JAPANESE BRIDGE engineers are conducting urgent safety checks on their most widely-used incremental deck launching technique following the release of a scathing report on the country's worst bridge accident in decades which killed seven workers.
The accident occurred in June on one of the world's largest bridge projects, the pounds5.5bn Kurashima crossing, near Imabari, southern Japan. Sections of a temporary steel platform supporting an already launched side span were being dismantled and lowered on cables. Three cables gave way, tipping the 50t platform section and plunging workers on it 60m to the ground.
Safety mechanisms controlling the dismantling operation were condemned by a government report. Shozo Yoshikawa, deputy general manager of the client, Honshu Shikoku Bridge Authority, said the system was widely used throughout Japan, but in the light of criticisms in the report drastic changes would be made.
'We have scrapped the system totally and will lower the remaining three sections using large ground-based cranes,' he added. The two 550t capacity mobiles should be in position next week, and Yoshikawa is confident that the 10 week delay can be made up and the crossing opened on schedule next spring.
Three massive back-to-back suspension bridges - each with a central span up to 1.03km and sharing common anchorages - make up the crossing, which forms part of an impressive 60km sea crossing between Honshu mainland and Shikoku island.
Unusually, intermediate backspan decks between the main suspension spans are supported by conventional piers rather than suspension cables. These sections are incrementally launched from anchorages. Steel box deck units slide over a previously positioned temporary steel platform beneath, already spanning the full side span.
To dismantle the platform after the launched deck is in position, 18m long sections are unbolted and lowered on four cables operating through a jacking frame on the deck above. The first of five platform sections had been successfully lowered but, during the second operation, three cables parted or slipped through double sets of clamps as the unit was being jacked down.
'We may never know the exact failure sequence, but suspicion rests on the clamps, the jacking hydraulics and the back-up safety mechanism,' said Yoshikawa.