Contractor is harnessing technology backed by intricate planning to deliver a towering mixed-use scheme and avoid any ‘nightmares’ cropping up on site.
Project: One Crown Place
Contract value: £240m
Contract type: Major Project Construction Contract
Main contractor: Mace
Client project manager: CBRE
Concrete frame contractor: Getjar
Steel contractor: Severfield NI
Start date: April 2018
Completion date: Q1 2021
Extensive digital design work during Mace’s longest ever preconstruction services agreement has helped smooth out a complex residential and commercial scheme on the edge of the City.
The contractor was brought in to work on One Crown Place back in May 2016, but only started on site this spring after almost two years of detailed and state-of-the-art planning and preparation.
Backed by Malaysian developer AlloyMTD, the project will create 235 apartments across two towers of 30 and 34 storeys. These will include a shared seven-storey podium that will be mainly used for office space, with shops at street level and plant buried in a double basement.
Creating a concrete frame for the residential towers above an open-plan office podium has created a major engineering challenge for the contractor, compounded by the task of getting materials in and out of the highly constrained site as demanded by the programme. Technology has been a fundamental part of the solution.
“This is my first project working at BIM Level 2 and I’m seriously considering not doing anything without it now,” says Mace senior design manager Gareth Berridge. “I look back at projects from the past and realise how difficult it was. This is a watershed moment for me.”
BIM Level 2 requires all major parties on a project to share 3D models using a common data environment with the aim of reducing risk and improving useful collaboration.
One Crown Place Mace 7
Source: Agnese Sanvito
On One Crown Place, it allowed Mace to design an intricate steel truss load-transfer system, capable of supporting the weight of the concrete residential towers while leaving the office podium below column-free.
These huge steelwork trusses weigh a total of 650 tonnes and sit from levels seven to nine of the scheme, taking the load across to columns at the edge of the steel podium frame and down onto the piles below. Even with elements of the trusses to be left exposed as an aesthetic feature within some of the apartments, their location was extremely tricky to get right.
“It’s a headache in terms of services design,” Mr Berridge says. “Trusses are set inside party walls and we also have to ensure apartment doors don’t clash with them.”
Tech takes over
Mace didn’t stop at shared BIM models when it came to the use of digital tools to support delivery.
“We used BIM 360 Field, so guys on site have iPads and record quality and progress digitally,” Mr Berridge says. “We also use Disperse software that uses artificial intelligence and computer vision to take 360-degree photos of individual apartments each week and convert information from these into Excel-style progress reports.”
Mr Hutton says the technology is “very, very impressive”.
“It shows me where we actually are on each apartment rather than someone saying, ‘Oh, it’s nearly finished’. It lets me see trends and focus on problems and fix them.”
Drones have also been sent above the site to produce a point cloud. “This allows you to check spatial planning from a one-day survey,” Mr Berridge says. “We can check whether a mobile crane can fit through a certain space between existing buildings. As a bonus, CBRE was able to take panoramic imagery from apartment level to use in marketing.”
Working through the different permutations for this and other elements of the design digitally prevented major problems on site. But this was itself no simple process.
“We employed a model manager [firm] called Ibsecad, based just down the road in Barbican, and they checked co-ordination of consultants’ models on a fortnightly basis throughout the preconstruction services agreement period,” Mr Berridge says. “We had long meetings going through clashes and agreeing solutions throughout that period.”
There were many added benefits of the painstaking shared-model BIM work. “We were able to reduce the risk margin we needed to build into our price once we had certainty that the design was co-ordinated across consultants,” Mr Berridge explains. “We could verify everything in 3D in advance so didn’t need to account for reworks on site.”
The contractor started with a model handed to it by the client’s project manager CBRE. “We built on what the client gave us and modelled the tower cranes, their sway zones, the gantries, the hoists, the hoarding, props and so on to ensure temporary works were co-ordinated with permanent design.”
Depth of detail
The level of detail analysed before a pair of boots set foot on site included checking that specific hand tools would fit into gaps to tighten bolts.
A more obvious challenge was that the lift and stair cores for each tower had to be built to a point above the level of the ground-floor slab, before that slab could be poured. This necessitated the careful design of props to retain the secant basement wall so that the core could pass through, thereby allowing the slab to be cast later.
“We have found lots of things in the model that would have been a nightmare to discover on site”
Gareth Berridge, Mace
Once cast, operatives left holes in the ground-floor slab to allow for the removal of props. “We have found lots of things in the model that would have been a nightmare to discover on site,” Mr Berridge says.
When work did eventually begin for real in April this year, the contractor had a tight and highly refined programme to canter through.
With a previous contractor having flattened the site and created the basement box, Mace could then carry out further excavation for drainage before adding the pile caps and creating the lowest basement slab.
“We had a very large pile cap underneath the two towers,” says Mace construction manager Ian Moores. “It was 1,800 mm deep; about 1,000 cu m of concrete. It took three weeks to fix reinforcement and then we poured it in one go. After that we went round the rest of the basement putting in the rest of the B2 slab.”
one crown plaza divide
Source: Agnese Sanvito
The ground-floor slab came after B2, to allow the super-structure to begin as soon as possible. But missing out the intermediary B1 slab at this stage brought further challenges.
“The height you are then working at for the ground floor is about 12 m, so there was a lot of falsework – it was a bit of a forest,” says project director Keith Hutton.
After six concrete pours for the ground-floor slab, the B1 level could be tackled, which meant taking down temporary frameworks for the concrete that were already up in the basement and starting again. “You strike the falsework and rebuild it,” Mr Moores says. “The whole driver was to get the steelworks started as early as possible, so we had a challenging programme.”
All nine 600 mm-diameter tubular steel props came out after the B1 floor was cast. “We supported them on falsework, accessed them with a scissor lift and cut them using oxy-propane tools,” Mr Moores says.
Slipform was chosen for speed of concrete core construction, targeting a vertical rise of 2.4 m per day, a rate described by Mr Moores as “challenging”.
Once the cores reached the 12th floor, they became thinner and had also moved off the critical path, so a jumpform method was used for the remainder.
Steelwork started taking place at ground level in mid-August, with a hard deadline of getting the office podium frame to the point where the transfer trusses could be installed as planned two months later. This was critical to release construction of the concrete-framed residential towers above.
With no area for storing materials on the tight City of London site, the trusses were booked to start arriving in October in two deliveries per day containing various five-tonne sections. These had come straight from where they were manufactured at steel contractor Severfield’s facility in Northern Ireland.
“Logistics needs to be tightly managed,” Mr Hutton says. “The right piece needs to turn up at the right time and be fixed in a timely manner.”
One Crown Place Mace 9
Source: Agnese Sanvito
Cranes bring the steel elements into place where site workers bolt them together to form the mammoth trusses. This work will continue for most of the remainder of this year.
Meanwhile stairs are going into the cores, blockwork has started in the basement and internal fire protection is under way. External cladding will begin in early 2019, with panels of extruded terracotta arriving from Holland and being hoisted to the second floor, before a spider crane manoeuvres them to workers inside the building.
“This method is for safety and to avoid using tower cranes, as ours will be busy with the concrete structure and there won’t be space on site for any more,” Mr Hutton explains. “Again the logistical operation is massive, things have to arrive in the right sequence at the right time.”
Fit-out will begin at ground level in spring 2019, with the concrete frame scheduled to complete by the end of that year. There will be 600 people on site at peak from the third quarter of 2019 busily implementing the slick plans drawn up using the BIM model.
Prefabricated bathroom pods will come in from Italy and be manoeuvred in to the right floors before cladding is installed, then plumbed in afterwards.
As Mr Hutton says: “It’s all about sequence.”
“Logistics is one of the challenges of the project,” Mr Hutton says, and it’s not hard to see why.
Basing tower cranes to lift elements in and out of both the basement, and later on the emerging building, was a big puzzle for the site team.
Three luffing jib tower cranes – each of which will rise to up to 100 m, with a lifting capacity of about 12 tonnes – are needed to bring rebar into the basement slabs right at the start and will continue to be used on the main structure long into the job.
But there is no suitable place for them outside the building footprint and putting them within it could cause an obstruction to permanent works.
The solution was to support the cranes on temporary steel gantries, which will themselves be converted to become reinforcement for permanent concrete columns.