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Last Updated: Monday, 20 June, 2005, 07:59 GMT 08:59 UK
Vibration engineers feel the rock
Image of a rock concert in a stadium
Engineers need real people, of all shapes and sizes, to rock the place
Spectators could be spared the sickly, wobbly sensation of crowded stadiums thanks to a movement monitoring system using CCTV and sensors.

The University of Sheffield, UK, system monitors, in real-time, the behaviour of crowds in stadiums and the vibration levels that they can produce.

The system has been in place for a year in one stadium and was tested at a gig.

It will help engineers collect real data for a better idea of how stadiums act under the stress of swaying people.

It is not just about engineers wanting to stop the rock though.

"One of the big problems is that there is a general lack of understanding of how stadiums vibrate when crowds are on them," Dr Paul Reynolds, who heads up the project, explained to the BBC News website.

As soon as you get humans in the equation, everything becomes variable. People sitting down behave differently from people standing up from a dynamics point of view
Dr Paul Reynolds, Sheffield University
"When you have a source of vibration, like people jumping up and down, the way the structure responds, vibrates, and what levels of vibration are acceptable - all of these three things are not really well understood."

Every structure vibrates at a number of different natural frequencies. Problems crop up when resonance frequency occurs - that is when vibration, often caused by excited humans, matches the natural frequency of the structure.

With a better understanding, backed up by hordes of data, the engineers can build predictive computer simulation models that can guide future structural designs.

The system has been in place at Valley Parade, the home of Bradford City, and has monitored a total of 20 football games and nine rugby matches.

It has also been installed at the City of Manchester ground (built for the Commonwealth Games and now home to Manchester City), where it has just been put through its paces by two U2 concerts. It will measure vibrations at an Oasis gig in July.


Since the Hillsborough Stadium disaster in 1989, stadiums must be all-seated. This has led to the use of more cantilever designed seating areas, which tend to wobble more.

"Stadiums in recent years have been getting more slender," said Dr Reynolds.

"There has been increased use of cantilever tiers so that you can fit more people in and increase revenue. They tend to be prone to vibration because they have low vibration frequencies," he explained.

The UK has more wobbly stadiums than the rest of the world because they are mainly built from steel, a more "lively" material, rather than reinforced concrete.

Previously, engineers have relied on mathematical models and have not been able to build effective computer simulation models because they have not had the behavioural data.

It is also easier to model how an empty stadium might behave; but one full of thousands of excitable people is a different matter.

Stadiums are also used now for a far more diverse range of events - with rock concerts being the obvious example.

This inevitably means there are crowds of people bouncing up and down in time to the music.

Wobbly world

The monitoring system consists of accelerometers - special electronic sensors that measure acceleration - and CCTV cameras.

The data is then sent back to the researchers' base in Sheffield over the net.

Image of a researchers monitoring a concert in a stadium
The kind of data the systems shows has not been gathered before
The team wants to gather as much data as possible around different types of crowds that may behave in different ways. Sports fans move in different ways to music fans, for example.

Coldplay fans might even move in different ways to U2 fans. These are part of the "unknowns" structural engineers have faced with current models.

"As soon as you get humans in the equation, everything becomes variable. People sitting down behave differently from people standing up from a dynamics point of view," said Dr Reynolds.

Even whether a crowd is male or female can make a difference, he says.

The problem is particularly bad when seas of people move rhythmically in time with each other, such as at a concert.

But they also move with the music at slightly different intervals. This is because sound takes time to travel from one end of the stadium to the other.

Calm control

The researchers will also keep an eye on the effects of new sound systems which try to synchronise acoustic delivery to overcome this time lag problem.

Most stadiums built before the 1990s were not designed with rock concerts in mind, but increasingly that is what they are hosting.

"The general feeling is that the structures are safe," said Dr Reynolds. "There have been no cases of a permanent stadium collapsing under crowd overloading.

"It's really a matter of perception. The biggest worry is that if they feel unsafe they might try to get out the stand quickly in a panic situation."

There is a comfort issue, too. Even if people do not panic because they feel unsafe, they might still be uncomfortable about the level rocking and vibrating under them.

There already exists government guidance, set out in 2001, to ensure the design of stadiums takes vibration levels into account.

The Sheffield researchers, backed by the Engineering and Physical Sciences Research Council (EPSRC), hope the monitoring system could be used to better inform that advice.

It could eventually let engineers monitor the structural health of a building remotely, without having to carry out detailed building inspections.

Service marks Hillsborough deaths
15 Apr 05 |  Merseyside

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