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

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 center 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.

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.

At 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 centers for accelerated bridge construction is right on the FIU campus.

Martin Gordon is Professor of Manufacturing and Mechanical Engineering Technology, Rochester Institute of Technology. This article is published courtesy of The Conversation (under Creative Commons-Attribution / No derivative)