Building control: the Shard
Reaching the heights
11 September 2014
Peter Card describes the particular challenges, triumphs and lessons from working on western Europe’s tallest tower
Visitors to London can hardly miss the impressive Shard of Glass on the south side of the River Thames. In a short time, the building has become one of the most iconic on the London skyline. Designed by architect Renzo Piano, the 'vertical city' contains world-class office space, exclusive residential apartments (the highest living accommodation in the UK), a 5-star hotel with publicly-accessible mid-level restaurants and high-level viewing galleries offering unprecedented 360º views of the capital.
Completed in July 2012, the Shard is currently the tallest building in western Europe at a height of 310m (1,016 feet), with a total of 72 occupied floors, while an extended floorplate, the 'backpack', sits on one side for 18 storeys.
Complex engineering and logistical planning was needed for a small city centre site, requiring all work to be carried out within the footprint of the new building to avoid interrupting the operations of the adjacent London Bridge Station and bus terminus and Guy's and St Thomas' hospitals. London Underground's Jubilee Line also crosses one corner of the site.
Completed in July 2012, the Shard is currently the tallest building in western Europe at a height of 310m (1,016 feet), with a total of 72 occupied floors
Innovative solutions were devised by structural engineers WSP Cantor Seinuk using a variety of construction methods. The result is a reinforced concrete basement, steelwork to level 40, concrete from levels 40 to 72, and steel above.
Composite steel and concrete floor slab was chosen for the office floors because the plethora of services could be accommodated within the depth of the steel beams. On the residential floors, post-tensioned concrete was specified. By using concrete, not only was the height per floor reduced, but the additional mass helps its dynamic properties, removing the need for additional dampers to combat building sway. It also gives better performance in terms of vibration and acoustics.
A massive central concrete core forms the basis of the Shard's lateral stability system. The core is linked to perimeter columns by a 'hat' truss, which crowns the building. If the building tries to sway due to wind, it stiffens up by engaging the perimeter columns.
The statistics of the Shard are staggering:
- 82 storeys
- 11,000 glass panels
- 57,000m3 of concrete
- 5,000 tonnes of reinforcing bars
- 44 lifts, including double-decker lifts
- 306 flights of stairs.
The authority responsible for ensuring the project meets compliance with the Building Regulations is the London Borough of Southwark through its Building Control service. The scheme was managed in-house by Major Projects senior surveyors Warren Lapper and Simon Harvey.
Southwark Building Control's commitment to the Shard project has been long term and comprehensive. First discussions took place in 2002, with ongoing meetings as the design evolved, even though the application was not lodged with the council until 2008.
The unique design is not without many challenges; the height of the building being only one aspect. In Building Regulations terms, in addition to its structural complexity, fire safety issues predominate.
It was realised from an early stage that outside assistance would be necessary to help vet the designs. WSP services engineers checked the Part L submission, while structural checking was outsourced to Eastwood & Partners.
Fire safety was a major risk and fire consultant Beryl Menzies was appointed, along with Professor Colin Bailey from Manchester University to check structural fire engineering design, with support from London Fire Safety and Engineering Service.
It was essential that all checking consultants kept Southwark Building Control informed at all times and this was fed into the building control tracker.
Experienced building control officers will recognise the difficulties in resolving the following:
- mixed-use tower sharing stairs for escape
- 3 staircases serving floors up to level 18; 2 up to level 52; and 1 up to the external viewing gallery at level 72. The single stair continues externally to a higher viewing platform at level 78 and beyond to the cleaning crane at level 87
- hotel sleeping accommodation over non-sleeping occupancies, and higher levels apartments
- public access to reach the mid-level restaurants and the viewing gallery
- 3 basement levels housing storage, plant and a car stacker
- mechanical pressurisation systems to the stairs
- hotel corridors requiring a section 20 smoke extraction system
- apartment corridors requiring a 'push-pull/pull-pull' smoke extraction system
- temperature-controlled smoke ventilation system for viewing gallery
- lift evacuation for public viewing gallery.
Solutions are not readily found in Approved Document B and this called for a holistic whole building approach encompassing a combination of fire engineering and prescriptive measures.
Key fire safety features include:
- 'fire break' floors between occupancies and plant rooms to limit vertical fire spread externally
- full sprinkler coverage
- L1 smoke detection throughout
- 24-hour manned building management and fire control centre maintains overall control of the building
- comprehensive smoke control systems
- large fire resisting refuges at high and mid-levels
- stair pressurisation
- 3 diverse power supplies and 2 standby generators.
There are a total of 44 lifts, with some only running 1 or 2 floors and others having a major role in the fire access and evacuation of the building. Kone was responsible for lifts with lift speeds ranging from 1m/s-6m/s. Getting from bottom to top takes less than 60 seconds.
To save time, the construction of the core started with the ground-floor slab being poured before the basement excavation, progressing top down into the 3-storey basement as the superstructure was constructed upwards.
First, 900mm-diameter secant piles and 1,500mm and 1,800mm-diameter individual bored piles were taken 50mm through Lambeth Group Clays, Thanet Sands and chalk to act as a cofferdam.
The plunge columns support the core while the basement is excavated. The large braced plated columns weigh around 800kg/m with a capacity of 23,500kN.
Figure 1: Progress of the Shard by September 2010 (Image ©Southwark Council)
At the same time, slipforming of the core started. The thickness varies from 900mm to 450mm at the top. Excavation continued until the core reached level 23.
Next, a 3m-4.5m thick raft was cast beneath the core with 1,000 tonne rebar in 4 layers and with 5,500m3 of concrete poured over Easter 2010. Some 700 concrete trucks were involved in what is believed to be the largest pour in the UK. When the raft foundation was in place, the core continued its rise at 3m/day.
Building control issues
The Shard is unique in terms of building control and among the issues to resolve were:
- security access to site
- structural inspection of the cores took 30 minutes some days due to queuing to get to the top (unlike the 60 seconds now)
- design changes: the inclusion of a swimming pool at level 54
- checking emergency lighting to the stairs took over 4 hours and 600 flights of stairs
- problems with telephone communication; mobile phones not working above 100m
- 11,000 fire safety items to check
- dealing with overnight/out of hours fire safety system testing.
The success of the Shard has been marked in being named winner of the best large commercial building in the 2013 LABC National Building Excellence awards. For Southwark Building Control it has brought a raised profile, additional income, an agreement to be preferred BC provider in the Shard and an enhanced relationship with the developer. After more than 12 years the department is still very much involved.
The lessons learnt along the way include:
- Communication: regular meetings with developers and design team is essential
- Outsourcing: vital for structural checking
- Maintain comprehensive records: Trackers were used to discharge Conditional Approval issues and site issues; at one stage there were 400+ outstanding items
- Third party support: do not be afraid to seek additional resources
- Be proactive: take a lead role in bringing all parties together when issues arise to find a solution, ensuring that you include only those who can authorise or make decisions.
Peter Card is Head of Building Control at the London Borough of Southwark