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THE RISE OF CLT

Although CLT has been around as a building material for over two decades, it is still a relative newcomer when compared to steel and concrete. Michelle Gordon talks to Gavin White, structural engineering director at Ramboll, who has long been a pioneer of CLT, about pushing boundaries and bringing it into the mainstream

Timber as a building material is nothing new, in fact it is one of oldest known to man, but in modern day construction it has all too often been cast aside in favour of steel and concrete, with a widespread lack of understanding over what can be achieved.

However, attitudes have started to change in recent years and mass timber products such as cross-laminated timber (CLT) have been used in the construction of several high-profile buildings, showcasing its versatility.

Murray Grove, a nine-storey tower block in Hackney, London, designed by Waugh Thistleton Architects, was the first tall urban housing project to be constructed entirely from pre-fabricated solid timber, back in 2009.

Last year Dalston Works, a 10-storey, 121-unit building, also in Hackney became the world’s largest CLT building.

The landmark project was designed by engineering consultancy Ramboll and Waugh Thistleton Architects on behalf of developer Regal London.

Ramboll has a history of building with CLT and projects in its portfolio include Open Academy in Norwich – the largest cross-laminated timber panel structure in the country on completion – and William Perkin High School in London – a four-storey complex, containing 3,800 cubic metres of timber.

It is currently working on a building in East Ham which is going to be similar in size to Dalston Works and is engineering volumetric CLT modules for Swan Housing Group, which are being used in two-storey and low-rise housing.

Structural engineering director Gavin White leads a team of 35 structural and mechanical and electrical engineers at the company’s Cambridge office, where 30-40% of the projects are timber related and mainly CLT.

He has long been a pioneer of timber having worked with the material since the early days of his career and has worked with CLT for some time.

It was his interest in sustainability which led him to learn more about the material at a time when Ramboll was just starting work on some pioneering projects but with timber not being covered during his time at university he did all of his learning on the job.

“When I did my university degree I didn’t do any timber design and then when I joined Ramboll I did a couple of years of doing steel and concrete and then moved into the timber side,” explained White. “I really learnt the timber design aspect on the job from colleagues and other people and courses.

“We don’t teach enough timber engineering in universities and we need to be teaching people more about it, so they can at least come out of university with an appreciation of it.

“Most engineers are quite happy to work with timber. It is a natural material and is more interesting than some of the other materials to learn about, so it’s a shame that we don’t do more but that is something that we can hopefully change over time and get more done.”

While CLT has got some way to go before we are seeing its use on every street corner, progress is being made in the battle for hearts and minds and schemes such as Dalston Works are getting people talking and highlighting what can be achieved.

“The key thing is trying to have good example projects of where CLT is being used to provide solutions for the client in a cost effective and programme effective manner,” said White.

The benefits of building with CLT are plentiful including a site that is cleaner and safer for those working on it, as well as quieter for nearby residents. There is also less waste on site and minimal disruption to residents due to fewer deliveries. During the construction of Dalston Lane there were just 90 truck deliveries to site, whereas a comparable building in concrete would have required 800 deliveries.

Buildings are much more airtight, therefore using less heat and CLT is lightweight, with buildings tending to weigh 30% less than they would if constructed from concrete.

Speed of build is another huge advantage and typically, CLT projects are around 10-15% quicker programme-wise than steel or concrete, reducing time on site, therefore bringing down costs.

There are benefits to residents too with CLT buildings being much more airtight and therefore not requiring as much heat.

“If we can educate everyone about the benefits of it and then at least people can make an informed choice. That is key for me,” said White.

“Some projects CLT is just not suitable for and that is fine but as long as people are exploring it and testing the viability of it then we will get more projects built from it and people will see it and it will add to the number of projects that are being built in CLT.”

There are challenges, of course, when building with CLT, mainly centred around a lack of research and guidance but these should also be viewed as opportunities said White.

“There are different challenges every day and there isn’t as much guidance and research done on CLT, so if you are designing in steel most of the things that you will do every day have been done before and then there is a design guide or something that tells you how to do it and it is a case of working through that,” he explained.

“With CLT you have generally got a basic design, anything beyond that, you tend to have to have more lateral thinking and do a bit of research and testing.”

While CLT has been around for 20 years or so it is still relatively new in terms of research and developments are constantly being made.

“We are still learning every day in terms of how we can push the material, how we understand the material’s properties better, what we can do with it, how we can engineer it, how we can be more efficient, how we can create longer spans etc,” explained White

People’s preconceptions about timber around areas such as acoustics, fire and durability are also a challenge.

“Challenges happen in nearly every project, but it is something we need to work through and the more we can engage with the industry and the wider industry and clients, developers and contractors and educate them on the benefit of CLT construction then the better,” said White.

There has been growing interest in CLT from architects, clients and development colleagues in recent months he said, and the material is becoming more prevalent in the market although there is some way to go before it can be classed as mainstream.

CASE STUDY: DALSTON WORKS (PREVIOUSLY KNOWN AS DALSTON LANE)

  • Client: Regal London
  • Architect: Waugh Thistleton Architects
  • Structural engineers: Ramboll (CLT), PJCE (Concrete)
  • M+E: XCO2
  • Contractor: Regal London
  • Developer: Regal London
  • Timber supplier: B&K Structures

Designed by engineering consultancy Ramboll and Waugh Thistleton Architects on behalf of developer Regal London, Dalston Works is a 10-storey building in Hackney, London.

It comprises 121 apartments for rent alongside two ground level courtyards flanked by 1500m2 of retail and restaurant space.

Its sustainability credentials are impressive with a reduction of approximately 50% in the embodied carbon of the structure compared to a traditional concrete frame building. The timber also acts as carbon storage with over 2,600 tonnes of C02 locked into the material, effectively making it carbon negative for the first years of usage.

“Dalston Works reveals the future of low carbon construction,” said Andrew Waugh, of Waugh Thistleton Architects. Our building offers an exemplar solution to a demand which will only increase: the construction of high density, affordable and environmentally sustainable homes.

“Dalston Works demonstrates the possibilities of engineered, cross-laminated timber as a cost efficient, and desperately necessary, viable alternative to the polluting technologies of concrete and steel. The collaborative effort between our progressive client and passionately innovative design team has resulted in something truly special: Dalston Works sets a seminal precedent for the continuous innovation of engineered timber construction.”

Using offsite construction shortened the construction programme allowing the frame to be completed in 374 days; while 80% fewer site deliveries reduced disruption, lowering the impact of the development on residents and the environment.

“Dalston Works is a real landmark project, and a testament to the versatility of CLT,” said Gavin White structural engineering director at Ramboll. “It showcases what can be achieved when a forward-thinking client enables you to demonstrate the benefits of offsite construction methods.

“It has been exciting to work with a team who are as passionate about the benefits of offsite construction as we are, and we look forward to seeing many more such schemes emerge across the UK. The height and size of Dalston Works, and its excellent performance in terms of sustainability and efficiency, demonstrate what can be achieved with this dynamic material.”

The building was subject to weight restrictions due to an exclusion zone for a planned Crossrail route underneath the site. Using timber for the external walls, party walls, floors, ceilings, stairs and lift cores, meant that Dalston Works weighs a fifth of a comparable concrete structure. It also allowed the construction of a further 15 units without breaching the restrictions.

CASE STUDY: THE CUBE

  • Client/contractor: Regal Homes
  • Structural engineer: Pringuer James Consulting Engineers
  • Architect: Hawkins/Brown Architects
  • Timber engineer: Engenuiti
  • Timber supplier: B&K Structures

The Cube is a 10-storey mixed use development in Hackney comprising 50 residential flats built above three commercial/retail units.

The original brief for the development was to maximise the value of the site and gain planning permission by creating something a “little bit different”.

The uniquely shaped building is a hybrid construction with the basement and ground floor built in concrete and a first-floor concrete deck. The main superstructure is formed of a central reinforced concrete core with a hybrid CLT superstructure wrapped around. Steel elements were introduced to help overcome large CLT spans and provide the most efficient structural solution.

“The residential flats are arranged in a cruciform plan surrounding a highly efficient central core. This plan geometry creates four courtyards with direct visual connections to the city and dual or even triple aspect apartments,” explained Hawkins\Brown architect Andrew Tindale.

“Flats benefit from dual or triple aspects, which as a result provides excellent natural daylight and ventilation into all habitable rooms. By twisting the form from floor to floor we have been able to create generous terraces with oblique views along Wenlock Basin and across Shepherdess Walk Park. The unusual form also helps reduce residents’ awareness of their neighbours and gives the building a more dynamic appearance from the street.

The “Cross” is protected by a dark brick grid, providing a sense of defensible space to residents inside and giving much-needed definition to Wenlock Road.

“The design of the CLT hybrid frame further allows future flexibility within apartment layouts by limiting internal structural walls and columns which tend to ‘fix’ or hamper alternative layouts.

CASE STUDY: ALFRISTON SWIMMING POOL

  • Client: Alfriston School
  • Architect: Duggan Morris Architects
  • Contractor: Feltham Construction
  • Structural engineer: Elliot Wood Partnership
  • Environmental engineer: Skelly and Couch
  • Cost consultants & CDM coordinator: Appleyard and Trew
  • Approved building control inspector: Butler & Young
  • Timber prefabricated structure: Cowley Timberwork
  • Timber cladding: Pratley carpentry

Photo © Jack Hobhouse

Duggan Morris Architects was commissioned in January 2008 to replace a redundant swimming pool structure at Alfriston School with an up-to-date facility.

The pool, which faces directly onto greenbelt land, was designed to be a contemporary expression of the local pitched roof vernacular.

The raised timber structure, articulated by three repeated fold roof segments, creates a dramatic internal landscape and serves as an acoustic baffle, preventing a high reverberation of sound – a crucial aspect of the brief.

A 1m narrow glass strip around the edge of the building, separates the roof from the rest of the base, protecting the privacy of pool users while preserving the panoramic view.

The prefabricated roof was manufactured offsite by Cowley Timber in 12 triangulated truss modules, which were delivered with a weather tight membrane ready for roof dressing and the interior face is factory finished.

The entire assembly took place over four weeks with no need for temporary internal or external scaffolding.

A standing seam roof system with integrated cladding clips supports the roof timber allowing the entire roof to be clad in the same material, with precise spacing and alignment.

The challenge was to provide a clear spanning structural solution that allowed maximum offsite pre-fabrication without the aesthetic compromise of appearing that it had been simply bolted together. The solution was to pre-fabricate 12 roof, 12 wall and two gable stressed skin panels constructed with primary glulam framing members and secondary glulam infill members working compositely with a cross-laminated timber skin as an efficient engineered solution that could be achieved under factory controlled conditions.

A major ambition was to conceal all traces of the galvanised metalwork and bolting which was not only necessary to connect the offsite assembled primary glulam frame of the individual panels but also to connect the interfacing panels as they were assembled to each other on site. The sequence of build was an important consideration as was limited tolerance. Trial assemblies were a luxury that space could not afford and so the architects were reliant on the accuracy of their 3D modelling and the CNC 5 axis router machine.

Reducing the amount of time required to attend to the fully installed panels was another important consideration and so close liaison with M & E service engineers to provide concealed but accessible service routes was vital as was pre-finishing the timber surfaces with a three-part coating system before the panels arrived on site to provide both temporary and permanent protection. Mock up sections were manufactured to assist in the process. The result is an efficient, elegant and functional example of an off-site prefabricated, highly engineered stressed skin panel solution to create an open space.

MAIN PICTURE: The Cube, a 10-storey development in Hackney

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