Following the collapse of the westernmost pylon of a 51-year-old bridge in Genoa that killed 43 people, efforts to pinpoint the cause have centered on what was described as the bridge’s “unusual stay cables” design.

Crossing the Polcevera Valley, the 1.1-km-long viaduct included three cable-stayed sections on the east side, with spans ranging from about 142 m to 207 m. The westernmost pylon (span 9) collapsed during a storm before noon on Aug. 14. About 240 m of deck dropped some 45 m onto a highway, rail track and buildings.

Per a report in Engineering News-Record, Paris-based cable-stayed bridge engineer Michel Virlogeux—who studied the structure’s design and saw a video of the collapse—suspects cable failure. The bridge’s design was unusual in that it had a small number of stays, which provided no redundancy. Also, their prestressing created a “dynamic effect” when ruptured, hastening collapse, he said.

The report said that the stays of span 11 were reinforced over their full lengths with external steel plates in the 1990s, and that the tops of stays of span 10 were reinforced, but no retrofitting appears to have been made on the collapsed span 9. Following the collapse, the infrastructure and transportation ministry ordered an urgent review of an estimated 10,000 bridges older than 50 years.