The failure during construction of the West Gate Bridge in Melbourne, Australia in 1970 was one of world’s worst bridge collapses. Thirty five people died and, as with most failures, a creeping barrage of poor decision making dominated events.
‘Error begat error … and the events which led to the disaster moved with the inevitability of a Greek Tragedy.’ Royal Commission
The bridge was a box girder bridge, made up of thin sheets of steel that were bolted together to form a hollow box. Work had commenced on the east and west spans of the bridge by April 1970, and the contractor’s construction methodology was to fabricate half of each span on the ground. Each of these half-spans was then lifted 50 m up in the air and slid into position.
Then, one of the half-spans on the east side developed a problem. When it was lifted off its temporary trestles at ground level, it suddenly developed a buckle on the top free flange. (The top free flange is the top plate of the bridge that runs down its centre line when it’s connected to the other half-span.)
The buckle occurred because of the decision to lift each half-span separately – each half-span was more flexible than when they were connected together. This flexibility or lack of stiffness allowed the buckle to occur. (While the free flange was stiffened, there were problems with the stiffeners: some didn’t have the necessary stiffness; and some had joints every 16 m, which created points of weakness.)
So now there was a buckle in the flange plate, but rather than lowering the span back onto its trestles and removing the buckle while it was still at ground level, the decision was made to continue with the lift and somehow attempt to remove the buckle when the span was in its final position – at a height of 50 m. But this buckle was a significant 380 mm, and once the span was placed in position there was no way it could be unloaded.
Despite this, the lift went ahead.
Now they had to straighten the buckle in the air, and the method chosen was to remove bolts from some of the joints in the top flange. Then, with the stress thus relieved, they could let the two flange plates slide over one another and flatten out the buckle. Once flattened, new holes were drilled or existing holes were widened in the overlapping plates, and new bolts installed. But once you start removing bolts from the top flange of a bridge, its ability to carry load decreases. This time they got away with it.
Attention then turned to the west span and, in order to prevent buckling of the free flange of these half-spans, they stiffened the flange itself with an extra stiffener, and they also added cross beams to prop the free flange and prevent buckling. This arrangement worked and no buckling occurred during the lifts.
But when they went to connect the two spans, they discovered there was a vertical gap of 115 mm between them. They had faced this sort of issue on the east span, but they’d been able to remove it with hydraulic jacks. But the gap of 115 mm on the west span was too large for the jacks to close, so they made the decision to place 51 t of large concrete blocks on one half-span to weigh it down and close the gap. It worked, and the two halves were brought into line.
Suddenly, the entire top flange buckled across its width.
The one thing they’d been trying to prevent had occurred. (While there had been sufficient strength to prevent buckling during the lift, the extra loading from the concrete blocks was too much.)
The assembly of the span then sat dormant for one month while they decided what to do next.
Eventually, they selected the same method to relieve the buckle as they used on the east span – they would remove bolts from the joints, relieve the stress, and straighten the buckle.
They began removing bolts.
The stresses in the remaining part of the top flange began to increase – with every bolt removal reducing the bridge’s load carrying ability.
They removed 16 bolts.
And kept going.
When 37 bolts were removed the bridge had had enough. It failed locally and the remaining bolts in the top flange sheared.
The left-hand half-span began to drop downwards. Then the load shifted to the right-hand half-span – it was partially connected. The entire span then collapsed 50 m to the ground below.
‘Rescuers worked all afternoon and far into the night, always in horrifying conditions, often in peril of death or injury themselves…All that was humanly possible to save life and mitigate the suffering of the injured, was undoubtedly done.’ Royal Commission
The Royal Commission into the failure found that while the removal of the bolts had caused the failure, it was misleading to attribute the failure to this one cause. Instead they pointed the finger at two primary causes.
The first was a problem of design. The Commission found that the designers, Freeman Fox & Partners, had ‘failed to give proper and careful regard to the process of structural design’. They failed to undertake a proper check of the erection methodology used by the contractor, and the margins of safety during the bridge’s construction, and for the finished bridge, were inadequate.
The second cause was that the contractor had adopted an unusual erection methodology that required them to exercise more than the usual care in its execution. The contractor did not exercise this care, and the designer failed in their duty to prevent the contractor using procedures that were liable to be dangerous. Ultimately, the Commission would find that the tragedy of the 35 deaths was utterly unnecessary.
But the story doesn’t end there….
The collapse in Melbourne was not an isolated incident. More box girder bridges around the world were also collapsing…
Images kindly provided by Public Record Office Victoria.