Causes of Taiwan Bridge Collapse Investigated
At the beginning of the month, investigators from the Taiwan Transportation Safety Board announced that they were looking at several possible causes of last year’s Nanfang'ao Bridge collapse.
Occurring in October 2019, the failure crushed multiple fishing boats, injured dozens of people and caused six fatalities.
About the Bridge
Completed in 1998 by MAA Consultants (Bangkok, Thailand), the Nanfang'ao Bridge was built to replace a lower preexisting bridge that prevented large fishing boats from passing underneath, TIME reports. It measured 140 meters long (460 feet long) and was designed as a single-span arch bridge.
According to MAA, the bridge was the only single-span arch bridge in Taiwan to be supported by cables and the second single arch-cable steel bridge in the world. Prior to its collapse, the structure stood 18 meters high and was a popular tourist attraction in Yilan, Taiwan.
Rust damage a potential factor in last year's Nanfang'ao Bridge collapse: TTSBhttps://t.co/KuuJiX82FT— Focus Taiwan (CNA English News) (@Focus_Taiwan) May 6, 2020
However, the Central News Agency reports that in 2016 a report on bridges in Yilan had discovered problems with the structure, stating that there were issues with the expansion joints. Designed to absorb changes in temperatures, motorists reported that they could sense an alteration in levels on either side of the joints, possibly a consequence of warping or other complications.
These issues were stated to be resolved by Taiwan International Ports Corporation, Ltd.—the company responsible for managing the bridge—in 2017 and 2018 when the joints were cleaned, along with the maintenance and repair of rusted steel reinforcements, guardrails and other issues.
On Oct. 1, 2019, the structure collapsed around 9:30 a.m. local time. The evening before, the bridge was reported to have endured Typhoon Mitag, although reports claim that 137 kilometers per hour (85 mph) wind gusts affected the island before moving to the northeast, where the bridge is located. Disaster relief officials wouldn’t say if the storm had weakened the bridge.
A Taiwanese oil tanker driver and nine Filipino and Indonesian fishermen were sent to nearby hospitals, six of whom were reported to have serious injuries.
Following the incident, Taiwan’s military deployed a floating platform to help rescue workers, remove debris and extract boats. Two workers were also reported to have experienced injuries during rescue efforts.
Following the collapse, an investigation was opened, focusing on possible corrosion in suspension cables caused by saltwater ingress. At the time, independent bridge consultant Simon Bourne, suggested that problems with the suspension cables could have triggered the catastrophic collapse.
“There are only three [main] elements to the bridge—the compressive arch, the tension hangers that hold the deck up and the tension tie in the deck itself. The arch looks fine, even up to the point it hits the water,” Bourne said.
“Halfway through the collapse, all (or most) of the tension hangers seem to have failed—it is too early to tell if this was the cause or the result of the collapse.”
However, Bourne also went on to predict that the bridge collapse could have been the cause of corroded hangers, which would undo the support system serving the deck, leaving it to bend in the middle.
Dean of the School of Engineering at National Taipei University of Technology Sung Yu-ch also concluded with the same suggested cause. However, Sung believes that the sequence of snapping cables caused the arch to fall, as the original balance force between the steel reinforced concrete at the base and the steel girders was also compromised.
“It’s all about good maintenance regimes,” Bourne added. “Bridges used to only really fail during construction … but sadly nowadays, failures are increasing due to poor maintenance.”
Since the collapse, the Taiwanese government has announced that it is committed to building a replacement bridge within three years. Costs for the project are expected to reach almost $17 million.
What’s Happening Now
Although investigations of the bridge’s collapse did find corrosion in various steel cables, along with rust and other deficiencies on the structure’s steel strands, the TTSB says that investigators still have to look into the possible impacts of other factors, such as road construction projects and overloading.
However, in expanding on the rust and corrosion, senior TTSB investigator Wang Hsing-chung explained to Focus Taiwan CAN English News that although rust and corrosion damage on the steel strands that had “escaped” their anchor on the bridge’s arched and deck wouldn’t have been able to be seen by the naked eye during a safety feature.
According to the investigator, the bridge’s design failed to establish additional inspection standards when regarding these visibility issues.
While that is being considered, TTSB reports that surveillance camera footage also revealed some abnormal movement in one of the structure’s arches, prior to the collapse, causing the dislodge of five of the bridge’s 13 suspenders. The report goes on to say that due to the dislodge, the west end of the bridge dropped by eight degrees, which caused the deck to fracture.
TTSB is also continuing to look into the structure’s possible improper construction and the extent of the aforementioned rust damage. According to New Civil Engineer, between 2005 and 2019, the structure underwent 60 road construction projects, nearly doubling the thickness of the road in some portions.
From 2016 to 2019, port authorities also reportedly transported concrete loads across the bridge four times to use for the construction of a nearby seawall. The Taiwan International Ports Corporation is conducting an internal review of its maintenance regimes following the collapse.
Since announcing these confirmed corrosion results and other possible causes, the TTSB has yet to conclusively state what exactly caused the collapse. A draft report on the bridge collapse cause is slated to be complete in August.