Timber Bridges Closed in Norway After Collapse
Last week, a Norwegian laminated timber bridge built to last a century collapsed into the river below just 10 years after it was completed. No injuries were reported. The 150-meter-long (500-foot-long) Tretten Bridge connected the west bank of the Gudbrandsdalslaagen River and the village of Tretten.
Following the incident, the Norwegian Public Roads Administration has also closed 14 timber structures while investigating the cause of the collapse.
Shortly after 7:30 a.m. on Aug. 15, police were notified that the bridge had collapsed. At the time, a truck and a car were crossing over the structure. Both drivers were rescued and were unharmed.
Nearby witnesses told local news stations they heard an “intense crash,” and then saw the bridge “laying in the river.”
“It is completely catastrophic, completely unreal,” local mayor Jon Halvor Midtmageli told reporters. “It is also a fairly new bridge.”
Tretten bridge crossing Gudbrandsdalslågen collapsed this morning. Hopefully no injuries, as the vehicles seen seem to have gotten away with little or no damage pic.twitter.com/dDyXi3u0h9— Bernie’s Mittens and the Battery Electronic Car (@torbenbrenden) August 15, 2022
Originally opened in 2012, the bridge was built to replace a single-lane steel bridge that had been there since 1894. The bridge was constructed of glued laminated timber (glulam), Corten steel beams and stone pillars, with a two-lane road and pedestrian walkway.
It was designed by architect PLAN Arkitekter with engineer Norconsult and contractor Contexto. The team had reportedly noted the structure was designed with a life expectancy of 100 years at the time.
“We who travel on the roads must be able to trust that the bridges are safe to drive on,” said the Norwegian Automobile Federation’s spokeswoman, Ingunn Handagard.
In a statement, Norway's road director Ingrid Dahl Hovalnd said: “We must have a complete and independent review of the incident. It must be safe to drive on Norwegian roads. That is why it is important to get to the bottom of this matter.”
The Perkolo Bridge in Norway, also made from glued laminated timber, collapsed in 2016. Following that collapse, the Tretten bridge and 10 others were temporarily closed. A report following the incident said that “the direct cause of the bridge collapse is a defective joint in the framework.”
The 2016 inspection for the Tretten Bridge noted that it found “no significant errors,” but did recommend one of the joints be strengthened and some dowels replaced.
However, when the bridge was checked again in 2021, the Norwegian Automobile Federation raised concerns about the safety of these kinds of bridges. A further inspection was also completed in June this year.
According to reports, the Norwegian Public Roads Administration and Innlandet County Council, who own and maintain the bridge, have ordered an external investigation of the collapse. All other timber bridges in the region will also be inspected.
“We have designed most of the larger wooden bridges in Norway as well as many smaller wooden bridges,” said Plan Arkitekter partner Oddmund Engh. “Our contribution to the bridge design is only concept and aesthetics, not calculations or construction drawings. This was a bridge that we were proud of.
“We have no information or knowledge about the cause of the collapse. Investigations will reveal what failed so we will not speculate on the cause.”
A spokesperson for Norconsult, who was involved in the bridge's design, said that the firm had also launched its own investigation. Norconsult head of communication Njå Bjørkmann said: “First and foremost, our thoughts go to those affected. We can confirm that Norconsult was involved in the design of the Tretten bridge. Shortly after the accident became known, the company created an emergency organization that will look into the incident.”
On Aug. 17, work to demolish the bridge began, with plans for the structure to be cut up, lifted onto land and transported away from the site.
“Now, we have a good plan for how the work can be carried out safely,” Aud M. Riseng in Innlandet County Municipality said. “It is important to get E6 opened as soon as possible, but the work is very demanding. The workers must get this done in a safe way.”
In the meantime, 14 other timber bridges in the country have been closed for safety concerns. Eight are located in Viken county, five in Innlandet county and one in Nordland county. Two of the bridges were reportedly built in the mid-1990s, while the remaining 12 were built since 2000.
The Norwegian Public Roads Administration has began installing temporary crossings at two of the sites to mitigate disruptions caused by the closures, with preparations for the remaining temporary bridges ongoing. The administration’s national bridge emergency department will deliver the temporary bridges, which will have the same load-bearing capacity as permanent bridges with narrower access.
Some engineers have suggested what they thought could have been the cause of the collapse, as well as stressing the importance of designing timber bridges properly. Andrew Lawrence, of design, engineering, architecture, planning and advisory firm Arup, speculated the bridge's timber parts looked to have been exposed to the rain and at risk of decay, with its main load path relied on timber tension connections.
“If for whatever reason those connections fail, due to design errors or decay they will fail suddenly in a brittle way,” he said, adding that the collapse was surprising especially given the bridge had been treated with “very effective and very toxic” creosote-based preservatives.
“It's got those vulnerabilities, but there are likely to have been some design or workmanship errors added to those to lead to the collapse,” he said.
“Timber is really important as part of making structures more sustainable. But it is a material which is vulnerable to decay and rot and vulnerable to brittle failures. Therefore a collapse like this, where nobody was injured, is actually very useful to help improve safety across the industry.”
Daniel Ridley-Ellis, head of the Centre for Wood Science and Technology at Edinburgh Napier University, said while decay and lack of performance are definite possibilities with timber, the circumstances of the collapse suggest they are not the main cause in this case. He said the element of the bridge he suspects most is one of the connections in the steel truss, above a support column.
“This happens to be steel, but this also doesn’t mean that the wood wasn’t also somehow involved,” Ridley-Ellis explained. “If I was investigating, one thing I would begin with is the possibility that the river’s action on that round column caused fatigue failure.”