The engineering team behind The Shard in London have described its development as nothing short of an extraordinary journey in structural design.
Standing 310 metres high and consisting of a total of 72 habitable storeys, the tower was designed by Renzo Piano to be the tallest building in Europe as well as London’s only true mixed-use building with full public access.
Designing a structure of this scale on a constricted site while ensuring it is still resistant to high winds or terrorist attack meant the building’s engineer, WSP, had to overcome some significant challenges.
“The Shard presented a number of unique engineering challenges, the solutions to which have set precedents that will influence engineering for generations,” said WSP head of building structures Kamran Moazami. “From controlling the sway of the tower in the wind for the comfort of its occupants, to minimising the weight of the materials used to cut costs and carbon, it has been one of the most extraordinary journeys in structural design.”
Described as a vertical city, the multi-use nature of the tower, which includes office space, apartments and hotel facilities, public piazzas and a triple-ceiling-height viewing gallery, meant the engineers had to reconcile the need for entirely different floor systems required for the different uses.
The non-repetitive nature of the floor plan geometry, produced by the unique shape of an array of glass shards that merge to form an elongated pyramid, added another level of complexity.
The solution is a hybrid approach that stacks a series of different floor framing systems on top of each other. The office levels and public spaces use composite steel floors, while the hotel and apartment levels utilise post-tensioned concrete floors.
Layering concrete levels on steel levels provided additional, beneficial dynamic effects. The added mass and stiffness of the concrete framed levels significantly reduced the swaying induced by expected lateral acceleration movements caused by wind.
The spacing of perimeter columns also changes between levels, diminishing from six metres at the office levels to three metres at residential levels, to 1.5 metres in the spire. These changes in spacing coincide with changes in material type, from steel to concrete, while kink points resolve the changes in shard alignment from sloping to vertical.
A 60-metre steelwork spire, complete with a public viewing gallery, comprises the highest part of the structure. Serving as the housing for building maintenance units, the spire is stiffened by a concrete core containing the central lift shaft. The shaft and a scissor stair are enclosed in a glazed mast framed in architectural steel.
The topmost parts of the spire, where the shards extend upwards beyond the main structure, are stiffened with steel trusses. Given the height of the spire frame, this element was designed so it could be fabricated in modules.
The overall design provides a high level of robustness as well as a number of safety-related innovations, including state-of-the-art blast protection and structural fire engineering design.
The project also included an innovative approach to construction. Although top-down construction had been employed before, at The Shard the core was also built top-down, staring with the ground floor and proceeding upwards and downwards simultaneously.
Using plunge columns to support the super structure, strategically placed to miss existing piles and in a sequence carefully coordinated between the contractor and WSP, enabled the core’s initial 23 storeys and much of the surrounding tower to be built before the basement levels were fully excavated. This technique took four months off the schedule.
Unique in its appearance, usage and engineering, The Shard is bound to become a dominant part of London’s rapidly changing skyline.