2020 Vision – Where CLT is going

Andrew Lawrence, ‘Global timber specialist’ at mighty Arup engineers, examines the increasing popularity of CLT and ventures his line on what the future holds for the material, highlighting part of his last few years work focus, the arrival of hardwood CLT.

Just over twenty years since it was invented and only fifteen years since its introduction into the UK, CLT is becoming a mainstream construction material for housing, schools and even offices.

At the same time, new products are emerging such as Tulipwood CLT that combine the speed of construction and fire resistance of CLT with the strength and appearance of hardwood, helping to open up new uses for the material as combined structure and finishes in high end applications.

Why has CLT become so popular?

CLT was developed in the mid-1990’s as a use for waste “waney-edged” sideboards.

By gluing the boards together, in a cross-laminated fashion, it was possible to make very large panels that could be used for building. CLT made from square edged boards was originally thought too expensive for the mainstream market. Then advances in CNC fabrication enabled the CLT to be accurately cut and therefore quickly assembled on site, like a giant piece of flat-packed furniture. For instance Alison Brooks Smile Pavilion took just seven days to go up. At the same time high strength self-tapping screws emerged, enabling fast strong on-site connections. CLT now offered an incredibly easy way to make the walls and floors of entire buildings. The resultant programme and site set-up savings offset the higher material costs of the CLT. CLT was now able to match the overall cost of a steel or concrete building, but with the added advantage of a faster income stream thanks to the earlier completion date. At the same time there was increasing demand for offsite construction; as a relatively lightweight material, machined to high tolerances, CLT was the ideal solution. Waugh Thistleton’s Murray Grove Stadthaus in 2008 was a world first, with eight storeys of CLT - see this Unstructured Extra’s Hackney feature here.

First off – the first UK CLT building -
dRMM's Kingsdale Sports hall and and music rooms (Photos drMM)
Inner Smile, the hardwood CLT Pavilion designed by Alison Brooks

Building with CLT walls rather than steel or concrete columns makes cellular spaces that are best suited to residential applications. To create more open plan spaces for offices, CLT slabs can be combined with a timber or steel frame. Compared to pouring wet concrete onto a metal deck, CLT slabs offer significant programme savings. This must be balanced against the need for deeper beams to reduce vibration of the lighter weight timber floor; deeper beams are also needed because it is hard to make the timber and steel work compositely together in the way that a concrete slab and steel beams work together to make an overall deeper and stiffer element.

Sky structures – Arup’s CLT slab combined with a steel frame at
the Sky Sports Centre -  Photo Arups

Can CLT be left exposed in the finished building?

So far, the majority of CLT has been hidden behind plasterboard to limit sound transmission between apartments. However, as CLT finds more uses in non-residential applications, with lower acoustic demands, it is increasingly being left visually exposed. Even in residential applications, as the visible use of wood starts to become a selling point, architects now want to expose the CLT ceilings and so engineers are developing ways to hide all the acoustic treatment on top of the CLT slab

CLT experimental slab render - Image Arup
Kertowood “stressed skin” floor panels

As more of the CLT becomes exposed, the increased fire load needs to be considered. However, this is more of an issue for high-risk structures such as taller buildings with a ‘stay put’ fire policy.

Are there alternatives to CLT?

Where the CLT is hidden behind plasterboard, is it really the best choice? As timber becomes more popular and prices increase, it needs to be used as economically as possible. One way to do this is by the use of hollow “stressed skin” floor panels instead of solid CLT panels. Apart from using less material, a stressed skin panel can be made from laminated veneer lumber. LVL is fabricated from peeled veneers enabling 80-90% of the log to be utilised. By comparison, sawing and planing the planks for CLT utilises only about 40-50% of the log. LVL is also stronger than CLT because the veneers are so thin that the knots in any one layer have little effect on the overall strength. Thus, there is a real possibility that in the future, hidden floor and wall construction will be increasingly made from laminated veneer products and that floors will be increasingly made from stressed skin panels, especially for longer spans. While such hollow stressed skin floor panels are lighter and therefore more prone to vibration, this can easily be overcome by pumping sand into the voids to add weight.

Laminated Veneer Lumber
Endless endless - dRMM's Endless Stair, outside London's Tate Modern, the first
hardwood CLT project – Photo Toothpick Nation

Why is there a sudden interest in hardwood CLT?

Over the next 10 years, as the demand for timber increases and approaches the sustainable annual cut of the traditional softwood species, it makes sense to look for other species that are suitable for structural use. A third of the temperate forests in North America and Europe are actually hardwood, including some of the most beautiful and strongest timbers.

The concept of hardwood CLT was pioneered by Arup and dRMM in the Endless Stair, built for the 2013 London Design Festival. The team was looking for a low-density hardwood species. Low density was important not only for ease of machining and transport but also to ensure that the self-tapping screws, really intended for use with low-density softwood, could still be used without slow and expensive predrilling. Tulipwood appeared to be the ideal choice. It is easy to glue and machine; it is close to the density of softwood but with the strength of a hardwood; it is also relatively cheap and plentiful.

Like any new product, it takes time to create a supply chain and the relevant product approvals, but the first two projects have now been built using large Tulipwood CLT panels and more are sure to follow. With increasing amounts of CLT now being left exposed, hardwood CLT really comes into its own, offering an enhanced appearance as combined structure and finishes. The ability to leave off the finishes also offers additional programme savings.

Team V's Haut tower in Amsterdam, Holland, which Arup are engineers on

What is the most efficient way of building in timber and CLT?

As well as using all the forest species, and making more efficient products such as stressed skin LVL panels, it is also important to develop more efficient ways of building in timber. For the Haut project in Amsterdam, Arup and Team V Architectuur are seeking to build the tallest timber building in the world. The team also want to free up the facades and to make adaptable internal spaces. The solution is to use long thick CLT panels for the loadbearing party walls between the apartments; these will carry the vertical weight of the floors (which will span 6m between the party walls) and will stop the building twisting under an eccentric wind load (although the main lateral resistance is provided by a concrete core). Internal walls will be cheap lightweight timber stud partitions. Unlike the early CLT buildings where all the walls were loadbearing, these partitions will be non-structural, reducing cost and also enabling them to be adapted in the future. This should help to prolong the life of the building, which must be one of the primary goals for truly sustainable construction.

How tall can you build in CLT?

It should be remembered that CLT construction is still in its infancy, dominated by proprietary products much like the early concrete industry 100 years ago. The industry is currently undergoing a period of standardisation, which will help to make costs lower and more predictable. This in turn will help to grow the market. At the same time, research is ongoing into the performance of whole timber buildings (rather than just elements), to ensure reliable performance under extreme events such as earthquake, fire, explosion and strong winds.

Haut too

Research is also ongoing into the maximum heights achievable in timber. As work for the Haut project has shown, the height of fully timber buildings is limited to about 10 storeys or so because a timber “shear” wall, made from many small pieces of CLT is simply not stiff enough to limit the sway or vibration of a taller building. But there is no reason why taller timber structures cannot have a concrete core as being used on Haut and many other taller timber buildings such as ActonOstry Architects Brock Commons , student dorm building currently the world’s tallest timber tower at the University of British Columbia, Vancouver, Canada – for an overview of timber towers see this edition of Unstructured Further section.

What does the future hold for timber and CLT?

The future is very likely to see increased use of LVL in hidden areas with CLT being increasingly reserved for visually exposed applications or shorter spans. There will be increased use of different species, including hardwoods and increased use of timber slab systems on steel frames. There will also be increased standardisation of components and connection systems, which in turn will help create lower and more predictable costs. All of this will help grow the market and help engineered timber become a standard construction material alongside steel and concrete. All the materials have their advantages and there is no doubt that the real advantage of engineered timber is speed, being easy to machine to high tolerances and also light and therefore easy to transport. While entirely timber buildings will be limited to about 10 storeys, timber will also be used for the floors of taller structures, combined with concrete cores and steel frames. Composite floor systems are also being developed which combine the advantages of timber and concrete in a single composite element. It is indeed an exciting time for timber!

Andrew Lawrence is head of timber engineering at Arup Engineers, or, as he is currently titled, is their Global Timber Specialist. He has been collaborating on CLT projects since the late 2000’s, including on the Hardwood CLT experiments outlined in this feature.

© Andrew Lawrence

WaughThistleton's Vitsoe factory in Leamington Spa, which was designed to optimise CLT and other engineered timber, such as Baubuche hardwood LVL – Photo WaughThistleton