Structural Design Gymnastics Enable Use of Abandoned Foundation
The new London Tower bears little resemblance to the original building design
The architecture and structure of London’s emerging 62-story building at 22 Bishopsgate bear no resemblance to its predecessor, called the Pinnacle, initially intended for the same site but abandoned soon after coming out of the ground. Yet, with some structural gymnastics below street level, the replacement is using all the Pinnacle’s foundations, plus much more.
Compared with the 63-story Pinnacle, 22 Bishopsgate, on the same footprint, has almost 1.5 times the net area and a structure that costs almost half, says Kamran Moazami, managing director of property and buildings for WSP Group, London.
The structural design strategy is “all about how to bring the loads from the new [structure] to existing foundations, to minimize the number of transfers,” he adds. “Every type of transfer is being used,” says Moazami.
Structurally, the two designs could hardly be more different. With its small, 260-sq-meter core, the slender, 288-m-tall Pinnacle would have been stabilized by 20 heavily loaded perimeter-steel megacolumns, each founded on its own large pile.
The 278-m-tall building currently under construction, with an average floor plate nearly 30% bigger, has a core 2.7 times larger than the Pinnacle’s, reducing the need for such heavy structural-steel megacolumns.
Planned in the heady days before the banking crash, the Pinnacle was to be the tallest, most visually striking tower in London’s financial district. In 2012, after the market mood changed, the project was abandoned, with only nine floors of its core built. Soon, the Pinnacle became known as The Stump.
For 22 Bishopsgate, “we started afresh,” says Peter Rogers, partner with Lipton Rogers Developments. The design was new, but not the architect.
Karen Cook, then a partner in the local office of Kohn Pedersen Fox Architects (KPF), started working on the ill-fated Pinnacle in 2003, when London was still a low-rise city. The owner wanted a landmark building, so Cook produced a dramatic, but expensive, scroll-like design. Sitework started in 2007, with Cook still at the helm of Pinnacle’s design.
In September 2009, when the Pinnacle’s basement was underway, Cook and four other KPF partners broke away to set up PLP Architecture. KPF retained most of its projects in construction, including the Pinnacle, which then faced “no significant problems,” says Cook. However, from the distance of her new firm, she later saw the project flounder.
Four years ago, “Lipton Rogers approached me to do some studies,” adds Cook. Having recently completed a project with Moazami, Rogers recruited WSP for engineering support to develop a different building on the same site.
During some 18 months, Moazami’s team developed about 80 structural models. “Existing drawings were available” and the Pinnacle’s original structural firm, Arup Group, was also helpful, says WSP’s technical director, Ross Harvey.
The new team, including Multiplex Construction Europe Ltd., the Pinnacle’s contractor, came up with a building that is bigger, less radical and structurally more efficient and has greater internal quality, says Cook. “This building is designed with the inhabitants [more] in mind,” rather than as an icon, she adds.
During the new building’s development and early construction, the designers and contractors worked directly for Lipton Rogers. The team had yet to find investors to buy the project from the owner, Arab Investments Ltd. “We wanted to get going quickly,” says Rogers.
In early 2015, AXA Real Estate Investment Managers acquired the project for investors and retained Lipton Rogers as developer. That October, with the new core a few floors above ground, Multiplex signed the main design-build contract, and PLP and WSP became subcontractors.
The building is scheduled for completion in May 2019. The core is more than 20 floors above the Pinnacle’s three-level, 100-m x 60-m basement. Only a few perimeter columns will need to be tilted for the building to fit the old, 200-pile foundation.
On the front elevation, for example, four columns incline inward by up to 3 m at various levels between floors eight and three. They are tied to the core with horizontal trusses buried in the floor plates.
Nearly all the structures that transfer new column loads to piles are below grade. The Pinnacle’s legacy comprises only about fifty 2.4-m-dia, 65-m-deep piles. Many shallower, 1.5-m-dia belled piles from a previous building share the site.
Transfer structures include, in one corner, two-story steel A-frames, spreading loads from three perimeter columns to the piles through a 1.4-m-thick concrete overlay on the lowest basement slab.
On the south side, a 15-m-long, 3-m-deep steel girder weighing 75 tonnes diverts loads from two columns via a 3-m-thick concrete overlay. Basement walls at five locations are thickened to act as deep girders, spreading column loads to piles.
Workers also bored 85 piles to support the new structure. For those 1-m-plus-dia piles, the team first planned to replicate the Pinnacle piles. But to bore bentonite-supported megapiles from the ground level would have required “a huge amount of propping,” says Varnava Christofis, Multiplex’s design manager.
Instead, the piling contractor used two small rigs in the basement, operating with only 6 m of headroom. The rigs needed to be dismantled to fit into the small opening in the ground-floor slab, says Neill Stratton, the project’s construction logistics manager who had been Multiplex’s piling construction manager at the time of the work.
To speed work, the 50-m x 14-m core has been split in two. The northern half is rising directly from The Stump’s foundations. The other half extends beyond the original core and needs new piles.
To prevent the piling from delaying the work, the contractor is using top-down construction to jump-cast the south core from the ground level. Rising through the basement, five of the Pinnacle’s megacolumns supported the core when casting began. As the core reaches its final height, workers are progressively replacing the megacolumns with concrete walls.
For the replacement, the designer retained almost all of the Pinnacle’s ground-floor slab. For the new configuration, workers have removed all of the old core, the first basement-floor slab and 40% of the second basement slab.
Crews had to excavate a quarter of the 6,500-sq-m bottom slab for new foundations. But of the numerous original piles, “there was no question of not reusing them,” says Rogers. “The big bonus was we could get the core [started] early.”