Creating Principal Tower

At 163m high Principal Tower will be one of London’s tallest residential buildings with space for 301 apartments, with views stretching from Canary Wharf to St Paul’s Cathedral and the Houses of Parliament.

The neighbouring 600,000 sq ft landmark office space, which is also part of the Principal Place development, is occupied by Amazon and is a bridge to the creative industries just to the north of the financial buzz of the city, in Shoreditch.

Both buildings are fronted by a half-acre piazza, also constructed over the railway and surrounded by cafes and stores. It’s a new city hub for the future and, when Crossrail opens, will be roughly 10 minutes from the capital’s other financial centre in Canary Wharf.

Buildings have been erected over railways before, many times, but the really tricky part was that almost half of the Principal Tower’s foundation had to leave space below the structure in a protected corridor for a potential further two tracks in and out of Liverpool Street station – known as the 8 track corridor.

At the same time, the architect Foster & Partners did not want the conventional, visible, massive arches or A-frames that are the usual options for bridging rail lines. For Principal Tower it was felt that no one would understand the link to the railway usually made by exposed structural elements, so a primary objective was to develop a design that appeared to be built on solid ground.

Our solution had to be a hidden gem.

We devised a design that involved forming the sides of the protected rail corridor out of 1.5m diameter piles and reinforced concrete capping beams 6m deep, effectively creating a tunnel.

On the eastern side, because of the orientation of the building, eight 50m deep piles were needed where there was little room for them. But there was not enough depth available to carry loads from the eastern columns of the tower directly on to the 8-track roof structure. Proximity of a protected building also meant we had no space for an angled deep transfer slab.

As a result, our approach was instead to create a maze of load paths, using transfer structures from level 7 of the tower down to direct loads away from the rail corridor and toward the heavy substructures. The combined solution uses a stack of reinforced concrete walls gradually transferring loads through multiple storeys called “walking walls”. It also uses inclined concrete-encased steel columns, steel Y-frames, and 50 tonne steel girders spanning the width of the rail corridor. The out of balance lateral forces resulting from the various transfer structures are resisted by the reinforced concrete core, which is located to one side of the 8-track corridor, through flat steel plates embedded in the post-tensioned slab.

“This unique combination of structural solutions is what makes Principal Tower such a prime example of precision engineering,” says project lead structural engineer Nello Petrioli.