General principles of the Tuneable Vibration Absorber:
All structures can be made to vibrate when excited by an oscillating force. The amplitude of the induced vibration is a function of the applied force and its frequency. An exciting force has the greatest effect when applied at the fundamental frequency of the structure. The structure is then excited at resonance, and in the case of a lightly damped structure the induced movement can be many times greater than the deflection caused by the equivalent static force. The ratio between the two effects is called the magnification factor.Vibrations in a structure have two effects. First, the very high peak accelerations can mean that the effective weight of the vibrating structure increases several-fold, and this may cause its destruction. Secondly, people on or inside the structure feel these accelerations, which can be uncomfortable or even dangerous.
In civil engineering structures, the greatest problems of vibration are encountered with structures whose fundamental frequency is a small multiple of some human activity, e.g. walking, running or jumping. It has been found that such activities can excite structures with fundamental frequencies of up to 8.4 Hz. This includes a wide range of bridges, grandstands, etc. In order to avoid such problems, structures have hitherto been made stiffer, in order to increase the fundamental frequency, or the mass of the structure has been increased, to keep accelerations low. However, both these approaches suffer from disadvantages, not least that they lead to increased cost.
The Tuneable Vibration Absorber is
advantageous primarily in that it reduces the amplitude of vibrations in
the structure by, for example, people walking over or within the structure.
Vibrations of the structure are transmitted to the pivoting arm and it
oscillates in opposition to the structure at its fundamental frequency
so that no peak in the amplitude of the vibrations of the structure is
reached.
Instead, a first peak is reached at a frequency slightly below
the fundamental frequency of the structure, and a second peak at a frequency
slightly above the fundamental frequency. However, the magnification factors
of these peaks are much lower than that at the fundamental frequency, depending
on the ratio of the equivalent masses of the device and the structure,
and so the amplitude of the vibrations induced in the structure is much
lower. The use of the device obviates the need for structures to have increased
stiffness or mass, thereby reducing the cost of construction and allowing
lighter weight structures to be used. The Absorber
itself is relatively light, compact and easy to incorporate into, or attach
to, a structure. In general, the stresses to which the device is exposed
are low, which results in low requirements for maintenance and long life
expectancy, in many cases as long as the structure on or in which the device
is used. The Absorber can be produced in any of a number of configurations,
to suit the structure to which it is to be applied.
Particularly, the position of the mass on the pivoting arm is adjustable. This renders the Vibration Absorber in effect tuneable, the fundamental frequency being adjustable simply by altering the position of the mass on the pivoting arm. In general, it is found that satisfactory results are achieved using a mass corresponding to from 2% to 8% of the equivalent mass of the structure with which the Tuneable Vibration Absorber is to be used, and more preferably from about 2% to 4%.
The Tuneable Vibration Absorber may be used on any of a wide range of civil engineering structures.
View a slideshow on the Tuneable Vibration Absorber