AN INCORRECTLY positioned section of travelling formwork was this week thought to have triggered a catastrophic temporary works collapse during construction of a motorway bridge in Spain in November.
Failure to position the formwork properly is assumed to have caused it to overbalance and crash to the ground, killing six workers and injuring three others.
The collapse took place near the town of Almuñécar in southern Spain at 3:35pm on 7 November. Five Portuguese workers and one Spanish worker died falling from the structure.
Ahead of the collapse, deck contractor Puentas y Calzadas was preparing to move a massive travelling formwork system that was being used to cast a 60m section of the 500m viaduct deck.
The travelling formwork system was moving from west to east as contractors cast the deck. It comprised a railmounted mobile shutter resting on parallel steel trusses, which spanned 130m between the piers on either side of the central arch.
Each pier was fitted with supporting saddles. A specially adapted saddle was also slung around the crown of the arch.
British bridge experts who examined photographs taken before, during and after the collapse said they believed the contractor was moving the temporary works to the east across the central arch section ahead of a concrete pour.
Usual practice would allow the travelling shutter to move to the far end of the support trusses before they are, in turn, moved beneath it into their next position (see box). Piers would normally be close enough to each other to allow the trusses to straddle three at once.
This is thought to have been the case for the rest of the viaduct, where the piers were spaced closer together at 100m intervals.
But at the arch, the distance between the piers and the arch crown was thought too great to allow the trusses to rest on all three supports at once.
As a result, the contractor would have to align the central load-bearing section of the support trusses to span from the western pier to the arch crown, so that it could cast the western half of the arch-supported deck (see diagram).
Photographs taken just before the collapse show that the contractor was attempting to centre the travelling shutter above the arch crown before moving the support trusses forward to bridge the next span.
This would have unloaded the first span and allowed the trusses to be moved without overstressing the tapered ends.
British engineers agreed that while performing the difficult load transfer sequence, a jam may have prevented the shutter from moving, or it may have been incorrectly positioned above the crown.
Photographs taken during the collapse and a first-hand examination of the wreckage show that the west ends of the trusses slipped off their saddles first. They buckled in contact with the ground, with the eastern ends toppling to the south of the structure.
This collapse sequence suggests that the formwork was positioned to the west of the arch crown, causing it to overload the trusses as they were moved to the east. This is thought to have caused the trusses to slip off the saddles at the western pier.
Damage to the bridge deck box girder's side cantilevers indicates that the eastern ends of the trusses were lifted by the weight of the wrongly positioned shuttering.
Launching noses at both ends of the trusses were largely undamaged and showed no signs of buckling or shearing.
This suggests that they came off the western saddle, allowing the trusses to tilt under the weight of the shutters.
Police and Public Works Ministry investigators were this week continuing investigations into the causes of the accident.
Inspectors were unable to comment on the causes of the collapse as NCEI went to press.
The site may remain closed for a year while the wreckage is studied for signs of fatigue and failure.