One of the final pieces has been dropped into place on the Olympic Park’s East Village masterplan.
Portlands Place, designed by Hawkins\Brown, comprises 524 private rental homes across two towers, of 26 and 30 storeys respectively, and two connecting 10-storey blocks. And “dropped into place” is pretty much accurate – the scheme has been built by Mace using a new approach to modular high-rise construction, based on the precision placing of precast-concrete floor slabs.
Mace has a track record in deploying innovative pre-assembly methods on high-rise – and you don’t have to stray too far from Portlands Place to see the evidence. Two blocks along the East Village, the contractor built Lifschutz Davidson Sandilands’ Victory Plaza residential towers using a “jump factory” – essentially applying the principles of jump-form core construction to a whole floor (see CQ 270, Winter 2019).
Portlands Place offers a variation on this theme, using a system that Mace calls High Rise Solutions (HRS). One of the key aspects of this approach is that the perimeter floor units are delivered with the facade attached, fixed to the bottom and supported with temporary props. This means that as soon as the units are lowered into place, the perimeter becomes a safe, enclosed working environment to complete the internal floor modules. These are fabricated complete with precast columns and include provision for other factory-assembled or hybrid elements, such as twin walls, lift shafts, vertical riser modules, and bathroom and kitchen pods. Stairwells are also precast and installed floor by floor, enabling access between working levels. Each new floor unit is lowered onto the previous level, using docking pins in the temporary propping to ensure millimetre-precise placement.
One of the advantages that Mace claims for HRS is that it can be adapted to an architect’s design, rather than requiring the design to be based on the constraints of the system. Hawkins\Brown had already taken Portlands Place to RIBA Stage 3 by the time Mace suggested HRS, and the contractor then worked with the architect, structural engineer Walsh and offsite specialist PCE to fit the system to the design. “It was a perfect trial project for Mace,” says Roland Reinardy, partner at Hawkins\Brown. “We had quite repetitive layouts in plan – although they are different on the lower and upper levels.”
The facade design too, lent itself to HRS, both in its “Jenga-like” articulation and in its use of lightweight materials. It appears to be more conventional precast concrete, but is actually a curtain wall with glass-reinforced concrete panels, cast with deep returns to create a sense of solidity. These 50mm-thick elements proved easier to fix and prop in place on the floor units prior to transportation, and easier to manoeuvre once on site.
The HRS system chimed with the digitised approach of Hawkins\Brown’s specialist design studio, which Reinardy co-leads. The development of a detailed BIM model meant that each element could be tracked from design to manufacture to delivery to positioning within the structure. The studio also wrote algorithmic scripts to generate the randomised colour patterns for the facade and for elements of the internal layout. “It was a bit of a 3D chess game,” says Reinardy, “working out which module to assign to which location, where to place sockets, and so on.”
On site, the buildings rose at the rate of a floor a week. Each 625m2 level consisted of 12 perimeter floor-facade units, 12 internal floor units and 22 other precast-concrete units comprising columns, twin walls, solid walls, stairs and landings. The assembled floorplates were stitched together with in-situ concrete, prefabricated reinforcement cages and high-strength structural grout. Concrete was also poured in situ to complete the twin-wall core. Towards the end of each week, the bathroom pods and utility cupboards were lowered into position and the network of 240 temporary props for the next level put in place. This entire process was carried out using just two cranes.
At the same time as the first ten floors of the towers were rising, the linking blocks were also being built, but with more a more traditional in-situ reinforced concrete structure. “It was quite fun watching them go up side by side,” says Reinardy. “It became a bit of a competition to see whose system was best.”
Beyond the tenth floor is a communal area, designed to be one of the biggest draws for PRS tenants, featuring lounges, a wellness studio, cinema room and dining rooms. This interconnected space flows from garden terraces on the roofs of the linking blocks to a double-height zone running through the towers via a skybridge. The showpiece structure required four concrete transfer beams, two of which were precast and two cast in situ due to their depth. The level 12 beam weighed in at a massive 70 tonnes, with an 8-tonne reinforcement cage tied into the HRS floor system. While this was being built, the main superstructure work was able to progress unhindered.
The fact that a prefabricated skybridge could be threaded diagonally between two towers is testament to the precision engineering of the modular system. This has a number of benefits on a high-rise scheme, points out Reinardy. “The whole superstructure went up to cladding tolerances,” he says. “Normally you might have a 20-40mm tolerance in your superstructure, and then you have to achieve 5-10mm for the cladding. So that's the real beauty of this system.” There were advantages for the services too, with the exact tolerances meaning that vertical risers could be slotted in with ease as the towers gained height.
Mace says the use of HRS led to 25% programme savings, a 40% reduction in site traffic, 75% less waste and 15% reduction in embodied carbon across the project. Reinardy is certainly impressed with the possibilities of the system from a design perspective. We like to go on these adventures,” he says. “It’s all part of democratising knowledge about delivery.”