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World's biggest CLT structure relies on collaboration

The world’s largest CLT building both by volume and height is under construction in east London and has required close collaboration between client, engineer, contractor and architect to get it right and hit the tight programme required.

Project: Dalston Lane
Location: Hackney, London
Client: Regal Homes
CLT engineer: Ramboll
Timber erector: B&K Structures
Architect: Waugh Thistleton
Concrete designer / civils: Pringuer-James Consulting Engineers

Most successful projects these days rely on close collaboration between the different parts of the supply chain.

But sometimes, when the boundaries are being pushed and contractors are innovating, this collaboration has to happen much earlier and last much longer than normal.

It has to be genuine, not just simply paid lip service, for it to be effective.

One such project is a new residential development is Dalston Lane in Hackney, east London.

The scheme is a 12,500 sq m 122-unit Regal Homes development, with more than 3,460 sq m of retail and commercial space also included.

On the face of it, this may not sound too unusual. But when you take into account the fact that the project is being constructed almost entirely out of timber, the need to collaborate becomes clear.

Choosing CLT

The use of cross-laminated timber in construction has been on the rise, with more clients and contractors turning to it for its speed of erection and sustainability benefits.

Hackney Borough Council became the first local authority in England to throw its weight behind timber as a construction material when it proposed its ‘wood first’ policy in 2012.

While the authority doesn’t specify timber first, it has been keen to use it where possible and this was one of the factors behind choosing to build the Dalston Lane scheme in CLT.

Ramboll is the project’s CLT engineer, working closely with architect Waugh Thistleton, timber subcontractor B&K Structures and client Regal Homes.

The engineering consultant has extensive experience in using and designing CLT in buildings, working on projects such as the four-storey £20m William Perkin High School in London, which contained 3,800 cu m of timber, making it the largest CLT project by volume at the time.

“We started working on Dalston Lane a year-and-a-half ago, at the start of 2014,” says Ramboll director Gavin White.

“The client was looking at whether they could do this in CLT, so we did a lot of work in the early stages to see if it was feasible – to see if it was the right type of structure.”

As Mr White explains, there are a number of things to take into account when deciding whether CLT was the right method of construction.

“You’ve got to have your load-bearing walls stacking all the way up, and you’ve got to get your spans at about 4-5 m, so it works really well for residential use”

Gavin White, Ramboll

“There are a few key things that really make it economical – you’ve got to have your load-bearing walls stacking all the way up, and you’ve got to get your spans at about 4-5 m, so it works really well for residential use,” he says.

“And you have to try to get the top of one CLT slab to the underside of the next to be 2.95 m or less, as then you can use one long panel like this rather than lots of individual panels.”

The largest panels in the development are 13 m long by 2.8 m high. “It is possible for the panels to be up to 3.5 m wide but makes it cost twice as much for transport,” explains B&K Structures pre-construction manager Greg Cooper.

Ramboll had worked with client, contractor and architect before on different projects, so brought in the other parties to work on the design as early as possible.

“The supply chain instantly knows what we want,” Mr White says. “We worked with B&K’s designers and the architects to make sure the detailing was all as it needed to be.

“With CLT, it has to be economical to build. As an industry, we’re learning all the time as the use of CLT is going up and up, and there’s more people using it. It’s understanding these nuances that help you make the most of it.”

Rapid erection

When Construction News visits the site, the team is still erecting the first of two residential towers, with commercial office and retail space to connect the buildings.

The team has reached six storeys, with the second, taller tower to be 10 storeys and 33.8 m high – beating the current record holder of Forte in Melbourne, of 32.2 m and making Dalston Lane the tallest pure CLT building in the world, according to Ramboll.

“In the time they’ve taken to build two floors of the concrete podium next door, they’ve built all six storeys here”

Gavin White, Ramboll

The programme is a quick one, with B&K Structures set to be finished with the first tower by the end of this year – a total erection time of just 12 weeks. The entire development will be complete in 2016.

“This is one of the main reasons we are seeing people go for CLT – it’s a very fast programme,” Mr White says.

The team did face a six-week delay at the start of the build due to a crane shortage, using a mobile crane initially, but have been able to get back on track thanks to the speed of erection.

“In the time they’ve taken to build two floors of the concrete podium next door, they’ve built all six storeys here. It makes it a lot faster for the follow-on trades as well,” Mr White says.

Foundations of success

Another factor behind the use of CLT lies below ground, in the building’s very foundations.

Underneath the development is a tunnel that carries High Speed 1, as well as another protected tunnel route for Crossrail 2 – which meant the team faced restrictions over how much weight it could build above ground.

The team looked at the various options for the structure, including concrete and steel, and worked out that with timber, it could build 121 units and leave room to add 20 flats on top in future (although there is currently no planning permission for this yet).

“We’ve pre-engineered the structure and the groundworks, and given the loads to HS1 and Crossrail, so they could build an extra 20 flats in future,” Mr White says.

“To get to the same weight [in concrete], we’d have to have 15 fewer flats than we do now. From the client’s point of view he gets extra flats, and potentially even more in future”

Gavin White, Ramboll

“To get to the same weight [in concrete], we’d have to have 15 fewer flats than we do now. From the client’s point of view he gets extra flats, and potentially even more in future.”

Mr Cooper adds: “In total, the building contains only 7 tonnes of steel – it’s not got much steel in it at all.”

The walls, floors, lift shafts, risers and staircases are all built out of timber, with the steel beams only helping to support the structure as it steps back while going upwards.

“That just means the detailing’s simpler, the erection’s simpler, and we don’t have beams blocking vent paths for M&E, those kind of things,” Mr White says.

Stable speed

One of the clever pieces of engineering that has enabled the team to build so high in CLT has been the development of the building’s stability walls.

“We can’t have a column underneath every wall as that would be uneconomical and make the retail space at the bottom unusable,” Mr White says.

“So we’ve got quite a thick concrete slab with some discreet column locations, and then we’ve picked certain walls to be stability walls.”

The stability walls are essentially the same as the building’s standard walls – either 100, 120, or 140 mm thick – but what sets them apart are the connections to the floors above and below.

“Normally we just connect with a 100 mm angle bracket,” Mr White says. “Where we have stability walls, we have a thicker bracket with a gusset plate, a bolt through it, and a stiffener inside.

“As the wind blows, you get pull up one side of the wall and push down on the other side. We’re trying to resist the pull up by taking the force out of the wall, take it through the bolt and into the wall below, which stops it prying up.”

Dalston Lane in numbers

  • 10,850: Sq m of residential space
  • 4,400: Cu m of timber used, which the team claims is the most of any development in the world
  • 3,450: Sq m of non-residential space
  • 2,400: Estimated amount of carbon saved in tonnes compared with an equivalent concrete-framed building
  • 33.8: Height in metres – tallest pure CLT building in the world
  • 20: Extra flats that could be added to the structure in future
  • 2: Railway tunnels, existing and proposed, running underneath the site

The connection’s stiffener element then stops the floor from crushing as it pushes down on the other side.

“We looked at doing that as a hardwood dowel or a sawtooth taking some of the wall down below,” he says. “But in the end we actually routed out a pocket and put in non-shrink grout, which transfers the vertical load straight through the grout and into the vertical panel.”

Into the void

Close collaboration over the building’s design is also evident in the detailing as we walk around the site.

Voids have been left on each floor for M&E services and vents, while small detailing has also been cut out for steel balcony connections, some of which will be cantilevered.

“These are steel brackets that will come along and be screwed into place,” Mr White says.

“That took a lot of co-ordination, as when you put the bracket on it bends the slab and you have to look at how it interacts with the structure, and it’s got a thermal break on it too.

“One of the key things we tried to do was stick to pure use of CLT – not out of an esoteric point of view, but just to get the efficiencies we needed”

Gavin White, Ramboll

The team carried out various load testing on the designs, with variable loads to represent different numbers of people and items standing on the balconies, to ensure they would be strong and stable.

These voids and details were all carefully co-ordinated in advance, so that the panels could be pre-cut in the factory and just slotted into place for maximum efficiency.

“One of the key things we tried to do was stick to pure use of CLT – not out of an esoteric point of view, but just to get the efficiencies we needed,” Mr White says.

“If you start trying to put too much glulam or steel in, you lose that speed of erection.”

With 4,400 cu m of timber to be used in total, Dalston Lane will be comfortably the largest CLT project by volume in the world, as well as the tallest of its type.

And with any short programme requiring careful panel design and fast erection, it has been vital for all parts of the supply chain – client, architect, contractors, engineer, and timber supplier – to work closely together to keep the project on track.

Moving façade

One area of complication was a local planning and client requirement to have brickwork for the building’s external façade.

“It isn’t ideal for a CLT frame as we’ve got a nice, lightweight frame and you’re putting heavy solid brickwork on the outside,” Mr White says.

The extra weight caused concern that the façade might move too much over time.

“We worked out that we would see a drop in the roof of around 60 mm over the lifetime of the building,” Mr White says.

“Obviously when you’ve got brickwork moving, and timber expanding while it’s coming down, the detailing around windows is not going to work properly and survive in the long term.

“So we did a grout pocket on the external façade as well as on the stability walls so we could limit that movement.

“We got it down from 60 mm to about 10 mm movement over the lifetime of the building.

“The client is used to concrete frame – and that’s not dissimilar to what you’d get with a concrete frame, so they could understand that.”

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