Sub-report 2 on the collapse of Tretten bridge on 15 August 2022
Road traffic report 2024/03
On the morning of 15 August 2022, Tretten Bridge collapsed, and fell into the river Gudbrandsdalslågen and onto the E6 highway, after having been in operation for approx. 10 years. Tretten bridge was an architect-designed truss bridge in glulam and steel. Sub-report 2 focuses on the management and follow-up of Tretten Bridge throughout its life cycle. The investigation has shown that inadequate caution was exercised in the planning, design, control and approval of Tretten bridge, and that the follow-up of the bridge failed in 2016. Inadequate knowledge and learning after previous incidents is also an important explanation for the collapse of Tretten Bridge. With respect to future bridge management, the investigation raises some fundamental questions.
Main conclusion of technical investigations
Tretten Bridge collapsed most likely as a result of block shear failure in one of the timber diagonal members near the river foundation in the western part of the river. It is predominantly likely that the fracture location in question has been exposed to weakening over time (fatigue). This is described in more detail in sub-report 1.
Main conclusion of investigation of the bridge's life cycle and history
Tretten bridge was an architect-designed truss bridge in glulam and steel, and was constructed by the NPRA in 2012 as part of the project E6 highway Øyer-Tretten. The investigation has shown that inadequate caution was exercised in the planning, design, inspection and approval of Tretten Bridge, with respect to the risk factors linked to its unconventional design. A short construction period, span lengths and reuse of existing foundation were framework conditions for the planning of the bridge. These framework conditions, combined with the choice of material and a strong focus on aesthetics, contributed to the structure not being robust.
Tretten Bridge was designed during a transitional period between old and new regulations. According to the transitional provisions, the project had the opportunity to relate to the old regulations. In retrospect, all the involved parties can be criticised for selecting and accepting regulations that proved to have a serious shortcoming (block shear failure). However, the transition from national standards to European standards for structural design (Eurocodes) was extensive and complex.
Block shear is a form of fracture that has been included in the Eurocodes since 2004, as a result of a building collapse in Finland in 2003. However, block shear failure was not covered in the previous Norwegian standard that formed the basis for the design of Tretten Bridge in 2009–2011. The investigation has shown that there was insufficient transfer of knowledge about the criticality of block shear failure from its implementation in the Eurocodes until the collapse of Tretten Bridge.
After the collapse of Perkolo Bridge in 2016, Tretten Bridge was re-calculated according to the current regulations with check against block shear failure by the NPRA. The NPRA identified serious faults in Tretten Bridge, but it was not evaluated and followed up in a systematic manner. This represents a system failure in the NPRA, reflecting a lack of knowledge about block shear failure, as well as shortcomings in the bridge management system and risk management. An important basis for managing risk is that the precautionary principle should be applied when the consequences are potentially extensive and there is inadequate knowledge about the issue. The NSIA believes that Tretten Bridge should either immediately have been closed or traffic restrictions imposed in 2016 when finding over-utilisation in relation to block shear in several of the bridge’s connections.
The regional government reform of 2020 has posed some challenges related to the clarification of responsibilities between the NPRA and Innlandet County Authority. However, the resources, expertise and systems employed by the county authority’s bridge management have had no bearing on the collapse of Tretten Bridge.
Safety learning
The external review of bridge management and the history of Tretten Bridge, which PwC has conducted on behalf of the NPRA, has identified several areas for improving safety. The NPRA has launched several relevant and important measures that will address many of the safety issues identified in the investigation.
Inadequate knowledge and learning after previous incidents is one important explanation for the collapse of Tretten Bridge. The NSIA therefore considers it important that this investigation provides safety lessons for relevant stakeholders, both in Norway and internationally. With respect to future bridge management, the investigation also raises some fundamental questions related to crisis management in the road sector, the supervisory authority for county roads, the Directorate of Public Roads' exercise of authority, as well as the management and application of regulations.
The NSIA submits six safety recommendations following the investigation.
Published 20.03.2024
Safety recommendation
Safety recommendation Road No 2024/07T
Tretten Bridge collapsed on 15 August 2022 after having been in operation for approx. 10 years. An external review of the NPRA’s bridge management and the history of Tretten Bridge has identified several areas for improving safety. The NPRA has launched a number of measures to ensure that there is sufficient capacity, expertise and independence to perform its official authority and control duties, and procedures to build experience when new types of materials and methods are used.
The Norwegian Safety Investigation Authority recommends that the Norwegian Public Roads Administration implement the measures it has identified:
- A review of the organisation, roles/responsibilities, processes and regulations/legislation for design, control and approval.
- Regulatory development/preparatory work related to ‘ductility’ and resilience.
- Regulatory development and decision-making structure for new structures and material choices.
- Mitigating measures should be implemented for shortcomings in the current bridge management system Brutus related to logging and data extraction.
Safety recommendation Road No 2024/08T
Tretten Bridge collapsed on 15 August 2022 after having been in operation for approx. 10 years. An external review of the NPRA bridge management and the history of Tretten Bridge has identified several areas for improving safety. The NPRA has launched several measures to ensure that an inspection and follow-up regime is in place for new and existing bridges that safeguards operation and maintenance, and identifies any existing faults and defects.
The Norwegian Safety Investigation Authority recommends that the Norwegian Public Roads Administration implement the measures it has identified:
- The bridge manager’s role in approving the information for necessary approval and maintenance must be clarified, including by ensuring that all necessary documentation for the design life cycle assumptions are in place.
- A process and emergency response procedure should be established for conducting an investigation and/or follow-up of findings showing that structures may be subject to structural faults and defects, including those that cannot be identified during inspections.
- Risk-based follow-up inspections of existing bridges must be systematically incorporated into the management regime in addition to the inspection regime.
- Complete the introduction of the FDV system including a documentation system for bridges. The documentation system is prioritised and will be completed during 2024.
- Increase expertise in the management and maintenance of wooden bridges.
Safety recommendation Road No 2024/09T
Tretten Bridge collapsed on 15 August 2022 after having been in operation for approx. 10 years. The bridge led County road 254 over the E6 highway and Gudbrandsdalslågen river. After the collapse, it was initially unclear to Innlandet County Authority who ‘owned the crisis’ and was responsible for the incident, and there was uncertainty about the county authority’s and the NPRA’s mandate to make various decisions. Good crisis management is contingent on roles and responsibilities being clarified and clear before a crisis occurs.
The Norwegian Safety Investigation Authority recommends that the Ministry of Transport clarify and establish the road operators’ roles and responsibilities for crisis management, safety and emergency preparedness.
Safety recommendation Road No 2024/10T
Tretten Bridge collapsed on 15 August 2022 after having been in operation for approx. 10 years. The bridge led County road 254 over the E6 highway and Gudbrandsdalslågen river. The resources, expertise and systems employed in Innlandet County Authority’s bridge management had no bearing on the collapse of Tretten Bridge. However, the NSIA considers that the county authorities are not in a better position than the NPRA to identify and correct any shortcomings in their own bridge management and inspection regime, and therefore points to the need for an external supervisory body.
The Norwegian Safety Investigation Authority recommends that the Ministry of Transport authorise the Road Supervisory Authority to conduct supervisory activities to ensure that safety requirements related to the county road infrastructure are safeguarded by the county authorities.
Safety recommendation Road No 2024/11T
Tretten Bridge collapsed on 15 August 2022 after having been in operation for approx. 10 years. The Directorate of Public Roads appeared to be passive both in the control and approval of the design basis for Tretten Bridge, as well as in connection with the review of timber truss bridges in 2016 and the associated safety-critical findings at Tretten Bridge. The NSIA raises the question of whether control and authority duties relating to bridges should be a supervisory task outside the NPRA. The NSIA is also uncertain about whether the current organisation of the exercise of authority, where Authority and regulations are organised under the Directorate of Public Roads in the NPRA, ensures sufficient independence to make decisions relating to safety.
The Norwegian Safety Investigation Authority recommends that the Ministry of Transport elucidate the control and authority duties assigned to the Norwegian Public Roads Administration, and whether there is sufficient independence to make decisions relating to safety.
Safety recommendation Road No 2024/12T
Tretten Bridge collapsed on 15 August 2022 after having been in operation for approx. 10 years. The bridge was designed during a transitional period between old and new regulations. In retrospect, all the involved parties can be criticised for selecting and accepting regulations that proved to have a serious shortcoming. It is unfortunate that the legislation is so complex that it can lead to uncertainty and incorrect application by authorities and users. The investigation has raised questions about whether this issue may also be relevant today.
The Norwegian Safety Investigation Authority recommends that the Ministry of Transport clarify and communicate to the users of the regulations, the relationship between the Road Act and the Planning and Building Act in relation to the design, control and approval of bridges. This includes the ranking of regulations and the eventual retroactive effect of the regulations for existing structures.
Facts
| Location | Tretten bridge, county road 254, Øyer |
| Occurrence date | 15.08.2022 |
| Accident type | Other accident |
| Road class | County road |
| County | Innlandet |
| Vehicle type | Passenger car, Truck and trailer |
| Type of Transportation | Freight transport, Private transport |
| Accident category | Annen ulykke |