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Timber structures: Loaded lumber

Britain’s tallest hybrid timber tower was recently topped out in east London.

Taking shape by a basin off the Regents Canal in Hackney, London, is the UK’s tallest “timber” building. Regal Homes’ 10 storey, £10M Banyan Wharf mixed use development is due to claim the record from the nearby nine-storey Graphite Apartments building in Murray Grove in April next year. It will also be one of the tallest timber towers in the world.

A complex hybrid steel/cross laminated timber (CLT) structure also makes it one of the most unusual. Architect Hawkins Brown project architect Alex Smith says the distinctive “twisted” design is more than mere architectural whimsy.

“Twisting half the floors of a basic cruciform shape towards the sun creates generous sunny terraces and means all flats have at least two aspects and some have three.

Banyan Wharf

Core and frame: The concrete core provides a strong spine for the composite frame

“So there’s more natural light and better views of the city and over the Wenlock Basin.”

Timber was always going to be the prime structural option, given the London Borough of Hackney’s enthusiasm for more sustainable buildings. But specialist timber design, supply and erect subcontractor B & KStructures specification manager Craig Liddell says that this project “didn’t lend itself obviously to CLT construction.”

He adds: “Although we’ve done curved and even circular CLT buildings, most CLT structures are basically rectangular “flat pack” designs, like the Graphite Apartments, with all walls and floors CLT. At Banyan Wharf the best option was a hybrid design.”

Concrete was chosen for the basement and ground floor (see box) and for the central service shaft, which provides all the necessary lateral stability. From the first floor upwards, CLT and steel form the intricate structure, where one major design challenge had to be addressed.

“Few elements line up vertically. We also had to look very carefully at the progressive collapse requirements”

Marcos Armas, Engenuiti

Specialist timber consultant Engenuiti senior engineer Marcos Armas explains.

“Figuring out sensible load paths wasn’t easy, especially around the perimeter.

Few elements line up vertically. We also had to look very carefully at the progressive collapse requirements.”

Steel fabrication was simplified by the choice of steel box sections for all internal columns and beams.

Columns are only 200mm by 200mm, remarkably slim for such a tall building. Around the perimeter, 400mm by 200mm universal beams and columns are used, mainly to carry the façade loads. Intumescent coatings provide the necessary fire resistance.

CLT wall and floor panels up to 200mm thick and weighing up to 4t were produced in Austria by Binderholz, Europe’s largest CLT manufacturer.

The panels arrive with all openings precut, and can be lifted straight into position, a boon on such a tight site.

Banyan Wharf

Banyan Wharf artists impression

One or two layers of plasterboard add fire resistance.

Unlike earlier CLT towers, however, at Banyan Wharf not all walls are CLT.

“A 100% CLT design would have been too heavy, and wasn’t necessary structurally,” explains Smith. “By using stud partitioning for some internal walls, we saved weight and gave future owners the option of reconfiguring their apartments in due course.”

Two thirds of the 50 units on the development are aimed at the upper end of the residential market, 17 are affordable.

All require the same very high standards, not least in the levels of acoustic insulation.

Some earlier timber framed developments have been criticised for being poorly insulated, but Smith says the real problem is the lack of relevant research in this area. “We specified an insulation layer under the floor, but there was really only one way to be sure we had the overall acoustic design right. And that was to build a test flat on the ground floor before we finalised the details.”

This provided the necessary reassurance. “It turned out that if we complied with the fire regulations we would also meet our acoustic requirements,” Smith reports.

On such a constricted site, with buildings close at hand on both sides, the speed of erection of the hybrid frame minimised disturbance. Another benefit was the significantly lower noise and dust generated.

Cladding on the CLT elevations away from the Wenlock Road façade will be slatted western red cedar. A dark brick screen will face the road, endowing the “much larger than normal” terraces behind with an enhanced sense of privacy and security.

“It will also help the building blend into the streetscape,” Smith adds.

Banyan Wharf’s overall sustainability is boosted by 24 photovoltaic arrays on the roof. There is also a section of “brown roof” - basically an area of plants above a waterproof membrane which differs from a “green roof” in that it has an effective substrate for the plants to root into and flourish.

Growing popularity

Cross laminated timber (CLT) is an increasingly popular structural option, particularly for schools, retail outlets and apartment blocks.

Its speed of erection and the “solidity” of the finished structure have contributed to this popularity, but its real selling point has been the perception that it offers a greener, more sustainable alternative to traditional materials.

There are those who question this perception, pointing to the need to transport the CLT panels from mainland Europe. Lobbyists for the concrete industry also claim that the significantly higher thermal mass of a concrete building gives it the edge when it comes to regulating internal temperatures, thus saving energy over the long term.

Some recent buildings, however, such as the Julian Study Centre at the University of East Anglia, combine CLT with precast concrete floors, to obtain the best of both worlds.

CLT production requires significant investment and access to high quality spruce, larch or pine trees. Panels are built up from individual boards ranging from 20mm to 80mm thick and up to 240mm wide stacked in three, five or seven layers with sequential layers orientated and glued orthogonally.

Panel size is limited more by limitations on transport logistics, with 3m wide by 13.5m long being the usual maximum.

CLTs accuracy of manufacture means establishing airtightness is relatively easy.

Fixing items to the panels is also much quicker and quieter, without the health risks associated with the silica dust generated by concrete drilling.


Basement challenge

Banyan Wharf’s single storey basement will provide car parking and office space, and will have two light wells.

Designing a basement so close to a canal basin was always going to present a significant challenge, but lead structural engineer Pringuer-James Consulting Engineers (PJCE) project manager John Lange says there was a particular problem on this constricted site.

“We discovered that the site included reclaimed land where, sometime in the past, part of the canal basin had been filled in. So the basement footprint actually extended beyond the original canal bank, and the water level was only 500mm below ground level.”

PJCE’s solution was to install a secant piled wall to act as a temporary water barrier. This allowed the basement excavation to be dewatered and a concrete raft foundation to be installed.

Everywhere except in occupied areas, an internal concrete wall inside the secant piled wall provides water resistance. For the office area a drained cavity system was selected.

A 500mm thick concrete slab at first floor level acts as a transfer deck, supporting the vertical loads from the hybrid structure above and allowing a change of grid to something more conventional, to suit the office and parking space below.

Lange says the choice of a CLT/steel hybrid main structure significantly reduced vertical loads, resulting in savings throughout the concrete frame and particularly in the foundations.

“We also designed the central concrete core, which carries all lateral loads, including wind. The hybrid frame simply bolts to it.”

Such has been the success of the Banyan Wharf approach that PJCE is already working on a similar, but significantly larger design for the same developer, again in Hackney.

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