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Site clearance hampers Korean bridge collapse investigation

NEWS

AN INTERNATIONAL investigation is under way following the partial collapse of the £300M Seo-Hae Grand Bridge in South Korea.

No one was killed or injured in the collapse but 37 precast concrete bridge segments from one partially complete and two completed spans crashed to earth on 5 August.

Investigations are focusing on the possible failure of the steel launching truss being used to assemble the 80t deck segments. Two representatives from erection subcontractor Freyssinet and an assessor from insurance firm Veritas flew in from France three days after the incident.

No date for an official report or assessment of the cause of the collapse has been given. Freyssinet Korea site manager Michel Monballiu told NCE this week: 'We do not know the cause yet, but we have a team of experts here.'

He added: 'It's possible that the launcher collapsed. Maybe one of the structural elements failed, but we do not yet know.'

But it is understood that the main contractor moved in quickly to break up the collapsed units and clear the site shortly after the collapse. This highly unusual move is expected to have left the investigation team with little physical evidence to examine on arrival.

Construction of the 7.31km long bridge started in November 1993 and it was due to open to traffic in December next year as the longest bridge in Korea. It is the key link on the new six-lane Seo-Hae Coastal Highway and spans the Asan Bay, some 90km south west of Seoul.

Two main contractors, Daelim Industrial and LG Construction, are working on the bridge. It is designed by Daewoo Engineering, with the support of TY Lin International and the Danish consulting engineer Cowi.

A 470m span steel cable-stayed bridge - the longest span in the country - crosses the main channel to the south. Another section to the north over the second channel is a 500m long free cantilever insitu concrete box girder bridge, with a main span of 165m.

The collapse occurred during construction of the 5.82km precast concrete segmental twin box girder southern viaduct. This has 60m long spans founded on twin 2.5m diameter insitu concrete piers and was being built by Freyssinet under subcontract to main contractor Daelim Industrial. The accident happened as work was approaching pier 55 in the middle of the link.

Segments are precast on site in a prefabrication yard on the bank adjacent to the bridge and transported along the previously completed deck before being positioned on a steel launching truss. Built by Italian specialist Deal, this truss spans adjacent piers and supports the segments making up a span until they are all in position.

Epoxy adhesive is then applied to the segment faces and external post- tensioning stresses the segments together. The launching truss then slides forward to span the next pair of piers.

An identical truss is being used to construct the viaduct on the northern side of the central cable stayed bridge. This section is due for completion in mid-September.

Failure of the truss is likely to have caused the incomplete span to collapse, and then overload and collapse the two preceding spans.

Monballiu said the incident was likely to lead to a three-month delay on the project. Only five more segments needed to be placed to complete the span that was being worked on as the collapse occurred.

TY Lin senior consulting engineer Dr Kookjoon Ahn said the equipment being used on the northern approach will be dismantled and re-erected to replace the wrecked truss on the south as part of the recovery operation. 'They can finish the north approach bridge and then move the launching truss to where the accident happened.

'The cause has not been established,' Dr Ahn added. 'This will not affect the project that much. The contractor will lose some time, two months or so, but they can meet the final opening day, I believe.'

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