The official investigation into November’s catastrophic bridge collapse in Indonesia has found that a catalogue of engineering errors triggered the failure.
The report, commissioned by Indonesia’s Ministry of Public Works, said a lack of engineering knowledge and understanding of the suspension bridge structure was the primary cause of shear-failure in the connection between the steel hanger and the bridge truss near its deck.
One of the key investigators, Bandung Institute of Technology structural engineering research group professor Bambang Budiono told NCE that the failings occurred during repairs to the 10 year old structure.
“The main reasons why the bridge collapsed was lack of knowledge [about the forces running through the bridge], an over simplified approach [to repairing the bridge] and lack of maintenance and repair,” he said.
The three-span, 710m long Kutai Kertanegara suspension bridge collapsed into the River Mahkam in East Kaliminta province last December leaving only the bridge cables and tower in place. At least 20 people were killed and many more injured (NCE 1 December 2011).
A team of nine engineers from three Indonesian universities − Bandung Institute of Technology, University of Indonesia in Jakarta and Yogyakarta’s Gadjah Mada University - along with an official from the Ministry of Public Works investigated the collapse.
Failure occurred when engineers where jacking underneath one side of the bridge deck at mid span. The jacking procedure caused extra stresses on the hanger connection triggering the shear failure of the bolt connection between the steel hanger and the bridge deck.
Once one connection failed, the increased stress on the other connections caused a chain-reaction resulting in all the other bolts failing with 20 seconds, according to the report.
Budiono said that a witness statement from the only surviving bridge worker - four of his colleagues died in the collapse -said that a maintenance firm was in the process of jacking the bridge to restore the bridge’s lateral camber of 3.7m after it had dropped to just 3m. The work was commissioned by the bridge’s owner, the Kutai local authority.
Workers first began jacking on the upstream side of the bridge two days before the collapse on 24 November, according to the witness. They raised it by 150mm, before beginning on the downstream side. It was at this point that the collapse occurred.
Failure to jack both sides
Budiono said the contractors should have jacked-up the bridge deck at both sides, and closed the bridge to traffic.
However, they shut just one lane, and jacked each span individually - which added to the death toll on the bridge.
But there were numerous errors prior to the repair work being carried out on site, according to the report.
Engineers working on the repairs failed to follow numerous standard practices, says the report.
“They did not follow any guidelines or good engineering principles,” said Budiono.
Before carrying out the maintenance, the company failed to make structural checks on the bridge to analyse what impact the jacking procedure would have on the bridge. The bridge had also not been checked during the first three years after its opening in 2001 due to lack of funds, alleges the report.
“Engineers did not follow any guidelines”
Prof Bambang Budiono
However, because of the dramatic drop in camber, engineers checked the bridge and discovered the foundation of its western tower had moved horizontally 200mm, which changed the forces within the bridge structure, but no further investigation was carried out to explore the effects on the forces of this or the proposed jacking.
The other major contributing factor in the collapse was the use of poor quality bolts connecting the hangers to the bridge deck and suspension cable, says the report.
“They used a very brittle material manufactured locally which was not covered by the codes,” added Budiono.
Tests on the connections following the collapse revealed the bolt was made of using a cast-iron grade that was brittle and sensitive to impact loading. “This was the wrong choice of material,” said Budiono. “There should have been redundancy in the structure.”
But the choice of material was approved during design, he said.
Indonesia does not have any suspension bridge design codes and engineers used the Japanese equivalent, but it is unclear how accurately these were adhered to.
Indonesia’s Department for Public Works has accepted the conclusions of the report and is reviewing its procedures.
NCE was unable to contact the consultants and contractors involved in the bridge’s construction and maintenance at the time of going to press.