We report from the construction site of the Shard – the UK and Europe's tallest skyscraper, to be completed in 2012.
At 310 metres, it's London's tallest building and Europe's, too. In skyscraper-rich New York and Hong Kong, it is a struggle for recognition but here its solitary, luminous presence dominates the skyline.
Ten years ago property developer Irvine Sellar took the idea to architect Renzo Piano, best known for co-designing the Pompidou Centre in Paris. Piano wanted to create 'a shard of glass, a spire tower of angled planes of glass that will reflect light and the changing patterns of the sky so that it will change according to the weather and the seasons'. Piano also wanted to change the concept of the skyscraper and its function.
He tapered the building because 'I don't believe it is possible to build a tall building in London by extruding the same shape from bottom to top' it would be too small at the bottom and too big at the top. Likewise, symbols are dangerous. Often tall buildings are aggressive and arrogant symbols of power and ego, selfish and hermetic'.
He saw the 87-storey building as a 'Vertical City', with about 8,000 people living and working in premium office space, a world class hotel, luxury apartments, a spa, five restaurants and cafes, retail areas and space for exhibitions and performances. Walking around the public viewing gallery on Level 72, people will see as far as the North Downs, Heathrow and Hampstead, and on clear days have views as far as 30 miles.
Visions and concepts are exciting but the shape of the building and its site bring challenges and problems; it is the task of architects and engineers to realise them.
The site and foundation
The site for the Shard is an area that has been built on for centuries. It's next to major railway station London Bridge, close to the tunnels of the Jubilee underground line and was home to a 24-storey building that had to be demolished before work could begin. The first challenge was to ensure that ground movement during demolition and excavation did not compromise neighbouring buildings. This was done by carrying out rigorous tests at the foundation stage.
Work on the foundations was complicated and a bit risky, but not without precedent. It is called 'top down' or 'jump starting' construction. Tony Palgrave, project director of the main contractor, Mace, describes it as casting the ground floor slab on top of piles, so that the superstructure floors could be built upwards at the same time as lower basement levels were excavated.
This process had never been used on a building of this size and it also cut three months off construction time. To get to this stage:
- The perimeter of the building had to be secured by a wall of concrete secant (touching) piles, 900mm and 1,500mm in diameter. These piles were driven 50m through layers of Lambeth Group Clays, Thanet Sands and chalk to act as a coffer dam to hold back water and soil, to allow basement excavation and also to balance structural steel girders.
- The next operation was to install 68 plunge piles. First, 1,800mm diameter piles were excavated to 60m and then reinforcement cages were inserted and filled with concrete. While the concrete was still wet, hydraulic frames were employed to position 25m plunge columns into the piles. These plunge piles support the ground floor and Basement 2 levels slabs, internal columns of the steel frame and – a first for the UK – the main core.
- The 400mm ground floor was then cast, tower cranes deployed and excavation begun.
- The B2 slab, 350mm thick, was cast and work began on constructing the core using the 'slip form process' of pouring concrete into a mould that is moved vertically by hydraulic jacks. As the concrete hardens, fresh concrete is poured on top.
- Excavation continued until the B3 slab was cast, with thickness varying from 3m beneath the core, 4.5m in the lift pits and 1.5m towards the perimeter. A thousand tonnes of reinforcement bar was used and 5,500 cubic metres of concrete was poured in 36 hours to create the largest single pour in the UK. Extractor fans were employed to cool heat (exothermic) generated by the setting concrete. When the slab or raft foundation was in place, work began on the support walls between the slab and the underside of the core, which then began to rise at a rate of 3m every 24 hours.
- The 300mm-thick B1 floor does not cover the whole basement footprint and was cast last on falsework (temporary supports).
Structure and core
Piano says: 'The shape of the tower is generous at the bottom and narrow at the top, disappearing into the air like a 16th century pinnacle of the mast of a very tall ship.' This idea was realised by designing perimeter columns that reduce in weight, size and spacing with height. Most noticeably, the space between the columns is 6m at base and office level, 3m at hotel and apartment floors and 1.5m at the top radiator levels.
The structure of the tower frame is composed of both steel and concrete columns: steel to L39, concrete L40-50, then steel again. For safety, many of the 12,500 tonnes of steel girders are coated with fire protection paint. The floors are of two types – composite and post-tensioned concrete.
John Parker, technical director of structural engineers, WSP Cantor Seinuk, says at office and hotel levels the floors are 130mm concrete and 500mm steel beams, forming a 630mm structure. Other levels have post-tensioned concrete floors (PTC) of 200-250mm thickness. The PTC floors have a thinner overall construction because servicing needs are less, floor heights can be reduced and more floors fitted in. The use of concrete in pillars and floors at these levels stiffens the whole structure, adds weight to the upper building and does away with the need for additional dampers to minimise sway. A sway tolerance of 400mm has been allowed for in the Shard. The tapered shape is also beneficial because it has a lower centre of gravity than a rectangular building of that height.
The heart of the structure is the reinforced concrete core. 'It is a rugged and robust structure and is the main stabilising factor of the Shard,' says William Matthews, architect associate of Renzo Piano Workshop. 'It acts as a cantilever and provides all the strength required to resist lateral loads.'
The core houses lifts and stairs, three to L18, two to L52, one to viewing gallery L72, continuing externally to L78 and beyond to the cleaning crane at L87.
Constructors of tall buildings are now aware of the recommendations of the US inquiry into the collapse of the World Trade Centre buildings, the cores of which were clad only in plasterboard. Among its findings were the difficulty fire-fighters had climbing lots of stairs and ineffective evacuation without the use of lifts. They recommended fire-protected, structurally hardened lifts, as well as stairwell capacity and door widths adequate to accommodate counter flows in emergencies. 'UK building regulations are very different from the US,' says Matthews, 'and as a result all life-safety equipment, circulation, stairs and lifts are within the central concrete core. Fire-fighters have dedicated access routes and lifts. Public areas are evacuated using passenger lifts.'
Lifts are absolutely necessary for tall buildings. Kone is responsible for the design, manufacture, delivery and installation of five escalators and 33 lifts in the Shard. Contract manager James Clark says lift speeds range from one to six metres a second. Depending on the usage and chosen operation of the lifts to 72 floors, getting from bottom to top will take less than 60 seconds.
Clark points out that passengers should not be affected by any speed or air pressure during transit. The lifts also come in pairs, not just singly and there are a number of double-deck ones specifically for hotel and office floors, with one car on top of another. 'A specialised destination control system manages traffic flow of each lift ensuring that passengers are directed to the correct lift and the required floor designation.'
In emergencies, 'there are six lifts in groups of two that are specifically used for evacuation. Features include command control panels and special bespoke indication at required floor levels, to indicate that evacuation is in progress. These lifts can be used by the emergency services. Alternative power supplies are provided for evacuation lifts'.
Kone also brought its own improvisation, the 'Jump Lift', to the construction of the building. Clark says: 'As soon as the elevator shaft was waterproofed, technicians started installing the permanent lift equipment, forming an integral part of the Jump Lift solution.
'The Jump Lift then uses the building's permanent hoist-way for construction time, putting completed floors into use while allowing the installation to continue above.
'The Jump Lift functions by using the building's permanent shaft, temporary car and machine room mounted on a deck positioned part way up the shaft during construction phase. It also houses a temporary machine room that moves upward – 'jumps' – in the hoist way under its own power as construction work progresses. When the structure building is completed, changing over to the permanent elevator is a straightforward matter of installing the final machinery and finishing the material surfaces of the elevator car, landing doors and signalisation.'
Piano's tower was to be no ordinary tall building – not static or conventionally rectangular, but rising aspirationally, transparent like a shard of light. It would reflect the changes of the day, the year and the weather. This lightness starts with the construction of eight elongated shards of glass, sloping 6° from the vertical all the way to the top. The shards are not tightly joined because there are 'fractures' at the junction of the planes which are then 'stopped off' with conventional vertical glazing. Sheer glass facades reflect the changes in the sky but it is the 6° angle that makes this reflection more apparent.
'The cladding panels,' says Mace's Palgrave, 'are based on a 1.5m by 3.8m module and all glass is low-iron, with high-performance coatings and argon-filled double-glazed units for thermal performance. The panels are hung from load-bearing brackets at slab level and interlock vertically for water tightness and wind load transfer.'
A colourless low-emissivity coating on the panels reduces infrared radiation but the main system to control the effects of the sun is the use of roller blinds of woven glass-fibre. Flush finish is achieved by the external panes 'oversailing' (projecting over) polyester-coated aluminium glazing beads and butting up against one another.
There may not be balconies on the Shard but office workers can get out into the open air if they feel the need. There are two or three winter gardens found in the 'fractures', the spaces between the planes of the shards and enclosed behind conventional vertical walls, consisting of the same sealed double-glazed units of the inner section of the shards but without the screens and blinds.
Attached to the eastern corner of the Shard is the 'Back Pack' or London Bridge Place, a 55,740 sq ft office and retail block. This block, also designed by Renzo Piano, was built to compensate for the floor space lost by the tapering shape of the Shard. Access will be through a lobby located on a piazza with an entrance to London Bridge underground station and a new bus station that are part of the overall plan for development of the area.
The Shard is expected to be completed in 2012 and cost in the region of £430m.