~ Structural isolation for historic buildings ~
Mason UK – Structural isolation is common for new buildings where vibration will be a problem. You lay your isolation bearings and build the structure on top of them. However, isolating a pre-existing structure is far more complicated and therefore rarely attempted. Here, Adam Fox, director at vibration control engineering company Mason UK, explains how Mason helped isolate a Grade I listed building in Mayfair.
Building isolation bearings are used to isolate entire buildings or elements such as walls, floor plates or columns. A typical application would be prevention of vibration from a nearby railway or tube line carrying into a building structure, resulting in undesirable noise.
Isolation bearings can usually be elastomeric, typically natural rubber, or helical metal spring. The nature of the vibration will dictate which type is preferrable for an optimal solution. However, bearings would usually be installed before a building is erected.
We recently faced the challenge of isolating a pre-existing historic building. Cambridge House is Grade I listed building in central London. Built between 1756 and 1761, the building boasts a distinguished list of former occupants. In the first half of the nineteenth century, it was the London residence of Prince Adolphus, Duke of Cambridge, and it is from here that it gets its current name. It was later purchased by Viscount Palmerston, who was Prime Minister in the mid-1800s, before it was sold to the Naval and Military Club in 1865.
It was purchased in 2013 by David and Simon Reuben, and the developers planned to transform the building into a luxury hotel. However, the noise generated from nearby trains, which transmitted into the building via the basement structures, would have been intolerable to the hotel’s guests. Effective acoustic engineering was therefore a prerequisite for this project. Knocking the building down and starting again was not an option. For listed buildings, you are also limited in the changes you can make to the building.
Slicing through
Hoare Lea devised an acoustic specification to provide the necessary levels of isolation and we were brought on board to help deliver the specification through the provision of bearings. However, design flexibility was key for a project like this where we were dealing with lots of unknowns. One key unknown from the outset was the distribution of loads across the bearings.
Making things work architecturally was also challenging. For example, if you have an old wall, you cannot simply use twice as much space, as you have to fit things within the space of the old wall. Challenges like this necessitate lots of iterative design processes with the architect and structural engineers to make things work. Collaboration like this is normal with acoustic engineering, but the complexity of this project meant a step change in its magnitude.
At the basement level of the building, a network of rooms and walls under the ground were acting as a transmission path for vibration. We therefore needed to slice through these walls to remove the rigid connections and replace them with a series of rubber connections.
Approximately 1500 Mason 6Hz isolation bearings were supplied for this purpose. Each one was designed bespoke, to take a specific load, and came with a unique identifier to ensure its intended location was adhered to during the installation phase. Once we had sliced through the foundations, a rigid beam was installed, before load was gradually transferred from this to the bearings using hydraulic jacks.
One of a kind?
Providing isolation to an existing building of this scale and prestige is unprecedented. However, what has been learned on this project can and will be applied elsewhere. The success of the Cambridge House project demonstrates that these techniques can allow for the isolation of existing buildings, large and small.
Another factor worth highlighting is the longevity of the bearings. This is not simply a bonus feature of the product, but something that is a prerequisite in making the redevelopment commercially viable. It is essential that bearings in a project like this are designed and engineered to last for as long as the structure.
For elastomeric materials, the risk is they gradually crush under the weight of the building, requiring their replacement. Mason bearings are different, as they are designed to experience a low level of stress which is essential to providing longevity. A simple commercial warranty does not suffice, as replacement can be extremely costly and in some circumstances impossible. Most importantly, an engineering-led justification for lifespan is first required, which is then supported by insurance to cover the unexpected. For this project we provided an industry-leading 100-year warranty.
There will be many development projects in future, especially in London, where the viability of redeveloping an existing, historic building is dependent on the kind of vibration isolation and acoustic engineering described above. However, very few projects will match Cambridge House in terms of scale and complexity. In other words, if this can be achieved for a building like Cambridge House, there are many more potential projects in the pipeline now developers can see this is a realistic option.
Mason UK specialise in acoustic engineering and vibration control for high profile building projects. If you are an acoustic consultant or property developer looking at isolating a pre-existing structure, visit mason-uk.co.uk or call 01252 716610.
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