Trelleborg’s design manager, Ashley Haines, discusses noise and vibrations in buildings.
Building noise and vibration isolation is becoming increasingly important as cities become more densely populated. Premium building demand is driving the need for higher performance specifications, not only for specialist buildings like concert venues, but also for commercial and residential buildings.
Vibration and noise Isolation bearings installed within a building’s base and body significantly reduce the effects of ground vibration, which is a major source of noise in buildings. However, since the British Standard (BS 6177:1982) was withdrawn in August 2013, there are no regulations in this area. As a result, there is a risk that standards will deteriorate across the industry.
A forcing frequency is vibration that passes through the ground and into a building, and it will take advantage of surfaces such as walls or cupboards, effectively turning it into a speaker. There are acceptable levels of disturbance based on a building’s function to ensure that occupant comfort is not compromised and that machinery or apparatus functions properly.
There are numerous types of vibration isolation bearings, each with its own set of specifications. Unfortunately, there are some products on the market that do not meet basic and important performance standards.
The industry previously relied on BS 6177:1982 for guidance, despite the fact that the standard was over 30 years old, until it was withdrawn last year. The regulation, titled ‘Guide to the Selection and Use of Elastomeric Bearings for Building Vibration Isolation,’ covered design considerations, acceptable level of disturbance, bearing type, testing, and bearing identification. The deflection of bearings was one factor covered, though not in prescriptive detail.
According to the regulation, bearings are frequently installed early in the construction process and deflect progressively as the structure’s weight is applied to them. As a result, it is critical to understand the weight distribution during and after construction.
The British Standard, on the other hand, did not address the bearing’s performance under stress, leaving the industry to decipher it for themselves, sometimes with negative consequences.
Ground vibrations, also known as forcing frequencies, cannot be stopped, but they can be controlled. The amount of vibration entering a building can be controlled, but doing so requires an understanding of the proper processes.
To begin, an acoustic consultant evaluates the site and determines the forcing frequency. The bearing manufacturer must then use this data to ensure that the natural frequency at which the building vibrates on its bearings is set correctly. This must be significantly less than the forcing frequency, resulting in the greatest possible ratio between the two. The ratio must be greater than or equal to 2 / 1.41, or the bearing will amplify the vibration.
In an ideal world, the ratio would be three. This results in a transmissibility of 0.1, indicating that 90% of the vibrations are detuned. So, for a forcing frequency of 30hz coming through the ground, we should aim for a natural frequency of 10hz.
To achieve the desired natural frequency, we must control the bearing deflection, which is the distance the bearing is compressed by the weight of the building. This is controlled by specifying the correct bearing positioning and stiffness.
This cannot be applied to all bearings because the mass of the building varies, causing bearings to experience different strains depending on their location in the structure. Each bearing must be evaluated individually to ensure that they all deflect equally.
It is critical to test each bearing to ensure its stiffness characteristics and structural integrity, as once installed, it cannot be easily replaced or corrected.
Once the performance requirements and deflection values have been determined, the bearing design can be modified to meet those requirements. Structure and strength will be provided by a rubber bearing with steel shim plates inserted within it. This enables the manufacturer to tune the rubber block and influence its behaviour, allowing performance to be predicted and calculations to be met. Using these processes and techniques, the isolation bearing will respond as it should when in use.
Our environment will continue to evolve, and building designs must become more sophisticated as a result of more stringent regulations in the construction industry. Buildings must last, so we cannot allow inferior products and techniques to take root.
Bearing specification should not be feared because the scientific principles are simple enough. A reintroduced, valuable guidance outlining this will ensure best practise among manufacturers as well as assurance for architects, contractors, and building owners.