A series of interrelated causes led to the collapse of a concrete element on the northern abutment on the Sundsvall Bridge in Sweden on 4 August, a report by the contractor concluded.

The report, which was submitted to Vagverket, the Swedish Transport Administration on 18 September, concluded that five mistakes contributed to the failure, the most serious being that the designer made the wrong calculations. In addition, subsequent changes that were made on the site were never notified to the Swedish Transport Administration and the designer. The bridge was built by contractor Sundsvallbron, a joint venture led by E Pihl & Son with Max Bogl and Josef Mobius. Pihl's design office was reponsible for the design of the piers and north abutment.

"It is an honest and considered report, which fits well with our own analysis," said Sundsvall Bridge project manager Magnus Lundberg. He explained that five interconnected mistakes led to the accident last month, in which a 30t concrete element fell onto a contractor's car. No-one was injured in the accident as the staff were working inside the abutment at the time (Bd&e issue no 80).

The first finding was that the designer used too simple a calculation, which underestimated the forces involved. Rather than use three-dimensional load cases, the designer relied on a two-dimensional load case, which is clearly insufficient. The second error was to undersize the connections that were used to hold the element in place, resulting in only four bolts being specified for a 30t piece of concrete. Lundberg said that neither the contractor, who will review the internal structure to the smallest detail, nor the Swedish Transport Administration, which makes spot checks, discovered the shortcomings during the construction process.

Lundberg said that the designer had chosen an inappropriate solution, which required a very accurate fit and was difficult to carry out in practice. The final error was that changes were made on the site without being reported to the client. An incident report should always be submitted, and changes must be approved before they may be implemented, Lundberg said.

After the incident on August 7, all bridge elements were examined and only three were found to have the same shortcomings. The concrete elements, which are not structural, but are designed as cladding for the abutments, were taken down and the bridge was confirmed to be completely safe. "The next step is that we, the contractor and the Work Environment Authority must reconcile and agree on the content of the report," Lundberg said. "Before the end of the year we hope to have new elements in place."