Westminster Bridge Park Plaza

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.

LONDON

POST-TENSIONING / PRESTRESSING

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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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 constructionThe 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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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.

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POST-TENSIONING / PRESTRESSING

was complete, the ground-floor slab was cast, which allowed the work to continue simultaneously above and below ground level.

Temporary worksThe 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 designStructural 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 zoneThe 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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