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The collapse of a wooden bridge in southern Norway was caused by a "significant overload", according to a preliminary investigation into the incident.
One car plunged into the river while a lorry became stranded when Tretten Bridge in Gudbrandsdalen Valley broke apart in August morning while two vehicles were crossing it.
The 150m structure was only built in 2012, leading to questions over why the timber and steel truss bridge failed so dramatically.
According to the Norwegian Safety Investigation Authority (NSIA)'s preliminary investigation, the collapse was caused by "a break in one of the diagonals in the main span towards the western river foundation".
The report explains: "The fracture form is identified as block shear failure between the wooden part and steel/dowels at the junction. This is a momentary form of failure that can cause overloading of other elements in the truss upon subsequent loading, and then with the collapse of the bridge as a result. The fracture form is supported both by technical examinations of fracture parts, photographic material and witness observations, as well as carried out structural analysis and calculations.
"The cause of the fracture form has so far been assessed to be a significant overload in relation to the bridge's load impact and bearing capacity, with regard to block shear failure in the connections to the said diagonals."
After the collapse, a number of bridge experts considered the possible causes.
Daniel Ridley-Ellis, head of the Centre for Wood Science and Technology at Edinburgh Napier University, identified streel truss issues as a possible factor.
He said the element of the bridge he suspected 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," he said. "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."
Meanwhile, Richard Fish, an independent bridge management consultant, said one area of "potential concern" with Tretten could be the transition and connections between the bridge's timber and steel sections.
Whether the capacity has been reduced as a result of repeated loads or fatigue is still to be clarified, but the investigations have not revealed any signs of a reduction in the bridge's load-bearing capacity as a result of rot or corrosion.
The NSIA emphasised that the report is a preliminary one and not a complete presentation of the technical investigations that have been carried out in connection with the bridge collapse. The final report will contain more detailed information about the technical investigations carried out, results and conclusions.
The Tretten Bridge, which was designed by Norconsult with architect Plan Arkitekter, was inspected along with all other wooden truss bridges following the collapse of a different Norweigan bridge in 2016.
According to a report by Norweigan broadcaster NRK, some maintenance issues were discovered on Tretten Bridge, but nothing that led to it being declared unsafe for traffic.
Further investigations
The main areas for the NSIA's further safety investigation are as follows (the list is not exhaustive):
- The NSIA, together with the professional group, will continue the work of carrying out technical investigations into the collapse of the Tretten Bridge, and as far as is practically possible, try to uncover the underlying mechanisms for block shear failure as a form of failure.
- Regulations for the design of wooden structures, bridge management and follow-up of the Tretten Bridge after the collapse of the Perkolo bridge in 2016, will constitute three main areas in the further investigation of the underlying causal factors for the bridge collapse. In this context, The NSIA is well underway in obtaining relevant information and documentation, and meetings and interviews with relevant actors are being held on an ongoing basis.
- Further investigations will also aim to try to uncover how Tretten Bridge could have been in operation for around 10 years before it collapsed. In this context, the investigation will try to uncover whether there have been other causal factors, beyond low capacity against block erosion, which may have contributed to a gradual weakening of the bridge over time.
The scope and complexity of the investigation means that The NSIA cannot estimate a date for publication of the final report.
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Something ‘lost in translation’? I found the headline and article a little misleading or ambiguous. When it headlines ‘overload’, that may be taken as excessive imposed load to the structure, if read by an English speaker. It seems, however, that ‘overload’ applied to the connection mentioned or even what we often call ‘load shedding’ to other components. So, it could do with some clarification – for me at least.