Engineering the Shard
26 January 2016
Authors: Dr Richard Mawer BEng PhD CEng MIStructE, associate director WSP|PB John Parker MA CEng FICE FIStructE, senior technical director WSP|PB
The Shard sits in the heart of the newly created London Bridge Quarter. The building has achieved one of its main goals, to revitalise the surrounding area. It has brought investment to the quarter as well as playing a part in the redevelopment of London Bridge station.
The vision of client Irvine Sellar and architect Renzo Piano was of an architecturally outstanding building featuring high-quality commercial, residential and public spaces. The Shard is the realisation of this, a mixed used building with public spaces at the base. At 306m, the Shard is the tallest building in western Europe. It accommodates 8,000 workers, residents and hotel guests each day; the viewing gallery has more than a million visitors each year.
The Shard is a ‘vertical city’ comprising 25 storeys of offices, three levels of restaurants, 18 storeys of hotel and 13 floors of apartments. At the top is a 65m tall steel and glass ‘spire’ where carefully detailed steel surrounds viewing gallery visitors as they appreciate the views over London.
The development promoted sustainable travel by including only 48 car parking spaces and including a major refurbishment of the adjacent London Bridge station. The team, client and Southwark Council also put together a vocational programme associated with the project to create local employment and apprenticeships.
The Shard is on the south side of the River Thames, in an area which was occupied from Roman times until the 19th century by small-scale buildings and narrow roads. In 1836, the first railway into the capital, the London to Greenwich Railway, built a terminus at London Bridge. There was rapid expansion of the station, and by 1893 it occupied an area over 300m long and 130m wide. The Shard site was used by a succession of railway buildings, including offices, a hotel and a Royal Mail depot.
During the Second World War the area was heavily bombed, destroying an irregular-shape corner of the station. This area was redeveloped in the 1970s by the 26-storey Price Waterhouse Coopers headquarters. The building was a reinforced concrete frame with masonry cladding. Demolition of this building made way for the Shard.
The site geology is typical of London. Chalk is present at depth, overlain by Thanet sands and then the Lambeth Group beds. Over this is London Clay followed – because the area was once part of the Thames – by layers of river terrace gravel and alluvium. The water table is around 4m below ground level, at the top of the gravel, and the high permeability of this stratum presented the greatest challenge for keeping the basement dry. The presence of water in the Thanet sands meant piles required bentonite.
It was not possible to re-use the existing piles because they extended only a few metres below the level of the underside of the new raft slab. Coring through the unreinforced under-reams was possible, but not through the reinforced shafts. To avoid this, the existing pile locations were determined from record drawings and the design allowed for some repositioning in case the as-built locations were different. In the event this was necessary in only a small number of places.
Being surrounded by roads, utilities and railway infrastructure meant that the design and construction of the substructure was challenging, and critical to the programme. The team discussed methods to achieve time and cost savings in the construction of the core, basement and piled raft. The result was top-down construction for the three level basement, including the first major core constructed top-down in the world.
Top down construction was achieved by using steel plunge columns, embedded into the wet concrete of the pile. Once cured they were used as temporary supports for the core and the floor slabs while excavation took place below. The carefully optimised piled raft was 3m deep beneath the core and 1.5m deep elsewhere: shallow for a building of this scale.
The use of ground granulated blast furnace slag eliminated nearly 800t of carbon in the raft slab. It also reduced absolute and differential temperatures during the curing of the concrete, minimising the risk of cracking. A secant pile wall retained the ground, minimised movement in surrounding assets, and prevented water ingress.
The sequence for top down construction was as follows:
- Existing condition: Southwark Towers piles, under-reamed and approximately 20m deep.
Secant pile wall and bearing piles (1,800mm diameter) were bored from ground level and steel plunge columns installed. Extremely tight tolerances (±10mm in position and ±1:400 vertically) were required on the plunge columns to ensure that the 1,000kg/m sections stayed within the finished core walls.
- The ground slab was cast on a slip membrane so that blinding concrete did not adhere to the underside. Excavation of two levels of basement then took place. The slipform was set up at level B2 on the plunge columns, and the level B2 slab was cast.
- Excavation continued beneath B2 to formation level. The slipform was not allowed to climb above level 21 while the core was supported only on plunge columns.
The raft slab was installed in a single 5,500 m3 pour taking 32 hours. Up to this point, all loads were carried on the secant wall and the piles containing plunge columns. Subsequently, the other piles, and bearing pressures under the raft slab, were also mobilised. The core walls in the basement were then completed using self-compacting concrete pumped from the base of the shutters. This rendered the plunge columns redundant – but they were, of course, left in place.
- Superstructure construction continued.
This carefully sequenced construction of the sub-structure and core ensured stability and safety at every stage of the build. It led to a three-month programme saving.
The core provides the lateral stability for the whole building. It acts as a cantilever from level B3 to level 72. Lateral forces are transferred through the ground floor into the basement perimeter wall; vertical push-pull forces in the core continue down to the piles and basement raft.
The height and mixed-use nature of the tower requires 21 lift and stair shafts in the core at ground level. The number of shafts gradually reduces with height in order to maximise the net lettable area of the floor plates. In order to achieve maximum efficiency some shafts are shared by lifts at different levels, separated by slabs capable of withstanding buffer forces.
At level 28 there are three levels of shaft walls designed as a grillage and referred to as the ‘egg crate’. These transfer stability forces and release space inside the core for toilets, providing additional lettable floor area.
As the core reduces in size with the height of the building, a number of strategies are adopted for maximising its stiffness. ‘Wing walls’ on each side of the service risers extend outside the main perimeter of the core, increasing its inertia. Near the top of the core, in the high level plant rooms at levels 66-68, a ‘hat truss’ of diagonal steel members connects the core and the perimeter columns.
This increases the stiffness of the structure and ensures that lateral accelerations in the apartments remain below the 0.015m/s2 limit recommended by the Council for Tall Buildings and the Urban Habitat (Isyumov, 1993). To prevent differential axial shorting of the core and perimeter columns causing large axial forces in the hat truss members, the members were not connected until the main structure was completed.
Wind tunnel testing was required to ascertain the lateral forces applied to the core. The high-frequency force balance technique was used: a rigid 1:400 scale model was connected to its base by a system of springs. The stiffness of the springs provided the model with appropriate lateral and torsional frequencies based on computer model calculations.
The forces at the base of the model were measured using load cells and the full-scale values were determined by calculation, using an overall damping ratio of 1.5 per cent. The model was mounted on a turntable and 36 wind directions were tested. Sensitivity checks were carried out by varying the damping ratio and the natural frequencies.
Wind pressures on the cladding were determined using a second model containing 800 pressure taps. A larger (1:150) scale model with 123 pressure taps was used to investigate the loads on the Spire. A fourth wind tunnel test investigated pedestrian comfort and wind speeds at ground level.
The spire structure is open to the elements and contains the viewing gallery, plant and building maintenance units. The architect’s vision of the spire was to ‘allow the building to merge into the sky’ and this was achieved by gradually reducing the density of the structure. Above level 72, the concrete core was replaced by a steel mast, floor plates were only provided at every third level and open grids were used instead of solid floors.
The ‘shards’ – the planes of façade – were terminated in jagged points at different levels and gave the building its name. They cantilevered beyond the top floor (level 87) by up to 18m and were supported by trusses. The compression booms were restrained by u-frame action from the trusses acting together with the frames in the plane of the facade
The biggest challenge in designing the spire was to ensure that it could be built safely and quickly at height. In order to achieve this, the entire structure was split into modules that conformed to two constraints: firstly, each module weighed less than eight tons (the lifting capacity of the crane) and secondly, it was small enough to be brought to London from Yorkshire without police escort.
Each prefabricated module was designed to be self-stable so it could be lifted without temporary bracing. Edge beams were fabricated channels so that, when one module was bolted to its neighbour, the appearance was of an I-beam. Building the spire in modules rather than ‘piece-small’ minimised both the number of crane lifts required, and the number of connections to be made in-situ, at extreme height.
Despite 70 per cent of crane time being lost due to weather conditions, and the significant risks in working at this height, the modular system proved to be a remarkably efficient and safe way of assembling the UK’s highest pieces of steel and glass.
The Shard achieved the client’s and architect’s aims and is now one of the buildings that defines the London skyline. It was designed efficiently and was built rapidly with an exemplary safety record.http://www.engineersjournal.ie/2016/01/26/engineering-the-shard/http://www.engineersjournal.ie/wp-content/uploads/2016/01/aaashard1-1024x682.pnghttp://www.engineersjournal.ie/wp-content/uploads/2016/01/aaashard1-300x300.pngCivilconstruction,United Kingdom