The Westminster Bridge Park Plaza development in London required complex engineering to construct a column-free ballroom basement. Two giant Vierendeel trusses support the upper floors and allow an atrium, with the floors hung from the trusses. Post-tensioning was used in transfer beams and slabs to support the 12 upper floors without columns and provide the necessary column-free zone for the ballroom. The construction involved a top-down method and temporary works to support the trusses during erection.

1. POST-TENSIONING / PRESTRESSING
Westminster Bridge
Park Plaza
LONDON
Richard Gaskill of Structural Systems explains the complex
engineering needed for a column-free section of a new residential
development in the heart of London.
A
s the need for executive-style accommodation
in central London becomes greater, so do the
physical attributes of the building. Constructed
on the south bank of the River Thames, Westminster
Bridge Park Plaza is just such a scheme. This exclusive
ApartHotel development has over 960 spacious rooms
including 54 suites and penthouses, and over 500 studio
rooms, all of which boast the latest facilities expected
of a top London establishment. Situated at the end of
Westminster Bridge, it has spectacular views of the
capital and is within walking distance of the city’s many
iconic attractions, including Big Ben, the Houses of
Parliament, the London Eye and the London Aquarium.
Rising 16 storeys, the hotel also has a full-height atrium
and four basement levels, one of which includes one of
the city’s largest ballrooms.
The hotel was completed in 2010 and is situated in the
centre of what was a busy roundabout, which was the
site of the former County Hall Annex, demolished in late
Aerial view showing the two massive Vierendeels that run either
side of the atrium and provide support to the floors.
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2. POST-TENSIONING / PRESTRESSING
The quantity of the
reinforcement was quite
dense and congested
around the multi-strand
ducts containing the post-
tensioning strand. The
depth of the beam is quite
extensive.
Penthouses and Suites
are available on the 13th
and 15th floors.
2006. The scheme forms part of a larger regeneration
programme, which covers much of the river front. This
included the part-pedestrianisation of the roundabout
and redesign of the road layout to allow limited access to
only buses and taxi around the hotel.
Complex construction
The construction of the building was complex due to the
need to keep the 1200m2 basement ballroom column-
free. Therefore, a large steel truss was used to form the
roof of the ballroom and this structure also incorporated
the first basement level within its depth.
Two giant Vierendeel trusses are positioned centrally,
either side of the full-height atrium and these support
the upper floors of the hotel, which are hung from them.
The Vierendeels also provide a column-free reception
lobby and allow for a 6m cantilever to the perimeter of
the building.
The construction programme was critical and
the main contractor, Gear, opted for a top-down
construction method to allow the ballroom roof to be
installed once the basement excavation and piling was
complete. Once the installation of the steel Vierendeels
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3. POST-TENSIONING / PRESTRESSING
was complete, the ground-floor slab was cast, which
allowed the work to continue simultaneously above and
below ground level.
Temporary works
The trusses spanned from the second up to the 13th
floor and require a considerable amount of temporary
works to support them. They could only be released from
their temporary supports once they have reached the
ninth floor. The bottom lower level of these trusses had
hangers to support the first floor, which was hung from
them. The trusses are V-shaped to mimic the profile of
the hotel’s central atrium. Once finally in place, they
were encased in concrete. The floors of the structure,
coupled with the diaphragm walls, are connected via
studs to the trusses.
Concept design
Structural Systems (UK) worked closely with the
consulting engineer at Taylor Whalley Spyra, who
carried out the concept design for the building, to
provide a solution to the north-east corner of the
structure where columns could not be accommodated.
Post-tensioning was required in several locations on
Level 1 and seven pours were needed to complete the
level. The structure required several post-tensioned
transfer areas at Level 2, which were either 1000mm or
800mm thick, with the latter carrying a single wall point
load in the middle of the slab. The slab was stressed from
one end via 1.8m-long × 0.7m-wide temporary access
holes left in the slab and completed in three pours.
The two large transfer beams, required to overcome
the lack of columns, had to be accommodated at Level 1
and 2. This complex design also allowed for the transfer
deck to be partly supported from above by hanging wall
supports, which were designed as beams to cantilever
from the columns to support the post-tensioned slab.
Column-free zone
The beams were required to support the 12 upper levels
of the hotel and to assist in providing a column-free zone
for the ballroom in the lower levels, with the smallest
beam being 4.55m deep, 800mm wide and 12m
long. The larger beam was 4.55m deep, 1.60m wide and
21.3m long.
The smaller beam had two 1906 CMI multi-strand
tendons – with the larger beam requiring eight multi-
strand tendons, each made up of nineteen 15.7mm
diameter strands – to control the deflection to 7mm
maximum. Both beams were designed to carry
significant live and dead point loads.
The beams were also heavily reinforced with six layers
of longitudinal T40 bars. The beams were built in three
pours, split horizontally and incrementally stressed.
The perimeter of Level 13 also required post-
tensioning due to the large cantilever, along with
localised areas on Levels 15–16.
Structural Systems was the only company willing to
undertake such a complex design and without its input
the construction of the building in its current form
would not have been possible. ●
From top: The post-tensioning duct was faceted to allow it to curve
around the north-east corner of the building.
The hotel exterior nearing completion with the full-height atrium
hidden behind the scaffolding.
CGI illustration of the top of the building, which houses the
exclusive penthouses.
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