In order to meet the needs of the industry in these times of development, sufficient standards must be reinstated to define exact specification parameters, so that only the highest performance bearings are used. Richard Hepworth, president of Trelleborg’s marine and infrastructure operation discusses rate of deflection; one parameter that isn’t always considered in bearing designs. This can be significantly affected by the varying weight distribution of a building and have an impact on the performance of the bearing.
A changing infrastructure
As urbanisation and resulting infrastructure continue to grow, construction environments are becoming more complex to build in. Not just in terms of space, access or proximity, but also in the effects this growth has on the behaviour of our buildings. Specifically, the vibration caused from traffic and railways, which can transfer directly through a building’s structure to cause noise discomfort throughout.
Therefore, building designs have to incorporate strategies that meet the change in demands from the environment; isolation bearings are one example of this.
The vibration that passes through the ground and into a building is called a forcing frequency. This vibration will take advantage of any surface, be it a wall or a cupboard, to effectively turn it into a speaker to amplify sound. There are specified acceptable levels of disturbance dependant on the function of the building to ensure that occupant comfort is unaffected and machinery or apparatus works as it should.
There are many types of vibration isolation bearing, built to different specification requirements. Unfortunately, there are also products on the market which are not meeting simple and important performance ideals.
Calling for guidance
The industry previously took guidance from BS 6177:1982 - albeit that the standard was over 30 years old - until it was withdrawn last year.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 at an early stage of construction and deflect progressively as the weight of the structure comes on to them gradually. The overall static deflection of a bearing is always significant (sometimes amounting to 20 mm or more) so it is important that the distribution of weight both during and after construction be understood appropriately, and due allowance made for changes in the relative levels of any adjacent un-mounted parts of the structure.
Where individual bearings or mounting systems are incorporated at significantly different levels, precautions should be taken to ensure that loading is imposed on both the bearings and the structure in a manner that does not introduce unacceptable stresses. Allowance should also be made for any additional deflections that may occur due to creep or as a result of wind loading during the life of a bearing.
However, the British Standard failed to go into detail about the bearing’s performance when placed under stress, leaving the industry to decipher it for themselves; sometimes with negative consequences.
What does the ideal bearing specification look like?
All buildings and structures are subjected to ground vibration, or forcing frequencies, which cannot be stopped, but can be manipulated. The amount of vibration coming into a building can be controlled, but a full understanding of the right processes is required to do this efficiently.
Firstly, an acoustic consultant will assess the site where the building is to be constructed, to establish the forcing frequency. The bearing manufacturer must then use this information to ensure that the natural frequency that the building vibrates on its bearings is at just the right level. This has to be considerably less than the forcing frequency, making the ratio between the two as big as possible. As an absolute minimum, 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 the forcing frequency of 30hz coming through the ground, a natural frequency of 10hz is what should be aimed at.
To achieve the desired natural frequency, the bearing deflection must be controlled. This is the distance by which the bearing is compressed by the weight of the building, and deflection is controlled by specifying precisely the right positioning of the bearing and its stiffness.
However, this is not a value that can be broadly applied to every bearing installed, as the mass of the building differs, meaning that bearings can be placed under different strains depending on their location in the structure. Each bearing must be assessed on the strain that will be upon it, 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.
Raising the standards
Once the performance requirements and desired deflection values for the bearings has been established, the bearing design can be tweaked to suit. A rubber bearing which is designed with steel shim plates inserted within it, will give it structure and strength. This design allows the manufacturer to tune the block of rubber and influence its behavior, so that the performance can be predicted and the calculations met.
It is only as a result of these processes and techniques, that an isolation bearing will respond the way it is required when in situ. And it is this sophistication that enables experienced manufacturers to provide rubber bearings which perform to an optimum and competitive level, every time.
Our environment will continue to evolve and develop; couple that with more stringent regulations in the construction industry, and building designs must become even more sophisticated. The nature of our infrastructure is that it is built to last, so we cannot allow substandard products and techniques to take hold.
The issue of ground vibrations certainly won’t disappear; and given that an installed bearing is extremely difficult to refurbish or replace, it is vital that the industry gets this right first time. The specification of bearings should not be feared, as the scientific principles are simple enough. A reinstated, valuable guidance which details this, will help drive best practice amongst manufacturers and provide assurance to architects, contractors and building owners.