Moved to tiers

UK universities collaborate on a project to understand better the dynamic loads crowds exert on grandstand structures to help make future venues safer. Siobhan Wagner explains.

Anyone lucky enough to get tickets to this year’s FA Cup Final between Chelsea and Manchester United at the new Wembley Stadium, might still be shaking from vibrations caused by the foot stamping and chanting of 90,000 fans.

For many spectators, the rumbling, pulsating grandstands of crowded sports stadiums can enhance the game experience. But in some extreme cases it can cause panic about the safety of the structure.

The way modern-day stadiums like the new Wembley and Twickenham are constructed there’s almost no chance of structural collapse. But a group of civil engineering researchers at Oxford University claims that they are still designed with a lack of scientific information on the behaviour of crowds and the vibration levels they can produce.

This doesn’t mean that stadiums can’t handle crowd-induced shaking, they say, but they are over-designed in a way that is too conservative and costly.

The researchers hope to change that by gaining a better insight into the dynamic loads that spectators exert on grandstand structures. Their findings could then be used to influence government regulations in the designs of future stadiums.

The team will focus its studies on cantilever tiers, which are being increasingly used in stadiums with the twin aims of housing more people and therefore generating more revenue.

The problem with these tiers, however, is that they tend to be prone to vibration because they have low-vibration frequencies — some as low as four to to five Hz.

All structures vibrate at a number of different natural frequencies. The time when problems occur is when resonance frequency occurs, which is when vibration, often caused by excited crowds, matches the natural frequency of the structure.

‘We’re not talking about the likelihood of structural collapse, but a stadium’s cantilever tiers can move alarmingly,’ said Martin Williams, a co-investigator with Tony Blakeborough in the Oxford project. ‘We’re talking about crowd discomfort.

The likelihood is that you’ll have a crowd where part of it is moving and part of it is stationary. The moving ones won’t particularly feel the vibrations, but the stationary ones certainly will. If the crowd discomfort gets severe, we think it could reach panic levels.’

Williams said it could be a scary situation for not only spectators who are sitting in the cantilever tier, but also those who are seated beneath it looking up at a structure moving up and down. ‘The worst-case scenario is you get some sort of stampede as people are desperate to get off the structure, thinking, “This isn’t safe. I want to get out of here”,’ he said.

The project involves a moving, instrumented grandstand rig that can hold up to 15 people in a realistic stadium configuration and provide continuous output of forces at the feet of each individual.

The rig is supported on air-springs and driven by four electric actuators that can either provide a pre-defined input motion or enable it to respond dynamically as though it were part of a larger, flexible structure.

This mode of operation is achieved through an Oxford-developed technique known as real-time hybrid testing.

The experiment is intended to replicate the way crowds behave during a rock concert, as many stadiums are now used as both music and sports venues. Williams said the problem with crowd loads is exacerbated at concerts because seas of people move rhythmically to the music in time with each other.

While it is hard to replicate a rock concert inside a laboratory, Williams said the information they get will be much more detailed than they would gather from on-site testing.

The tests involve a group of people who are asked to jump or bob in time to a metronome beat. Additional data is gained by analysing digital video footage of tests using video tracking techniques, so that individual head motions can be tracked and correlated with foot forces.

‘We’re hoping that will be a way of quantifying the loads in a real crowd,’ said Williams. ‘Then we’ll be able to determine the dynamic load a person is generating on the structure by the motion and speed of his or her head. Then from films of pop concerts at Wembley Arena, or whatever, we can estimate what the loads are.’

Other universities around the country are conducting experiments that will help feed into the overall purpose of creating more informed design guidelines for stadium structures. Sheffield, for example, has been using field-testing equipment on several stadiums in the UK, such as City of Manchester Stadium, to measure its structures’ dynamic properties and their response to crowd loads.

The Oxford team is filling in some of Sheffield’s blanks by determining how people interact with a structure and quantifying the loads they’re putting in. ‘That itself raises various issues,’ said Williams. ‘Quite a few people measure the load a person generates just by jumping up and down on a force platform. but with a crowd of people, do you just multiply that individual load by how many people you’ve got, or do you say in fact they won’t be perfectly synchronised, so the load will be less than that?’

There are other issues, he said, such as if the structure moves how does that affect how people themselves move? ‘Is there a sort of feedback effect where they either lock in to the structural vibration — like London’s Millennium Bridge?’

‘Pedestrians found the only way they could walk across the structure was by adjusting their pacing frequency so that it was in time with the lateral sway of the deck, and that in turn amplified the lateral sway of the deck, so you’ve got this positive feedback which made the structure dangerous.’

Williams said it is possible this sort of feedback response could happen on a cantilever deck in a stadium. ‘It’s rather different because we’re talking about vertical vibrations, whereas with the Millennium Bridge it is horizontal sway, which is a very different load case in terms of how the person feels.’

‘I think people are much more sensitive to horizontal sway, because it causes much more disturbance to you and balance difficulties. But a similar sort of phenomenon could occur vertically.

‘There is even a possibility that it might be beneficial and people will find that under vertical vibrations they can’t move in time with the structure. There is some experimental evidence to suggest that’s the case, but we’re trying to look at that in more detail.’

While one of the goals of the project is to provide better design guidelines for future stadiums, the Oxford team also believes the research results will help provide more informed safety assessments of current structures.

Williams said a lack of information was a problem seven years ago with Cardiff’s Millennium Stadium because people were unsure as to how to carry out a safety assessment for the stadium’s eve-of-opening party.

‘They couldn’t guarantee the safety of the structure because they didn’t know how to estimate how it would respond,’ said Williams. ‘They had no idea what the loads generated would be. They couldn’t do it, so they had to make a very conservative estimate.’

The researchers hope that by sharing their knowledge, that won’t be the case in the future.