Pretoria. A truck hitting a supporting pier of the bridge over the N3 caused the collapse, the SA National Roads Agency Limited revealed on Wednesday.
Acting Engineering Executive Louw Kannemeyer said: “The collapse of the Geldenhuys Pedestrian Bridge, B1187, on the N3 in the early hours of 9 August 2017 was caused by an impact force on the pier in the median of the highway, our investigation has established.
“Following the incident, Sanral commissioned Superstructure Consulting Engineers to perform an independent investigation into the cause of the collapse. The firm has completed the investigation and handed its report to Sanral.”
Sanral's evidence suggests that the impact was caused by an 18.1 ton coil of steel that was found lying some 100 metres north of the collapsed bridge on the N3 northbound road. Marks on the road led directly from the point of impact to the position where the steel coil came to rest.
The high weight of the coil of steel, and moving at speed, can induce an impact force far in excess of the pier’s capacity. Debris that scattered radially away from the pier that was hit to a distance exceeding 40 metres confirms an external impact force was of very large magnitude. The report indicated the force involved was far in excess of the design requirements of the bridge.
The report further stated that statistically it is an extremely rare occurrence for a bridge support to be subject to an impact force of such an extreme magnitude at a height. It must be noted that the pier was protected by the median concrete barrier but that the impact was higher up.
The coil was afterwards inspected at ArcelorMittal premises in VanderBijlPark and the marks on the coil indicated substantial impact with concrete. Sanral insurers are pursuing the matter with the third parties involved while the SA Police Service is performing its own investigation into the coil and the transport vehicle involved.
The bridge was constructed in 1978 to provide pedestrian access between the mining operations on the Germiston side to the East and the Geldenhuys Railway Station to the West. The mining operation ceased some years ago and the bridge was closed for pedestrian traffic since.
Although the structure was no longer in use, it remained on Snaral’s Bridge Management System and it was inspected as part of the roads agency’s general bridge inspection programme. The last inspection was conducted in July 2016 and picked up no items of concern on the main structural elements. All defects were designated as minor and irrelevant (Minimal relevancy) by the bridge inspector who is a professional engineer with several years of bridge design experience. The handrails were missing but as the bridge was no longer in use and cordoned off, this was of no concern.
“Several speculations about the cause of the collapse were proffered immediately after the incident had occurred and these included seismic effects, explosives and illegal underground mining in the area. The consulting engineers identified all possibilities and then ruled them out systematically until the actual cause was identified,” said Kannemeyer.
Louw thanked motorists who use that road, the Gauteng Provincial Government, SAPS, Department of Labour and Ekurhuleni Metro for their cooperation in the aftermath of the incident, especially during the site clearing period.