CONTRACTORS STAGED the controlled collapse of a 50 year old 140t post tensioned concrete beam at the heart of London's Heathrow airport last week.
Almost exactly as planned, the giant 46m long concrete beam plunged suddenly to the ground last Thursday after a tense two hour operation to weaken the concrete and sever reinforcement.
Vibration from the collapse could have damaged highly sensitive computers at a nearby data processing centre within the crowded British Airways concession area.
There were also fears for BA's biggest maintenance facility, a giant hangar handling four planes or two 747s at once. It adjoins Technical Building E (West), a second wing of which remains standing.
The beam was the first of five to come down on a post war hanger being demolished to make room for large aircraft parking in the congested airport.
Technical Building E (East), with five bays for small Lockheed Constellation planes, was built in 1950 to a design by Sir Alan Harris, one of the best known pioneers of post tensioning in Britain. Analysis shows impact and vibration had been well contained, said Dr Susanne Buchner, a specialist demolition civil engineer with Peter Linsell & Associates which calculated the 'erosion' sequence for the box beam.
The beam was nibbled at six hinge forming points along the top with a hydraulic crusher jaw mounted on a long reach excavator. Buchner directed demolition contractor Hughes & Salvidge (Portsmouth)'s operation from an aerial platform.
Buchner designed the collapse to minimise impact on the hangar's base slab, as this will be retained following superstructure demolition. The idea was to make the whole beam fall at once to maximise the spread of impact loads.
'The maintenance hanger is crucial to BA operations,' said project manager Ralph Goldney from Vector Management, which is representing client British Airways for the over £2M conversion contract.
'It turns around up to ten planes in one evening. And it could be affected by vibration or flying object debris.'
FODs are a major risk during demolition at airports. High velocity debris can easily damage nearby aircraft unless contained.
FODs are a possible consequence of the sudden release of energy in wires held in tension with grout and anchor wedges at each end.
'And since we have a £150M South African airlines 747 parked within 200m, the end of the main runway on the other side with a queue of planes, the workers themselves of course, and nearby buildings as well as other planes and vehicles close by, the danger is significant,' said Mike Wakely of consultant Ross & Partners. His firm is in charge of overall design for the project.
'We did not want the beam landing with a point impact,' said Wakely. He thought it had fallen well in the event, in a kind of 'pancake landing' which spread the load.
It was thought grouting of the post tensioning ducts was free from voids and therefore energy release would be contained.
A parked excavator held steel shields in place over the beam ends as a precaution. This had helped with smaller 25t secondary beams which had been found to have only partial grouting.
A number of these were successfully brought down over the past two weeks.
'Dust is another problem. It does not do jet engines any good at all,' said Wakely.
Four more beams must now be taken down and the remainder of the building demolished before main contractor Costain can complete the parking areas by May.