The Nanfang’ao bridge in eastern Taiwan suffered a catastrophic collapse on October 1, 2019, killing six and wounding a dozen others.
The bridge suddenly fell apart when an oil tanker was crossing over the structure. The cause of the failure is still under investigation, but preliminary evidence points to corrosion in the bridge’s suspension cables as the possible cause. The 320-ton arch crashed down onto the truck and boats below the bridge, killing six people and injuring a dozen more.
Video footage shows that the vertical cable at the centre of the bridge snapped first. Then the broken cable set off a domino effect of more snapped cables, leading to the bridge’s rapid collapse.
Should the bridge have suffered a catastrophic collapse when a tension cable broke?
Should there have been enough reserve capacity in the other cables?
The structure is a complex one involving tension cables and arches and definitely deserved a design that considered the possibility of an element weakening over time. Perhaps it did. If a corroded cable was to blame, then it’s feasible that adjacent cables were also corroded and hence had less capacity than when new.
The catastrophic collapse of bridges is all too common. Somewhere between 100 and 200 bridges collapse in the US each year apparently. In Italy, following my research into the 2018 collapse of the Morandi Bridge in Genoa, I discovered that 15 – 20 bridges in Italy collapse every year! In Australia, Wikipedia says we’ve had (only) 4 “famous” bridge collapses since 1960. Can you name them? I’m sure there’s been more. There always is!
The takeaway for me is that bridges – and more specific to our work – mining structures – are not static structures. They are subjected to harsh environments. They shake in response to vibrating equipment and dynamic loads. Loads come and go, creating the possibility of fatigue. Essentially, from day one onward, they get a little bit weaker.
Designing a structure that “decays” means considering the effects of corrosion, damage, modification and let’s be honest – abuse -and ensuring that failure – at least catastrophic and sudden failure – is eliminated through good design.
Access to critical structural elements is another consideration, lest those are neglected until it’s too late.
Often it’s the unseen detail that makes a structure more successful in use than another, and significantly lower maintenance and remediation costs is the result. Good structural engineers are rarely responsible for poor design!
You’d think that with so many collapses, there’d be more attention being paid to inspections and structural health monitoring. The latter is an entire industry of knowledge and equipment but who amongst us in the mining industry are right across “structural health monitoring”? In my experience, most of the structural audits we’ve gotten involved with are “reactive” engagements – either the structures have scared somebody enough to take action, or it’s the result of a mine inspector’s visit to site. And forget knowing if a structure has any spare capacity or not – in the reactive phase, even suitable structural elements are ripped out and replaced or unnecessarily strengthened at high cost and with lost production.
Fortunately, we’re starting to see more clients coming to us before problems occur for regular proactive structure care. That’s definitely a good thing (prevention is always better than the cure), and technology has allowed us to better monitor and inspect structures during the life of the plant, minimising disruption and shutdowns.
Proactive maintenance would have been needed to guard against rust, wear and tear of the Nanfang’ao bridge and in that case six people may still be alive. The maintenance regime was an inspection every four years. But a mere three years after its last inspection report, the bridge failed—casting doubts on the effectiveness of those inspections and their frequency.
It’s my vision that structures are carefully designed and then monitored and maintained throughout the design life to provide safe access and successful operation of the plant.
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