In a world that demands absolute certainty, it comes as a big disappointment when the precise cause of a catastrophe remains a mystery. Such was the case last month when the UK safety authority, the Health and Safety Executive (HSE) admitted it was unable to reach any firm conclusions about how three people died when a crane collapsed in London in the summer of 2000.
Despite a five-year probe that reportedly cost more than £300,000, the HSE decided to take no legal action over the collapse of the Wolff 320 BF luffing jib tower crane on a construction site at Canary Wharf in London’s Docklands. It concluded there was insufficient evidence to provide a realistic prospect of conviction, and said it would not be in the public interest to proceed with criminal charges.
HSE offers theories
However, although it offers no conclusive answers, the HSE’s new report into the crane collapse does contain a number of theories about what caused the crane to collapse at 4pm on that fateful Sunday afternoon on May 21, 2000.
Michael Whittard, an erection supervisor, Martin Burgess an erector, and Peter Clark, a crane driver, were part of a team using a large ‘climbing frame’ incorporating a hydraulic lifting device to raise the height of the tower crane when the collapse occurred.
They fell more than 120m when the top of the crane and the climbing frame overturned as they were approaching the end of a weekend of climbing operations.
Two other members of the crew escaped onto the tower of the crane and survived.
At the time of the incident, the erectors had just inserted the final mast section, and were starting to lower the crane top to make the final connection to the mast. At this point, says the HSE, an erector noticed that an upper guide wheel was twisting around the corner of a crane mast leg. The mast began to shake violently before the ‘climbing frame’, hydraulic cylinder, new mast section, and top of the crane overturned and fell.
The question is:How? After all, there was no evidence of any pre-existing defects, and the HSE did not believe that the erection team had deliberately slewed the crane to move the last mast section into position. Indeed, the Executive concluded that the physical evidence suggested that the crane was probably not slewed.
Backwards movement
The Health and Safety Laboratory (HSL), which operates as an agency of the HSE, established that a substantial backwards overturning movement must have acted on the top of the crane. But it remains unclear precisely how this occurred.
The HSE has, however, proposed a number of theories.
One – called the ‘double wind hypothesis’ – considered the possibility that the wind speed exceeded the maximum permitted at the start of the climb then reversed direction by 180 degrees during the course of the climb.
This was later dismissed by the UK’s Building Research Establishment as unlikely.
However, there were other theories too. External specialists commissioned to review the HSL’s work included a professor of engineering, who suggested test loading an undamaged guide wheel. This test was performed and the guide wheel supports were found to be stronger than had first been thought. The professor claimed that the overturn must therefore have been caused by an “abnormal event”.
He suggested what became known as the ‘yoke disengagement hypothesis’ where the climbing frame had lost its support on one side of the mast. The mast would have sprung forwards, causing the top of the crane to move backwards, increasing the guide wheel forces, and leading to the overturn.
A tower crane consultant recommended by OSHA, the US safety authority, put forward a third theory – the ‘multi factor hypothesis’.
Several factors to blame?
This contended that the main factors in the accident included the degree of misalignment of the mast, the ‘balance’ of the crane assessed by the erection crew before each climb, non-uniform clearances between the guide wheels and mast, the stiffness of the climbing frame and mast, wind forces, tolerances of various components, and the condition of the guide wheel assembly.
The OSHA consultant agreed with the HSL that the overturn probably started at the upper guide wheel that failed.
A further review of all the specialist work by an HSE lifting specialist resulted in a fourth theory – ‘the jammed guide wheel hypothesis’ – but, again, the evidence did not tend to support this explanation.
Philip White, head of the HSE construction sector, told Cranes Today that it was a big disappointment that nothing conclusive had come out of the HSE’s investigation. He said the hardest part was not being able to provide answers to the families of those who died.
But, he added, this was not for want of trying: “A considerable amount of time, effort, and resources were put into this investigation including the involvement of a number of experts.”
Like many others, White expected the investigations to result in the identification of both an immediate and a root cause of the tragedy. “Initially, we thought there might be a straightforward answer to the crane collapse, for example, the failure of a load-bearing element due to metal fatigue. This obviously wasn’t the case. But we never imagined that we would not be able to find the cause.”
Investigations into the accident have now ceased. However, White did not rule out a return to the incident room. He said that the HSE would certainly consider any new evidence brought to light.
‘Not a waste of time’
The HSE refuses to concede that the investigation was a waste of time. White said the key issues outlined in the Executive’s discussion paper on tower crane climbing, published in February 2003, had reduced the likelihood of such an incident happening again.
But, he added, the crane industry could not look at any single theory in the HSE report as being a more likely cause of the accident than any other. “As with most accidents,” White said, “there are a number of factors that come into play. Each theory as outlined in our report has a number of different elements associated with it which make it difficult to provide an absolute answer.”
So the investigation does not represent a turning point in the way tower cranes are erected. White explained that the type of tower crane that collapsed at Canary Wharf was not widely used in the UK. He also emphasised that both the discussion document and the report of the investigation outlined steps the industry, in conjunction with the HSE, was taking to improve the safety of tower crane erection.
“We would also hope that HSE’s broader programme designed to bring about a cultural change in the way the construction industry manages health and safety will have an impact on the safety of tower crane erection,” White concluded.
