WEDNESDAY, DECEMBER 14, 2022
A preliminary report recently released by the Norwegian Safety Investigation Authority found that “significant overload” caused the main span of a laminated timber bridge in Tretten, Norway, to break and collapse in August.
While the investigation is ongoing, the NSIA is seeking to determine how and why the bridge collapsed, identify safety problems and map the incident's underlying causal factors. Based on the results, they will then consider areas for improving safety.
What Happened
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.”
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.
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A preliminary report recently released by the Norwegian Safety Investigation Authority found that “significant overload” caused the main span of a laminated timber bridge in Tretten, Norway, to break and collapse in August. |
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.
At the time of its collapse, the laminated timber bridge, which was built to last a century, collapse just ten years after it was completed. Following the incident, the Norwegian Public Roads Administration also closed 14 timber structures while investigating the cause of the collapse.
According to reports at the time, the Administration and Innlandet County Council, who own and maintain the bridge, ordered an external investigation of the collapse. All other timber bridges in the region would also be inspected.
A spokesperson for Norconsult, who was involved in the bridge's design, said that the firm had also launched its own investigation.
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.
Preliminary Report
Together with an external expert group, the NSIA carried out technical investigations of the Tretten bridge collapse, reviewing relevant technical material, videos and pictures of bridal parts, and witness observations.
NSIA reports that uncovering the technical causal factors was challenging due to significant consequential damage to the bridge structure from the collapse itself, the impact with the ground and the salvage work.
According to the preliminary report, the technical investigations substantiated that the initial cause of damage to the collapse was a break in one of the diagonals in the main span towards the western river foundation. The fracture form was reportedly identified as block shear failure between the wooden part and steel/dowels at the junction.
The cause of the fracture form so far, NSIA states, appears to be “significant overload” in relation to the bridge’s load impact and bearing capacity. Based on recommended calculation methods used for regulations in the design of wooden structures, the capacity of the connections was half of what they should have been when calculating the bridge’s failure limit state.
It is currently unclear if the capacity was reduced as a result of repeated loads or fatigue. Additionally, technical investigations reportedly did not reveal any signs of reduction in the load-bearing capacity due to rot or corrosion.
According to NSIA, the main areas for further investigations will include:
Due to the scope and the complexity of the investigation, the NSIA reports that it cannot provide an estimated date for the final report. The investigation is ongoing with a “high level of activity.”
Tagged categories: Accidents; Bridges; Bridges; cross-laminated timber; Health & Safety; Health and safety; Infrastructure; Infrastructure; Inspection; Program/Project Management; timber
