Picture caption: London’s growing cluster of skyscrapers – Picture credit: Tadas Petrokas on Unsplash

As London’s cluster of skyscrapers grows ever larger, one construction project in particular stands out for a unique piece of engineering – the “floating” crane.

One Leadenhall is due to be completed in 2024, and the speed of the build can partly be put down to not having to work around a traditional crane. Most city sites are very tight on space, and this one is no different as it is next to Leadenhall Market, with the busy Gracechurch Street running along one side.

From a distance the “floating” tower crane appears to be hovering on the outside of the partially built structure, as if by magic. But, in fact, the crane sits on a bespoke steel H frame which is cantilevered off the new building’s partially erected steel frame.

The structural engineers and construction engineering firm are one and the same – Robert Bird – and they enhanced the permanent steel to be able to take the additional forces from the crane.

The mast of the crane is tied into the building at levels 8, 17, 23 and 29. These have been carefully positioned to make sure they don’t interfere with the building’s façade, which has mullions 1.5m apart.

The site has two additional cranes – climbing cranes – one of which was used to erect the “floating” crane on the outside of the building.

The new tower will create 40,000m² of offices, and includes a free public terrace on the 4^th floor, and ground floor retail space. The first 4 storeys of the 35-storey building are comprised of a masonry ‘street block’ which has been designed to fit in sympathetically with the neighbouring buildings, including the Grade II* listed Leadenhall Market, which has been used in numerous films, including Harry Potter and the Philosopher’s Stone. (The market is being closely monitored during construction.) A glass tower will rise above the masonry block but is set back to make it less obvious from street level.

From a structural point of view, other points of interest are the column-free corners, to maximise views from the tower, and the extended basement. The previous building had a one-storey basement, which is being extended to two-storeys, so 100 new piles have been added, avoiding the existing shorter piles. These new piles are 30m long and dry bored, and they’ve been designed to reach the bottom of the London Clay. Dry piles avoid the need for a bentonite plant on site which, again, maximises site space and speeds up the build.

Like a lot of the work we do, as engineers, architects or other professionals, it’s all about solving problems and putting our heads together to figure out how we can do our work more efficiently – whether we’re saving time, space, budget or the planet.

Of course, as interesting as elements of this project are, we might question exactly how much more office space we need in the City of London. But the public terrace will at least bring new views of surrounding architecture, including the ironwork rooftops of Leadenhall market and the spires of St Michael’s Church.

Meanwhile, if you need assistance with the structural elements of an upcoming project, please do get in touch.