Offsite construction, technical innovations and BIM were all vital in constructing the Leadenhall Building, but front-end planning at the beginning of the project was just as important.
Project: Leadenhall Building
Client: British Land and Oxford Properties
Main contractor: Laing O’Rourke
Architect: Rogers Stirk Harbour + Partners
Structural engineer: Arup
Steelwork subcontractor: Severfield
Cladding and glazing subcontractor: Yuanda
Engineer: Crown House Technologies
Start date: September 2011
Completion date: June 2014
The assortment of high-rise buildings springing up across London have a variety of unusual shapes: the Shard, the Walkie Talkie and the Scalpel are all interesting and unusual additions to the capital’s skyline.
But the unique shape of the Leadenhall Building, more commonly known as the Cheesegrater, is down to more than striking architecture.
It was designed by architects Rogers Stirk Harbour + Partners to fit in with the surrounding area and in particular not obstruct the view of St Paul’s from the historic Cheshire Cheese pub on Fleet Street.
Architect Graham Stirk designed the building with a sloping south face so as not to block this view, which gives the building its unique shape and its nickname.
It also provided a series of technical and engineering challenges for main contractor Laing O’Rourke, not least the extremely confined site and the ever-decreasing floor space as the levels increase.
The 53-storey, 224 m-high tower includes 41 floors of office space, restaurants, a ground-level seven-storey landscaped public gallery and seven floors of plant rooms at the top of the building.
The building is a joint venture between British Land and Oxford Properties.
To maximise internal space, the engineering team designed the building without an internal concrete core, opting instead for an external steel frame to provide structural stability.
“It’s a pretty unique building,” says British Land construction executive Matthew White. “There is not the central concrete core that most towers have. The mega-frame provides the lateral stability.”
“The architects like to express how well the building works structurally, mechanically and electrically, so it’s a very transparent building”
Matthew White, British Land
This design also led to the double-skin cladding system. “The main structural stability for the Leadenhall Building is provided by the exoskeletal steel structural frame,” says Laing O’Rourke section engineer Natasha Eversley-Robertson.
“The client wanted an exposed finish, which is why you have the internal and external cladding system.”
Every seven levels, beginning at level five, is what the team call a mega storey, where the steel components are connected by mega nodes. “Everything on this structure is mega,” Ms Eversley-Robertson says.
“Internally we have a series of columns, which is the strongbox of the structure. That part was built up before the outside frame. The internal beams were connected to the internal columns in different zoned areas.”
The structure was designed to showcase the internal working of the building – a trademark of the architects – including the exposed yellow steel.
“It’s a Rogers building, Graham Stirk was the lead architect on it, and they like to express how well the building works structurally, mechanically and electrically, so it’s a very transparent building,” Mr White says.
“You see all of the structure and the lifts; it’s part of the design to use the primary colours and to show the internal works of the lifts and the escalators.”
The design provided another unique engineering challenge: due to the shape of the structure, the dead load and the settlement, the building had a natural lean to the north.
Instead of compensating for this in the design and hoping it would end up even, Laing O’Rourke instead used a process called active alignment.
“We had to monitor the position of the structure from the start,” Ms Eversley-Robertson explains.
“If we hadn’t used active alignment, the building would have been 160 mm to the north”
Natasha Eversley-Robertson, Laing O’Rourke
“We couldn’t do active alignment until we got to a certain part of the build because of the loading requirements, and it had to be stressed at different points as well.”
The process involved adjusting the lean of the building at every mega level by either removing or adding in packing pieces where the nodes meet the columns.
“At each stage we monitored it and decided how much to add or remove based on where the structure was,” Ms Eversley-Robertson says.
Using this method meant the team knew exactly how much movement they needed to compensate for and exactly what stage the structure was at during any phase of construction.
“If we hadn’t used active alignment, the building would have been 160 mm to the north,” she says.
The Leadenhall Building in numbers
- 224 m high
- 85 per cent of the building manufactured offsite
- 18,000 tonnes of steel
- 300 miles of cables
- 97 per cent of construction waste diverted from landfill, to date
- 7,191 precast planks
- 8m/s is how fast the passenger lifts can travel
- 410 tonnes of plant housed in the attic
- 141 table-shaped components, each weighing 35 tonnes, in the north core
- 4 cranes weighing up to 250 tonnes each
- 45 tonnes – weight of the largest steel components
- 610,000 sq ft of lettable floor space over 46 floors
- 26 m – length of the longest steel components – longer than Wimbledon’s Centre Court
It was pre-tender innovations such as this from Laing O’Rourke that impressed the client at tender stage.
“The biggest thing Laing O’Rourke brought to the table was the ability to face those structural engineering challenges and resolve them,” Mr White says.
“The whole building had to be built within the footprint rather than having space around and adjacent to the building, so it was a huge logistical challenge”
Matthew White, British Land
“They were excellent at dealing with the issues as they arose. The active alignment was one of the ideas they came up with.
“They did a lot of due diligence pre-tender to satisfy themselves that the method they were going to propose would work.
“They used external engineers SOM to do the views at pre-tender stage, so they covered a lot of ground in a short space of time in terms of due diligence and the technical verifying of the existing design.”
Aside from the active alignment, the client was also impressed with Laing O’Rourke’s solution to an extremely tight site and ever decreasing floor space; the building is 85 per cent manufactured offsite.
“Given where the site is, it’s extremely confined and the building fills the whole ownership footprint of the site,” Mr White says.
“Therefore the whole building had to be built within the footprint rather than having space around and adjacent to the building, so it was a huge logistical challenge to build that scale of building in that position.”
“Because it’s such a small space, one of the biggest challenges was getting these huge pieces of kit more than 200 m in the air”
Nick Jarman, Laing O’Rourke
Laing O’Rourke used its offsite manufacturing company Crown House Technologies and design for manufacture and assembly techniques to deliver the building.
Prefabricating the majority of the building offsite means that components arrive and are fitted into place which saves time, decreases the workers needed, reduces working at height, increases safety and adds a level of programme certainty.
This level of offsite prefabrication is unprecedented on a commercial building of this size. One area where offsite manufacturing was particularly important in the Leadenhall Building was the attic, where the plant is housed.
The building’s unique design sees the floor area decrease as the building rises, meaning the top floors were too small for usable office space and so house the building’s plant.
“Because it’s such a small space, one of the biggest challenges was getting these huge pieces of kit more than 200 m in the air,” says Laing O’Rourke project engineer Nick Jarman.
“The build sequence on this project was slightly different too: typically you build a structure, bring the plant in, then clad. But in this case we had to clad the façade first before bringing the rest in.
“We only had a couple of options for bringing the plant in, so we went for a letterbox in the top of the building we could lower the plant through.”
The team left a hole in the top of the structure to allow the plant to be craned in, before being skated into place.
“It’s so small up here you couldn’t do this off 2D plans; you wouldn’t see everything, so it had to be done in 3D”
Nick Jarman, Laing O’Rourke
“In some instances we had only 100 mm underneath the structure to slide some of the instillations in, so the measurements had to be exact,” Mr Jarman says.
The larger plant was split into sections before being brought up and then reassembled in-situ. “The biggest piece that came in at once was the engine, which is just less than 20 tonnes,” Mr Jarman explains.
“The enclosure was split down into five sections, so we’d land each then have a team of workers to slide them into place.”
The team also did extensive testing of the components and some of the processes at offsite facilities before bringing them to site.
BIM to the fore
Although it sounds like a simple solution to make components offsite then slot them into place, it requires a huge amount of work upfront with the design team.
Building information modelling was vital in the successful delivery of the offsite components on this project and particularly the attic section of the build.
“We started space planning the attic in November 2011 and it was about 18 months in planning and design,” Mr Jarman says.
“We knew the size and weight of all the elements within millimetres. When we started planning the attic instillations we knew the model was going to be vital.
“It was all that pre-planning that paid off when they came to put it together onsite; you can’t afford for it not to when you’re 50 storeys in the air”
Matthew White, British Land
“It’s so small up here you couldn’t do this off 2D plans; you wouldn’t see everything, so it had to be done in 3D.”
Construction on the lower levels of the building began in 2011, but as the structure didn’t top out until June 2013, it allowed the team working on the attic 18 months of planning time to get the sequencing correct for bringing in the plant.
“The level of planning was the secret to getting the attic right,” says Laing O’Rourke project leader Paul Kennard. “The attic was part of the critical path activity, so work had to go like clockwork otherwise everything we’d done previously would have been wasted.”
Early subcontractor engagement
This not only meant getting the main contractor on board early, but most of the subcontractors too.
“A big part of the planning stage was early engagement with subcontractors, making sure we had their buy in as well,” Mr Jarman says.
“It’s not every day you’re going to be up here installing plant, so it was important they understood what it was going to be like, what would be expected of them and that we got their feedback.”
Using BIM meant that before any of the team set foot onsite, they had built and installed the plant many times before. “I tried to build this in my head a hundred times,” Mr Jarman says.
The attic installation went as planned and caused no delays to the programme.
“It was all that pre-planning that paid off when they came to put it together onsite; you can’t afford for it not to when you’re 50 storeys in the air and in a small footprint,” Mr White says.
“We couldn’t have done it without BIM,” Mr Jarman adds.
Technical innovations, BIM and offsite manufacturing were all crucial in successfully constructing the building, but for Mr Kennard successful delivery began much earlier than that.
“The secret for us on the Leadenhall Building was making sure we looked at it and its design differently, and with that come up with new initiatives on how we construct it,” he says.
Laing O’Rourke started on site in September 2011, but the project had been at various stages of design since 2006.
However, construction couldn’t begin until after the demolition of the existing building on the site, which was similar in design to the Aviva building that stands next door.
“It was about 16 storeys high and the first job was to demolish that,” says British Land construction executive Matthew White.
“One of the challenges was that the floors were suspended from a post-tensioned concrete beam system at roof level, so we demolished the building from the bottom up rather than the top down.”
This approach allowed the project team to begin piling underneath while the demolition was still going on.
“We started to procure the various packages of works up to 2007/08 when the financial downturn came along and the decision was to put the project on hold,” Mr White recalls.
It was picked up again in 2010, and at that stage the basement works were finished. While these were being completed, the development company procured Laing O’Rourke for the main build contract; these works started in September 2011.
“They’re due to deliver by mid June 2014 and they’re very much on track for that,” Mr White says. “We also hand over to various tenants at that date for their fit out to commence.”
More than half of the building has already been let to companies Aon, Amlin and Servcorp.