Why a permitted traffic load brought down a ten-year-old bridge

No exceptional overload was identified when the Tretten Bridge collapsed. A highly stressed timber-steel connection probably failed by block shear after repeated traffic loads had weakened its capacity. The most important lesson is that the critical overutilisation had already been identified analytically six years earlier.

Observed damage

On 15 August 2022, a heavy goods vehicle and a passenger car were on the Tretten Bridge in Norway. As the truck reached approximately midspan, the structure began to deform without a local warning. The glulam-and-steel truss bridge fell into the Gudbrandsdalslågen river and onto the E6 below. Nobody was seriously injured.

The Norwegian Safety Investigation Authority found that the load at the time was well below the design load. There was no evidence of an immediately preceding illegal heavy transport. The collapse therefore cannot be explained by an exceptional traffic load.

The local load transfer

In a timber truss, diagonals primarily transfer axial forces. At their ends, these forces are introduced through slotted steel plates and dowel-type fasteners.

The fasteners do not act alone. They press against the timber and distribute forces into the material. Sufficient timber must remain around the fastener group to transfer the load through shear parallel to grain and tension perpendicular to grain.

If the effective timber block is too small, a block bounded by the fasteners can tear out. This combined, predominantly brittle mechanism is known as block shear. A fastener check alone is not sufficient; the surrounding timber must be able to retain the complete group.

Established technical findings

The investigation considered calculations, collapse scenarios, witness evidence, images and recovered components. The most probable initiation point was block-shear failure in a timber diagonal close to the western river pier.

Reassessment under the newer rules produced utilisation ratios of approximately 200 per cent for relevant connections. This figure includes partial factors; it does not mean that the truck applied twice the physical capacity. It demonstrates that the connection had no adequate design reserve.

After the local connection failed, the truss lost an essential part of its load path. Progressive failure of further diagonals, chords and cross-beams followed. The system had little robustness for load redistribution.

Probable but not fully proven deterioration

The bridge remained in service for about ten years and then collapsed under a load below the design value. The authority considers progressive weakening from repeated heavy traffic to be more likely than not. A fatigue reassessment supports this scenario, but the exact time history was not measured or reconstructed completely.

Fatigue is therefore a well-supported cause hypothesis, not a directly observed finding in every detail. Research data on fatigue of dowel-type timber connections remain limited.

The decisive QA issue

After the collapse of the Perkolo Bridge in Finland, Tretten was reassessed in 2016 under the newer block-shear rules. The same high utilisations were identified and strengthening was recommended. The work was not carried out and no traffic restrictions were imposed.

The problem therefore did not lie only in the original design. A serious analytical finding did not become a consistently managed safety case.

Visual inspections could not close this gap. Block shear can occur in a brittle manner and was not expected to announce itself through a reliably visible local defect. Inspection intervals cannot replace a missing calculation or a decision on a known capacity deficit.

Transferable lessons

Fasteners, embedment, net section, tension perpendicular to grain and potential block tear-out must be assessed as a connected system.

When reassessment identifies safety-critical overutilisation, the finding requires a documented decision: immediate restriction, detailed investigation, strengthening or a robust demonstration that no action is necessary.

For brittle mechanisms, it must also be checked whether the assumed deterioration can be detected by the proposed inspection method before failure.

Standards context

The original design used Norwegian standard NS 3470-1:1999, which did not include the relevant block-shear check. The mechanism was covered by NS-EN 1995-1-1, but that newer standard had not been contractually specified for the project.

For German context on 5 August 2026, DIN EN 1995-1-1:2010-12 including A2:2014-07 and the National Annex remained current. A project-specific German bridge assessment would also have to consider the applicable Part 2 and National Annex. No retrospective German non-compliance is inferred.

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