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The reason for the collapse of the pedestrian bridge on the N3 highway in Germiston last month has been found to have been a truck hitting the supporting pier of the bridge. This was established by an investigation conducted by the Superstructure Consulting Engineers, the South African National Roads Agency SOC (Sanral) said in a statement.

“"All evidence suggests that the impact was caused by an 18.1 ton coil of steel that was found lying some 100m 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. Debris that scattered radially away from the pier that was hit to a distance exceeding 40m confirms an external impact force was of very large magnitude,” Sanral said.

N3 bridge collapse explainedThe report indicated the force involved was far in excess of the design requirements of the bridge, with the combination of the weight of the coil of steel, measured at 18.1 tons, and the speed at which it was moving, induced an impact force far in excess of the pier’s capacity. The coil was inspected at ArcelorMittal premises in VanderBijlPark, with the marks on the coil indicating substantial impact with concrete.

“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,” says Sanral’s Acting Engineering Executive Louw Kannemeyer.

According to the report, it is statistically an extremely rare occurrence for a bridge support to be subject to an impact force of such an extreme magnitude at a height. The pier was protected by the median concrete barrier, but that the impact of this accident was higher up.

The bridge was constructed in 1978 to provide pedestrian access between a mine and the Geldenhuys Railway Station. It was closed for pedestrian traffic after mining operation ceased a few years ago. Despite it not being in use, it remained on Sanral’s Bridge Management System and was inspected as part the general bridge inspection programme. The last inspection was conducted in July 2016.

This inspection found no items of concern on the main structural elements, and all defects were designated as minor and irrelevant. The handrails were missing, but as the bridge was no longer in use and cordoned off, this was of no concern.

Reports indicate that the estimated final cost for the demolition of the collapsed bridge and clearing of the site is R2.9 million.

Image credit: https://twitter.com/traveller24_sa/timelines/895337934349627392

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