The Nanfang’ao bridge suddenly fell apart when an oil tanker truck 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 12.
The Nanfang’ao bridge was 460 feet long and 59 feet high. It was the only steel single-arch bridge in Taiwan, the first bifurcated single-arch bridge in Asia, and one of only two bridges of its kind in the world.
Video footage shows that the vertical cable at the center of the bridge snapped first. While the other cables should have been able to absorb and redistribute the extra load evenly, they didn’t—instead, the broken cable set off a domino effect of more snapped cables, leading to the bridge’s quick collapse.
Sung Yu-chi, dean of the Taipei Technology College of Engineering, theorized that the collapse of the arch could have been caused by the dissolution of the force equilibrium between the bridge and the arch.
Sung suspected that the cables were so weakened that when the first one snapped, it triggered a chain reaction of other cables breaking in quick succession—rapidly leading to the bridge’s collapse. When the deck of the bridge gave way, the force between the base’s steel-reinforced concrete and the steel girders was also compromised, causing the arch to fall.
In essence, corrosion wore down the cables to the point where one single truck could bring down the entire bridge.
The bridge was continuously exposed to sea water and airborne salt, which could have caused the cables to rust. Long-term exposure to wind could also have worn down the bridge’s anchor points. These factors would have compromised the cables’ ability to bear their own load, much less compensate for the extra weight caused by a broken cable.
It’s also possible that the bridge was further weakened by a recent earthquake as well as a typhoon that hit the region mere hours before the collapse—though the weather was calm when the bridge fell. Typhoon Mitag’s strong winds and heavy rain would have caused the cables to sway dramatically, potentially forcing the cables to the point of snapping.
Proactive maintenance would have been needed to guard against rust, wear and tear. Such maintenance is proving to be an essential component of bridge longevity, particularly when it comes to suspension bridges like the Nanfang’ao. While this may seem obvious, it’s not always implemented: maintenance problems related to corrosion have recently led to the closure of the Hammersmith Bridge in London earlier this year and the fatal collapse of the Ponte Morandi bridge in Genoa in 2018.
“We know from Hammersmith, Morandi and the many suspension bridges around the world that are in difficulty that thousands of individual wires can fail bit by bit, if there hasn’t been a good maintenance regime in place,” said Simon Bourne, an independent bridge consultant.
The maintenance regime for the Nanfang’ao bridge has been called into question. A 2016 report found that the expansion joints—which are designed to absorb changes in temperature—were warped, damaged and sagging. The report recommended reinforcing the body with concrete, conducting regular rust removal from the cable beams, repairing the downspouts, and replacing expansion points.
Taiwan International Ports Corporation (TIPC), which maintains the bridge, said it had conducted remediation work on the joints and other rust-related problems. TIPC has stated that it conducted routine annual maintenance on the bridge since it was built, and that each cable cord on the bridge was inspected every four years. The last inspection was documented in the 2016 report.
But a mere three years after its last inspection report, the bridge failed—casting doubts on the effectiveness of those fixes.
“It’s all about good maintenance regimes,” said Bourne. “Bridges used to only really fail during construction … but sadly nowadays, failures are increasing due to poor maintenance.”
While the bridge was only built in the late 1990s, making it fairly new for such a structure, it was based on an older design that didn’t factor in the sophisticated modern protections against natural erosion than are needed to protect the bridge.
The Taiwanese government has committed to building a replacement bridge within three years at a cost of almost $17 million.
Hopefully the investigation will pinpoint exactly what happened and offer recommendations for other bridges facing similar environmental challenges.
Engineers have been theorizing about the bridge’s collapse on engineering.com’s discussion forum, where participants have also identified corrosion and possible fatigue damage as factors that could have contributed to the bridge’s failure. Head over to the discussion forum to share your thoughts about the accident.