The Tuneable Vibration Absorber
As installed at the Arsenal Football Club:
Background:
Problems with uncomfortable vibrations were encountered in the North Bank at the Arsenal Football Club in some circumstances, such as the crowd jumping to a particular rhythm. This was due to the natural frequency of the stand (the frequency at which the response to an excitation is maximum). This frequency was close to that of some human activities, which meant that the crowd could excite the structure to resonance. Resonance is the name given to the phenomenon when the structure is fed a force when it is going down, and unloaded when it is coming back up, which amplifies its movements up to a point where they are high compared with the exciting force.This behaviour was confirmed by dynamic testing.
Possible solutions:
The first possible solution to such a problem is to modify the structure itself. Its mass can be reduced or its stiffness increased, to bring its natural frequency above that of human activities. In practice, a mass reduction is nearly always impossible to achieve in sufficient proportions to have a significant effect. A stiffness increase can be achieved, generally at great additional cost, when a new structure is designed. It may be possible to do so on existing structure, but the cheaper solutions are rarely acceptable for architectural or practical reasons, and the cost of 'elegant' solutions makes them unaffordable.In the instance of the North Bank, the only possibility to increase the stiffness of the stand at a reasonable cost would have been the addition of columns to support it. Unfortunately, these would not have been acceptable, since they would have partially blocked the view from the lower tier.
The alternative is to use a vibration absorber.
Principle of a vibration absorber:
These generally consist of a mass, linked to the structure by a spring and a damper. They are set up so that the mass can vibrate at a specific frequency, close to that of the structure. Thus, when the structure starts to move under an excitation, the mass of the vibration absorber move as well, which creates a new system with a different behaviour. It is then impossible to reach the uncomfortable accelerations reached with the structure alone.Choice of the vibration absorber:
One of the requirements for the North Bank was that the vibration absorbers should be tuneable, meaning that the frequencies at which they operate could be changed. They could therefore be adjusted to suit the structure if the initial measurements proved incorrect. The easiest way to do so without reducing the efficiency of the devices is to vary the spring rate.The simplest way of achieving this is to put the mass at the end of a flexible arm or blade: The closer the mass is to the support, the higher the spring rate and thus the frequency. Such a vibration absorber was initially designed.
However, this very simple form of vibration absorber poses problems in the long term: Their life expectancy is linked to the fatigue in the metal constituting the blade, which is difficult to evaluate accurately. From there comes the need of regular maintenance inspections, to check the state of the devices.
This simple form of vibration absorber also poses the problem of its acceptance by the crowd: The combination of the mass and frequency required meant that it had to be very large. This led to the design of the current Tuneable Vibration Absorber.
The Tuneable Vibration Absorber:
The Tuneable Vibration Absorber, for which a patent has been applied for, is a more mechanical version of the simple device described above: Instead of a flexible blade, the mass is mounted onto a rigid arm. This arm is connected through an articulation to a spring and a damper within a sealed cylinder.
There are two advantages to this: The vibration absorbers are much more compact, and their useful life is dictated by that of manufactured components, which is well known. This means that the vibration absorbers installed will outlast the stand, and should not require any maintenance.
Results:
The behaviour of the North Bank was tested before and after the fitting of vibration absorbers. Measurements showed a distinct change of behaviour in the stand: Before the fitting of the absorbers, it was possible to excite it to resonance. Afterwards, the structure would start to resonate, and then the absorbers would start working and calm it down. This could even be felt by simply standing on the structure during the test.
The result of the fitting of Tuneable Vibration Absorbers to the North Bank is that it can no longer be made to resonate in an uncomfortable fashion. A crowd jumping to a rhythm will only make it vibrate perceptibly for a short period, before the absorbers start working and restrict the movement.
The absorber in movement.
General principles of the Tuneable Vibration Absorber
See a slideshow on the Vibration Absorber