Published: 16 November 2017
Nigel Annereau explains the engineering behind London’s Leadenhall Building, which has been shortlisted for The Structural Awards 2017.
The Leadenhall Building is a 52 storey steel tower whose architecture expresses its structural engineering to a degree rarely seen in a commercial office building. Its primary structural system is fully external and unclad, with engineering and construction celebrated throughout by use of clear, integrated systems and legible, refined details. The team behind Leadenhall was the same as that behind London’s Lloyds Building (Richard Rogers and Arup) and shared the ethos of pure, exposed structure.
The iteration of the building’s megaframe was a collaborative exercise involving both architect and engineers and has delivered a truly innovative work environment to the client. The combination of the megaframe with the north core provides completely open and flexible floor plates, better suited to delivering adaptable internal spaces, while the megaframe’s steel members distribute vertical and horizontal loads to the tower’s foundation system and provide overall stability to the building. The columns in the building take nearly 80MN of vertical force, equivalent to 50 Boeing 747’s.
Much of the building’s innovation was in its construction methodology. We used a solid, pre-cast floor system that eliminated the need for wet concrete, utilising a series of grouted dowels that connected planks together and to the underlying steelwork - made possible by very accurate pre-casting techniques. This saved time on construction, improved safety by requiring less man power, and reduced noise in a densely populated area. This also avoided any ‘splatter’ down the sloping façade which would have cost a lot of money to keep clean.
Active alignment was another important element of Leadenhall’s construction: due to the triangulation of the megaframe we expected the building to lean to the north during construction. To compensate, the diagonal members in the east and west faces were deliberately made short and had their length initially adjusted by the introduction of shims (metal plates used to make up a gap). As the building moved, the shims were gradually removed in a series of planned operations, shortening the diagonals thereby bringing the building back to a vertical position.
Off site manufacture was also a crucial challenge that tested the logistics planning around the Leadenhall Building’s constricted site. Many of the deliveries were very large and were brought to site at night.
One important element in meeting this challenge was the ability to deliver more at once, thereby reducing truck movements in the City. The North Core (the yellow steelwork) was assembled as complete “tables” in a factory, allowing one delivery lorry to contain the steel columns and concrete floors, MEP services, and edge guardrails. Each floor of the North Core was built in just three deliveries and hence three crane lifts, greatly reducing the traffic needed to deliver all components of the building.
We’re really proud of the engineering behind this new London landmark, affectionately nicknamed The Cheesegrater by Londoners, and are delighted to be shortlisted for The Structural Awards 2017.
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