One Leadenhall: Multiplex brings its goods to market

Towering over a historic City of London landmark and one of Harry Potter’s stomping grounds, One Leadenhall has presented a raft of building challenges

Factual and fictional cultural icons, old and new, have left their mark on the areas around Leadenhall Market in the City of London. Along its east side runs Whittington Avenue, named after Richard Whittington, the three-time lord mayor of London 600 years ago. Four centuries later, a gander named Old Tom became the market’s de facto mascot after evading the butcher’s knife, and spent the rest of his 37 years in and around the market being fed by friendly innkeepers. Latterly, the stunning market, which dates back to 1321, stood as the fictional Diagon Alley in the first Harry Potter movie.

Multiplex is hoping its adjacent One Leadenhall development will be just as iconic. It will replace Leadenhall Court, Royal Sun Alliance Insurance Group’s former headquarters – a seven-storey structure built in 1988 and bought by developer Brookfield Properties in 2012.

Fitting in

The new building is 36 storeys (176.6 metres) tall and will provide 40,000 square metres of office space, two basement levels, ground floor retail units, and a restaurant and public terrace on the fourth floor with views of St Paul’s Cathedral. The first four storeys match the aesthetic of the nearby older and lower-level buildings, with masonry separating the glass panes. From level five upwards, it becomes a quintessentially modern glass tower. One Leadenhall is aiming for BREEAM Outstanding, WELL Gold and EPC A ratings.

Planning permission was granted by the City of London in 2018. The consent came after years of having to contend with a planner who, according to Peter Clarke, vice president of development for the client, Brookfield, was adamant that the site would never host a tower, but times changed.

Multiplex couldn’t start the building work until an anchor tenant was secured. Once a lease was signed in mid-January 2020 with global law firm Latham & Watkins, Erith began to demolish the previous building on the site the next working day and continued for five months. The project site was physically constrained by major roads on two sides, which meant finely calibrated logistics and just-in-time deliveries were needed.

The Grade II-listed market had leant against the Sun Alliance building so, as Erith began its demolition, Multiplex undertook digital monitoring that would alert the team to movement of more than 5mm. “There was no way we were going to be allowed to let that [movement] happen,” Clarke says. When the old building was demolished, the bearing pads went with it. So Multiplex reinstated the bearing pads that were on Leadenhall Court and replaced the degraded bearing pads that were on the market building to absorb any movement.

One Leadenhall has an integrated piled raft foundation, for which Erith combined piles and rafts into a single system. Andrew Feighery, project director for Multiplex, says Erith filled in the big hole left by demolition, then covered it with a 5,750 cubic metre concrete raft slab, 3.5 metres thick. It then dug down using the semi-top-down method to create a two-storey basement with a floorplate of approximately 4,600 square metres. This involved using the slab as a lateral support to help the excavation to go faster. Gaps were left in the ground floor slab and the slab below it, allowing the jump-formed core to rise through them. “The engineering gymnastics involved were quite something in such a constrained site,” says Feighery.

High forces

The basement slabs are supported by 23 reinforced concrete columns rising from the foundation slab. In turn, this sits on 103 bearing piles, 1,200mm in diameter and 33 metres deep, with the volume of concrete totalling 3,825 cubic metres. The piles were installed with a Soilmec SR-75 hydraulic drilling rig and a Bauer BG 33 rotary drilling rig. Due to the high forces in the foundation, the concrete mixes for the foundation and basement slab are typically C50/60 strength. “It’s a significant amount of concrete that’s carrying the load. And, again, all that load from the building frame’s 18 columns is transferred back down into those slabs,” says Feighery.

To create a larger area for the typical office floor plate, nine columns were erected at an incline from the basement to level 6. Five columns were inclined at 88 degrees along the south side of the building and four columns were inclined at 82 degrees on the west side. This allowed the typical office floor plate from level 6 and above to be nearly 10 per cent larger than the basement footprint.

Steel frame

The building has a traditional steel frame, with a total of 5,960 tonnes of steelwork. The jump-formed concrete core’s area is approximately 427 square metres, incorporatingthe lift shafts, lift lobbies, stair shafts, toilets and internal risers. The height of the core from basement 2 to level 34 is roughly 162 metres. But at levels 5 and 22, the core narrows with the building’s floorplates. The lift shafts are still under construction but contained the jump lifts used by the contractors. The mobile machine room, which hosts the lift controls and hoisting machine, is jumped to the top of the most completed level at the time and can then serve the floors beneath it. Two of the hoists were also located in two lift shafts.

The internal climbing tower cranes 1 and 2, located in two lift shafts, were both Terex CTL 430-24 luffing jibs, with an overall mast height of 48 metres. Clarke says: “The mast of the crane never actually changed in height, it just climbed up within the lift shaft.”

Chocking frames allowed the internally climbing cranes to operate and jump up the structure by supporting the cranes in the lift shafts, providing horizontal restraint to limit sway or deflection of the crane. Once the jump form rig climbed up from the basement, Andun Engineering Consultants constructed the chocking frame underneath it. When the frame came within a certain distance of the ballast of the crane, the crane was released from its anchors, the load was transferred into the core walls, and the crane was pulled up into its first jump position. The load was then transferred back to the base of the crane. This process was repeated 12 times with tower crane 1 and seven times with tower crane 2.

Feighery says: “The core jumped, the cranes jumped and the lifts jumped. So we had three key elements of a project that we were all jumping in tandem. It’s very orchestrated, very planned, which is critical to getting the rig. The rig was on a six-day cycle; every three weeks we had to jump the cranes and then every eight weeks we had to jump the lifts.”

Janet Paul, senior project manager at Multiplex, says: “Everyone was chasing everybody. It’s a race to the top but it’s got to be slick and if something goes wrong, [the project] falls down like a domino effect.”

The external tower crane 3, a Terex CTL 282-18 with an overall mast height of 140 metres, seemed to be hovering parallel to the structure.

In fact, it was sitting on a cantilevered H frame, supported on just one side by the structure’s steel frame. This was an innovative space-saving solution on a construction site with such a tightly constrained footprint.

Laing O’Rourke subsidiary Explore Manufacturing made the ground-floor colonnade system, a unitised facade with precast mullions and precast spandrels made of concrete. Each mullion weighs roughly 8,500kg, and each spandrel spanning the columns weighs roughly 6,000kg.

To complete the exterior, Multiplex contracted Permasteelisa, which designed, engineered, manufactured and installed 27,100 square metres of facade. The facade for the street block from the ground floor to level 4 is glass reinforced concrete, the panels of which were made offsite and brought to site with the glazing units pre-installed. At levels 4 and 5, and from level 30 and above, vertical plant louvres resembling metal fins frame air-intake slots. Plant rooms are in basements 1 and 2, levels 4 and 5 and levels 30 and 31, with cooling towers on level 33. The entrances are formed of single, double and revolving doors. The facade from level 4 to 36 takes on the archetypal look of a modern tower. It is predominantly a unitised glass and aluminium system with the aluminium given a polyester powder-coated application. According to Permasteelisa, the structural silicone glazing is made of double-insulated glass.

Multiplex staff who worked nights to make up for time lost to high winds were kept company by a fox. The creature seeking shelter left paw prints in the concrete. Another unlikely mascot, like Old Tom, left its mark in the fabric of London’s latest landmark.