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Bridge sway feared cause of Cambodian festival tragedy

Inadequate crowd control on a new footbridge has been identified as the main cause of a festival stampede last week that left over 350 people dead in Cambodia.

Cambodian investigators have said that the stampede occurred after rumours rippled through the crowd that the suspension bridge was about to collapse.

An estimated 7,000-8,000 people were on the structure when the stampede started. Bridge experts said it was possible that stampede was triggered by the bridge starting to sway.

The pedestrian suspension bridge opened in August this year. It links the Cambodian capital Phnom Penh to Diamond Island, one of the festival’s main event sites. It is around 100m long and just 7m wide.

Sway “entirely plausible”

“It’s entirely plausible that the bridge could sway under that type of loading,” said Cambridge University structures reader Alan McRobie. “It appears a lightweight bridge.”

The bridge appears undamaged after the crush.“When you get a critical number of people on a bridge it can begin to sway,” said Bristol University senior lecturer in structural dynamics John MacDonald.

“Once it’s triggered the oscillations grow and grow until something stops it, for example a dampener.”

MacDonald said all types of bridges are susceptible to similar types of movement due to pedestrian loading.

London’s Arup-designed Millennium bridge was famously closed soon after opening due to pedestrian loading causing unexpected movement.

There, movement of the bridge was caused by the sideways loads generated by walking. The small oscillations caused people to sway in step with the bridge increasing the amplification, a phenomenon known as “synchronous lateral excitation”.

“We were careless. This was a joint mistake”

Cambodian prime minster Hun Sen

MacDonald said swaying on the Cambodian bridge would have been due to vertical movement caused by the high density of people on it, and not lateral excitation.

“The key thing is the natural frequency of oscillation of the bridge – the lower it is the more likely it will be susceptible to movement.

“Longer and lighter bridges tend to have lower natural frequencies,” added MacDonald.

UK design codes call on engineers to consider whether a bridge is likely to be used as an observation point and take adequate measures such as installing

On older bridges where crowd build up is possible precautions are now taken.

No action taken

During the Oxford and Cambridge Boat Race the Hammersmith bridge in London is closed to pedestrians whilst the competition takes place.

And in Bristol the Clifton Suspension Bridge is now closed during key periods during the International Balloon Fiesta after crowds noted movement in the bridge.
Cambodian authorities have promised that a full report on last week’s stampede would be released “in the coming days”.

But Cambodia’s prime minister Hun Sen has already said that no legal action will be taken against festival organisers or the bridge builders.

“Nobody will be punished for the incident,” said Sen.

“The incident that happened was the responsibility of the government,” he said, describing it as “a historical lesson that we must remember. We were careless. This was a joint mistake that nobody expected.”

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