many cities around the world. More and more private companies and city authorities are investing in green roofs, drawn to their wide-ranging benefits which include savings on energy costs, mitigating the risk from floods, creating habitats for urban wildlife, tackling air pollution and urban heat and even producing food.

A recent report in the UK suggested that the green roof market there is expanding at a rate of 17 per cent each year. The world’s largest rooftop farm will open in Paris in 2020, superseding similar schemes in New York City and Chicago. Stuttgart, in Germany, is thought of as “the green roof capital of Europe”, while Singapore is even installing green roofs on buses.

These increasingly radical urban designs can help cities adapt to the monumental challenges they face, such as access to resources and a lack of green space due to development. But buy-in from city authorities, businesses and other institutions is crucial to ensuring their success – as is research investigating different options to suit the variety of rooftop spaces found in cities.

A growing trend

The UK is relatively new to developing green roofs, and governments and institutions are playing a major role in spreading the practice. London is home to much of the UK’s green roof market, mainly due to forward-thinking policies such as the 2008 London Plan, which paved the way to more than double the area of green roofs in the capital.

Although London has led the way, there are now “living labs” at the Universities of Sheffield and Salford which are helping to establish the precedent elsewhere. The IGNITION project – led by the Greater Manchester Combined Authority – involves the development of a living lab at the University of Salford, with the aim of uncovering ways to convince developers and investors to adopt green roofs.

Ongoing research is showcasing how green roofs can integrate with living walls and sustainable drainage systems on the ground, such as street trees, to better manage water and make the built environment more sustainable.

Research is also demonstrating the social value of green roofs. Doctors are increasingly prescribing time spent gardening outdoors for patients dealiong with anxiety and depression. And research has found that access to even the most basic green spaces can provide a better quality of life for dementia sufferers and help prevent obesity.

An edible roof at Fenway Park, stadium of the Boston Red Sox. Image: Michael Hardman/author provided.

In North America, green roofs have become mainstream, with a wide array of expansive, accessible and food-producing roofs installed in buildings. Again, city leaders and authorities have helped push the movement forward – only recently, San Francisco created a policy requiring new buildings to have green roofs. Toronto has policies dating from the 1990s, encouraging the development of urban farms on rooftops.

These countries also benefit from having newer buildings, which make it easier to install green roofs. Being able to store and distribute water right across the rooftop is crucial to maintaining the plants on any green roof – especially on “edible roofs” which farm fruit and vegetables. And it’s much easier to create this capacity in newer buildings, which can typically hold greater weight, than retro-fit old ones. Having a stronger roof also makes it easier to grow a greater variety of plants, since the soil can be deeper.


The new normal?

For green roofs to become the norm for new developments, there needs to be buy-in from public authorities and private actors. Those responsible for maintaining buildings may have to acquire new skills, such as landscaping, and in some cases volunteers may be needed to help out. Other considerations include installing drainage paths, meeting health and safety requirements and perhaps allowing access for the public, as well as planning restrictions and disruption from regular ativities in and around the buildings during installation.

To convince investors and developers that installing green roofs is worthwhile, economic arguments are still the most important. The term “natural capital” has been developed to explain the economic value of nature; for example, measuring the money saved by installing natural solutions to protect against flood damage, adapt to climate change or help people lead healthier and happier lives.

As the expertise about green roofs grows, official standards have been developed to ensure that they are designed, built and maintained properly, and function well. Improvements in the science and technology underpinning green roof development have also led to new variations on the concept.

For example, “blue roofs” increase the capacity of buildings to hold water over longer periods of time, rather than drain away quickly – crucial in times of heavier rainfall. There are also combinations of green roofs with solar panels, and “brown roofs” which are wilder in nature and maximise biodiversity.

If the trend continues, it could create new jobs and a more vibrant and sustainable local food economy – alongside many other benefits. There are still barriers to overcome, but the evidence so far indicates that green roofs have the potential to transform cities and help them function sustainably long into the future. The success stories need to be studied and replicated elsewhere, to make green, blue, brown and food-producing roofs the norm in cities around the world.

Michael Hardman, Senior Lecturer in Urban Geography, University of Salford and Nick Davies, Research Fellow, University of Salford.

This article is republished from The Conversation under a Creative Commons license. Read the original article.