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A 50-storey tower on the corner of two of the most prominent streets in London, 8 Bishopsgate offers 52,900m2 of offices, with elevated outdoor terraces, a café and deli, a 200-person auditorium and a public viewing gallery at the top of the building. Arup provided multidisciplinary services for the EPC A-rated building, which has become the tallest commercial building in the country to achieve BREEAM Outstanding.
Developed by Stanhope for Mitsubishi Estate London and designed by Wilkinson Eyre and Arup, 8 Bishopsgate is formed as a series of stacked blocks. This maximises the lettable floor area while protecting views of St Paul’s. Arup was chosen to provide a range of services based on our international experience designing tall buildings, local knowledge and sustainability capabilities, helping the client and architect set and achieve ambitious goals.
Conceptual alignment to reduce carbon and cost
At the earliest stage of the project, a form finding collaborative design approach between architect and engineer allowed the team to maximise the development area on the constrained site and arrange the massing around lean component building blocks. In essence, the form was developed to be conceptually efficient from day 1.
Minimising materials use to cut embodied carbon
Using advanced analytical 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 is more granular than the usual approach 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 also 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 the number of piles from 89 to 28, saving two months on the construction programme, and required 3,000 cubic metres less concrete. This achievement was delivered thanks to our integrated team of geotechnical and structural experts who challenged themselves to go beyond the usual: our optimised design avoided another 680 tonnes of CO2.
Project Summary
5,000tonnes CO₂ saved through steel optimisation
25%reduction in structural steel
3,000cubic metresconcrete saved in foundations
8 Bishopsgate is formed as a series of stacked blocks, maximising the lettable floor area while protecting views of St Paul’s. Credit: Paul Carstairs
Optimising operational 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 brightness and position of the sun. 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 also reducing the embodied carbon and cost of the air conditioning plant.
The positioning of building systems affects how efficiently they function so we completed a tipping point service analysis to finetune locations of equipment, considering floor demands and distances. With the reduced size of plant and risers needed, in addition to the energy saving benefits, we managed to achieve a slimmer core resulting in an increased net lettable area and lower embodied carbon.
Introducing floor by floor air handling plant rather than a centralised ventilation system will enhance energy efficiency and give greater flexibility to occupiers. Working closely with the client and architect, we located air plant in areas with less desirable outlook, balancing the need for access to external air with the objective to deliver high quality office space to occupiers. CO2 sensors mean air flow can be adjusted according to local demand rather than a static amount per m2, giving flexibility for multiple occupancy scenarios and empowering the building team to manage fresh air for efficiency and wellbeing.
© Dirk Lindner
Overcoming challenges and unlocking opportunities through innovation
At the start of the project, we launched an international design competition for the most efficient building scheme within the site constraints. These best practice ideas from around the world influenced plans for 8 Bishopsgate.
In early 2018, we validated the feasibility of adding 13 extra storeys to the building, following changes to the 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 mid-rise section of the building, called the braced box. The braced box rigidly links the two cores, allowing them to become more slender on the mid and high rise levels, ultimately delivering significant additional net lettable area.
Going even further, we proposed that the braced box would also support a cantilever on the west face, extending over Bishopsgate, still within the site ownership boundary. Working with the architect, we progressed the cantilever to 9m on the south and 5m on the north and extended it up the building. This cantilever accounts for 15% of net internal floor area, key to commercial viability and accentuates the iconic stacked block design. It is structurally efficient too, using the existing braced box.
The level 6 cantilever over Bishopsgate represented a particular challenge during construction. In the completed building, the braced box supports this cantilever, yet during construction, the braced box is built after the cantilever. We worked with Lendlease and steelwork contractor William Hare to develop temporary works that would support this load and ensure that the tip of the cantilever remained within 10mm of its permanent position. To achieve this, we developed a procedure that used 5 hydraulic jacks to support 700 tonnes of building load (equivalent to 6 blue whales) before transferring this load to the permanent structure. During this suspenseful load transfer, the movements stayed within 4mm of modelled predictions – a real achievement given the scale and complexity.
As the project was already on site when Covid hit, we swiftly identified opportunities to futureproof the building for future pandemics. This included introducing touch-free lifts and fully ventilated WCs with direct fresh air rather than recycled office air typically used in pre-pandemic office buildings. Our flexible initial design made these relatively easy to action.
