LONDON – The CTBUH United Kingdom Chapter conducted an under-construction tour of 22 Bishopsgate. Developed by Lipton Rogers Developments and AXA, it is the tallest building under construction in the UK.
Renos Charitou of PLP Architecture, alongside Diego Padilla Phillips and Alex Black-Roberts of WSP Structures, provided an overview of the complex history of the development, with particular focus on how the design responds to the needs of its occupants. The presenters explained how the use of “structural gymnastics” helped overcome the challenges inherent to the complex construction project. Following the presentation, the delegates were taken on a tour of the project.
|The view as delegates tour the inside of 22 Bishopsgate, currently under construction.|
|Construction in London is not an easy task. From underground railways to Victorian sewers, from Roman ruins to unexploded WWII bombs, London’s building sites can present serious challenges. 22 Bishopsgate is no exception. The new tower sits on the site of a previous development of similar scale called The Pinnacle, which stopped after only a few levels had been completed. The challenge was that instead of building entirely new foundations, the constructors had to re-use as much of the existing building as possible, as well as a three-level basement that had already been constructed.|
Through mega transfer girders, inclined columns with high strength steel cable ties, triple-story A-frames, and partial demolition of the existing basement, the approach to the challenges faced at 22 Bishopsgate pushes the limits of traditional structural engineering whilst providing a sustainable and environmentally conscious building that reuses 100% of the existing foundations. When it is complete, 22 Bishopsgate will become the tallest building in the city of London. Not only will it look impressive from the outside and be a landmark on the London skyline; it will also possess some of the most efficient structural engineering in tall building design, and, hidden below ground in the basement, one of the most challenging foundation reuse strategies ever implemented.
At the building's north side, A-frames carry the load from three of the new megacolumns
||The lowest basement was excavated, retaining the megacolumns and pile caps from the previous building. This was carried out simultaneously to the tower being built above, using a technique known as top-down construction, whereby 20 levels of the concrete core were built before completing the basement raft. The building above was temporarily supported by using the existing foundations of the Pinnacle.|
Despite this challenge, almost 100% of the old Pinnacle foundations and 50% of the existing basement were reused. To achieve this, inventive engineering gymnastics were implemented. For example, the steel mega-girder used is 14 meters long and 4 meters deep, and it carries one of the main perimeter columns. Sixty stories of building (weighing the equivalent of 1,000 African elephants) are being supported by one beam hidden within the basement, transferring the weight to the position of the existing Pinnacle piles.
|Next, another main perimeter column needs to be cranked at the base to move the weight to existing foundation elements. To resist the overturning of the weight on an inclined column, high-strength pre-stressed steel bars will be embedded within the ground floor slab, tying the system back to the main concrete core.|
|Various challenges with the foundation were seen.
|Main perimeter columns cranked at the base.|
|About 40 attendees were duly impressed with the amount of engineering work underway on the site, and were grateful to have seen it before it is enveloped by the finished building.
|Giorgio Carlo Roberto Albieri
||Lipton Rogers Developments|
|Diego Padilla Phillips
||Zaha Hadid Architects|