Ashley Haines, design manager at Trelleborg talks noise and vibrations in buildings
Urbanisation is resulting in a growing requirement for noise and vibration isolation in buildings. Demand for premium buildings is driving the need for higher performance specifications; not only for specialist buildings such as concert venues, but for commercial and residential buildings too.
Noise and vibration isolation bearings installed within the base and body of a building dramatically reduce the effects of ground vibration; a primary cause of noise in buildings. However, as the British Standard (BS 6177:1982) was withdrawn in August 2013, there is now an absence of regulations in this area. This has led to a danger of standards falling across the industry.
Vibration passing through the ground and into a building is called a forcing frequency and it will take advantage of surfaces like walls or cupboards, effectively turning it into a speaker. There are specified acceptable levels of disturbance dependant on a building’s function, to ensure that occupant comfort is unaffected and machinery or apparatus works as it should.
There are many types of vibration isolation bearing, which are built to different specification requirements. Unfortunately, there are also products on the market which are not meeting simple and important performance ideals.
The industry previously took guidance from BS 6177:1982 – albeit that the standard was over 30 years old – until it was withdrawn last year. The regulation, titled ‘Guide to selection and use of elastomeric bearings for vibration isolation of buildings’ included design considerations, acceptable level of disturbance, type of bearings, testing and identification of bearings. One factor which it covered, though not in prescriptive detail, was the deflection of bearings.
The regulation stated that bearings are often installed early on in construction and deflect progressively as the structure’s weight comes on to them. Therefore it is important that the distribution of weight during and after construction is understood appropriately.
However, the British Standard did not discuss the bearing’s performance when placed under stress, leaving the industry to decipher it for themselves; sometimes with negative consequences.
Ground vibrations, or forcing frequencies, cannot be stopped but can be manipulated. The amount of vibration coming into a building can be controlled, but an understanding of the right processes is required to do this efficiently.
Firstly, an acoustic consultant assesses the site and establishes the forcing frequency. The bearing manufacturer must then use this information to ensure that the natural frequency, at which the building on its bearings vibrates, is at the correct level. This has to be considerably less than the forcing frequency, making the ratio between the two as big as possible. The ratio must be no less than √2 / 1.41 otherwise the bearing will in fact amplify the vibration.
Ideally the ratio should be three. This gives a transmissibility of 0.1 meaning that 90% of the vibrations are detuned. So for a forcing frequency of 30hz coming through the ground, a natural frequency of 10hz is what we should aim for.
To achieve the desired natural frequency we must control the bearing deflection – the distance by which the bearing is compressed by the weight of the building. This is controlled by specifying the correct positioning of the bearing and its stiffness.
This cannot be applied to every bearing, as the mass of the building differs, causing bearings to face different strains depending on its location in the structure. Each bearing must be individually assessed, to ensure that they all deflect equally.
It is essential to test each bearing to verify its stiffness characteristics and structural integrity, as once it has been installed, it cannot easily be replaced or rectified.
Once the performance requirements and deflection values have been established, the bearing design can be tweaked to suit. A rubber bearing designed with steel shim plates inserted within it, will give it structure and strength. This allows the manufacturer to tune the block of rubber and influence its behavior, so that the performance can be predicted and the calculations met. By using these processes and techniques, the isolation bearing will respond the way it is required to when in situ.
Our environment will continue to evolve; couple that with more stringent regulations in the construction industry and building designs must become more sophisticated. Buildings have to last, so we cannot allow substandard products and techniques to take hold.
The specification of bearings should not be feared, as the scientific principles are simple enough. A reinstated, valuable guidance which details this, will guarantee best practice amongst manufacturers and assurance for architects, contractors and building owners.