Building Foundations

A SQUARE PEG IN A ROUND HOLE WEALTH CREATION A SQUARE PEG IN 22 Bishopsgate is the tallest building in the City of London and the second tallest in western Europe. However, the most extraordinary feature of this new skyscraper is not its sheer size, but the ingenious structural engineering – or ‘structural gymnastics’ as the team A ROUND HOLE describes it – that has enabled the remains of an earlier, failed development to be transformed into a large efficient modern office block. Hugh Ferguson talked to Peter Rogers CBE FREng, co-founder of the project’s developer Lipton Rogers Developments, about the building’s innovative engineering. Modern high-rise work spaces The Pinnacle had severe was a chance to put the exercise in the City of London have led drawbacks. It was inefficient into practice. The challenge was to a succession of instantly and expensive to build: its to create a building the same recognisable iconic structures tapering, spiralling shape height as The Pinnacle, but with with quirky nicknames – ‘The allowed little opportunity for a third more lettable space, no Gherkin’, ‘The Walkie-Talkie’ and repetition in construction with more weight and at less cost. ‘The Cheesegrater’. The tallest its highly complex engineering and most eye-catching of all and irregular floor spaces. Also, was set to be ‘The Pinnacle’, post-2008 developers wanted BUILDING a twisting, tapering tower, efficient buildings, attractive FOUNDATIONS provisionally nicknamed to both businesses and their The basic design of the new The Helter Skelter. However, employees, which would help 62-storey block, provisionally following the 2008 property draw activity back into the City. dubbed Twentytwo, is crash, the investors pulled out A very different building was conventional by modern and the project stopped. By then required, one that used the size skyscraper standards: a large London’s deepest piles had been and shape of the plot more central concrete core holding sunk, most of the three-storey efficiently, and was affordable the lifts, steel columns and basement had been built and the to build. beams around the periphery first seven stories of the building’s The challenge was taken up and concrete floors in between. concrete core had been erected by Lipton Rogers Developments The difficulty came at the above. Efforts to revive the (LRD), with its architect PLP, bottom. The columns and core scheme came and went, and the engineers WSP and contractor were not in the same place as desolate, abandoned concrete (as for The Pinnacle) Multiplex. The Pinnacle’s existing piles. soon acquired a new nickname: LRD had jointly run an exercise Installing new piles would be ‘The Stump’. with the City on how to expensive and would delay the A render demonstrates how 22 Bishopsgate dominates the City of London’s earlier tall buildings, including 122 Leadenhall Street – The Cheesegrater – just to Although widely admired as construct high-rise buildings start of construction by many the building’s left, with The Shard across the River Thames to the south © 22 Bishopsgate an architectural masterpiece, quicker and cheaper, and this months. 14 INGENIA INGENIA ISSUE 80 SEPTEMBER 2019 15 A SQUARE PEG IN A ROUND HOLE WEALTH CREATION Columns along the west edge of the building slope inwards between levels six and three, with the huge horizontal forces countered by horizontal trusses connecting the columns to the concrete core © WSP Sequence of ‘top-down-bottom-up’ construction for the south core, showing the additional strengthening measures to allow the core to be increased to level 20 AIR TRAFFIC CONTROL before the foundations below were completed © WSP Structural engineers rarely get involved with air traffic control, but a very tall building under busy The core of the old stump had do this by sloping the columns included two lorry lifts, which flight paths is an exception. to go, but much of the existing inwards from level six to level was the main means of access The flight path to London City Airport is directly over the City of London, with planes normally at FIRE SAFETY basement and all the old piles three, returning to vertical for to the lower levels during Twentytwo has all that would be expected in terms of fire safety more than 600 metres but with a safety zone down to 305 metres to allow, for example, for failure of could be re-used. This required the entrance lobby down to construction and not easy to in a modern tall building, including careful choice of building a plane’s engines. The building was designed to fit within this limit, but the designers had assumed a series of engineering measures ground level, and then sloping relocate, so the structure above materials, compartmentalisation to prevent fire spread, a fire- that cranes would be allowed to rise higher during construction – as was the case with The Shard to transfer the huge loads from again from ground to basement had to bridge over it. Engineers engineered structure to allow at least a four-hour safe period for across the river to the south (‘Building The Shard’, Ingenia 52). When planning permission came the core and columns of the three. Sloping introduced designed a ‘mega truss’ to evacuation, smoke control, four dedicated lifts for firefighters and through with a surprise 309.6 metre limit on crane height, a quick redesign of the top of the building new building on to more than massive horizontal forces that transfer the load from three of a conventional ‘down-the-stairs’ evacuation procedure. But, as a and how it would be assembled was required, with the topmost crane only allowed to operate with 300 existing piles – 25 mega- pushed outwards at the top the main columns to the pile first in the UK, the lifts will also convert in a fire to ‘vertical tube its jib horizontally. Eventually through negotiation, the restriction was lifted outside flying hours so piles installed for The Pinnacle of the sloping column and caps via columns either side of trains’ for rapid evacuation. that the jib could be raised – between 11pm and 6am Monday to Saturday and all day on Sundays. A plus other smaller piles from inwards at the bottom. To resist the lift. Elsewhere, basement When the external cladding of the Address Downtown tower ‘hotline’ was installed with City Airport so that the site could check that all flights had stopped before previous developments – these, large steel Warren trusses space was needed for delivery in Dubai caught fire in 2015, the remainder of the building’s fire they lifted the jib, and to confirm that the jib was locked down before flights could restart. using their bearing capacity to (steel members arranged in a or waste-collection lorries to safety strategy worked well enough to prevent serious injuries The flight paths in and out of Heathrow are at a much higher level, but the issue here was the full without overloading triangular grid) were installed in turn, requiring one column to be – apart from one person who suffered a heart attack during interference with the radar operated by Britain’s air traffic control service NATS – either shadowing, any. In the end, engineers only a horizontal plane just beneath removed. A massive 15 metre- the evacuation. Hurrying down some 60 flights of stairs can be which would hide some aircraft, or reflection, which would cause aircraft to appear on the radar in needed 85 new piles, most the floor to connect the line of long, 3.8 metre-deep, 97-tonne exhausting for fit people, and with the UK population getting less the wrong place. An impossibly low height limit was imposed on the building until a solution could to lend support to the larger columns with the solid concrete steel ‘mega girder’ was inserted fit, using the lifts makes sense. However, this is usually ruled out be found. concrete core, and these were core – a truss at the top of the just below the ground floor slab as too expensive: using lifts in fires requires fire-rated shafts, dual LRD quickly identified that small enough to be sunk with sloping column to take the to transfer the load to points power supply, cars pressurised against smoke ingress and large the 112-metre-high 1960s a small piling rig that could fit tension and another at the where there was sufficient pile lift lobbies free of flammable material. Hyde Park Barracks was directly inside a basement storey. bottom to resist compression. capacity. LRD’s ingenious solution has avoided these costs. The building in line between the NATS In many places, the transfer For one of the columns near The north side of the new is divided vertically into zones, with fire-hardened floor slabs radar station at Heathrow and of load to the existing piles was a corner of the building, the concrete core sits over the with a two-hour fire rating separating them at levels 26, 42 and Twentytwo, thereby casting its achieved by installing massive truss solution would not work adequate foundations of the old 58, which coincide with the levels served by each of the groups own shadow. This allowed the beams within the basement area for the tension, so instead the core, but the south side required of lifts. Each set of lifts has a motor room just above the fire- height restriction to be raised to – effectively one-metre-thick top of the sloping column was new piles and a new heavy hardened slab, itself encased in concrete. the top of the ‘shadow’, at 126 reinforced concrete walls up to anchored in to the core with concrete raft to be constructed A fire on the lower floors will still require a conventional metres, which bought time for a three storeys high.

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