The collapse of a new lifting bridge over Manchester Ship Canal could be have been down to a systems or control failure, according to engineers.
Unusually for an accident like this, the failure was caught on camera so engineers have been able to examine the footage and draw some initial insights into what might have happened and the areas the investigation might cover.
Bridge consultant Simon Bourne told New Civil Engineer that it seems the problem was not in the main bridge deck, but related to the supporting ropes.
“It (the deck) seems to fail quite slowly at first with both ropes on the side away from the existing bridge slowly going down whilst the ropes on the other side do not move – this would suggest a system or motor failure rather than a structural failure,” he said.
“Once it has started to collapse on that side, it then dramatically fails on the other side, firstly with one corner breaking and then the final one goes too. This is all confirmed in the final images, where the side that started to fail first still has its ropes intact and is simply resting on the ground, whereas the side that then fell more dramatically has its ropes broken and missing and is resting much lower within the ground due to the final impact.”
Bourne concluded that it initially appears to be a system failure, where two ropes lowered on one side at a different rate to the two on the other side.
His view is supported by Allan Mann, a member of the Structural-Safety panel, who said: “From what I can see, the deck drops on one side but not in free fall. That suggests the lifting cables were still attached and controlling the rate of fall and so the incident points to some sort of mechanical / control system failure.”
Because the bridge is a moveable structure, it requires a mix of the structural, mechanical and control system engineering disciplines.
“The design of the control system is especially demanding since the design has to anticipate all potential modes of failure – say uneven lift, cable jam, loss of power etc – and configure the sensors and control system overall to prevent catastrophic consequences to the requisite level of security,” said Mann.
“Although you might think this points to some weakness in the control system that observation would be highly speculative. It is unusual for there to be one single cause of a catastrophic failure,” he added.
As both engineers pointed out, these rare failures happen during construction and rigorous testing, not in service.
“It is in all our interests that there is a proper investigation and the causes are then made public so that the community can learn from it,” said Mann.
A full investigation is underway into the deck collapse and the results of this investigation have not yet been published. The bridge is part of the £32M Western Gateway Infrastructure Scheme, where work is being carried out by a joint venture of Hochtief and Buckingham Group. The new £1.1M lift bridge over the Manchester Ship Canal is to the east of the existing M60 bridge.
Howard Civil Engineering is carrying out civils works for the bridge with Cass Hayward as designer. It has extensive experience of moveable bridges and has been involved in the design of vertical lifting bridges, swing bridges, bascule bridges, dutch drawbridges, retractable bridges and roll on roll off link spans.