When construction of the Pinnacle tower in London was stopped in 2012 the basement structure was completed and the core had reached the seventh floor. The site is now being redeveloped as “22 Bishopsgate” a 62 storey office tower.
The new development has required the demolition of the existing part core and much of the basement structure. The new concrete core will sit half on and half off the foundations for the old core, requiring new piled foundations to be installed under half of the proposed core.
When PJ Carey was appointed to construct the core, it was tasked with jump forming both halves of the core together and programme constraints required this to start before the new foundations could be installed. In effect some 16,000t of the reinforced core were to be suspended at basement one level to allow works to progress underneath it.
ByrneLooby had worked on a number of schemes with PJ Carey and had been responsible for the top down design of the Shard basement. PJ Carey therefore appointed ByrneLooby to undertake the design of this temporary core support, allowing the maximum height of core to be constructed before the new foundations were engaged. All the structure was to be contained between the level two basement and ground floor slabs and had to be constructed while basement demolition proceeded around it.
ByrneLooby identified five steel columns from the original scheme in the vicinity of the new core and developed a grid of walls to transfer the core loads into these columns. The columns and their supporting piles thus determined how high the core could be built.
The transfer structure and new core were modelled on Tekla to enable the loads, stresses and deflections to be assessed throughout the core construction.
As the existing level two basement slab could not take the construction load from the transfer structure it was split into three horizontal bands. The lowest band was designed to be carried by the slab until the concrete cured, after which it could support itself and the next lift. The situation then repeated for the final lift.
The interaction of foundation and transfer structure programme was critical for the superstructure programme. As the works progressed it was determined that, with a revised programme, the transfer structure would need to support up to 22 storeys to enable core construction to progress without a break.
A system of additional props was therefore developed to enable the core to reach level 22. This avoided delays to the core progress which would have caused the same overall delay to project completion.
The project shows the importance that temporary works can have on project outcomes. The transfer structure allows the core construction to be brought forward by around six months which translates to major cost savings and a similar overall saving on the project programme.
In association with Byrne Looby