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Diaphragm walls cleared of blame for Injaka disaster

INVESTIGATORS IN South Africa have ruled out the failure of concrete diaphragm walls used to anchor external prestressing tendons as the cause of this month's horrific Injaka bridge collapse.

Roped access engineers carried out a full inspection inside the bridge's failed concrete box girder deck last Wednesday. They had hoped to discover why the partially constructed, incrementally launched bridge suddenly collapsed, killing 14 and injuring 13 others (NCE last week).

But specialist adviser to the South African Health & Safety Inspectorate, Andries Oosthuizen, said 'nothing abnormal' had been found inside the deck. The walls had acted as anchor points for external prestressing tendons running through the middle of the deck segments.

'This is not clear cut,' he said. 'I suspect the cause of the collapse will be a combination of design and construction problems.'

The bridge was thought to be only the second constructed in South Africa with both launching and permanent prestressing tendons outside the concrete deck.

The more common method is to grout tendons into the concrete.

Oosthuizen added: 'There is quite a difference in behaviour between bonded cables and unbonded cables. Once the [Injaka] cables had lost some of their force, there was a strength loss over the whole length of the deck because the launching cables run for a very long way.'

The HSI investigators plan to take cores from concrete deck segments, and will also test the failed high tensile steel bars used to join a nose girder on to the front of the deck.

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