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Bridge brilliance | Sky pool

A birds eye view of the the Sky Pool

London’s sky pool is the first swimming pool in the world to bridge between two buildings 10 storeys up. However this isn’t its only unique feature – it is also a fully transparent structure.

“The challenge was set by the client,” says HAL Architects founder Hal Currey. It said: “If we’re going to do it then we’re going to do it as a transparent structure.”

The 5m wide pool is 200t when full and spans 14m between two residential buildings in the heart of the new American Embassy development in London. Swimmers brave enough to take the plunge, will have a dizzyingly clear and dramatic view of the ground below. It has been designed by structural engineer Eckersley O’Callaghan, which specialises in glass and transparent structures, and HAL Architects.

Initially the designers looked to develop on a previous swimming pool which Eckersley O’Callaghan director Brian Eckersley had worked on. The glass swimming pool in a building in north London had a 2.5m wide transparent floor, which used the sides as the beams to carry the weight with the base, stiffened by secondary beams underneath. But this approach did not lend itself to the simplicity of form the team was looking for.

The dramatic Sky Pool from the ground

The dramatic Sky Pool from the ground

Source: HAL architects Arup Associates

The team took their inspiration from aquariums which are normally made from acrylic, which behaves plastically compared to the more troublesome brittle nature of glass

“Actually things all start to get quite complicated when you do that,” says Eckersley. “All the connections start to get quite big and you need lots of lamination because you have these two glass beams which carry all the weight. So you need to build in quite a lot of safety.”

Instead the team looked to other water retaining structures for inspiration and found what could be described as a fishy solution – aquariums.

“Aquariums may look like quite different structures, but actually, they’re quite tall so the pressure of the water acting on the sides when they’re 10m tall are pretty enormous,” says Eckersley.

Acrylic aquariums

Aquariums are normally made out of acrylic which behaves plastically compared to the more troublesome brittle nature of glass says Eckersley.

“The thing about glass is that it’s brittle, it breaks quite easily which means that it introduces quite strict disciplines for the engineers who work with it,” says Eckersley. “If you put a bolt in steel and the hole is a little bit higher than it should be then it doesn’t matter too much, as the steel just relaxes a bit and redistributes the stresses.

“In glass that doesn’t happen, if you stress it then it breaks very suddenly.”

So the team turned its attention to designing the structure from acrylic instead of glass.

Acrylic versus glass

The transparency of glass and acrylic is one of the only similarities between the two materials. But acrylic has many benefits for the design of the Sky Pool.

  • Acrylic is much lighter than glass.
  • Joints in the structure are less visible – transparent bonded joints as strong as the original parent material can be made in acrylic. “This is a pretty amazing thing compared to what it takes to connect glass structures together,” says Eckersley.
  • Acrylic is more transparent than glass – glass has to be laminated to give redundancy to the structure. However with each lamination, the interlayer combined with the thickness of the glass reduces the visual transparency. This is not the case with acrylic.
  • Acrylic has less visual distortion. When you fill the acrylic with water and you look through it, the light travels through it in a pretty straight line between the water, acrylic and then air because the refractive index is pretty similar; whereas when you look through glass structure, you get quite a lot of distortion.
  • It’s not brittle, it’s plastic as the name suggests, so it doesn’t have that sudden failure behaviour and it also has better impact resistance.
  • On the negative side, it lacks the stiffness of glass, so for any given load it will deflect more than a glass structure. “But that’s ok,” says Eckersley. “It’s just a matter of sizing it correctly so that deflection is still within the limits that you need to work to.”
  • It is also much weaker. But as with the sizing of an element to account for the lack of stiffness, the element is designed using the properties of the material to give the correct size.

Turning the architect’s vision of the pool into a reality was the job of the structural engineer. To satisfy building regulations and ensure swimmers do not meet a soggy end, the sides of the pool between the buildings had to be 3.3m tall. This gave the team ample structural depth for a pair of 200mm thick beams to span between the buildings and support the base of the pool. In turn, the 300mm thick base is then bonded between the side walls with a virtually invisible mitred joint as strong as the parent material itself.

Each end of the pool is housed in stainless steel ‘tubs’ set into the buildings. To give the structure additional robustness, two tensioned cables will run beneath the acrylic base, one alongside each of the pool walls, tying the tubs to the acrylic, but not the building. This means the structure all moves together with no movement occurring at the joints.

“The cables add additional robustness to the design and ensure that the stainless steel and acrylic move together without over-stressing the sealant joints,” says Eckersley. “There are springs at each end of the ties to maintain a consistent level of tension when the acrylic expands and contracts due to temperature variation.

Brian Eckersley holding acrylic sample

Brian Eckersley holding acrylic sample

A sample of the acrylic and joint. The actual pool will be constructed from 200mm thick, 3.3m high acrylic beams. The base will be 300mm thick and the two bonded together as strong as the parent material.

“Acrylic has a really high thermal coefficient, so in the winter it will shrink a lot and in the summer it will grow.”

To analyse the forces in the structure the team carried out sophisticated computer analysis to look at the various natural frequencies of sloshing water and a finite element analysis was used to calculate the stresses within the structure, making sure to keep within the limits of the material.

Differential settlement

One of the main issues the team had to consider during the design phase was differential settlement between the two buildings. Although the two buildings will be built at approximately the same time, no guarantees can be made about how much each will move over time.

“They’re built on different foundations, they’re built at slightly different times, they will behave slightly differently and that is totally unavoidable,” says Eckersley.

Sky Bridge

Sky Bridge

“They may sink slightly on their foundations, they may tilt slightly, the concrete will shrink over time. All of these little movements add up to something which for the engineer is quite significant. If we rigidly joined them together, those movements would potentially induce enormous stresses into the structure, so you have to ensure that stresses are released.”

To release the structure, the pool has been built on bearings at either end to allow it to move independently of the buildings and avoid any build-up of stresses.

Any necessary services will be carried beneath a footbridge which is situated parallel to the pool.

The development is expected to be finished in late 2017, but only the lucky few living in the residential apartments on either side will be able to take a seemingly gravity-defying dip.

Readers' comments (1)

  • Without doing any calcs I can't say for sure, but I find it hard to believe those edge beams will be okay for lateral torsional stability. They don't seem to be restrained at their ends and there is no top flange. Call me old fashioned, but I go along with the notion that a structure needs to look right...

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