The Shard
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Images: courtesy of Sellar Group
The Shard will be the UK’s tallest building. It forms part of
the £2 billion Renzo Piano designed London Bridge Quarter
regeneration project near London Bridge and has been so named
because of its slender, spire-like shape covered with facets of
angled glazing that are intended to reflect light and the changing
patterns of the sky.
An innovative approach was taken to enhance the construction
programme and reduce programme risk. This involved ‘jump-starting’
the concrete core and steel structure to allow construction above
and below ground to start simultaneously. Such an approach would
have been challenging enough without taking into account the
constrained urban site and having to operate amongst the thousands
of commuters using London Bridge station every day. In addition,
the approach has never been used on a project of this size
before.
The initial plan was to construct a cofferdam at the centre of
the site and excavate so that work could start on the core from
base-up. Meanwhile, top-down construction would be used for the
area between the cofferdam and the perimeter secant piled wall.
However, jump-starting meant that work could start on the core and
steelwork whilst the core continued to be built downwards into the
basement. Jump-starting would also remove the need for installing
the cofferdam and had the potential to reduce the duration of the
construction programme by three months.
The 250m high core is being slip-formed and was started at
basement level B2, 9m above the lowest excavation level. This was
achieved by sinking large steel box columns into support piles
directly beneath the core, and constructing a grillage of
reinforced concrete beams at B2 level, connected to the piles with
welded couplers. This takes the load as the core is constructed
skywards. The basement below has now been excavated and the large
raft foundation poured. The 6m-high reinforced concrete core walls
from level B3, the lowest level, to B2 have been constructed using
self-compacting concrete pumped from the bottom of the form so that
a tight fit is achieved between the walls and the grillage of
beams.
The concrete for the slip-formed core is grade C50/60 concrete
achieving 30 MPa cube strength at 24 hours. There are 36 different
concrete mix types to allow for variations in weather, time of
year, height of pumping etc. The slip-form rig also holds one of
the cranes for the construction so that the crane is moved up the
building with the core. This has added an extra 180 tonnes to the
weight of the rig which has an area of 26m x 22m. The core itself
covers an area of 22m x 19m. The slip-forming was started in
January 2010 and 72 storeys of slip-forming should be complete at
the end of the year. The maximum speed that the slip-forming has
achieved is 3.6m in 24 hours.
The core had already reached 21 storeys high by the time that
700 truckloads of concrete were poured into the basement to form
the 3m deep raft foundation upon which the tower will sit. Carried
out over 36 hours, the 5,500m3 single concrete pour is one of the
largest ever undertaken in the UK. At the peak of the pour, trucks
were arriving on site every two minutes. Three concrete pumps were
installed to pump up to 150m3 an hour to ensure a fast truck
turnaround time. The concrete was poured in layers 750mm deep. This
helped to reduce heat build-up and regulated the concrete
placement. Reducing heat was important in order to limit the
potential for high levels of shrinkage and cracking. The C35/45
concrete contains a cement blend using 70 per cent ground
granulated blast-furnace slag (ggbs) to limit early heat gain. This
high level of cement replacement has the potential disadvantage of
low early strength gain. This was overcome by developing the
concrete mix so that it would achieve sufficient strength gain to
meet initial structural requirements within 14 days with the full
strength being achieved at 56 days.
Levels 40 to 72 are to be constructed in post-tensioned concrete
on high strength (C65/80) concrete columns, but these levels have
not yet been started. Pumping concrete up 250m is just another of
the challenges this project has posed for the concrete
subcontractor to overcome.
Project team
Client: Sellar Properties
Architect: Renzo Piano Building Workshop/Adamson Associates
Engineer: WSP Cantor Seinuk
Contractor: Mace
Concrete subcontractor: Byrne Bros