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Inexperience and errors led to fatal Injaka bridge collapse

INCORRECT POSITIONING of temporary bearings during incremental launching have been identified as the primary cause of the fatal 1998 Injaka Bridge collapse in South Africa.

But the official investigation report into the collapse, which killed 14 people and injured 19, concludes that bearing problems were made worse by a catalogue of design, monitoring, construction and organisational errors.

Inexperienced design and construction staff, poor construction quality control and a failure to react to a 'clear warning that all was not well' with the structure, led to the disaster.

The conclusions are contained in a report to South Africa's director of public prosecution by presiding inspector Larry Kloppenborg and made public last week.

The report recommends that criminal charges under the Occupational Health & Safety Act be brought against the bosses of designer VKE Consulting Engineers' Pretoria office, contractor Concor Holdings and client the Department of Water Affairs & Forestry.

Kloppenborg also recommends that similar criminal charges be brought against Johan Bisschoff, who directly supervised VKE's permanent design and against Rolf Heese of Concor who was in charge of the temporary works design.

The report recommends that any charges brought should also consider the fact that 'the extent of the catastrophic failure of the bridge equally could have caused the deaths of any of the 19 injured persons.'

The collapse of the bridge on 6 July 1998 was one of the worst construction accidents ever seen in South Africa (NCE 16 July 1998).

At 300m long, 14m wide and up to 37m above the river bed, Injaka Bridge was a major structure and the consultant and contractor had extensive experience with such incrementally launched post-tensioned structures.

The collapse occurred after the contractor had slid out five of the 20, 15m long sections of the 3m deep box section deck (see box).

The sixth segment was being jacked as the structure collapsed. At that point the concrete deck extended 24.4m beyond pier 2 with the leading edge of the 27m long launching nose projecting 7.1m beyond pier 3.

Among those killed was Maria Gouws, the VKE engineer responsible for designing the structure. She was on the deck with several guests celebrating progress on the structure.

The primary cause of the collapse was found to have been the positioning of temporary bearings on which the deck structure slid out during construction.

These were located inside the permanent bearing positions which were to be under the box section webs. As a result the temporary bearings punched through the structure and caused the collapse.

Kloppenborg concludes that Gouws and Heese lacked sufficient experience to have been left in charge of the permanent and temporary work for such a complex, incrementally launched structure. Gouws had only around two years' bridge design experience.

The report highlights the lack of calculation checks carried out on the permanent and temporary works designs, as well as lack of construction and production monitoring. It criticises the designer and contractor for allowing inexperienced staff such operational freedom.

INFOPLUS For the full report go to www. nceplus.co.uk/magazine

Likely sequence of collapse 1. Structure weakened some weeks before collapse by cracks in deck diaphragm, longitudinal cracks in the haunch area and longitudinal and transverse cracks in the underside of the deck.

2. West bearing punches through bottom slab at top of pier 2.

3.Full deck load transfers to eastern bearing causing similar punch-through of soffit.

4. Without support at pier 2, bridge now spans from abutment to pier 3 - some 82m.

5. Abnormal load causes welds to fail on the launching nose and leads to catastrophic buckling of girders.

6. Top slab above pier 2 fails followed by web-crushing.

7. Loss of axial strength in post-tensioning cables leads to failure of deck above the abutment and at midspan between pier 2 and abutment.

8. Launching nose to deck connection fails.

9. Pier 2 shears off at base due to horizontal forces as structure collapses.

Causes of the collapse

Principle cause 'Placing the sliding path in a position other than under the webs of the deck section.' This caused a 20mm punch-through in the bottom slab of the bridge deck by one or more of the bearing pads on the top of pier 2.

Other contributory factors 1. Designer and contractor using engineers lacking appropriate qualifications or experience in incrementally launched bridges.

2.Failure to comply with design specifications regarding:

lreinforcement used in the deck llack of automatic pier deflection monitoring devices lfailure to ensure proper checks of both temporary and permanent designs by competent engineers.

3.Incorrect feeding of slide pads at pier 2, reducing the surface area of the slide path.

4.Deviation from specified bearing height tolerances at pier 3.

5.Manufacturing defects in the welds of the launching nose.

Other problems highlighted during the inquiry 1. Failure to properly consider, appreciate and address the causes of cracking in segment 2, particularly the crack in the western haunch of the deck between diaphragms 3 and 4 as it passed over the abutment - clear signs of a problem.

2.Severe cracking of diaphragm 4 before the launch of segment 2, causing deformation in the web and concentrating stresses above the temporary bearings.

Additional factors that could have contributed but were not conclusively proved by the inquiry 1. Reinforcement bunching was seen in the deck structure, caused by inadequate construction control. This meant that there was incorrect separation between top and bottom reinforcement in the bottom slab. Although there was evidence of poor workmanship it is not clear that it contributed to the collapse.

2.Shutter oil was found on reinforcement and could have caused inadequate bonding between concrete and steel.

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