Sub-report 1 on the bridge collapse at Tretten on 15 August 2022

There was no local warning that the Tretten bridge was about to collapse before a lorry and a passenger car drove onto the bridge at 07:33 in the morning. It was only when the lorry traveling east was about halfway across the bridge that the bridge started to deform, which was observed by the driver of the passenger car behind the lorry. The driver of the passenger car evacuated the bridge, while the driver of the lorry was evacuated with the help of a helicopter. None of the persons involved were seriously injured in the incident. The fact that there were only two vehicles on the bridge when it collapsed and no cars under the bridge on the E6 highway was essential to the extent of the consequences not being more severe. The rescue work was also carried out in a satisfactory manner.

A number of hypotheses for the cause of the collapse have been investigated by the external expert group based on external input and available background material, as well as their own investigations and various testing of elements from the collapsed bridge. The NSIA has followed up the professional group's work closely and continuously, and fully supports conclusions of the expert group.

From this work, the most likely cause of the collapse appears to be so-called “block shear failure”  in one of the timber diagonal members near the river foundation in the western part of the river. This conclusion is supported by post-calculations of the capacity in the current connections in relevant diagonals, by calculated collapse scenarios, by witness observations, as well as by assessments of visual material and bridge elements from after the collapse.

Block shear failure as a form of fracture, can theoretically be triggered by a momentary overload from traffic, or from a gradual weakening of the capacity due to repeated heavy loading from traffic possibly together with other secondary effects (so-called fatigue). Based on known information about heavy vehicle traffic on the bridge, and based on the sequence of events prior to the collapse, it is predominantly likely that the fracture location in question has been exposed to weakening over time.

Calculation of potential fracture-connection in the relevant area shows a clear overutilization of the capacity in relation to the current design rules for timber structures. However, this set of rules had not been made applicable to the project at the time for designing Tretten Bridge. The failure mechanism in question was thus not checked for at that time. The control calculation gives a utilization rate of approx. 200% in relation to failure, including built-in safety factors in the regulations. A utilization factor of this order of magnitude indicates that a breach can occur for the largest dimensioning lorries or a collection of several such. However, there is no indication that the bridge has been subjected to such a load directly prior to the collapse.

The load on the bridge when it collapsed was significantly lower than what it was designed for. It is therefore likely that critical parts of the bridge were degraded as a result of fatigue in the timber. A calculation of the fatigue capacity at the connection in question based on the assumed volume of heavy traffic confirms that this is a likely scenario.

After the collapse of Perkolo Bridge in 2016, Tretten Bridge was re-calculated according to the current rules with check against block shear failure by the Norwegian Public Roads Administration. The same overutilization as calculated by the external expert group were then found, and it was recommended that the bridge should be strengthened at these connections. Such a strengthening was never implemented. The NSIA is critical that the Norwegian Public Roads Administration did not follow this up sufficiently as no measures or restrictions were implemented.

The NSIA believes that the collapse of Tretten Bridge highlights the necessity of performing block shear failure controls in the design of timber structures. With this as a background, NSIA issues a safety recommendation on recalculation and control of existing timber bridges against block shear failure.

Sub-report 2 will focus on bridge management and follow-up of Tretten bridge throughout the bridge's life cycle, based on the main findings identified through the technical investigations.

Published 15.08.2023