Engineering the tallest BREEAM Outstanding building in the UK

Unlocking opportunities through innovation

In 2018, we validated the feasibility of adding 13 extra storeys to the building, following the changes to planning context with neighbouring schemes. This opportunity led to a re-evaluation of the structural scheme, culminating in a perimeter steelwork bracing solution in the building’s mid-rise section, called the braced box. Rigidly linking the two cores, the braced box allowed them to become slender on the mid-rise and high-rise levels, delivering significant additional net lettable area. 

Arup proposed that the braced box would also support a cantilever on the west face, over Bishopsgate. With the architect, we progressed the cantilever to 9m on the south and 5m on the north, whilst extending up. The cantilever accounts for 15% of net internal floor area, key to commercial viability and accentuates the iconic stacked block design. 

The level 6 Cantilever represented a particular challenge during construction. In the completed building, the braced box supports this cantilever, yet during the construction the braced box is built after the cantilever. Working with Lendlease and steelwork contractor William Hare to develop temporary works that support this load and ensure that the cantilever’s tip remained within 10mm of its permanent position. We developed a procedure that used five hydraulic jacks to support 700 tonnes of building load, before transferring this load to the permanent structure. During this suspenseful load transfer, the movements stayed within 4mm of modelled predictions, a notable achievement given the scale.

As the project was on site when COVID hit, we swiftly identified opportunities to futureproof the building for future pandemics. This included touch-free lifts and fully ventilated WCs with direct fresh air, rather than recycled air typically found in office buildings. Out flexible initial design made these relatively easy to action. 

Decarbonisation through detailed design

Using advanced analytic modelling, we designed every structural element for the individual load it would bear across its design life, reducing the required steel weight by 25% and saving 5,000 tonnes of CO₂. This approach is more granular than usual for tall buildings, where elements tend to be grouped and designed for the combined load. We saved a further 140 tonnes of CO₂ by optimising the beam spacing for the higher levels. This brought cost and programme benefits, with fewer beams to manufacture, unload and lift into place.

Tall towers typically use piled foundations to transfer large vertical loads into the ground. We developed a better way to do this, adopting a pile assisted raft that reduced pile numbers from 89 to 28, saving two months on the construction programme and requiring 3,000 cubic metres less concrete. This was delivered by our integrated team of geotechnical and structural experts who challenged themselves to go beyond the usual.

Façade engineering for comfort and efficiency

To reduce solar gains and glare, we engineered a closed cavity façade system with automated blinds, controlled via a rooftop sensor that tracks the sun’s brightness and positioning. Using an automated system instead of relying on occupiers to use manual blinds reduced the building’s cooling load by half a megawatt, yielding not only operational energy benefits but reducing the embodies carbon and cost of the air conditioning plant.