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Thursday24 July 2014

Cladding Renzo Piano’s Shard

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The facade of Piano’s glass tower at London Bridge station is designed to be as transparent and flush as possible

Architect Renzo Piano Building Workshop
Location St Thomas Street, London SE1
Completion date May 2012

Two blocks south of London Bridge, Renzo Piano’s 72-storey, £416 million Shard is visibly blasting skywards. When completed in 2012, this tower, containing offices, apartments and a hotel, will be 306m high – 71m higher than Britain’s current tallest building, One Canada Square at Canary Wharf – making it the tallest habitable structure in Western Europe.

In truth, the Shard is not the geometrically perfect “sharp crystal pyramid” of its architect’s description but a loose bivouac of eight elongated, jagged shards of glass all seemingly propping each other up at the pinnacle.

The eight shards slope at a constant inclination of six degrees from vertical all the way up from pavement right up to the pinnacle. This loose assembly is expressed by leaving “fractures”, at the junctions of the planes and stopping these off with conventional vertical glazing.

The eight facades themselves could hardly be simpler in appearance – sheer curtain walls of clear glass, uncluttered by any form of external structure or solar shading.

TRANSPARENT FLUSH FACADE

Great pains have been taken by Renzo Piano Building Workshop to make the Shard’s facade as transparent and flush as possible, while also ensuring it is thermally efficient.

Transparency is increased by specifying low-iron laminated glass. “The glass just disappears, and all you see is the skeleton of the building,” says project architect William Matthews.
A colourless solar-control coating of Ipasol made by Interpane has been applied “to make the building look wonderfully glassy”.

In addition a colourless low-emissivity coating has been added to reduce the reflection of infra-red radiation back into the building. The main solar control comes from the roller blinds that are woven in glass-fibre by Hexcel to reduce solar radiation by 95% while still leaving the curtain wall semi-transparent. The total solar radiation passing through the facade – the G value – amounts to only 0.12%.

To achieve the immaculately flush finish, the external glass panes oversail the polyester coated aluminium glazing beads and butt up against each other.

Scheldebouw is propping the glass on timber blocks for 48 hours while the silicon that bonds it to the glazing beads sets. This, Matthews claims, eliminates the very slight dishing effect that can mar curtain walls of double glazing units.

WINTER GARDENS

For the occupants of this immense air-conditioned tower, access to fresh air is offered through two or three winter gardens on each floor. These are located at the “fractures” between the tower’s inclined shards.

The winter gardens are enclosed behind conventional vertical curtain walls that step back every sixth floor. The curtain wall is made up of the same sealed double-glazed units as the inner leaf of the inclined shards but without the rainscreen outer leaf and roller blinds. In fact, one of these glazing units in each winter garden is a conventional top-hung opening window.

ince the winter gardens are more exposed to the external environment, they are separated from the main habitable floor space by single-glazed partitions.

The floor plan shows a typical office level. The winter gardens are located in three corners and feature opening windows.

PASSIVE DESIGN SUN-SHADING

“What we are building here is a great big greenhouse,” explains William Matthews. “So the problem is how to stay cool inside. The simplest way would be to provide external sunshading. But you can’t do that 200 metres up in the air, where it would flap around in the wind.”

Instead sun-shading is provided by motorised roller blinds incorporated within the external envelope. The design team stuck rigorously to this principle over the 10 years that the building took to design and pass through a £4 million public inquiry. But this posed another major problem in technical design that was solved with a U-turn from what Matthews calls an active facade to a passive facade.

The active facade initially adopted by the design team involved mechanically ventilating the cavities in the double-glazing units that housed the roller blinds. But the increased energy efficiency brought in by the 2006 revision of Part L of the building regulations meant that low-velocity fans would now be needed to ventilate the cavities. This in turn called for bulky ducts to be housed within the cavities and affect the facade’s transparency.

So the team switched to a passive facade in which the roller blinds are protected from wind and rain by single glazing. Thermal insulation is provided by hermetically sealed double-glazed units making up the inner skin of the facade.

Each outer cavity housing the roller blind is now 250mm wide, unventilated and requires periodic maintenance by opening the internal double-glazed panel on side hinges. Because of the depth of this cavity, the aluminium window mullion has been split into two connected by narrow spacing bars.

Winds at the spire’s pinnacle are less of a problem than turbulence at lower levels caused by the neighbouring Guy’s Hospital tower, claims Matthews.

The slight inclination of the facades reduces updraft, while a 4m-wide glass canopy at first floor level shields pedestrians outside.

Matthews claims the passive facade helped the building exceed the 2006 revision of Part L by 25% and would also pass the increased standards of this year’s revision. He also expects the building to achieve a Breeam “excellent” rating.

Matthews admits that, though simpler, the passive facade is more expensive than its active antecedent. The total 55,000sq m
curtain wall package cost £60 million, which is just over £1,000 per square metre.

SPECIFICATIONS

Client Sellar Properties, Executive architect Adamson Associates, Facade consultant Emmer Pfenninger & Partner, Main contractor Mace, Cladding sub-contactor Scheldebouw (UK), Glass coating Interpane, Glass supplier Flachglass, Blind motors Somfy, Blind fabric Hexcel, Steelwork paint systems Leigh’s Paints

Original print headline - The Shard

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