London property developers made waves in the British media in recent months with the announcement that they plan to construct a transparent swimming pool that spans between the roofs of two ten story luxury apartment blocks. The towers in question are part of the Embassy Gardens development in Nine Elms, a rapidly reconstructing area on the Thames’s south bank. The media has mostly focused on the criticism the plans- some Londoners see it as a symbol of the excess and hyper-gentrification of modern London- but has unfortunately given little coverage to a more interesting aspect of the proposal: the engineering challenges that it poses.
Renderings of the pool seem at first unrealistic: the bottom and sides of the pool are made of a highly transparent material that is unbroken across the structure’s 25 meter span. Though ambitious, such a design is not impossible. Of course, the material used cannot be the standard window glass with which most are familiar; it will be constructed of the much stronger, transparent acrylic plastic that is now common in places like aquariums. For instance, a large fish tank at the Dubai Mall boasts a single acrylic panel that is 22 meters long and 8 high, which bears both its own weight and the tank’s water pressure. The London pool would require even longer panels that supported a greater load; several layers of acrylic, or even plastic trusses could brace the structure without affecting its transparency.
The Dubai Mall’s acrylic expanse
In addition to the task of creating a strong 25 meter span, engineers will face complications that are unusual for conventional bridges that are anchored to the ground. The swimming pool cannot be attached rigidly to the buildings it connects, as, unlike your average river banks, the apartment blocks are in constant, unsynchronized, structurally important motion. All high rises (and bridges) vibrate due to various causes: wind, expansion due to temperature changes, and the movement of people, to name a few. London is no stranger to problems caused by such structural resonance: Norman Foster’s Millennium Bridge, a shallow suspended walkway over the Thames, was a cause of panic when it “wobbled” when large crowds walked over it. Quickly closed and inspected, the bridge had vibrated due to the synchronized footfalls of pedestrians and was refitted with mass dampers.
The Millennium Bridge famously ‘wobbled’ under heavy traffic
In some cases skybridges can be useful structural devices; the one between the twin Petronas Towers in Kuala Lumpur connects the structures’ cores, stabilizing both buildings. In this case, however, it is unlikely that the acrylic surfaces will provide a strong enough connection between the structures to cause synchronized motion. It is more likely that the material, if it serves as a rigid connection, would fracture as a result of unsynchronized motion over time, which would be inconvenient for those in and under the pool. A good deal of ingenuity on the part of the engineers would be needed to overcome this obstacle. Solutions cold include complex hinges on each building- a strategy found in some bridges- or brute-force massive damping of the towers.
Nevertheless, the largest challenge facing the developers is one that is out of the hands of the engineers: the public’s reception of the scheme. In recent years, Londoners have become infuriated with the city’s whirlwind of new development. Block after controversial block of luxury apartments has gone up in the ever-growing business hub. Their designs are seen as uninspired; they are encroaching and sometimes entering the “view lines” between landmarks that the city council is supposed to protect. The general consensus is that gentrification has become a blight on the city, and nothing represents the trend of ostentatious real estate spending that a swimming pool in the sky, a symbol of wealth literally elevated above the masses. The project has already been a target of derision from the press. It may be that public opinion will be the vital element that determines whether the scheme will sink or swim.
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